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E.m.f. and internal resistanceEdexcel A-Level Physics: Flashcards

What these 13 flashcards ask

  • Define e.m.f.
  • Define terminal p.d.
  • What is meant by internal resistance?
  • Write the equation linking e.m.f., current and resistances.
  • Write the equation for terminal p.d.
  • What are 'lost volts'?
  • When is the terminal p.d. equal to the e.m.f.?
  • What is the short-circuit current of a source?
  • What does the y-intercept of a graph of V against I give?
  • What does the gradient of a graph of V against I give?
  • Why switch off between readings in the e.m.f. practical?
  • Give the power wasted inside a cell carrying current I.
  • Why do car headlamps dim when the starter motor operates?

Exam questions on E.m.f. and internal resistance

  1. A cell of e.m.f. 1.5 V and internal resistance 0.50 Ω is connected in series with a 2.5 Ω resistor and a switch.
    Calculate the energy transferred as thermal energy in the internal resistance of the cell in 60 s.2 marks
  2. A student measures the terminal p.d. V of a battery for different currents I drawn from it, using a variable resistor. She plots V against I and draws a straight line of best fit. The line meets the V axis at 6.2 V and has a gradient of −0.80 V A⁻¹.
    Explain why the terminal p.d. of the battery is less than its e.m.f. when a current is drawn.2 marks
  3. A 12 V car battery has an internal resistance of 0.020 Ω. The headlamps are connected across the battery terminals. When the driver starts the engine, the starter motor draws a current of 150 A from the battery.
    Calculate the terminal p.d. of the battery while the starter motor is operating.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).