Section 4 - Electricity and Magnetism - With Notes Page

Electricity and Magnetism

PAL (IGCSE) Single Science

Revision Book - Section 4

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NOTES PAGE

Syllabus

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4. Electricity and magnetism

4.1 Simple phenomena of magnetism

Core

• State the properties of magnets

• Give an account of induced magnetism

INDUCED MAGNETISM: If a piece of iron is brought close to a magnet

it becomes magnetic and is attracted to the magnet.

• Distinguish between ferrous and non-ferrous materials

FERROUS: Containing a large proportion of Iron (e.g. Iron, steel)

NON-FERROUS: Containing no iron

• Describe methods of magnetization and of demagnetization

MAGNETISATION: Stroking a magnet across a material (e.g. iron) DEMAGNETISATION: Hitting the material

Material

Type

Properties

Iron Soft magnetic material Magnetism is temporary Steel Hard magnetic material Magnetism is permanent • Distinguish between the design and use of permanent magnets and electromagnets

Type of magnet

Design

Use

Permanent Hard magnetic material For applications where magnetism is needed over long periods – fridge doors

Electromagnet Uses a solenoid to create magnetic field

For applications where the magnetic field needs to be turned on and off – Scrap metal moving

4.2 Electrical quantities

4.2 (a) Electric charge

Core

• Describe simple experiments to show the production and detection of electrostatic charges

GOLD LEAF ELECTROSCOPE



If a charge is moved close to the cap the gold leaf rises

• State that there are positive and negative charges

There are two type of charges; POSITIVE and NEGATIVE

• State that unlike charges attract and that like charges repel

LIKE CHARGES – Repel

UNLIKE CHARGES - Attract

• Describe an electric field as a region in which an electric charge experiences a force

• Distinguish between electrical conductors and insulators and give typical examples

ELECTRICAL CONDUCTOR: Charges are able to flow through the material

(metals)

ELECTRICAL INSULATOR: Charges are unable to flow through the material

NOTES PAGE

Supplement

• State that charge is measured in coulombs

Charge (Q) is measured in coulombs [C]

• State the direction of lines of force and describe simple field patterns, including the field around a point charge and the field between two parallel plates

• Give an account of charging by induction

 Charged rod brought close to top of electroscope

 The positive rod attracts electrons to the top of the electroscope (induced charge)

NOTES PAGE

• Recall and use the simple electron model to distinguish between conductors and insulators



In a conductor the charges are free to move (the electrons in a metal)



In an insulator the charges are not free to move

4.2 (b) Current

Core

• State that current is related to the flow of charge

CURRENT: Related to the flow of charge

NOTES PAGE

Supplement

• Show understanding that a current is a rate of flow of charge and recall and use the equation I = Q /t

• Distinguish between the direction of flow of electrons and conventional current

4.2 (c) Electro-motive force

Core

• State that the e.m.f. of a source of electrical energy is measured in volts

EMF is measured in Volts [V]

Supplement

• Show understanding that e.m.f. is defined in terms of energy supplied by a source in driving charge round a complete circuit

Electromotive force (e.m.f.): The total energy difference per unit charge around a

circuit

4.2 (d) Potential difference

Core

• State that the potential difference across a circuit component is measured in volts

NOTES PAGE

• Use and describe the use of a voltmeter

4.2 (e) Resistance

Core

• State that resistance = p.d./current and understand qualitatively how changes in p.d. or resistance affect current

an ammeter

• Relate (without calculation) the resistance of a wire to its length and to its diameter

Supplement

• Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire

R = Resistance



= Resistivity of material

L = Length of conductor

A = Area

 Increasing length = increasing resistance

 Increasing cross-sectional area = decreasing resistance

Supplement

• Recall and use the equations

P =IV and E = IVt

4.3 Electric circuits

4.3 (a) Circuit diagrams

Core

• Draw and interpret circuit diagrams containing sources, switches, resistors (fixed and variable), lamps, ammeters, voltmeters, magnetizing coils, transformers, bells, fuses and relays

Supplement

• Draw and interpret circuit diagrams containing diodes and transistors

NOTES PAGE

4.3 (b) Series and parallel circuits

Core

• Understand that the current at every point in a series circuit is the same

• Give the combined resistance of two or more resistors in series

• State that, for a parallel circuit, the current from the source is larger than the current in each branch

NOTES PAGE

• State that the combined resistance of two resistors in parallel is less than that of either resistor by itself

• State the advantages of connecting lamps in parallel in a lighting circuit

Parallel Circuit advantage: If one lamp fails the other lamps in parallel

continue to function

Supplement

• Recall and use the fact that the sum of the p.d.s across the components in a series circuit is equal to the total p.d. across the supply

NOTES PAGE

• Recall and use the fact that the current from the source is the sum of the currents in the separate branches of a parallel circuit

• Calculate the effective resistance of two resistors in parallel

4.3 (c) Action and use of circuit components

Core

• Describe the action of a variable potential divider (potentiometer)

 The output voltage can be varied by changing the position of the potential divider

NOTES PAGE

• Describe the action of thermistors and light dependent resistors and show understanding of their use as input transducers

NOTES PAGE

• Describe the action of a capacitor as an energy store and show understanding of its use in time delay circuits

NOTES PAGE

• Describe the action of a relay and show understanding of its use in switching circuits

Supplement

• Describe the action of a diode and show understanding of its use as a rectifier

show understanding of its use in switching circuits

• Recognise and show understanding of circuits operating as light sensitive switches and temperature-operated alarms (using a relay or a transistor)

Supplement

• Explain and use the terms digital and analogue

• State that logic gates are circuits containing transistors and other components

LOGIC GATES: Circuits containing transistors and other components

• Describe the action of NOT, AND, OR, NAND and NOR gates

• State and use the symbols for logic gates (candidates should use the American ANSI#Y 32.14 symbols)

gates

4.4 Dangers of electricity

Core

• state the hazards of – damaged insulation

DAMAGED INSULATION – risk of electrocution when handling wires etc

– overheating of cables

OVERHEATING OF CABLES – insulation will melt and wires become

exposed

– damp conditions

DAMP CONDITIONS – impure water conducts electricity and so risk of

electrocution

• Show an understanding of the use of fuses and circuit-breakers

FUSE: A thin piece of wire which melts and breaks if too much

current passes through it. Used to cut a circuit if the current is too high.

CIRCUIT BREAKER: Automatic electrical switch which will cut a circuit if

the current is too high

4.5 (a) Electromagnetic induction

Core

• Describe an experiment that shows that a changing magnetic field can induce an e.m.f. in a circuit

• Show understanding that the direction of an induced e.m.f. opposes the change causing it



As the magnet is moved in and out of the coil the magnetic field in the

coil changes



The direction of the induced EMF and so current opposes the change

causing it (i.e. makes a magnetic field opposite to the field of the

moving magnet

Supplement

• State the factors affecting the magnitude of an induced e.m.f.

FACTORS EFFECTING MAGNITUDE OF INDUCED E.M.F.

Increasing strength of magnet = increased induced E.M.F.

Increasing velocity of motion of magnet = increased induced E.M.F. Increasing # of coils in solenoid = increased induced E.M.F.

Core

• Describe a rotating-coil generator and the use of slip rings

AC GENERATOR

 If a coil is rotated in a permanent magnetic field the coil experiences a changing magnetic field

 If a wire experiences a changing magnetic field an EMF is induced  If the wire is connected into a circuit a current will flow



• Sketch a graph of voltage output against time for a simple a.c. generator

Core

• Describe the construction of a basic iron-cored transformer as used for voltage transformations

• Recall and use the equation (Vp /Vs) = (Np /Ns)

Supplement

• Describe the principle of operation of a transformer

• Recall and use the equation Vp Ip = Vs Is (for 100% efficiency)

for 100% efficiency

THE TRANSFORMER…

 An AC current is passed through the primary coil

 A varying magnetic field is induced in the coil and the iron core  The secondary coil experiences a varying magnetic field

 An EMF and so current is induced in the secondary coil  The output current is also alternating

electricity

TRANSFORMERS ARE USED IN ELECTRICTY TRANSMISSION BECAUSE...



The output of a power station is high current



The transformer is used to convert this to low current and high voltage

before passing through the overhead cables



A second transformer is used to lower the current and voltage before

the supply enters homes

• Give the advantages of high-voltage transmission

HIGH VOLTAGE TRANSMISSION = Low energy losses through heating in the

cables

• Explain why energy losses in cables are lower when the voltage is high

High voltage …. Low current …… Less heating of the cable …… Low energy loses

Core

• Describe the pattern of the magnetic field due to currents in straight wires and in solenoids

Supplement

• State the qualitative variation of the strength of the magnetic field over salient parts of the pattern

• Describe the effect on the magnetic field of changing the magnitude and direction of the current

GENERAL CONVENTION FOR MAGNETIC FIELD LINES...



The arrow points towards the south pole



The spacing of the field lines is proportional to the magnetic field

strength



If the current changes direction the magnetic field will reverse in

direction

NOTES PAGE

• Describe applications of the magnetic effect of current, including the action of a relay

ACTION OF A RELAY…

 With no current through the coil there is no magnetic field in the coil

 The contacts will be separate

 When a current passes through the coil a magnetic field is induced  The contacts will become magnetized and so close

4.5 (e) Force on a current-carrying conductor

Core

• Describe an experiment to show that a force acts on a current-carrying conductor in a magnetic field, including the effect of reversing:

(i) the current

direction

If the field direction is reversed – The force will act in the opposite direction

• State and use the relative directions of force, field and current

• Describe an experiment to show the corresponding force on beams of charged particles

When using the left hand rule for charged particles…

 For electrons the direction of current is the opposite direction to the electron velocity

 For protons the direction of current is the same direction as the electron velocity

NOTES PAGE

4.5 (f) d.c. motor

Core

• State that a current-carrying coil in a magnetic field experiences a turning effect and that the effect is increased by increasing the number of turns on the coil • Relate this turning effect to the action of an electric motor

Supplement

• Describe the effect of increasing the current

THE ELECTRIC MOTOR



A current is passed through a coil in a magnetic field



If a current carrying wire is placed in a magnetic field it experiences a force



The coil feels a force which rotates the coil



The commutator ensures that the current flows around the coil to give a force

which always acts to rotate the coil in the same direction

To Increase the rate of rotation...

 Increase the number of coils

 Increase the strength of the magnetic field  Increase the current

NOTES PAGE

4.6 Cathode-ray oscilloscopes

4.6 (a) Cathode rays

Core

• Describe the production and detection of cathode rays

DETECTION OF CATHODE RAYS

 Cathode rays can be detected by a fluorescent screen

 When the rays are incident on the screen the screen emit’s light • Describe their deflection in electric fields

The particles emitted in thermionic emission are electrons

4.6 (b) Simple treatment of cathode-ray

oscilloscope

Supplement

• Describe (in outline) the basic structure and action of a cathode-ray oscilloscope (detailed circuits are not required)

 Electrons are generated by the hot element

 The electrons are accelerated towards the anode and focused into a beam

 The deflection coils direct the beam at a certain position in the screen

NOTES PAGE

• Use and describe the use of a cathode-ray oscilloscope to display waveforms

KEY CONTROLS….



Time base: Time taken for beam to pass through one horizontal division

[Sec/div]