Electrical power and I-V characteristicsEdexcel International A Level Physics: Subtopic test

1. 1
   A 230 V electric heater has a heating element of resistance 46 Ω. It is connected to a 230 V mains supply and works at its normal operating temperature.

   (a)

   What is the power of the heater?

   [1 mark]

   • A1.15 × 10³ W
   • B5.0 W
   • C0.20 W
   • D2.4 × 10⁶ W

   (b)

   The heater is switched on for 30 minutes. How much energy does it transfer?

   [1 mark]

   • A1.2 × 10³ J
   • B3.5 × 10⁴ J
   • C2.1 × 10⁶ J
   • D0.64 J

   (c)

   Calculate the current in the heater and use it to show that P = I²R gives the same power as in part (a).

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician investigates an unmarked electrical component in a sealed box with two terminals. With the potential difference applied in one direction, the current is almost zero until the p.d. reaches about 0.7 V, after which the current rises very rapidly for small further increases in p.d. With the p.d. reversed, the current stays almost zero up to 5.0 V.

   (a)

   Which component is in the box?

   [1 mark]

   • AAn ohmic resistor
   • BA filament lamp
   • CAn NTC thermistor
   • DA semiconductor diode

   (b)

   Which statement describes the resistance of the component in the forward direction as the p.d. increases from 0.8 V to 1.0 V?

   [1 mark]

   • AIt increases
   • BIt decreases
   • CIt stays constant
   • DIt is infinite

   (c)

   At a forward p.d. of 0.80 V the current in the component is 20 mA. Calculate the power dissipated and the resistance at this p.d.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A filament lamp is labelled 12 V, 36 W. It is operated at its rated p.d. from a 12 V supply of negligible internal resistance. The resistance of the filament when cold, at room temperature, is measured as 0.40 Ω.

   (a)

   Calculate the current in the lamp at its rated p.d., the resistance of the filament at this p.d. and the energy transferred in 2.0 minutes.

   [3 marks]

   (b)

   Calculate the current at the instant of switching on and explain why it is much greater than the current in normal operation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student investigates three components, each connected to a variable power supply: a fixed resistor, a filament lamp and an NTC thermistor. She records current–potential difference data for each over the range 0–6.0 V, with the p.d. applied in both directions. She later connects the thermistor directly across a 6.0 V supply of negligible internal resistance. At the instant of connection the current is 15 mA; after a few minutes, once the thermistor has warmed up, it is 30 mA.

   (a)

   Describe the shape of the I–V graph for each of the three components and explain the differences.

   [6 marks]

   (b)

   Calculate the resistance and the power of the thermistor at the instant of connection and after it has warmed up. Explain the risk that this circuit presents and suggest how it could be reduced.

   [6 marks]

   Total for question 4: 12 marks

End of questions