Electricity
ICSE Class 8 · Physics · Chapter 8
What this chapter covers
- Household consumption of electricity
- Live, neutral and earth wires
- Safety devices: fuse, circuit breaker and earthing
- Dangers of electricity
- Static electricity and conservation of charge
- Charging by conduction and induction
- The gold-leaf electroscope
- Lightning and the lightning conductor
- Cells in series: the battery
Key points
- Electric power is the rate of using electrical energy, measured in watts; energy used = power × time.
- One kilowatt hour (1 unit) is the energy used by a 1 kW appliance in 1 hour; 1 kWh = 3 600 000 J.
- Electricity bill = units consumed × rate per unit; units = present meter reading − previous reading.
- The live wire (red or brown) brings energy at high voltage; the neutral wire (black or light blue) returns the current; the earth wire (green or green-yellow) is a safety path to the ground.
- A fuse is a thin, low-melting-point wire in the live wire that melts when the current is too large.
- A circuit breaker (MCB) switches off automatically during a fault and can be switched on again.
- Earthing connects metal bodies of appliances to the ground so that fault current flows into the earth, not through a person.
- Electricity can cause shocks, burns and fires; wet hands, damaged wires and overloaded sockets are the commonest dangers.
- Static electricity is charge at rest; rubbing transfers electrons, and like charges repel while unlike charges attract.
- Charge can neither be created nor destroyed, only transferred, so the total charge stays the same.
- Conduction (by touching) gives the same kind of charge; induction (without touching) gives the opposite kind.
- A gold-leaf electroscope detects charge: its leaves diverge when charged, and a greater divergence shows a like charge.
- Lightning is a giant spark between charged clouds and the ground; a lightning conductor gives it a safe copper path to the earth.
- A battery is a set of cells in series; the positive terminal of one joins the negative of the next, and their voltages add up.
Questions answered
An air conditioner is rated at 1500 W. How many units does it use if it runs for 8 hours?
Energy = 1.5 kW × 8 h = 12 kWh, that is, 12 units.
Define the kilowatt hour and express it in joules.
The kilowatt hour is the electrical energy used by an appliance of power 1 kW working for 1 hour. 1 kWh = 3 600 000 J.
A meter read 2 640 units at the start of a month and 2 790 units at the end. At ₹7 per unit, what is the energy charge?
Units used = 2790 - 2640 = 150. Charge = 150 × ₹7 = ₹1050.
Which wire is the switch placed in, and why?
In the live wire, so that when the switch is off the appliance is cut off from the high voltage and is safe to touch.
How does earthing protect a person from a shock if the live wire touches the metal body of a refrigerator?
A large current flows at once through the earth wire into the ground. It blows the fuse or trips the circuit breaker, cutting off the supply, so no current passes through the person.
Why must a blown fuse never be replaced by a piece of thick copper wire?
Thick copper wire does not melt at the current the fuse was meant for, so during a fault the wiring would overheat and could catch fire.
Give one advantage of a circuit breaker over a fuse.
After it trips, it only needs to be switched back on; nothing has to be replaced.
A glass rod rubbed with silk becomes positively charged. Explain this using the law of conservation of charge.
Electrons move from the glass to the silk. Charge is not created, only transferred, so the glass becomes positive and the silk gets an equal negative charge. The total stays zero.