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Mains electricity — KCSE Physics

KCSE Physics · 70 practice questions · 4 syllabus objectives · 4 revision lessons

24 easy23 medium23 hard

Last updated · Aligned to the KNEC KCSE syllabus

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

State sources of mains electricity and describe power transmission from generation to consumer

Explain the domestic wiring system including live, neutral, earth wires and fuses

Define the kilowatt hour, determine electrical energy consumption and cost

Select appropriate fuses, calculate current and power for mains appliances

Revision Notes

Concise lesson notes for Mains electricity, written to the KCSE Physics marking standard. Read the first lesson free below.

Sources and Transmission of Mains Electricity

Mains electricity is primarily generated from various sources, including:

  • Fossil Fuels: Coal, oil, and natural gas are burned to produce steam that drives turbines.
  • Nuclear Power: Nuclear fission generates heat, which is used to produce steam for turbines.
  • Renewable Sources: Solar, wind, hydroelectric, and geothermal energy contribute to electricity generation.

Once generated, electricity undergoes a series of steps for efficient transmission to consumers:

  1. Step-Up Transformers: After generation, voltage is increased using step-up transformers, reducing energy loss during transmission.
  2. Transmission Lines: High-voltage electricity travels over long distances through transmission lines.
  3. Step-Down Transformers: Near consumption areas, step-down transformers reduce the voltage to safe levels for residential or commercial use.
  4. Distribution Network: Finally, electricity is delivered through a distribution network to homes and businesses.

This entire process ensures that electricity is efficiently and safely delivered from power plants to end-users.

Key points to remember

  • Mains electricity sources include fossil fuels, nuclear, and renewables.
  • Step-up transformers increase voltage for efficient long-distance transmission.
  • Transmission lines carry high-voltage electricity to reduce energy loss.
  • Step-down transformers lower voltage for safe consumer use.
  • Distribution networks deliver electricity to homes and businesses.

Worked example

Question: Name two sources of mains electricity and describe how it is transmitted to consumers.
Answer:

  • Sources include fossil fuels and hydroelectric power.
  • Electricity is generated, stepped up in voltage, transmitted, stepped down, and distributed.

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More lessons in this topic

Lesson 2: Understanding Domestic Wiring System

Objective: Explain the domestic wiring system including live, neutral, earth wires and fuses

The domestic wiring system is essential for safely distributing electricity in homes. It consists of three main wires: live, neutral, and earth.

  • Live wire: This wire carries the current to the appliance. It is usually brown in color.
  • Neutral wire: This wire completes the circuit by carrying current away from the appliance. It is typically blue.
  • Earth wire: This wire is a safety feature that prevents electric shocks. It is green and yellow striped.

Fuses are also crucial components of the wiring system. They protect the circuit from overload by melting and breaking the circuit if the current exceeds a safe level.

In summary, the live wire provides electricity, the neutral wire returns it, and the earth wire ensures safety by directing excess current away from the user. Understanding this system is vital for ensuring electrical safety in homes.

  • Live wire carries current to appliances, usually brown.
  • Neutral wire returns current, typically blue.
  • Earth wire prevents electric shocks, green and yellow striped.
  • Fuses protect circuits from overload by breaking the circuit.

Explain the function of the earth wire in a domestic wiring system.

  • The earth wire directs excess current safely to the ground.
  • It prevents electric shocks, ensuring user safety.
Lesson 3: Understanding Kilowatt Hour and Energy Costs

Objective: Define the kilowatt hour, determine electrical energy consumption and cost

The kilowatt hour (kWh) is a unit of energy equivalent to the energy consumed by a power of one kilowatt operating for one hour. It is commonly used to measure electrical energy consumption in homes and businesses.

To determine electrical energy consumption, use the formula:

Energy (kWh) = Power (kW) × Time (hours)

For example, if a 2 kW appliance runs for 3 hours, the energy consumed is:

  • Energy = 2 kW × 3 hours = 6 kWh.

To calculate the cost of electrical energy, use the formula:

Cost = Energy (kWh) × Price per kWh.

For instance, if the price per kWh is Ksh 20, then:

  • Cost = 6 kWh × 20 Ksh/kWh = Ksh 120.

Understanding these concepts helps you manage energy consumption effectively and budget for electricity costs.

  • Kilowatt hour measures energy consumption over time.
  • Energy consumed = Power (kW) × Time (hrs).
  • Cost of electricity = Energy (kWh) × Price per kWh.

A 1.5 kW heater runs for 4 hours. Calculate energy consumed and cost if Ksh 15 per kWh.

  • Energy = 1.5 kW × 4 hrs = 6 kWh.
  • Cost = 6 kWh × 15 Ksh/kWh = Ksh 90.
Lesson 4: Understanding Fuses and Power Calculations

Objective: Select appropriate fuses, calculate current and power for mains appliances

In mains electricity, fuses protect appliances from excessive current, preventing damage or fire hazards. A fuse is rated by its maximum current capacity, measured in amperes (A). To select an appropriate fuse, consider the appliance's power rating, given in watts (W).

Calculating Current and Power:
Use the formula:

  • Power (P) = Voltage (V) × Current (I)
    For mains electricity in Kenya, the standard voltage is 240V. Rearranging the formula gives:
  • Current (I) = Power (P) / Voltage (V)

Example:
If an appliance has a power rating of 1200W, the current can be calculated as follows:

  • I = P / V
  • I = 1200W / 240V
  • I = 5A
    Thus, a 5A fuse would be suitable for this appliance. Always choose a fuse rated slightly above the calculated current to ensure safety.
  • Fuses prevent damage by breaking the circuit at high currents.
  • Select a fuse based on the appliance's power rating.
  • Use P = V × I to calculate current and power.
  • Standard voltage in Kenya is 240V for mains electricity.
  • Choose a fuse rated slightly above the calculated current.

An appliance uses 800W. Calculate the current and suggest a fuse.

  • I = P / V
  • I = 800W / 240V = 3.33A
  • Suggest a 5A fuse for safety.

Sample Questions

Read 3 questions and answers free. Sign up to access all 70 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER3 marks

List three factors that determine the appropriate fuse rating for an electrical appliance. (3 marks)

Answer & marking scheme

Part (a) — 3 marks
The power rating of the appliance in watts (1 mk)
The voltage supply of the circuit (1 mk)
The type of appliance and its usage patterns (1 mk)
2
easySHORT ANSWER2 marks

State two factors that determine the cost of electricity consumed in a household. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
The total energy consumed in kilowatt hours (kWh). (1 mk)
The cost per kilowatt hour charged by the electricity provider. (1 mk)
3
easySHORT ANSWER3 marks

State the definition of a kilowatt hour and how it relates to electrical energy consumption. (3 marks)

Answer & marking scheme

Part (a) — 1 mark
A kilowatt hour is the amount of energy consumed by a device of one kilowatt running for one hour. (1 mk)
Part (b) — 2 marks
It quantifies electrical energy consumption in households and industries. (1 mk)
It is used to calculate electricity bills by multiplying energy consumed in kWh by the unit cost. (1 mk)
4

State the colour coding of the three main wires used in domestic electricity and explain the role of a fuse in the system. (4 marks)

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Frequently asked questions

What does the KCSE Physics topic "Mains electricity" cover?

Power transmission, domestic wiring, kWh, cost of electricity

How many practice questions are available for Mains electricity?

HighMarks has 70 Mains electricity practice questions for KCSE Physics, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Physics syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Mains electricity for the KCSE exam?

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