Heating Effect and Electric Power
NCERT Class 10 • Science • Chapter "Electricity"
Heating Effect and Electric Power
When current passes through a resistor, electrical energy is converted into heat. This is called the heating effect of current, and it is both a nuisance in transmission lines and the whole basis of the heater, iron, geyser, toaster and electric bulb.
Joule's Law and Power
- Joule's law of heating states that the heat produced is proportional to the square of the current, to the resistance and to the time for which current flows.
- The heat produced therefore rises very sharply if the current is increased, since it depends on the square of current.
- Electric power is the rate at which electrical energy is consumed and is given by P = VI.
- Using Ohm's law, power can also be written as I²R or as V²/R.
- The SI unit of power is the watt, and one kilowatt equals one thousand watts.
- Electric energy consumed is power multiplied by time, and the commercial unit is the kilowatt hour.
Fuse, Appliances and Safety
- A fuse is a safety device made of a wire with a low melting point and high resistance.
- It is connected in series with the live wire, so that it melts and breaks the circuit when current exceeds a safe value.
- Excess current arises from overloading, when too many appliances draw current together, and from a short circuit, when live and neutral wires touch directly.
- Heating appliances use an alloy such as nichrome for the heating element, because it has high resistance and a very high melting point and does not oxidise readily.
- An electric bulb's filament is made of tungsten, which has a very high melting point.
- Earthing connects the metal body of an appliance to the earth so that any leaked current flows away safely.
| Formula | Gives | Unit |
|---|---|---|
| P = VI | Electric power | watt |
| P = I²R | Power from current and resistance | watt |
| Energy = P × t | Electric energy | kilowatt hour |
Fuse — a safety wire of low melting point placed in series with the live wire, which melts and breaks the circuit when the current becomes too high.
Exam me kaise aata hai
- The filament of an electric bulb is made of — tungsten
- The heating element of a heater is usually made of — nichrome
- A fuse is connected in which way — in series with the live wire
- Heat produced depends on the square of — current
UPSC/State PSC ke liye
- Power is transmitted at high voltage and low current precisely because heat loss depends on the square of the current, so lower current means far lower loss.
- A fuse protects the circuit while earthing protects the person, so the two serve different purposes and are both required.
Yahan confuse hote hain
✗ A fuse is connected in parallel | A fuse is connected in series with the live wire | ✓
✗ Earthing and fusing do the same job | A fuse protects the circuit; earthing protects the user | ✓
✗ Doubling the current doubles the heat | Heat depends on the square of current, so it becomes four times | ✓
Ek nazar me
- Heat produced depends on the square of current, on resistance and on time.
- Power equals VI, and energy is charged in kilowatt hours.
- A fuse melts on excess current; overloading and short circuit cause it.
- Nichrome is used in heaters and tungsten in bulb filaments.