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IGCSE · 4 · structured

Electricity and magnetism · Question 9

Electricity and magnetism · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
routine
Marks
8
Topics
1
Answer
Complete
An electric shower wired to the mains through a fuse, with its metal case earthed230 Vmainsfuselive wireneutral wireearth wiremetal caseheating elementelectric shower, 8.5 kW
Fig. 9.1A wiring diagram of the electric shower. A box on the left marked 230 V mains has two wires leaving it: the live wire, which passes through a fuse drawn as a small rectangle with a line through it, and the neutral wire, which runs straight across. Both enter the outline of the shower, where they are joined to each other by the heating element. The shower's outline is labelled metal case and the appliance is marked 8.5 kW. A third wire, the earth wire, runs from an earth symbol of three shortening horizontal bars across to the metal case, meeting it at a solid connecting dot.
structured8 marks

A household electric shower is connected to the 230 V mains supply through a fuse and an earth wire. The shower has a metal case and is marked 8.5 kW.

  1. (a)

    Calculate Calculate the current in the shower when it is operating normally.

    2 marks
  2. (b)

    Explain Explain how the earth wire and the fuse together protect a person who touches the metal case if a live wire comes loose inside the shower.

    3 marks
  3. (c)

    Calculate The shower is used for a total of 30 minutes each day. Electrical energy costs 24 cents per kilowatt-hour. Calculate the cost of using the shower for 30 days.

    3 marks
Ready to self-mark?Reveal the detailed answer guide
Answer overviewKey answer: I = P / V = 8500 / 230 I = 37 A the earth wire connects the metal case to earth through a very low resistance path
01

(a)

2 marks

Calculate Calculate the current in the shower when it is operating normally.

How to approach it

List the given quantities with units, identify the required quantity, write the governing relationship before substituting, and keep extra digits until the final line so rounding does not distort the result.

  1. 1

    I = P / V = 8500 / 230

  2. 2

    I = 37 A

02

(b)

3 marks

Explain Explain how the earth wire and the fuse together protect a person who touches the metal case if a live wire comes loose inside the shower.

How to approach it

State the outcome first, then link cause to effect with the relevant physical principle. Each link in the reasoning should be explicit enough to earn its own marking point.

  1. 1

    the earth wire connects the metal case to earth through a very low resistance path

  2. 2

    if the live wire touches the case, a very large current flows to earth rather than through the person

  3. 3

    this current melts the fuse, breaking the live wire and isolating the appliance

03

(c)

3 marks

Calculate The shower is used for a total of 30 minutes each day. Electrical energy costs 24 cents per kilowatt-hour. Calculate the cost of using the shower for 30 days.

How to approach it

List the given quantities with units, identify the required quantity, write the governing relationship before substituting, and keep extra digits until the final line so rounding does not distort the result.

  1. 1

    energy per day = 8.5 kW × 0.50 h = 4.25 kWh

  2. 2

    energy in 30 days = 127.5 kWh

  3. 3

    cost = 127.5 × 24 = 3060 cents = $30.60

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