Practical circuits

AS Level Physics · D.C. circuits. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A cell or battery gives energy to the charge that passes through it. Its e.m.f. is the energy transferred per unit charge in driving charge around the complete circuit. It is energy changed from other forms (such as chemical) to electrical. A potential difference is the opposite change: electrical energy per unit charge changed to other forms in a component.

A real source has internal resistance r. When current flows, some energy is wasted inside the source, so the p.d. across its terminals is less than the e.m.f. The difference, Ir, is often called the lost volts. A larger current gives more lost volts and a lower terminal p.d.

Rules to remember

Common mistake

Students use I = E/R and ignore the internal resistance. The current is e.m.f. ÷ total resistance, and the total includes r.

Worked example

A battery of e.m.f. 9.0 V supplies a current of 0.50 A to a resistor. The p.d. across the battery terminals is then 8.0 V. What is the internal resistance of the battery?

  1. Lost volts = E − V = 9.0 − 8.0 = 1.0 V.
  2. Lost volts = Ir, so r = 1.0 ÷ 0.50.
  3. r = 2.0 Ω.

Answer: The internal resistance is 2.0 Ω.

Practice questions

Try each one, then open the answer.

1. In the standard circuit symbols, a rectangle with an arrow drawn diagonally through it represents

  1. A
    a thermistor
  2. B
    a fuse
  3. C
    a light-dependent resistor
  4. D
    a variable resistor
Show answer

Answer: D. The diagonal arrow through the resistor rectangle shows that the resistance can be varied. An LDR is drawn as a resistor inside a circle with arrows pointing in.

2. A car battery delivers a very large current when starting the engine, and the headlamps dim. Why?

  1. A
    the e.m.f. of the battery falls
  2. B
    the large current causes a large p.d. drop across the battery's internal resistance, lowering the terminal p.d.
  3. C
    the headlamp resistance rises
  4. D
    the starter motor absorbs the light
Show answer

Answer: B. V = E − Ir. A large I makes Ir significant, so the terminal p.d. available to the lamps falls.

3. Electromotive force (e.m.f.) is defined as

  1. A
    the force on electrons in a wire
  2. B
    the energy transferred per unit charge in driving charge round a complete circuit
  3. C
    the p.d. across the external resistor
  4. D
    the current from a cell
Show answer

Answer: B. E.m.f. is energy converted (from chemical, for a cell) per unit charge, in joules per coulomb (volts).

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