E.m.f. and internal resistanceEdexcel International A Level Physics: Subtopic test

1. 1
   A technician connects a high-resistance voltmeter across the terminals of a new cell with nothing else in the circuit and reads 1.60 V. She then connects a 4.0 Ω resistor across the cell and the voltmeter reading falls to 1.50 V.

   (a)

   Which statement gives the correct definition of the e.m.f. of the cell?

   [1 mark]

   • AThe potential difference across the terminals of the cell when a current is flowing
   • BThe energy dissipated per coulomb in the external circuit
   • CThe force the cell exerts on each coulomb of charge
   • DThe electrical energy transferred to each coulomb of charge by the cell

   (b)

   Why does the voltmeter reading fall from 1.60 V to 1.50 V when the resistor is connected?

   [1 mark]

   • AThe e.m.f. of the cell decreases when a current flows
   • BEnergy is dissipated per coulomb in the internal resistance, leaving less across the terminals
   • CThe voltmeter resistance falls when the resistor is connected
   • DThe resistor uses up some of the charge in the cell

   (c)

   Calculate the internal resistance of the cell.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A cell of e.m.f. 3.0 V and internal resistance 0.50 Ω is connected in series with a lamp of resistance 5.5 Ω.

   (a)

   Calculate the current in the circuit.

   [1 mark]

   • A0.50 A
   • B0.55 A
   • C0.60 A
   • D6.0 A

   (b)

   What is the terminal potential difference of the cell while this current flows?

   [1 mark]

   • A3.0 V
   • B0.25 V
   • C2.75 V
   • D2.5 V

   (c)

   Calculate the energy dissipated in the internal resistance of the cell in 60 s.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In Core Practical 8 a student connects a cell, a variable resistor, an ammeter and a high-resistance voltmeter across the terminals of the cell. She records the terminal potential difference V for several different currents I and plots V against I. Her line of best fit has an intercept on the V axis of 1.58 V and a gradient of −0.62 V A⁻¹.

   (a)

   Explain how the e.m.f. and the internal resistance of the cell are found from the line, and state their values.

   [3 marks]

   (b)

   Calculate the largest current this cell could supply and suggest two reasons why the student should switch off the circuit between readings.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A designer is choosing between two cells for a torch. Cell X has e.m.f. 1.5 V and internal resistance 0.20 Ω. Cell Y has e.m.f. 1.5 V and internal resistance 2.0 Ω. Each cell is connected in turn across a lamp of resistance 3.0 Ω, and the designer wants the lamp to be as bright as possible.

   (a)

   Explain what is meant by e.m.f. and by terminal potential difference, and explain why the terminal potential difference of a cell falls as the current drawn from it increases.

   [6 marks]

   (b)

   Calculate the current and the terminal potential difference for each cell, and the power dissipated in the lamp. Hence evaluate which cell the designer should choose.

   [6 marks]

   Total for question 4: 12 marks

End of questions