Chap 19 Practical E 12 S

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09/22/20

Practical electricity

Syllabus

Syllabus

a.

a. Describe the use of the heating effect of electricity in appliances such as electric kettles, ovens and heaters.

b. Recall the relationships P = V I and E = V I t.

c. . Apply the relationships for electrical power and energy to new situations or to solve related problems.

d. Calculate the cost of using electrical appliance where the energy unit is in kWh.

e. . State the hazards of using electricity in the following situations.

i. Damage insulation

ii.Overheating of cables

iii. Damp conditions

f. Explain the use of fuses in electrical circuits and of fuse ratings.

g. Explain the need for earthing metal cases and for double insulation.. h. . State the meaning of the terms live, neutral and earth.

i. Describe how to wire a mains plug..

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Heating Elements



Heating elements

• found appliances using heating effect of electrical current.found appliances using heating effect of electrical current.

• Usually made of nichrome (high resistivity, melting point and does _{Usually made of nichrome (high resistivity, melting point and does} not oxidise easily) coiled round an insulating fire-resisting material. not oxidise easily) coiled round an insulating fire-resisting material.



Heating effect

• When electrical charge flows from a higher potential to a lower When electrical charge flows from a higher potential to a lower potential, energy is released.

potential, energy is released.

• Electrical energy is changed to light or heat energy._{Electrical energy is changed to light or heat energy.}



Examples of appliances

• Electric ironElectric iron

– metal base: to spread the heat evenly_{metal base: to spread the heat evenly}

– base insulate from heating element: heatingbase insulate from heating element: heating element element would round a flat sheet of mica and

would round a flat sheet of mica and

sandwiched between two thin insulating

sheets

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Heating Elements

• Electric kettle_{Electric kettle}

– heating element enclose in metal tube and insulated_{heating element enclose in metal tube and insulated} – when current flows, the heating element heats up and when current flows, the heating element heats up and

water around the element is heated by conduction



Electric lighting

• filament lamp_{filament lamp}

– use Tungsten for its high resistivity and melting point._{use Tungsten for its high resistivity and melting point.} – When current flows, filament becomes white hot_{When current flows, filament becomes white hot}

– higher the temperature of filament, greater _{higher the temperature of filament, greater}

proportion of energy is converted to light energy.

proportion of energy is converted to light energy. – Coiled coil to reduce convection current and to Coiled coil to reduce convection current and to

make it compact.

– Bulb filled with inert gas to prevent oxidization._{Bulb filled with inert gas to prevent oxidization.} • Fluorescent lamp_{Fluorescent lamp}

– By passing charges between the electrodes, the By passing charges between the electrodes, the mercury vapour emits ultraviolet light.

mercury vapour emits ultraviolet light.

– UV light converted to visible light by the fluorescent _{UV light converted to visible light by the fluorescent}

powder.

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Calculation of electrical power



Electric power is the rate at which energy is release

Power = energy change / time

P = E / t



Recall 1 J of energy is release when 1C of charge flows

through a p.d. of 1V.

E = V Q

= V (It)

E = V Q = (IR) x (It)

= I

22

Rt

E = I

22

Rt = (V/R)

22

R t

= (V

22

t

) / R



E.g. 19.1 (pg. 375)

P

= E / t

= V I

= I

22

R

= V

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COMMON HOUSEHOLD APPLIANCES

Appliance

Appliance Power (W)Power (W) Current under normal Current under normal conditions (A)

conditions (A)

Lamp

Lamp 100100 0.40.4

Hair dryer

Hair dryer 500500 2.12.1

Toaster

Toaster 120120 5.05.0

Kettle

Kettle 27502750 11.511.5

Air con

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SHORT CIRCUIT

Assume resistance is 1.0 Assume resistance is 1.0

Current = V/ R

Current = V/ R = 240 / 1.0= 240 / 1.0

= 240.0A = 240.0A

What do you think will happen, when there is a big increase in current ? What do you think will happen, when there is a big increase in current ?

Kettle: I = 11.5A Kettle: I = 11.5A

Household supply = 240V Household supply = 240V

Resistance = V/ I = 240 / 11.5 Resistance = V/ I = 240 / 11.5

= 20.9= 20.9

What if there was a short circuit?

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Measuring electrical consumption



Electricity meter measures the amount of electrical used.



Difference between this month’s and last month’s reading is

the total consumption for the month.



Units is usually in kWh (Eg. 19.6, pg. 379)

• 1 kWh = 1000w x (60x60)_{1 kWh = 1000w x (60x60)}

= 3600 000J= 3600 000J

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Dangers of electricity



Risk of electric shock and fire due to:

• Damaged insulation_{Damaged insulation}

– live wire has very high voltage while neutral wire _{live wire has very high voltage while neutral wire} has close to zero voltage. Exposure of live

has close to zero voltage. Exposure of live wire may cause electric shock when touched.

wire may cause electric shock when touched.

– Contact of live and neutral wire may cause short circuit._{Contact of live and neutral wire may cause short circuit.}

– Wires should be checked for damaged insulation and replaced if damaged._{Wires should be checked for damaged insulation and replaced if damaged.}

• Overheating of cables_{Overheating of cables}

– unusual large current flowing in the wire due to short circuit or overloading._{unusual large current flowing in the wire due to short circuit or overloading.} – cable may be very thin, with the corresponding high resistance will result in _{cable may be very thin, with the corresponding high resistance will result in}

more heat generated. This may melt insulation and cause a fire.

– Check that wires are of the correct power rating when fitting it to the _{Check that wires are of the correct power rating when fitting it to the}

appliance.

– Overloading the power outlet may cause the electric _{Overloading the power outlet may cause the electric}

cable(from mains to outlet) to overload and overheat.

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Dangers of Electricity

• damp conditions_{damp conditions}

– when water touches the live wire, current could pass through water._{when water touches the live wire, current could pass through water.} – Effect of electric current passing through a body:_{Effect of electric current passing through a body:}

1mA

1mA : threshold: threshold 5mA

5mA : frightening but harmless: frightening but harmless 10-20mA

10-20mA : uncontrolled muscular contraction: uncontrolled muscular contraction 50mA

50mA : pain and exhaustion, breathing affected: pain and exhaustion, breathing affected 100-300mA

100-300mA : uncoordinated contraction of the heart, : uncoordinated contraction of the heart, leading to death

leading to death

• size of current that will flow through the body depends on:_{size of current that will flow through the body depends on:} – size of voltage _{size of voltage}

– electrical resistance_{electrical resistance}

– dry skin has resistance of about100k_{dry skin has resistance of about100k}, , while wet skin may have about about 100

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Safe use of electricity at home



A typical house circuit.

• mains cable from substation_{mains cable from substation}

• mains cable passes through mains fuse, electricity meter and _{mains cable passes through mains fuse, electricity meter and} consumer unit

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Safe use of electricity at home

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

Miniature Circuit Breaker (MCB)

• Prevents current flow above rating from flowing by breaking _{Prevents current flow above rating from flowing by breaking}

circuit

circuit

• Can you recall?_{Can you recall?}



Earth leakage circuit breaker

(ELCB)

• Compares current flow in Live and Neutral wire_{Compares current flow in Live and Neutral wire}

• Able to switch off all circuits in the house in very short time when _{Able to switch off all circuits in the house in very short time when} earth leakage of more 25mA occurs

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

Fuse

• Tin coated copper wire connected _{Tin coated copper wire connected} to metal caps on the end of a short to metal caps on the end of a short tube.

tube.

• The thickness of the wire _{The thickness of the wire}

depends on the current permitted depends on the current permitted

• When the current flow is above the _{When the current flow is above the} fuse rating, the fuse melts and

fuse rating, the fuse melts and breaks the circuit.

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CHOOSING A FUSE



Rating: 1A, 2A, 5A, 10A, 13A (pg. 385)



E.g. 19.8, pg. 385



What happens

• If rating is lower than appliance requirement?_{If rating is lower than appliance requirement?} • If rating is higher than appliance requirement?_{If rating is higher than appliance requirement?}



choose a rating that is slightly higher than maximum current

allowed by the appliance.



Where should the fuse be placed, live or neutral wire? What

is the difference? Think about it.

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

Switches

• used to turn on or cut off the flow of current_{used to turn on or cut off the flow of current}

Safe use of electricity at home

•

Switches and fuse in a circuit

•located on the live wire

•live wire at same potential as neutral (zero

potential).

Hence safe to handle

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SCENARIO

•Casing may become

‘live’ due to faulty

heating elements

•No current flow as

metal case is insulated

from the earth, fuse

will not melt

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Earth wire



Attached to the casing of the

appliance



As soon as the live wire

touch the casing, there will

be a short circuit.



Fuse will melt and break the

break the live wire

connection.

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

Double insulation

• Some appliance do not have a _{Some appliance do not have a} earth wire.

earth wire.

• Live electrical components are _{Live electrical components are} isolated by means of double isolated by means of double insulation.

insulation.

– Electric cable is insulated from _{Electric cable is insulated from}

the internal casing

– internal metal part is also _{internal metal part is also}

insulated from the external

casing

• have non-metallic casing_{have non-metallic casing}

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THREE PIN PLUG

• live wire: To cause charges to flow when whenever the potential of the : To cause charges to flow when whenever the potential of the live wires varies between 240V and –240V.

live wires varies between 240V and –240V.

• neutral wire: Provides Earth potential. Completes: Provides Earth potential. Completes the circuit by forming the circuit by forming a path for the current to flow back to the supply

a path for the current to flow back to the supply

• earth wire : low resistance wire connected to the casing of the : low resistance wire connected to the casing of the appliance

appliance. When metal casing becomes live, the earth wires draws a . When metal casing becomes live, the earth wires draws a large current through its low resistance path. This breaks the fuse and large current through its low resistance path. This breaks the fuse and

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Wiring a three pin plug

 _{Remove outer}_{Remove outer}

insulation of the

wire and the outer

insulation of the

wire.

 Twist copper Twist copper

strands of each

wire together.

Clamp the wire to

the cord grip. Insert

each wire into the

correct terminal.

 Tighten the screwsTighten the screws

 Place the fuse in the Place the fuse in the

holder.

 Check the following:Check the following:

• rating of the fuserating of the fuse • cord grip firmly in _{cord grip firmly in}

place

• insulation extend to insulation extend to the terminal

the terminal

• the correct wire in _{the correct wire in}

the correct terminal

• no loose bare wiresno loose bare wires

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