physics-revise2.pdf

Measurement

Measurement of length Length can be measured by,

 A metre stick (straight lines)  An opisometer (small curved lines)  A trundle wheel (large curved lines)  A vernier callipers (diameters and small

widths)

Length is measured in mm, cm, m, km. Experiment: To measure the length of a curved line

 Roll the wheel of an opisometer back to the pointer.

 Place the pointer at the start of the line.

 Roll it carefully along the line, to the end.

 Now place the pointer on the zero of a metre stick and roll it backwards until the wheel stops at the pointer.  The reading on the metre stick is the

length of the line.

Measurement of area;Area is how much ground something covers. Area is measured in area mm2_{, cm}2_{, m}2_{, km}2_.

Experiment: To find the area of your hand  Place your hand, fingers together,

on squared paper.

 Draw its outline on the page.  Count all of the squares which are

completely inside or more than ½ inside the outline of your hand.  Discount any squares which are less

than ½ inside the outline of your hand.

 Multiply this number by the area of one square. This is the area of your hand.

Measurement of volume;Volume is the amount of space occupied by an object. Volume is measured in mm3_{, cm}3_{, m}3_.

Experiment: To measure the volume of a regular rectangular block

 Measure the length, width and height of the block.

 Multiply the measurements together. Experiment: To find the volume of an irregular object (a stone)

Method 1

 Add water to a graduated cylinder as shown.

 Gently slide in the stone.  The water level rises by 25 cm3_.

 The volume of the stone is 25 cm3_.

Method 2

 Fill the overflow can to the point of overflowing.

 Lower the stone gently into the water (use a thread).



The stone will displace its own

volume of water.

 The water which collects in the graduated cylinder is the volume of the stone.

Density

 The mass of an object is the amount of matter in it.

 The density of an object is the mass of 1cm3 _{of it. The unit of density is}

g/cm3 _{(grams per cm}3_).

) volume(cm

mass(g)

Density ₃

Mandatory experiment: To find the density of a regular rectangular block

 Find the mass of the block with an electronic balance.

 Find the volume by multiplying l x b x h.

) volume(cm

mass(g)

Density ₃

Mandatory experiment: To density of an irregular shaped object, such as a stone.

 Find the mass of the stone with an electronic balance.

 Find the volume with an overflow can or graduated cylinder (see chapter 37).

) volume(cm

mass(g)

Density ₃

Mandatory experiment: To find the density of a liquid (water)

 Find the volume of the water by reading the side of the graduated cylinder.

 To find the mass of the water make two measurements, (i) Get the mass of the graduated cylinder. (ii) Get the mass of the graduated cylinder and the water.

) volume(cm

mass(g) Density

 ₃

Flotation - an object will float in water if its density is less than that of water (1 g/cm3_{) and} will sink if its density is greater than the density of water.

Over flow can

Graduated

cylinder

Before

After

65 cm3 40

cm3

Stone

10.5g

Block

10.5g

peed, Velocity and Acceleration

1. Speed is the distance travelled by an object per unit time. This can be written

mathematically as,

taken Time

travelled Distance

Speed

Example: If a man walks 4 km in 1 hour, what is his average speed, in m/s?

Answer:

_1.11m/s

3600s 4000m

Speed 

To use the speed formula, it is sometimes useful to consider the triangle below. If you want to ‘find the distance travelled’, cover the ‘D’ and the answer is S xT. Speed, Velocity and Acceleration

 . Speed is the distance travelled by an object per unit time. This  If you want to ‘find the time taken’

for a journey, cover ‘T’ and the answer is D/T.

2

.

3. Acceleration is the change in velocity per second. This can be written mathematically as,

taken Time

velocity in Change on

Accelerati 

Example: The velocity of a car changes from 30 km/hr to 60 km/hr in 5 seconds, what is its acceleration?

Answer:

6km/hr/s 5

30 5

30 60 on Accelerati 

  

Pressure

The pressure exerted by an object is the force (in Newtons) which it exerts on 1m2_.

Mathematically, this maybe written as:

(Pa)] Pascal [unit Area Force

Pressure 

The connection between pressure and force

If you press down on the flat end of a thumb-tack with your finger, applying a certain force, you will experience no discomfort. If, however, you press, on the same thumb-tack, at the pointed end, with the same force as before you will feel considerable pain.

Why does the same force, applied to the same object, have such different results?

The force, in the first case, is spread over a larger area than in the second case and therefore is not as keenly felt.

Pressure exerted by a liquid

 Liquids exert pressure.

 This pressure increases with depth.  The pressure in a liquid is the same

in all directions. Atmospheric pressure

Above us we have 15 km of air pressing down on the surface of the earth. This air, like a liquid, exerts a pressure which is called atmospheric pressure.

To demonstrate atmospheric pressure – Method 1

Atmospheric pressure Glass of water

Cardboard

 Fill a glass to the rim with water.  Cover it with a cardboard.

 Hold the cardboard in place and turn the glass upside down.

 The water remains in the glass and the cardboard stays in place.

 Explanation; the pressure of the atmosphere, acting upwards, holds the water in the glass by pressing the cardboard upwards. This demonstrates that atmospheric pressure acts upwards as well as downwards.

Method 2 – The crushed can experiment

 Heat a small volume of water in a tin can until it starts to boil.

 As soon as steam is seen leaving the can, remove the heat and seal the can with a stopper.

 Allow the can to sit on a bench to cool.  As it cools, the steam in the can

condenses to water and a vacuum is created.

 The creation of this vacuum means the can will be crushed by the atmospheric pressure.

Measuring atmospheric pressure

The pressure of the atmosphere is measured with a barometer. There are two main types of barometer, a mercury barometer and an aneroid barometer.

What can we tell from atmospheric pressure? 1. Predict the weather – high atmospheric

pressure means fine, calm, sunny weather, with no winds. Low pressure means unsettled, windy, wet weather. Lines joining points on a map with similar atmospheric pressure are called isobars. If the isobars are close together the winds will be strong.

2. Measure altitude (height above sea level) – The higher you go above sea level the lower is the atmospheric pressure. An altimeter is a barometer which is adapted for making measurements of height-above-ground.

S

T

D

x

Force, Work and Power

A force is anything which changes the velocity or shape of an object. Forces are measured in Newtons (N). There are different kinds of forces – push, pull, friction, electric, magnetic.

Forces always occur in pairs.

Example 1: When a balloon is released, the air shoots out the back with a certain force and the balloon travels in the opposite direction with an equal but opposite force.

Example 2: When a gun is discharged, the bullet flies in one direction and the gun moves in the opposite direction with an equal force.

Friction is the force which opposes the motion between two objects in contact.

Examples of friction:

1. If you rub two pieces of sand paper together, there will be a very large force of friction between them. 2. The friction between rubber soles

on your shoes and the ground gives you grip and stops you slipping.

Preventing friction

To prevent or reduce friction we put lubricant between the two surfaces in contact. Grease and oil are common lubricants.

Advantages of friction

 The force of fiction between your shoes and the ground, prevent you from slipping.

 Friction helps tyres to grip the road.

 Friction generates heat when you rub your hands together.

Disadvantages of friction

 Shoes wear out.  Tyres wear out.

 Machine parts wear out.  Friction burns from a rope.

Experiment: To investigate the force of friction

 Set up the apparatus as shown.  Fix sand paper to the base of the

block and onto the bench surface.  Pull the block along the bench

surface with the spring balance.  Read the force applied to the block

from the side of the Newton-meter.  Repeat the experiment but use no

sand paper.

 Repeat the experiment with oil between the block and the bench.  You should find that the force

needed to move the block is greatest for the sand paper because of the large force of friction and smallest for the oil.

Work is done when a force moves an object. Work is measured in joules (J).

Work = Force (N) x Distance (m)

Example 1:

If a man uses a force of 300 N to move a wheel barrow a distance of 100 m, what work has the man done?

Answer;

Work = 300 N x 100 m = 30,000 J

Power is the rate at which work is done. It is measured in J/s (joules per second), or Watts.

taken(s) Time

done(J) Work

Power

Example; A man lifts a 200 N object from the floor to a table which is 750 cm above the ground, in 0.5 seconds. What is his power?

Answer:

Work done = (200)(0.75) = 150 J

Power = 150J/0.5s = 300 W

Mandatory experiment: To investigate Hooke’s law of spiral springs

 Set up the apparatus as shown.

 Measure the length of the spring and pan before any weights are added.

 Now add a weight to the pan.

 Measure the extension of the spring with the metre stick.

 Repeat the procedure by adding more weights and recording the extension each time.

 Record your results in a table, as shown.

Weight (N) Extensi on (cm)

 Now draw a graph of Extension versus weight placed on the spring.

 The graph should look like this,

 The graph is a straight line through (0,0). This means that the extension is directly proportional to the force applied to it.

 Note; When recording the force on the spring, you multiply the mass by 9.8 e.g. 200g (0.2kg) should be recorded as 0.2 x 9.8 = 1.960 N.

Pull

Newton-meter

Block

Bench top

Weights

Pan Spring

Metre stick

0

0

Weight on the spring (N)

A lever is a rigid body which is free to rotate about a fixed point called the fulcrum.

Everyday examples of levers

The Centre of Gravity (cog) is the point through which all of the weight of a body appears to act. This is usually its balancing point.

Experiment: To find the centre of gravity of an irregular piece of cardboard

 Hang the cardboard from a pin on a stand, so that it can swing freely.

 Attach a string with a weight as shown on the diagram below.  Draw a line behind the string to

mark its position.

 Move the cardboard to a new position and repeat the procedure.  The centre of gravity is where the

two lines cross.

Equilibrium and the stability of objects When an object is balanced and not moving it is said to be in equilibrium. There are three states of equilibrium,

Stable equilibrium – An object is in stable equilibrium if moving it raises its centre of gravity.

Stable equilbrium

Unstable equilibrium

Neutral equilibrium

Unstable equilibrium – An object is in unstable equilibrium if moving it lowers its centre of gravity.

Neutral equilibrium – An object is in neutral equilibrium if moving it has no effect on the centre of gravity.

Objects in stable equilibrium will have a wide base and a low centre of gravity. When designing objects this fact must be kept in mind.

The law of the lever - When a lever is balanced the sum of the clockwise moments* is equal to the sum of the anti-clockwise moments.

*The moment of a force = (Force) x (perpendicular distance from the force to the fulcrum).

Example 1: Is the metre stick below balanced?

Answer:

Left-hand side (anticlockwise) Right-hand side

moments 12 moments 12 0.3m 40N 0.4m 30N Distance Force Moment Distance Force Moments          

Since the moments are equal on both sides, the lever is balanced.

Example 2: Where on the metre stick must the weight, 20 N, be placed if it is balanced?

Answer:

Left-hand side Right-hand side

cm m 30 3 . 0            206 x 20x moments 6 (20N)(x) ) (15N)(0.4m Distance Force Moment Distance Force Moments

Load

Wheel barrow Weight

Light is a form of energy. We can say this because it can be made to do work.

 Solar cells produce electricity from sunlight.  Plants make food energy from sunlight in

photosynthesis. 

Luminous objects are those which give out their own light, e.g. the sun gives out its own light.

Non-luminous objects do not give out their own light but only reflect light, e.g. the moon reflects light from the sun.

Mandatory experiment; To show that light travels in straight lines.

3 Cardboards with holes screen

Line up three pieces of cardboard so that the holes in the middle of the cardboards are in a straight line.

Turn on the ray box.

As long as the holes in the cardboards are in a straight line light will shine on the screen.

This shows that light travels in a straight line.

Experiment : To show how shadows are formed.

 Set up the apparatus as shown.  The ray box emits a beam of light.

 Part of the beam hits the object and is stopped.  Part of the beam hits the screen.

 Where the beam is stopped, a shadow is left on the screen, the shape of the object.

Important shadows

 When the moon comes between the earth and the sun, a shadow of the moon falls on the earth. This is called a solar eclipse (because the sun’s light is blocked out).



Dispersion is the name given to the splitting of white light into its seven colours.

Example1: When sunlight passes through a shower of rain the seven colours separate out from each other to give a rainbow effect.

Experiment: To produce a spectrum of white light.

 Set up the apparatus as shown.  A beam of light hits the glass prism.

 As it passes through the glass the colours in the light are dispersed (scattered).

 The screen should show a rainbow effect.  The colours of the white light are red, orange,

yellow, green, blue, indigo and violet (Richard of York gave battle in vain).

Refraction is the bending of a light beam, from its original pathway, as it passes from one medium into another (from air to water or from water to air).

Experiment To show refraction of light.

 Set up the apparatus as shown.

 The ray box emits a beam of light.

 When the beam hits the glass block, it passes through, but it changes direction, and exits the block at a different angle.

 This change in direction is due to the refraction of the light beam.

Importance of refraction

In lenses, mirages, rainbow effect, extra daylight each day.

Mandatory experiment: To show that light can be reflected.

 Set up the apparatus as shown.

 The ray box emits a beam of light.

 Place a small mirror in front of the beam.

 The direction of the beam will change.

 The light beam has been reflected.

Important uses of reflection of light

1. Periscopes are used to see over tall objects. 2. Reflective mirrors in cars.

3. Shaving and make-up mirrors. 4. Microscopes.

5. Security mirrors.

Object

Ray box

Screen

Light beam

Prism of glass

Screen Ray

box

Refracted beam

Glass block Light

beam Ray

box

Mirror

Light beam

Sound is a form of energy How do we hear sounds?

 When a sound is made (hammer hitting a nail) the air molecules start to vibrate.

 These vibrations are passed from molecule to molecule until they reach your outer ear.  The outer ear acts as a funnel and directs these

vibrations to the eardrum.

 Once the eardrum starts vibrating, a signal is sent to the brain and the sound is registered.

Experiment: To show that sound is a form of energy.

 Hold the foam ball near the speaker of a stereo system when it is playing loud music.

 The ball should move because of the sound vibrations.

 If sound can move objects then it is able to do work.

 This makes it a form of energy.

How fast does it travel? Sound travels at a speed of 340 metres per second (in air) 1400 m/s in water

5,000 m/s in concrete.

Speed of light = 300,000,000 m/s.

Question: If an observer hears the sound of thunder three seconds after he sees the flash of lightening, how far is the lightening from the observer?

Answer:

away.

km

1.02

is

trhunder

The

1020m

3

340

Distance

Time

Speed

Distance

Time

Distance

Speed



















An echo is a reflected sound.

Experiment: To show that sound needs a to travel through a medium

 Start the alarm bell ringing on the clock.  Turn on the suction pump.

 The air is sucked out of the bell jar.  A vacuum has been created.

 The alarm bell can still be seen to be working but no sound will be heard.

Ultrasound waves (sound of very high frequency) are used in various instruments to locate objects or places. A machine sends out a burst of ultrasound waves and times how long it takes for the sound to bounce off an object and return.

 In medicine, ultrasound machines are used to ‘see’ inside the body and look at organs or even a baby in the womb.  Fishing vessels use ultrasound to locate the seabed and

shoals of fish.

 Doctors use ultrasound waves to smash kidney stones so that they can be passed without the need for surgery.

Loudness of sound

The decibel (dB ) is the unit used to compare the loudness of sounds. Jet plane (120dB), lawnmower(80dB), talk (50dB). Speakers

Foam ball on a light string

To vacuum Alarm

clock

Magnetism

A magnet is a metal which attracts other pieces of metal.

 Only three metals can be made into magnets, or will be attracted by magnets. These metals are nickel (Ni), iron (Fe), copper (cu). In fact, most magnets are mixtures of these metals.

 The first magnets were magnetic rock called lodestone, used as far back as 500 b.c.

 Magnets have two poles, a north and a south pole.

 Like poles repel,Unlike poles attract.  Magentic field – is an area around a

magnet where a magnet exerts an influence.

Experiment: To show the magnetic field of a bar magnet (method 1)

 Place a bar magnet on a bench.  Cover it with a sheet of paper.  Sprinkle iron filings over the sheet.  The filings will line up along the

magnetic field lines.

 The magnetic field of the magnet has become ‘visible’ (see diagram). Experiment: To show the magnetic field of a bar magnet (method 2)

 Place a magnet on a sheet of paper.  Place a number of plotting compasses

near the magnet so that the pointers follow each other as shown below.  Mark the positions of the dots with

a pen.

 Mark the positions of the dots with a pen.

 Remove the plotting compasses and join the dots to form one of the field lines.

 Repeat this procedure several times to construct more field lines. Experiment: To investigate the behaviour of magnets.

 Suspend a magnet from a piece of light thread, so that it can swing freely.

 Bring another magnet close to this.  If two north poles are brought near

each other, the magnets repel.  Same result, with two south poles.  If a south pole is brought a near a

north pole, the magnets will attract. Uses of magnets

 In speakers.  Fridge doors.  Electric motors. Earth’s magnetic field

The earth behaves like a large bar magnet, with two poles, one at the north and one at the south. The south pole of this imaginary magnet is in the northern hemisphere and the north pole is in the southern hemisphere. This is why the north pole of all compasses point to the north.

Storing magnets

 Magnets are stored in pairs, with opposite poles together.

 Two pieces of iron are placed at either end, as keepers. These close the magnetic fields and so preserve the magnetism.

 A piece of cardboard is place between the magnets as a spacer.

Static electricity

Static electricity is electric charge which is stationary. This kind of charge usually builds up on plastic materials and fabrics. Since they are insulators they don’t allow electricity to flow through them.

 Static charge builds up on materials when they are rubbed together.

 Electrons are knocked off one material and onto the other.

 When a material loses electrons it becomes positive.

 When it gains electrons it becomes negatively charged.

 When polythene is rubbed with a cloth it becomes negatively charged.

 When Perspex is rubbed with a cloth it becomes positively charged.

Earthing

When a large electric charge builds up on an object, it must be allowed to flow into the earth, to make the object safe for us to touch.

The charge will discharge itself, often with violent consequences. The most dramatic example of this is lightening. The charge on the thunder clouds builds up to an intolerable level.

Eventually, it will discharge into another cloud (sheet lightening) or into the ground (fork lightening).

Many structures and buildings are provided with lightening conductors to avoid damage by lightening strikes.

Experiment: To show the presence of static electricity.

 Rub a plastic biro with a cloth. It can pick up small pieces of paper.

 Rub an inflated balloon against your clothes. It becomes charged and will stick to the paint on walls and to your clothes.

 Hold a charged biro near a thin stream of water from a tap. The water will move towards the biro because water has tiny charges.

Sheet of paper

Current electricity

An electric current is a flow of electric charge.

To understand the behaviour of electricity we will look at a simple circuit.

When the switch is closed, electricity flows from the cell and the bulb lights. When the switch is open, as in the diagram above, no current flows from the cell. There are number of quantities which you must know to talk about electrical circuits.

Voltage(symbol is V) is the pushing power of the

power supply. It is measured in volts (V).

Current (symbol is I) is the flow of electrical charge.

It is measured in amps (A).

Resistance (symbol is R) is the ability of a substance to

resist, or slow down, the flow of electricity through a circuit.

Ohms law states that at constant temperature the voltage (V) is always proportional to the current (I) in a circuit i.e. V = I.R

Electrical Power = V.I

To show the heating effect of electric current

 Close the switch and allow current to flow through the circuit for a few minutes.

 Hold a thermometer against the bulb.

 The increase in temperature registered on the thermometer shows that heat has been produced in the circuit.

To show the magnetic effect of an electric current

 Set up a circuit as above with the switch open.

 Bring a compass near any part of the circuit wiring.

 Nothing happens.

 Now close the switch.

 The needle of the compass will be seen to deflect.

 This happens because the all electrical circuits are magnetic when carrying current.

To show the chemical effect of an electric current

 Set up the apparatus as shown below.

 When electric current is flowing in the circuit the water molecules are broken up and form hydrogen and oxygen gases.

 Hydrogen collects at the negative electrode.

 Oxygen collects at the positive electrode and is only ½ the volume of the hydrogen gas.

Bulbs in series

When the bulbs are connected in series,

 The more bulbs which are connected the dimmer the light given out by each one. This is because the voltage of the battery has to be divided amongst a greater number of bulbs.

 If you disconnect one, all the lights will go out.

Bulbs in parallel

When bulbs are connected in parallel as shown below,

 All the bulbs will shine equally brightly and there will be no dimming effect if you add more bulbs. This because all bulbs have the same voltage across them.

 If you disconnect one bulb, the others remain lighting. For this reason, parallel circuits are used in lighting circuits in our homes.

Mandatory experiment: To distinguish between conductors and insulators

 Set up the apparatus as shown.

 Connect a variety of substances across the wires at ‘x’.

 If the bulb lights,the substance is a conductor, if not, the substance is an insulator.



-

+

Bulb

Switch

Cell (power supply)

-

+

Bulb

Switch

-

+

Bulb

Cell (power supply)

Mandatory experiment: To verify Ohm’s Law

 Set up the apparatus as shown in the diagram.

 Record the current flowing through the resistor by reading the ammeter.

 Record the voltage across the coil by reading the voltmeter.

 Adjust the variable resistor to give a new voltage across the resistor.

 Record this new voltage.

 Record the current reading from the ammeter.

 Repeat this procedure for several different values of voltage and current.

 Make a table of your values.

 Plot a graph of Voltage versus Current.

 A straight line through (0,0) proves that the voltage is proportional to the current.

 If you divide V by I you should get the same value for R each time.

Electricity in the home

There are two types of current electricity; Alternating current (a.c.)

 Changes its direction of flow, constantly.

 The electrical supply to your home, provided

by the ESB, is alternating.

 The supply is called a 50 Hz (50 hertz)

supply. This means that the direction of the current changes 50 times every second.

Direct current (d.c.)

 This type of current flows in the same

direction all the time.

 The power supplied by a battery is direct

current.

Advantages of alternating current over direct current

 The ESB can transport it over long

distances without losing power.

 It can be converted to d.c. easily when

needed for appliances in the home.

Electricity is supplied to your home by two cables;

 The live cable (brown colour) and

 The neutral (blue colour).

A third type of cable is found in household circuits;

 The earth wire (green and yellow)

which is connected to the ground via a galvanised rod outside the house. This is a safety cable attached to all major circuits and to appliances with metal bodies.

Safety measures in the home 1. Fuses

A fuse is a thin metal wire housed in a ceramic container. In the event of a fault developing and too large a current flowing, the fuse wire melts preventing any major damage or fire. When using a fuse a number of precautions must be observed,

 The fuse must be of the correct rating.

The fuse rating is the maximum current that the fuse can carry without melting.

 The fuse must be in the live side of the

circuit for safety.

2. Earthing

 An additional measure must be taken to

safeguard a user. All electrical appliances, which have exposed metal parts should be made safe to touch even if a fault develops inside them.

 This is achieved by earthing, i.e. by

providing a wire (green and yellow) which connects the metal parts to a metal plate or rod, sunk deeply in the soil.

 Some electrical appliances are

manufactured with an all-plastic outer casing and do not require an earth connection.

Wiring a plug

 The live (brown) is connected to the

fuse on the right- hand side.

 The neutral (blue) is connected to

the pin on the left-hand side.

 The earth (green and yellow) is

connected to the top pin. This is also the longest pin.

 The earth pin, being the longest,

opens the holes of the socket and only then can the other pins be inserted. This ensures that the earth, and safety pin, is always first to be connected.

The units of electrical power

Electrical power is given by the following equation:

seconds

in

time

joules

in

done

work

Power



The unit of power is the watt (W).

The ESB calculates the electrical power used by your home in kilowatt hours (kWh).

A kilowatt hour is the electrical energy used by a 1kW appliance which has been running for one hour.

Example;

(i) Calculate the number of units of electrical power used by a 3 kW electrical heater over 4 hours of use.

(ii) If one unit of electrical energy costs 15c, how much will the heater cost to run, over the four hours?

Answer;

(i) Number of units used = (3kW)(4h) = 12 kWh = 12 units (ii) Cost = (12)(15) = 180c = €1.80

Example;

Calculate the cost of running a 50W television set for 10 hours.

Answer;

Number of units used = (0.05 kW)(10) Voltag

e

Variable

resistor

Resistor Voltmeter Ammeter

A

V

Sand

Fuse

wire

Metal

caps

Mandatory experiment: To verify Ohm’s Law

 Set up the apparatus as shown in the diagram.

 Record the current flowing through the resistor by reading the ammeter.

 Record the voltage across the coil by reading the voltmeter.

 Adjust the variable resistor to give a new voltage across the resistor.

 Record this new voltage.

 Record the current reading from the ammeter.

 Repeat this procedure for several different values of voltage and current.

 Make a table of your values.

 Plot a graph of Voltage versus Current.

 A straight line through (0,0) proves that the voltage is proportional to the current.

 If you divide V by I you should get the same value for R each time.

Electricity in the home

There are two types of current electricity; Alternating current (a.c.)

 Changes its direction of flow, constantly.

 The electrical supply to your home, provided

by the ESB, is alternating.

 The supply is called a 50 Hz (50 hertz)

supply. This means that the direction of the current changes 50 times every second.

Direct current (d.c.)

 This type of current flows in the same

direction all the time.

 The power supplied by a battery is direct

current.

Advantages of alternating current over direct current

 The ESB can transport it over long

distances without losing power.

 It can be converted to d.c. easily when

needed for appliances in the home.

Electricity is supplied to your home by two cables;

 The live cable (brown colour) and

 The neutral (blue colour).

A third type of cable is found in household circuits;

 The earth wire (green and yellow)

which is connected to the ground via a galvanised rod outside the house. This is a safety cable attached to all major circuits and to appliances with metal bodies.

Safety measures in the home 1. Fuses

A fuse is a thin metal wire housed in a ceramic container. In the event of a fault developing and too large a current flowing, the fuse wire melts preventing any major damage or fire. When using a fuse a number of precautions must be observed,

 The fuse must be of the correct rating.

The fuse rating is the maximum current that the fuse can carry without melting.

 The fuse must be in the live side of the

circuit for safety.

3. Earthing

 An additional measure must be taken to

safeguard a user. All electrical appliances, which have exposed metal parts should be made safe to touch even if a fault develops inside them.

 This is achieved by earthing, i.e. by

providing a wire (green and yellow) which connects the metal parts to a metal plate or rod, sunk deeply in the soil.

 Some electrical appliances are

manufactured with an all-plastic outer casing and do not require an earth connection.

Wiring a plug

 The live (brown) is connected to the

fuse on the right- hand side.

 The neutral (blue) is connected to

the pin on the left-hand side.

 The earth (green and yellow) is

connected to the top pin. This is also the longest pin.

 The earth pin, being the longest,

opens the holes of the socket and only then can the other pins be inserted. This ensures that the earth, and safety pin, is always first to be connected.

The units of electrical power

Electrical power is given by the following equation:

seconds

in

time

joules

in

done

work

Power



The unit of power is the watt (W).

The ESB calculates the electrical power used by your home in kilowatt hours (kWh).

A kilowatt hour is the electrical energy used by a 1kW appliance which has been running for one hour.

Example;

(i) Calculate the number of units of electrical power used by a 3 kW electrical heater over 4 hours of use.

(ii) If one unit of electrical energy costs 15c, how much will the heater cost to run, over the four hours?

Answer;

(i) Number of units used = (3kW)(4h) = 12 kWh = 12 units (ii) Cost = (12)(15) = 180c = €1.80

Example;

Calculate the cost of running a 50W television set for 10 hours.

Answer;

Number of units used = (0.05 kW)(10) Voltag

e

Variable

resistor

Resistor Voltmeter Ammeter

A

V

Sand

Fuse

wire

Metal

caps

Experiment; To show the action of a fuse

Set up the apparatus as shown. Close the switch.

A current flows through the thin fuse wire. Gradually increase the current delivered by

the power source.

At some point, the fuse wire will get red hot and break.

This illustrates the operation of a fuse.

Electronic devices Diodes

A diode is a device which will allow current to flow in only one direction through it. A diode looks like this,

but in a circuit it is represented by the symbol shown below.

A diode can be connected in a circuit in two ways, forward bias or reverse bias.

Experiment: To show the action of a diode in (i) Forward bias

 Set up the circuit as shown.  Close the switch.

 The light bulb will light.  The diode is in forward bias and

since the current always flows from the + to the – terminal, current will flow through in the direction of the arrow.

(ii) Reverse bias

 Set up the circuit as shown.  Close the switch.

 The light bulb will not light.

 The diode is in reverse bias and allows current through only in the direction of the arrow.

 But the current from the battery is going in the opposite direction and cannot pass through the diode.

Uses of diodes

 To control the direction of current in electronic devices.

 To change alternating current to direct current.

Light Emitting Diodes (LED)

A LED is a diode which gives out light when current passes through it. A LED looks like this,

but in a circuit it is represented by the symbol shown below.

Experiment: To show the action of a LED

 Set up the circuit as shown.  Close the switch.

 The LED is in forward bias and since the current always flows from the + to the – terminal, current will flow through in the direction of the arrow.

 The LED will give out light.

 The 330Ω resistor is placed in the circuit to protect the LED against a large current.

Light Dependent Resistor (LDR)

A LDR is a resistor in which the resistance decreases when the light intensity increases. This means that more current is allowed to flow through the LDR in bright light conditions.

Experiment: To show the action of an LDR

 Set up the circuit as shown.  Shine light on the LDR.  The bulb will get brighter.

 An increase in current flowing in the circuit will also be seen on the ammeter.

 Remove the light on the LDR and the current reading on the ammeter will fall again.

 The brightness of the bulb will decrease.

Variable power source

12 V

Fuse wire Switch

+

+

+

330

ΩΩ

LDR

Energy

Energy is the ability to do work. There are

many different forms of energy. Energy is measured in joules (J).

Forms of energy

 Potential energy (P.E.) – This is the energy

which an object has because of its mechanical condition or position above ground e.g. a coiled spring or a hammer held above ground.

 Kinetic energy (K.E.) – This is the energy

which a moving object has.

 Heat energy – Heat is a form of energy

because it causes things to move e.g. hot air balloon.

 Light energy – Light is a form of energy

because it causes things to move and it does work e.g. solar cells produce electricity to work appliances.

 Sound energy – sound can cause things to

move e.g. feel the vibrations near a speaker of a stereo.

 Electrical energy – Electricity can cause

things to move or do work e.g. electrical motor.

 Chemical energy – This the energy stored in

chemicals petrol or food.

 Nuclear energy – This is the energy stored

in the nucleus of an atom.

The law of conservation of energy states that energy cannot be created or destroyed but changed from one form into another.

Examples of energy conversion

Light bulb – electrical energy is converted

into light and heat energy.

Radio – converts electrical to sound energy.

Energy loss in the home

Energy is lost from your home in different ways. The main areas where heat is lost are, floors, walls, roof, windows, draughts.

Methods of preventing heat loss

 Glass fibre on the attic floor.

 Lagging the hot water tank.

 Air cavity in walls.

 Draught excluders on doors and windows.

 Double glazing on windows.

Energy supplies

Non-renewable sources – once used they gone

forever e.g. coal, oil, gas, turf. These are referred to as fossil fuels. They are costly to extract and transport, cause pollution, and there is only 300 years (at present usage) of known reserves left.

Renewable resources – are constantly being

replaced by nature. The main sources of renewable energy are,

 Solar - Solar panels turn sunlight

into electricity.

 Hydro-electric energy – Dams hold

back water and stored potential energy is released as kinetic energy to turn the turbine and produce electricity.

 Wind energy – Wind mills are used to

produce electricity.

 Wave energy – The movement of

large floats are used to produce electricity.

 Geothermal – The temperature of the

earth’s crust is used to heat water to steam and produce electricity.

 Biomass – Some quick-growing plants

are used to produce alcohol and methane gas.

Solar energy

 The sun is our primary source of energy.

 Plants absorb light for making food (photosynthesis).

 Animals eat plants and obtain food energy from digesting the food made in photosynthesis.

 The fossil fuels which run our cars, trucks, trains, planes, factories, homes etc. are formed from a build up of hundreds of millions of years of decaying plants and animals. The energy released when burn these fuels is a result of photosynthesis that happened millions of years ago.

 All of our heat energy, electrical energy, kinetic energy, food energy comes, either directly or indirectly, from the sun.

The warmth of the sun drives the winds on which wind generators depend

Nuclear energy

 Nuclear energy is the energy stored in the nucleus of an atom.

 When the nucleus of an atom disintegrates, a vast amount of energy is released.

 An uncontrolled release results in a devastating nuclear explosion.

 But controlled nuclear breakdown in a reactor, results in huge energy release which can be harnessed and used to produce electricity.

Advantages of nuclear energy

 In medicine, to kill cancer cells.

 Sterilise food (kills bacteria).

 Produce electricity.

 Will not run out.

Disadvantages of nuclear energy

 Waste produced by the nuclear industry is very dangerous.

 There is always a danger of explosion.

Experiment: To compare the insulating ability of different materials

 Place two beakers of boiling water on a bench. One is insulated the other is not.

 Take the temperature in each after 10 minutes.

 The water in the insulated beaker should be much warmer.

 This because the insulation holds in the heat.

 Try this experiment again but with a different insulating material and see which is best.

Experiment: To convert mechanical energy to heat energy and sound energy.

 Drill into a piece of timber with an electric drill (mechanical energy).

 After 1 minute, remove the drill and touch a thermometer off the tip of the drill.

 The temperature shoots up, showing that heat energy has been produced.

 The noise made by the drill shows that sound energy is also produced.

Experiment: To convert chemical energy to heat energy

 Light a candle.

 Hold a thermometer near the flame.

 Record the temperature change.

 The heat produced is due to the chemical energy in the wax being converted to heat.

Beaker of water

Thermomete rs

Mandatory experiment: To convert chemical energy to electrical energy to heat energy

 Close the switch and allow current to flow through the circuit for a few minutes.

 Hold a thermometer against the bulb.

 The increase in temperature

registered on the thermometer shows that heat has been produced in the circuit.

 The energy conversions which have occurred are,

 Chemical energy (in the battery) to

electrical energy (in the circuit).

 Electrical energy (in the circuit) to

heat energy (in the bulb).

Mandatory experiment: To convert electrical energy to magnetic energy to kinetic energy

 Close the switch and allow current to flow through the circuit.

 The electric current in the circuit is converted to magnetic energy, as the wires in the circuit are now magnetic.

 The magnetic energy of the wires is converted to kinetic energy when the compass needle moves.

Mandatory experiment: To convert light energy to electrical energy to kinetic energy

 The light hits the solar cell and is converted to electrical energy.

 The electrical energy is then converted to kinetic when the propeller turns.

-

+

Bulb Switch

Cell (power supply

)

Compass

-

+

Switch

Cell (power supply)

Light

Heat

Heat is a form of energy. The unit of heat energy is the joule (J).

There are three methods of moving heat from place to place.

1. Conduction – this is the transfer of heat from one place to another, through a solid, without the

particles, of the solid, moving out of position.

2. Convection – this is the movement of heat, through a liquid or gas, by the upward movement of heated

particles.

3. Radiation – this is the movement of heat, by invisible rays, from a hot object without the need for a medium to pass through.

An insulator – is a substance which will not allow heat to pass through it easily.

Examples of conduction:

Metal pots

Cooking pots are made of metal because they are good conductors and will allow food, placed in them, to heat-up.

A poker

A poker can get extremely hot if the end is left in the fire. The heat will travel out by conduction to the handle.

Examples of convection: Electric kettle

The element is placed at the bottom of a kettle. As the water is heated, the heated particles rise by convection and cooler ones take their place. In this way, all of the water will be heated.

Electric immersion heater

This works in a similar way to a kettle.

Convection heaters

These are usually called radiators, but this is not a good name for them, as they heat a room, mostly, by convection.

Examples of radiation:

Solar energy

Energy from the sun travels through space to the earth by radiation. There is no medium just a vacuum.

All hot objects

All hot objects radiate heat, in all directions. If you put your hand over a lighted candle you will feel great. This is heat by convection.

If you put your hands around a candle you will also feel heat. This is radiated heat and it is not as much as the convected heat.

Examples of insulators:

 Fibre glass wool – used for attic insulation.

 Polystyrene – used for burger boxes, pizza boxes (keeping food hot).  Polystyrene board – used for wall

insulation to prevent heat being lost from the walls of a house.

 Wool clothing – wool is a good insulator and prevents us from losing body heat.

The effects of heating

When solids, liquids and gases are heated they expand. The only exception to this rule is water, between 40 _{and 0}0_{C. W}

Mandatory experiment: To compare the conductivity of various metals

 Set up the apparatus as shown.  A thumbtack is attached with wax

to each of four metal strips with wax.

 A Bunsen flame is placed at ‘x’ and the four strips are heated evenly.  The thumbtack which falls first

indicates the best conductor.

Mandatory experiment: To show that water is a poor conductor.

 Fill a test tube with water.  Hold the test tube at the bottom

and heat the mouth with a Bunsen burner.

 The water at the mouth of the test tube will be boiling but you will still be able to hold the bottom of the tube.

 This is because the water is a poor conductor.

Mandatory experiment: To show convection in water

 Heat the water as shown.

 The hot water rises as a convection current and the dye goes with it.  The dye makes the current visible.

Mandatory experiment: To show convection in a gas

 The candle creates an updraft (convection current) of hot air.  The hot air rises and leaves through

the chimney on the left.

 Cold air is drawn in from outside, through the chimney on the right, to replace it.

Wooden

ring

Metals

x

Box with glass front

Candle Bunsen

Mandatory experiment: To show heat transfer by radiation

 Place two thermometers equal distances from a candle, as shown above.

 Thermometer 1 shows a small increase in temperature. This is due to radiated heat only.

 Thermometer 2 shows a large increase in temperature. This due to radiated heat and convection.

Solids, liquids and gases expand when they are heated. The following experiments are used to demonstrate this fact.

Mandatory experiment: To show that solids expand when heated.

 Put the ball through the hoop, to check that it fits through the hoop.  Heat the ball for 30 seconds with a

Bunsen burner.

 Try to fit the ball through now.  It cannot be done.

Mandatory experiment: To show that liquids expand when heated

 Heat the flask as shown.

 Since the flask is already full, any expansion of the water will be seen as the water rises up the tube.  The water level in the tube will fall

if the flask is cooled.

Mandatory experiment: To show that gases expand when heated

 Heat the flask as shown.  The air in the flask will expand.  The expanded air has only one

escape route, out through the top of the tube.

 If the tube is held under the water, the expanded air can be seen bubbling out.

 If the flask is allowed to cool, the air in the flask will contract and water will be sucked into the flask. Candle

Thermometer 2

Thermometer 1

Flask filled with water Heat