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Action and Use of Circuit ComponentsCambridge IGCSE Physics: Revision notes

Section 1

Recognising Circuit Components

Circuit diagrams contain a wide range of components, each with its own standard symbol and function: cells, batteries, power supplies, generators, potential dividers, switches, fixed and variable resistors, heaters, thermistors (NTC only), light-dependent resistors (LDRs), lamps, motors, ammeters, voltmeters, magnetising coils, transformers, fuses and relays.

  • Thermistors (NTC) decrease in resistance as temperature rises, so they are used in temperature-sensing circuits.
  • LDRs decrease in resistance as light intensity rises, so they are used in light-sensing circuits.
Key termsthermistorLDR
Exam tip

Diodes and LEDs also have standard circuit symbols and behave differently in a circuit — they conduct in forward bias and block current in reverse bias.

Section 2

The Variable Potential Divider

A potential divider is used to produce a variable output voltage from a fixed supply voltage.

  • It usually consists of two resistors (or a variable resistor) connected in series across a supply.
  • The output voltage is taken across one of the resistors, and its value depends on the ratio of the two resistances.
  • Moving a slider on a variable resistor changes this ratio, allowing the output voltage to be smoothly adjusted.
Key termspotential divider

Section 3

The Potential Divider Equation

For two resistors R1 and R2 used as a potential divider, the voltages across them are related to their resistances.

R1 / R2 = V1 / V2

This means the resistor with the larger resistance has the larger share of the supply voltage across it.

Example

If R1 = 2 x R2 in a potential divider, then V1 must also be 2 x V2, since the ratio R1/R2 equals V1/V2.

Section 4

Circuit Uses of the Magnetic Effect of a Current

The magnetic effect of a current has practical uses in circuit components.

  • In a relay, a small current in an electromagnet coil attracts a switch, allowing a small control current to switch a much larger current on or off — useful for safely controlling high-power devices.
  • In a loudspeaker, a varying current in a coil placed in a magnetic field causes the coil (and an attached cone) to vibrate, producing sound.
Key termsrelayloudspeaker
Think of it like this

A relay works like a small key that unlocks a much bigger door — a tiny control current does the 'unlocking' that lets a large current flow.

Must Know

  • Thermistors (NTC) and LDRs both decrease in resistance with rising temperature or light respectively, and are used as sensors.
  • Diodes and LEDs conduct in forward bias only.
  • A potential divider produces a variable output voltage from a fixed supply, using two resistors in series.
  • Potential divider equation: R1/R2 = V1/V2.
  • Relays use the magnetic effect of a current to let a small current switch a large one; loudspeakers use it to convert current into sound.

That's the notes covered.

Carry on to the next subtopic.