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Build a circuit · Physics

Lamp, resistor and switch

What it is for

A lamp lights, a resistor limits current, a switch breaks the circuit.

Mistake to avoid Using a lamp when you need a fixed resistance: its filament’s resistance rises as it heats, so its current–p.d. graph curves (Supplement).

Drawn in ink: the site's lab kit has no model of it yet, so the one at school may look a little different.

How it is asked

  • Draw and recognise the circuit symbols for a lamp, a fixed resistor and a switch, open or closed.
  • Say what each does: a resistor limits the current, a switch opens or closes the circuit, a lamp lights as current passes.
  • Calculate resistance with R = V ÷ I. Explain the lamp’s curved current–p.d. graph: a hotter filament has a higher resistance (Supplement).

Exam-style question

A student connects a cell, a switch, an ammeter and a fixed resistor in series, with a voltmeter across the resistor.

  1. Draw the circuit diagram, using standard symbols.
  2. The voltmeter reads 3.0 V and the ammeter 0.20 A. Calculate the resistance.
  3. The resistor is replaced by a filament lamp. Why does V ÷ I rise as the p.d. is increased? (Supplement)
Show what a good answer says
  1. One series loop: cell, switch, ammeter and fixed resistor, in standard symbols; the voltmeter in parallel, across the resistor only.
  2. R = V ÷ I = 3.0 ÷ 0.20 = 15 Ω.
  3. The filament gets hotter as the current increases, and the resistance of the metal rises with its temperature.

Teaching points for checking your own answer, not official Cambridge marks.

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