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Course: Science — Third Preparatory
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Science — Third Preparatory

Science — Third Preparatory

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Science — Third Preparatory 2026/2027

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Unit 1 - Force, Motion, Acceleration and Physical Quantities

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Text lesson

Unit 2 — Electricity and Magnetism

Science — Third Preparatory

Unit 2 — Electricity and Magnetism

1.4 Electric sources and electromotive force

1.4 Electric sources and electromotive force

  • An electric source supplies electrical energy and drives charges around the circuit. There are two kinds:
Key points
Direct-voltage sources, such as an electric cell, a battery. They produce direct current (DC), of constant intensity and in a single direction.
Alternating-voltage sources, such as an alternating-current generator. They give alternating current (AC), whose value and direction change with time.
  • The electromotive force (emf, ε) of a source is the total work done by the source to move one coulomb of charge through the entire circuit (inside and outside the source).
  • It is measured in volts.
  • Current also flows through the source itself, so the emf is divided into two parts: the potential difference across the battery terminals (V), which is used in the external circuit, and the potential difference lost inside the source to overcome its internal resistance.
  • So ε is slightly larger than V (ε > V).
  • In this unit we assume the internal resistance is negligible, so V (terminal) = ε.
  • This is why the question "the potential difference between a battery’s terminals is usually less than its emf" has the answer: because some energy is used inside the battery against its internal resistance.
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