Ch 9 · 4.4.1 Uses of Electricity · O Level Physics 5054

Topic 9 of 17 in Electricity and Magnetism — O Level Physics 5054 — 59 lessons.

Introduction

Every electricity bill charges you for energy, measured in a unit called the kilowatt-hour. To understand the bill you need to know how much power each appliance uses and for how long. (Syllabus section 4.4.1.)

Key ideas — What we use electricity for

🧑‍🏫 Teacher's explanation

Think of electricity as a flow of energy that can be converted into different forms depending on the appliance. When you use a kettle, the electrical energy is converted into thermal energy (heat). When you turn on a fan, it is converted into kinetic energy (motion). It is like having a versatile liquid that can be used to heat a room, light up a space, or turn a wheel, depending on the tool you pour it into.

📖 Definitions

🔍 Worked example — Easy

A 60 W lamp is switched on for 5 hours. Calculate the total energy transferred in kilowatt-hours (kWh).

🔍 Worked example — Standard

An electric heater has a power rating of 2.5 kW. It is used for 4 hours every day. If the cost of electricity is 0.20 \ per kWh, calculate the cost of using the heater for one day.

🔍 Worked example — Exam-style

A television is rated at 150 W. It is used for 3 hours a day. Calculate the energy transferred in Joules (J) for this period.

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Lamps in parallel

Lighting circuits in homes connect lamps in parallel. The advantages:

🧑‍🏫 Teacher's explanation

Imagine a water pipe system in a house. If you have one single pipe running through every faucet in the house one after another (series), the water pressure drops at every tap, and if one tap gets blocked, no water reaches any other tap. Now, imagine each faucet has its own separate connection to the main water supply (parallel). Each faucet gets the full pressure, and if you turn one off, the others keep flowing perfectly.

📖 Definitions

🔍 Worked example — Easy

A single 60 ext W lamp is connected to a 230 ext V supply. Calculate the energy transferred in 2 ext hours.

🔍 Worked example — Standard

An electric heater is rated at 2.5 ext kW. It is used for 3 ext hours every day for 30 ext days. Calculate the total energy used in $ ext{kWh}$.

🔍 Worked example — Exam-style

A television is connected to a 230 ext V supply and draws a current of 0.5 ext A. Calculate the cost of running the TV for 5 ext hours if the cost of electricity is \0.20 per $ ext{kWh}$.

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

(a) Calculate the current flowing through the lamp. (b) The lamp is used for 5 ext hours every day. Calculate the energy consumed in kWh after 30 ext days. [4 marks]

Key ideas — Electrical power

P = IV

🧑‍🏫 Teacher's explanation

Think of electricity like water flowing through a pipe. The voltage (V) is the water pressure, the current (I) is the flow rate of the water, and power (P) is how much work that water can do—like turning a water wheel. If you have high pressure and a high flow rate, you can turn a much larger wheel very quickly.

📖 Definitions

🔍 Worked example — Easy

A light bulb is connected to a 12 V battery and draws a current of 0.5 A. Calculate the power of the bulb.

🔍 Worked example — Standard

An electric kettle has a power rating of 2.5 kW. It is used for 4 minutes. Calculate the total energy transferred in kilowatt-hours (kWh).

🔍 Worked example — Exam-style

A heater is rated at 2000 W. It is used for 5 hours every day. If the cost of electricity is \0.20 per kWh, calculate the cost of using the heater for one day.

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Electrical energy

E = IVt

🧑‍🏫 Teacher's explanation

Think of electricity like water flowing through a pipe to turn a waterwheel. The voltage (V) is the pressure pushing the water, the current (I) is the amount of water flowing per second, and the time (t) is how long you leave the tap open. The total energy (E) used is simply the product of how hard it pushes, how much flows, and for how long.

📖 Definitions

🔍 Worked example — Easy

A 60 ext W lamp is switched on for 5 ext minutes. Calculate the energy transferred in Joules.

🔍 Worked example — Standard

An electric kettle has a power rating of 2.5 kW. It is used for 4 minutes. Calculate the energy used in kilowatt-hours (kWh).

🔍 Worked example — Exam-style

A heater is rated at 2200 W. It is used for 3 hours every day. If the cost of electricity is 0.25 USD per kWh, calculate the total cost for 30 days.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — The kilowatt-hour

The joule is too small for household bills, so electricity companies use the kilowatt-hour (kW h).

🧑‍🏫 Teacher's explanation

Imagine you are filling a bucket with water. The rate at which water flows from the tap is like 'power' (how fast energy is being used), and the total amount of water in the bucket is 'energy' (the total amount used). If you leave the tap running for a long time, you get a lot of water. In electricity, we don't use Joules to bill you because a single Joule is a tiny amount of energy—like a single drop of water. Instead, we use the kilowatt-hour (kWh), which is a much larger 'bucket' suitable for household bills.

📖 Definitions

🔍 Worked example — Easy

A 60 W light bulb is left on for 5 hours. Calculate the energy transferred in kilowatt-hours (kWh).

🔍 Worked example — Standard

An electric kettle has a power rating of 2.5 kW. It is used for 4 minutes. Calculate the energy used in kWh.

🔍 Worked example — Exam-style

A television with a power rating of 150 W is used for 4 hours every day. If the cost of electricity is 0.25 per kWh, calculate the cost of running the TV for 30 days.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Example 1. A kettle draws 10 A from a 230 V supply. Find its power, and the energy it transfers in 2.0 minutes.

🔍 Worked example — Easy

A kettle draws 10 A from a 230 V supply. Find its power, and the energy it transfers in 2.0 minutes.

🔍 Worked example — Standard

A 2.5 kW heater is used for 4 hours a day for 30 days. Electricity costs 40 cents per kW h. Find the total cost.

🔍 Worked example — Exam-style

A 60 W lamp is left on for 8 hours. How many kW h does it use?

⚠️ Common errors

Common mistakes

Practice

Answers: (1) Any two: each lamp gets the full voltage; each can be switched independently; if one fails the others stay on. (2) P = 0.50 × 230 = 115 W. (3) E = Pt = 1200 × 300 = 3.6 × 10^5 J. (4) The energy transferred by a 1 kW appliance in 1 hour (3.6 × 10⁶ J). (5) 3.0 × 2 × 7 = 42 kW h; 42 × 35 = 1470 rupees.

Ch 9 · 4.4.1 Uses of Electricity · O Level Physics 5054

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