CCEA A Level Physics

CCEA A Level Physics 1.11 Internal Resistance and E.M.F. and 1.12 Potential Divider Circuits Exam Questions

Free CCEA A Level Physics revision for 1.11 Internal Resistance and E.M.F. and 1.12 Potential Divider Circuits: what you need to know, common mistakes and exam-style questions with answers. Covers internal resistance, terminal p.d., lost volts, measuring e.m.f. and internal resistance, potential dividers, sensors and loading effects.

Practise 1.11 Internal Resistance and E.M.F. and 1.12 Potential Divider Circuits with past papers

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About CCEA A Level Physics 1.11 Internal Resistance and E.M.F. and 1.12 Potential Divider Circuits

These two sections complete the electricity content of Unit AS 1: Forces, Energy and Electricity. Internal resistance explains why the terminal p.d. of a real cell falls as it delivers more current, and you need to use V = E - Ir and describe how to measure E and r from a graph. Potential dividers show how a fixed supply gives a variable output voltage, and how adding a thermistor or LDR makes a circuit respond to temperature or light. You also need to calculate what happens when a load is connected across the output. Expect a graph-based experiment question and a two-step loaded divider calculation.

What you need to know

  • Internal resistance Real sources have internal resistance r. E = I(R + r), so terminal p.d. V = E - Ir. The term Ir is the lost volts: energy per coulomb transferred inside the source.
  • Effects of internal resistance As the external resistance decreases the current rises, more p.d. is lost inside the source and the terminal p.d. falls. A car battery has a very small r so it can deliver large starting currents.
  • Measuring E and r Connect the cell to a variable resistor with an ammeter in series and a voltmeter across the terminals. Record V and I for several settings and plot V against I: the y-intercept is E and the gradient is -r.
  • Potential divider Two resistors in series across a supply share the p.d. in the ratio of their resistances: Vout = Vin x R2/(R1 + R2).
  • Sensing circuits With an NTC thermistor or LDR in the divider, a change in temperature or light changes its resistance and so the output voltage. This can switch heating or lighting on and off.
  • Loading A load connected across the output is in parallel with that resistor, lowering the combined resistance and so the output voltage. Work out the parallel combination first, then use the divider equation.

Common mistakes in 1.11 Internal Resistance and E.M.F. and 1.12 Potential Divider Circuits

  • Treating the voltmeter reading across the terminals as the e.m.f. when current flows; it is E - Ir.
  • Saying the e.m.f. changes when the external resistor changes; E stays constant and the terminal p.d. changes.
  • Putting the wrong resistance on top of the fraction in Vout = Vin x R2/(R1 + R2); R2 is the resistor the output is taken across.
  • Ignoring the load resistance in a loaded potential divider; combine it in parallel with the output resistor first.

Try three 1.11 Internal Resistance and E.M.F. and 1.12 Potential Divider Circuits exam-style questions

1. A cell of e.m.f. 1.5 V and internal resistance 0.50 Ω is connected to a 2.5 Ω resistor. Calculate the current and the terminal p.d. (3 marks)

Show the mark scheme answer

I = E/(R + r) = 1.5/3.0 = 0.50 A; V = IR = 0.50 x 2.5; = 1.25 V (or 1.5 - 0.50 x 0.50).

2. A 12 V supply is connected across a 4.0 kΩ resistor in series with an 8.0 kΩ resistor. A voltmeter of resistance 8.0 kΩ is connected across the 8.0 kΩ resistor. Calculate the output before and after the voltmeter is connected. (4 marks)

Show the mark scheme answer

Before: Vout = 12 x 8.0/12 = 8.0 V; parallel combination of 8.0 kΩ and 8.0 kΩ = 4.0 kΩ; after: Vout = 12 x 4.0/8.0; = 6.0 V.

3. Explain how a potential divider containing an NTC thermistor gives an output that rises as temperature falls. (3 marks)

Show the mark scheme answer

The output is taken across the thermistor; as temperature falls its resistance rises; it takes a larger share of the supply p.d., so Vout across it rises.

1.11 Internal Resistance and E.M.F. and 1.12 Potential Divider Circuits FAQs

What does CCEA A Level Physics 1.11 and 1.12 cover?

Internal resistance and e.m.f., terminal p.d., measuring E and r from a graph, potential divider circuits with sensors, and the effect of a load on the output.

Which unit are 1.11 and 1.12 examined in?

Both are in Unit AS 1: Forces, Energy and Electricity.

What graph do I need for internal resistance?

Terminal p.d. V against current I. It is a straight line with y-intercept E and gradient -r, from V = E - Ir.