Electrical Power & Mains ElectricityEdexcel IGCSE Physics: Subtopic test

1. 1
   A hairdressing salon uses an electric hair straightener connected to the 230 V mains supply. The straightener draws a current of 5 A when it is heating up. The appliance has a metal outer casing, and a small section of insulation near the plug has worn away, exposing bare wire underneath.

   (a)

   Calculate the power of the straightener while it is heating up.

   [1 mark]

   • A1150 W
   • B46 W
   • C225 W
   • D1380 W

   (b)

   The straightener's metal casing is connected to earth. If the exposed live wire touches the casing, what is the immediate effect of the earth connection?

   [1 mark]

   • AThe casing becomes permanently magnetised
   • BA large current flows through the earth wire, blowing the fuse or tripping the circuit breaker
   • CThe current in the straightener's heating element doubles
   • DThe mains supply automatically switches from a.c. to d.c.

   (c)

   Explain how earthing protects the salon worker from electric shock if the exposed wire touches the metal casing.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A carpentry workshop's electric planer is fitted with a fuse in its three-pin plug. The planer operates from the 230 V mains and normally draws a current of 4 A, but a fault in the motor causes the current to rise suddenly to 13 A.

   (a)

   Calculate the energy transferred by the planer during 120 s of normal operation.

   [1 mark]

   • A27 600 J
   • B920 J
   • C110 400 J
   • D55 200 J

   (b)

   Which fuse rating is the most appropriate choice for the planer's normal operation?

   [1 mark]

   • A30 A
   • B1 A
   • C13 A
   • D5 A

   (c)

   Explain why a 5 A fuse protects the planer during the fault described.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An industrial bakery uses a large electric oven connected to the 230 V mains supply. The oven draws a current of 20 A and is switched on for 3 hours each working day.

   (a)

   Calculate the power of the oven and the energy it transfers during one 3-hour day of use.

   [3 marks]

   (b)

   Explain the difference between the a.c. mains supply used by the oven and a d.c. supply from a battery, and explain why the oven's heating element still transfers electrical energy to thermal energy regardless of which type of supply is used.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the late 19th century, engineers debated whether electricity distribution networks should use direct current (d.c.) or alternating current (a.c.). Today, national grids, including in the UAE, distribute electricity as a.c., which is stepped up to a high voltage for efficient transmission and stepped down again for safe domestic use.

   (a)

   Describe the difference between an alternating current (a.c.) supply and a direct current (d.c.) supply, and explain why domestic heating appliances transfer electrical energy to thermal energy regardless of which type of supply they use.

   [6 marks]

   (b)

   Explain how fuses, circuit breakers, earthing and double insulation each protect users and appliances connected to the mains supply, and evaluate which of these features would be most important for an appliance with a plastic outer casing compared with one with a metal outer casing.

   [6 marks]

   Total for question 4: 12 marks

End of questions