14.5Chapter 14: Electrical Circuits, Resistance and Power

Energy and power — Alhussan Physics - Stage 9 Revision Notes & Topical Questions

5 min read 7 parts Lesson 5 of 6 2 quizzes

📚 Alhussan Physics - Stage 9 Hub / Topical Revision

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Lesson summary

Physics 14.5 Energy and power — Watts (W), Kilowatt-Hours (kWh) and Electrical Appliances — Alhussan International Academy — Physics (Stages 7–9) notes: Describe the difference between energy and power, calculate power and electrical energy, and compare lamps.Power = the rate at …

Bright lamps
Lighthouse
Meaning of watts
Filament (incandescent)
Small size
Paying for electricity

Section 1 of 7Power

Power

Power the rate at which energy is transferred.

Units

Power is measured in watts (W) or kilowatts (kW), where 1 W = 1 joule per second and 1 kW = 1000 W.

Bright lamps

A bright lamp transfers more energy per second and emits more light per second than a dim lamp.

Lighthouse

Its lamp is much brighter than a torch lamp, which transfers energy more slowly, so it is far more powerful.

Section 2 of 7Power equations

Power

Power (W) = energy transferred (J) ÷ time taken (s), or P = E ÷ t.

Energy

Energy (J) = power (W) × time (s).

Units

Convert time to seconds before calculating in joules and watts.

Meaning of watts

A 60 W bulb transfers 60 J every second.

Section 3 of 7Types of lamp

Types of lamp

Filament (incandescent)

A very thin wire glows when it gets hot, and most of the energy is wasted as heat.

CFL

Compact fluorescent lamps are energy-saving lamps first sold in 1981.

LED

Light-emitting diode lamps are very efficient.

Lamp	Typical power for similar light	Efficiency
Filament	60 W	Low (about 5%)
CFL	12 W	Medium
LED	6 W	High

Section 4 of 7Uses of LEDs

Small size

LEDs produce a lot of light for their size and can be sewn into clothes.

Medicine

Doctors use their intense light in cancer treatments.

Displays

LEDs were used in the London and Rio de Janeiro Olympics to make patterns in the stadium.

Cost

LEDs cost more to buy but last a long time and save money.

Section 5 of 7The kilowatt-hour

Paying for electricity

A bill pays for fuel burned to produce a current that transfers energy.
The energy transferred depends on the power of appliances and how long they are used.

Why not joules

People transfer very many joules each month, so a larger unit is used.

Kilowatt-hour (kWh)

Kilowatt-hour (kWh) the energy transferred by a 1 kW appliance in 1 hour, which is 3 600 000 J.

Energy equation

Energy (kWh) = power (kW) × time (h).

Cost

Cost = energy (kWh) × price per kWh.

Section 6 of 7Key terms

Term	Meaning
Power	The rate at which energy is transferred.
Watt (W)	The unit of power, one joule per second.
Kilowatt-hour (kWh)	The energy transferred by 1 kW in 1 hour.
LED	A light-emitting diode lamp that is very efficient.
Filament lamp	A lamp with a thin wire that glows when hot.