Energy use at home and in the city
Power is how fast energy is used. Unit: watt (W). Energy = power × time. The electricity company counts in kilowatt-hours (kWh). One kWh is called one unit: a 1 000 W device running for 1 hour.
Units = watts × hours ÷ 1000. Cost = units × price per unit.
To save energy: switch off idle devices, use LEDs (same light, far fewer watts) and choose star-rated appliances. Cities also use solar, wind and hydro to cut coal use.
Light in fibre
An optical fibre is a glass thread thinner than a hair. Light enters one end and hits the wall at a shallow angle, so it is fully reflected (total internal reflection). It zig-zags to the other end with almost no loss.
Fibre carries very many calls and videos at once because light can be switched on and off extremely fast. It is thin, light and does not pick up electrical noise. Doctors also use fibre in an endoscope to see inside the body.
Lasers
Light from a bulb has many colours and goes in all directions. Laser light is one colour, moves in step (coherent) and spreads very little, so the beam stays thin over long distances.
Uses: sending data in fibre, barcode scanners, laser printers, eye surgery, cutting metal, measuring distance and aiming. Never look into a laser beam: even a small one can damage the eye.
The full story of how a laser makes this light is in the lesson Lasers.
Spaceflight: rockets and orbits
A rocket works by Newton's third law. It throws hot gas backwards (down). The gas pushes the rocket forwards (up). A rocket works in empty space because it carries its own fuel and needs no air to push on. Thrust = mass of gas per second × speed of gas.
A satellite is in orbit: it is falling towards Earth all the time, but it moves sideways so fast (about 7.8 km/s near Earth) that the surface curves away beneath it. Inside, objects float because everything falls together, not because gravity is absent.
Satellites give weather pictures, TV, phone links and navigation. See orbital speed.
Try it
At home: read the watt number on three devices. Write how many hours each is used per day. Work out units per day for each and find the biggest one.
Balloon rocket: blow up a balloon and let it go. The air goes one way, the balloon goes the other.
In the 3D: on Orbit, slide the speed down to the minimum. The satellite still circles in this model, but in real life too slow a speed means it falls to the ground.
Key formulas and definitions
- Energy (kWh) = power (kW) × time (h)
- Units = watts × hours ÷ 1000
- Cost = units × price per unit
- Thrust = (mass of gas per second) × (gas speed)
- Orbit time = distance round the orbit ÷ speed
Worked examples
1. A 60 W fan runs for 10 hours. How many units does it use?
Units = 60 × 10 ÷ 1000 = 0.6 units.
2. A 2 000 W geyser runs 30 minutes a day for 30 days. Price is ₹7 per unit. Find the cost.
30 min = 0.5 h. Daily units = 2 000 × 0.5 ÷ 1000 = 1. In 30 days: 30 units. Cost = 30 × 7 = ₹210.
3. A rocket throws out 100 kg of gas every second at 3 000 m/s. Find the thrust.
Thrust = 100 × 3 000 = 300 000 N.
Common mistakes
- Using watts as if they were units. A unit is a kilowatt-hour, so divide watt-hours by 1000.
- Thinking a rocket needs air to push against. It pushes against its own exhaust gas.
- Saying satellites float because there is no gravity. Gravity is still strong; the satellite is in free fall.
- Saying light in fibre goes straight. It reflects many times from the wall.