Practical Circuits

2026 What You Need to Know (Syllabus Objectives)

By the end of this topic, you should be able to:

  1. Recall and use the circuit symbols shown in section 6 of this syllabus
  2. Draw and interpret circuit diagrams containing these circuit symbols
  3. Define and use the electromotive force (e.m.f.) of a source as energy transferred per unit charge in driving charge around a complete circuit
  4. Distinguish between e.m.f. and potential difference (p.d.) in terms of energy considerations
  5. Understand the effects of the internal resistance of a source of e.m.f. on the terminal potential difference

1. Circuit Symbols

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When we draw electrical circuits, we use circuit symbols instead of drawing the actual components. These are simple diagrams that represent different electrical parts. You need to memorize these symbols and be able to recognize them in circuit diagrams.

Common Circuit Symbols

Here are the most important symbols you'll need to know:

Power Sources:

  • Cell — Two parallel lines (one longer representing positive, one shorter and thicker representing negative)
  • Battery of cells — Multiple cell symbols connected in series (one after another)

Resistors:

  • Fixed resistor — A simple rectangle
  • Variable resistor — A rectangle with a diagonal arrow through it (the arrow shows you can change its resistance)
  • Potentiometer — A rectangle with a diagonal arrow pointing to it (used to vary voltage in a circuit)

Special Resistors:

  • Thermistor — A rectangle with a line through it that has a flat horizontal section at the bottom left (its resistance changes with temperature)
  • Light-dependent resistor (LDR) — A rectangle inside a circle with two arrows pointing towards it (its resistance changes with light intensity)

Other Components:

  • Diode — A triangle pointing to a vertical line (allows current to flow in one direction only)
  • Light-emitting diode (LED) — A diode symbol with two arrows pointing away from it (lights up when current flows through)
  • Switch — Two dots with a line that can be open (off) or closed (on)
  • Lamp — A circle with an 'X' inside
  • Capacitor — Two parallel lines of equal length

Measuring Instruments:

  • Ammeter — A circle with the letter 'A' inside (measures current)
  • Voltmeter — A circle with the letter 'V' inside (measures voltage)
  • Galvanometer — A circle with an upward-pointing arrow inside (detects small currents)

Connection:

  • Junction of conductors — A dot where two or more lines meet (shows wires are connected)

How Components Work

  • Switch: Turns the circuit on (when closed) or off (when open)
  • Fixed resistor: Limits the flow of current and cannot be changed
  • Variable resistor: Can be adjusted using a slider to change resistance (used in dimmer switches and volume controls)
  • Thermistor: As temperature increases, its resistance decreases
  • LDR: As light intensity increases, its resistance decreases
  • Diode: Only allows current to flow in one direction (converts AC to DC)
  • LED: Works like a diode but also emits light
  • Ammeter: Must be connected in series with the component you're measuring
  • Voltmeter: Must be connected in parallel with the component you're measuring

Drawing and Interpreting Circuit Diagrams

When you draw a circuit diagram:

  • Use straight lines to connect components
  • Make sure switches are in the correct position (open or closed)
  • Check that diodes point in the direction current should flow (from positive to negative)
  • Remember: current flows from positive (+) to negative (−) terminal

Example: For a circuit to work with a diode, two conditions must be met:

  1. The switch must be closed
  2. The diode must be forward-biased (pointing in the direction of current flow)

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