Current, potential difference and resistanceAQA A-Level Physics: Subtopic test

1. 1
   A mobile phone is charged for 40 minutes with a constant current of 1.5 A. The pd across the phone's battery is 3.7 V while it charges.

   (a)

   What is the total charge that flows during the 40 minutes?

   [1 mark]

   • A3.6 × 10³ C
   • B60 C
   • C1.6 × 10³ C
   • D2.4 × 10³ C

   (b)

   The phone is now charged with a constant current of 0.75 A. How long does it take for the same total charge to flow?

   [1 mark]

   • A20 minutes
   • B40 minutes
   • C160 minutes
   • D80 minutes

   (c)

   State what is meant by a pd of 3.7 V across the battery, and calculate the energy transferred to the battery while 3.6 × 10³ C of charge flows through it.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student connects a resistor to a power supply. When the pd across the resistor is 6.0 V, the current through it is 24 mA. The resistor is an ohmic conductor and its temperature stays constant.

   (a)

   What is the resistance of the resistor?

   [1 mark]

   • A0.25 Ω
   • B4.0 × 10⁻³ Ω
   • C250 Ω
   • D144 Ω

   (b)

   The pd is changed until the current is 36 mA. What is the new pd across the resistor?

   [1 mark]

   • A4.0 V
   • B9.0 V
   • C12 V
   • D9.0 × 10³ V

   (c)

   The pd of 6.0 V is applied for 5.0 minutes. Calculate the charge that passes through the resistor and the energy transferred to the resistor in this time.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A car starter motor is supplied by a 12 V battery. During one start the motor draws a constant current of 150 A for 3.0 s. The battery can supply a total charge of 1.6 × 10⁵ C before it is flat.

   (a)

   Calculate the charge that flows through the starter motor during one start, the energy transferred to the motor, and the resistance of the motor circuit.

   [3 marks]

   (b)

   Calculate the maximum number of complete starts the fully charged battery could supply. The driver then leaves the headlamps on, which draw a constant current of 8.0 A from the fully charged battery. Calculate how long, in hours, the battery takes to go flat.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A bird perches on a bare overhead cable that carries a current of 400 A. The section of the cable between the bird's feet has a resistance of 2.0 × 10⁻⁵ Ω. The body of the bird, from one foot to the other, has a resistance of 5.0 × 10³ Ω. The pd across the bird's body is the same as the pd across the section of cable between its feet.

   (a)

   Calculate the pd across the bird's body and the current through the bird. Calculate the charge that passes through the bird in 10 minutes and the energy transferred to the bird in this time. Show that the current in the bird is a tiny fraction of the current in the cable.

   [6 marks]

   (b)

   Explain why the bird is not harmed even though the cable carries a very large current, but a person who touches the cable while standing on the ground could be seriously harmed.

   [6 marks]

   Total for question 4: 12 marks

End of questions