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Unit 9: Electricity

Practical Circuits — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 9 parts Lesson 11 of 12 1 quiz

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Practical circuits — HIA INTERNATIONAL notes: Use circuit symbols, variable resistors and meters to draw and build practical circuits.Symbols used — cell, battery, lamp, switch (open and closed), fixed resistor, variable resistor, ammeter, voltmeter, buzzer, fuse, motor, LED, LDR…

  • Draw wires as straight lines with no gaps
  • A switch next to the cell
  • Its symbol is a resistor symbol with an arrow through it
  • Increasing the resistance decreases the current
  • Decreasing the resistance increases the current
  • LDR (light-dependent resistor)

Part 1 of 7Symbols and meter connections

Symbols and meter connections
Symbols used
  • Symbols used cell, battery, lamp, switch, fixed resistor, variable resistor, ammeter, voltmeter, buzzer, fuse, motor, LED, LDR and thermistor.
    • (open and closed)
Draw wires as straight lines with no gaps
  • Draw wires as straight lines with no gaps a dot marks a junction.
Ammeter
  • Ammeter connected in series and has a very low resistance.
Voltmeter
  • Voltmeter connected in parallel and has a very high resistance.
A switch next to the cell
  • A switch next to the cell controls the whole circuit, whereas a switch in a branch controls only that branch.

Part 2 of 7Variable resistors

Variable resistors
Variable resistor
Variable resistor a component used to change the resistance in a circuit.
Its symbol is a resistor symbol with an arrow through it
  • Its symbol is a resistor symbol with an arrow through it a fixed resistor has no arrow.
Increasing the resistance decreases the current
  • Increasing the resistance decreases the current so a lamp gets dimmer.
Decreasing the resistance increases the current
  • Decreasing the resistance increases the current so a lamp gets brighter.
Uses
  • Uses dimmer switches for lamps and volume controls in music players; also fan or motor speed control.

Part 3 of 7Sensors in circuits

Sensors in circuits
LDR (light-dependent resistor)
  • LDR (light-dependent resistor) its resistance decreases as light intensity increases, used in automatic street lights.
Thermistor
  • Thermistor its resistance decreases as temperature increases, used in fire alarms and thermostats.
LED
  • LED a light-emitting diode that lets current flow in one direction only.

Part 4 of 7Everyday circuits

1Refrigerator light
  • Refrigerator light a switch in series with the lamp is closed when the door is open, so the lamp lights.
2Buzzer with adjustable loudness
  • Buzzer with adjustable loudness a variable resistor in series with the buzzer changes the sound.
3Car door and headlamp warning
  • Car door and headlamp warning the buzzer is in the same branch as the door switch S2, so it sounds only if the headlamp switch S1 is also closed.
4Battery voltage indicator
  • Battery voltage indicator a voltmeter in parallel across the battery with a switch in its branch.

Part 5 of 7Using resistors to get a lower voltage

1Two resistors of equal resistance in series with a battery
  • Two resistors of equal resistance in series with a battery share the battery voltage equally.
2
A 6 V lamp can be connected in parallel with one of two equal resistors across a 12 V battery — it then receives 6 V and works properly.

Part 6 of 7Fuses and safety

Fuses and safety
Fuse
Fuse a thin wire that melts when the current is too large, breaking the circuit and preventing overheating and fires.
Choose the fuse rating just above the normal working current
  • Choose the fuse rating just above the normal working current never replace a fuse with one of a higher rating.
A circuit breaker
  • A circuit breaker can be reset, whereas a blown fuse must be replaced.
UK mains wires
  • UK mains wires the earth wire is green and yellow, live is brown and neutral is blue.
Double-insulated appliances
  • Double-insulated appliances do not need an earth wire.

Part 7 of 7Designing and building circuits

1
Write a risk assessment for building electric circuits.
2
Draw the circuit diagram using standard symbols.
3
Ask the teacher to check the risk assessment and diagram.
4
Build the circuit with the switch open and the correct meter connections.
5
Close the switch, read the meters and switch off between changes.
6
Compare what happens with what you expected.

Key terms

TermMeaning
Variable resistorA resistor whose resistance can be changed.
Fixed resistorA resistor with a set resistance.
FuseA thin wire that melts when the current is too large.
Circuit breakerA switch that trips when the current is too large and can be reset.
LDRA resistor whose resistance falls as light intensity increases.
ThermistorA resistor whose resistance falls as temperature increases.
Dimmer switchA control that uses a variable resistor to change lamp brightness.
Earth wireThe safety wire that gives fault current a safe path.

Avoid these mistakes

1
Using a variable resistor symbol without an arrow — the arrow shows the resistance can change.
2
Putting a fuse of a higher rating in a circuit — this removes the protection.
3
Increasing the resistance to make a lamp brighter — a higher resistance makes it dimmer.
4
Connecting a voltmeter in series in a practical circuit — it goes in parallel across the component.

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