PHYSICS
PRACTICAL ELECTRICITY
Uses of Electricity
Electronic Heating
➔ Heating element is usually made of nichrome (alloy of nickel and chromium) coiled
round an insulating fire-proof material as it has high resistivity
Electric Iron
➔ Metal base so heat spread evenly over a large surface
area
➔ Uses a thermostat to control and maintain its
temperature – done by adjusting the temperature dial
(thermostat control) on the iron
➔ Energy in the internal store generated in the heating element quickly conducted to
the metal base – allows clothes to be ironed through the heat and weight of the iron
Electric Kettle
➔ Nichrome heating element(placed at the bottom of the
kettle) coiled round a fire-proof material and enclosed
in a metal tube — heats up as an electric current is
passed through it
➔ Water in the kettle to be heated through convection
and conduction
Electric Heating Appliances
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- Have a heating element and transfer energy electrically to the internal store of the
appliance
Electric Lighting
➔ An incandescent (filament) bulb has a wire
filament that is heated to a high temperature by
passing an electric current through it
➔ Energy from the chemical potential store of the
battery is transferred electrically to increase the
energy in the internal store of the bulb
➔ Subsequently transferred to internal store of surrounding air by:
● Propagation of electromagnetic waves(propagation of infra-red radiation and
visible light waves)
● heating due to a temperature difference
➔ Hot filament protected from oxidation through a glass bulb filled with inert gas
➔ Incandescent bulb is much less efficient than other types of lighting such as
light-emitting diode (LED) as it converts less than 5% of the supplied energy into
visible light
Measuring Energy Transferred Electrically
Electric Heating
➔ P = IV → E = IVt
➔ P = I2R → E = I2Rt
2 2
𝑉 𝑉𝑡
➔ P = 𝑅
→E= 𝑅
➔ SI unit
● Power: Watt(W)
● Energy: Joule(J)
2
Cost of Electric Consumption
➔ Kilowatt-hour (kWh): Energy used by a 1kW appliance in 1h
➔ 1 kWh = (1 kW) × (1 h)
= (1000 W) × (3600 s)
= 3.60 × 106 J
*Power rating measures the amount of energy used by component PER HOUR
*State what is meant when a lamp has a rating of 𝑥 V, 𝑦 W and find its resistance
Response
- The lamp works normally (100% brightness) when a potential difference of 𝑥 V is
applied across it
- 𝑦 J of energy per second from the chemical potential store of the battery
transferred electrically to the lamp by an electric current and subsequently
transferred by propagation of electromagnetic waves (infra-red radiation + visible
light waves) to increase energy in the internal store of the surrounding air
Dangers of Electricity
- Live and neutral wires needed to form complete circuit with main supply
- Live wire is dangerous due to high potential
- Neutral wire is at low/zero potential
- Both wires are:
- normally insulated by vulcanised rubber
- housed together in circular PVC sheathed cable or a braided-rubber
insulated cable
Damaged Insulators
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- Insulating material deteriorates and wires becoming exposed due to damage
insulation over time and use
- Touching exposed wire may lead to an electric shock and even death
- Direct contact between two wires causes short circuit → large current flows
through wires → may produce enough heat to cause a fire
Overheating of Cables
➔ Overly large current flowing through the conducting wires due to:
- plugging too many appliances into the same power outlet → overloading
→ excessively large current drawn from the main supply as all appliances are
connected in parallel
- electric cable carrying current exceeding maximum current its meant to carry
➔ Cables become overheated and lead to a fire
Damp Conditions
➔ Water provides a conducting path for a large current to flow from a live wire to a
person + electrical resistance of the human body reduces drastically when the skin
is wet → electric shocks or even death
➔ Ways to protect ourselves from electric shocks
- Must not handle electrical appliances with wet hands
- Keep hands dry when coming in contact with electrical appliances and / or
wearing rubber shoes (especially when carrying out electrical repairs)
Safety Features in Home Circuitries
Circuit Breaker
➔ Switches off electrical supply to a circuit when large currents exceeding their current
ratings flow through them
➔ Connected to the live wires of the main electrical wiring to prevent current surges
damaging appliances or causing electrical fires
*Prevents overheating
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*Home appliances should be connected in parallel as entire section will not work if one is
not in use
*Operates faster by disconnecting appliance to the circuit and does not need to be
replaced
Fuse
➔ Safety devices inserted into electrical circuits
➔ Melts when current flowing through it is larger
than rated value → breaks circuit
➔ Always connected to the live wire
➔ Fuses should have rating slightly more than current drawn from
an electrical appliance (nearest whole number upwards)
➔ Switch off mains before charging fuse
➔ Ensures appliance is at low potential and thus safe to touch
*Prevents overheating
*Normally rated in whole numbers at 1A-13A
Switches
➔ Used to break or complete an electric circuit
➔ Fitted to the live wire – high potential from
supply disconnected from appliance when the
switch is turned off → ensures appliance is at
low potential and safe to touch
*Two-way switch used to control a single lamp
*Prevents overheating
Earthing
Live Wire (Brown)
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➔ Connected to a high potential and delivers current to the appliance
➔ Circuit breakers, fuses and switches are placed on this wire
Neutral Wire (Blue)
- Completes the circuit by providing a returning path for the current
- Usually has a potential of 0 V
Earth Wire (Yellow/Green)
➔ Electrical safety feature: Earth wire connected to metal casing
➔ Low resistance wire connected to metal casing of appliances
➔ Earth pin longer to open up covered live and neutral holes of a power socket
➔ Live wire touches metal casing → metal casing of an electrical appliance may
become live (ie. have a high potential) → user touching the casing may be
electrocuted if no earth wire is connected to the casing due to the large p.d.
between the casing and earth
➔ Large current flowing through live wire will not flow through the user but to ground
instead through low-resistance earth wire → lower resistance increases current
with constant e.m.f → large current exceeds fuse rating of the fuse on the live
wire → fuse blows → circuit opens → electrical supply to the appliance cut off →
no current flowing through the appliance
➔ Cartridge fuse protects against an overly large current flowing through the circuit
when there is an electrical fault
*Prevents electric shock
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*Wires in plug should be fully connected and not have exposed wires
The cable grip should secure the outer cable and not the wires
Double Insulation
➔ Can be substitute for earth wire
➔ Electrical cables insulated from internal components of appliance
➔ Internal parts insulated from external casing
➔ Casings of appliances with double insulation typically made of non metals
➔ Two layers of insulating materials to enclose all metal framework and live parts
