Level: JHS, Basic 8 | Subject: Integrated Science | Strand: Forces and Energy | Topic: Electricity and Electronics (Ohm’s Law)
Curriculum reference: B8.4.2.1 Demonstrate understanding of current electricity and Ohm’s Law. Indicator B8.4.2.1.1: Apply Ohm’s Law in solving simple electrical circuits. All the exemplars are merged into one set of notes.
1. Parts of a Simple Electric Circuit
A circuit is a complete path along which electric current flows. It needs a source of energy, wires and a load. A switch controls the circuit.
| Part | Job |
|---|---|
| Cell or battery | Supplies the energy (voltage) that pushes the current |
| Wires | Carry the current |
| Switch | Opens or closes the circuit. The bulb lights only when the switch is closed. |
| Bulb | Changes electrical energy to light and heat |
| Resistor | Reduces the current |
| Ammeter | Measures current. It is connected in series. |
| Voltmeter | Measures voltage. It is connected in parallel across the part being measured. |
2. Current, Voltage and Resistance
| Quantity | Meaning | Symbol | Unit |
|---|---|---|---|
| Current | Flow of electric charge | I | ampere (A) |
| Voltage (potential difference) | The push that drives the current | V | volt (V) |
| Resistance | Opposition to the flow of current | R | ohm |
3. Ohm’s Law
Ohm’s Law states that the current through a conductor is directly proportional to the voltage across it, provided the temperature and other conditions stay the same.
V = I x R
From this: I = V / R and R = V / I.
If you plot voltage against current, you get a straight line through the origin. The slope is the resistance.
4. Worked Examples
Example 1 (voltage). A current of 2 A flows through a resistor of 6 ohms. V = I x R = 2 x 6 = 12 V.
Example 2 (current). A 12 V battery is joined to a bulb of resistance 4 ohms. I = V / R = 12 / 4 = 3 A.
Example 3 (resistance). A voltage of 9 V drives a current of 0.5 A through a wire. R = V / I = 9 / 0.5 = 18 ohms.
5. Series and Parallel Circuits
| Series | Parallel | |
|---|---|---|
| Path for current | One path only | More than one path |
| Current | Same everywhere | Shared between branches |
| Voltage | Shared between components | Same across every branch |
| If one bulb breaks | All the others go off | Others stay on |
| Total resistance | R = R1 + R2 + R3 | Less than the smallest resistor |
Example 4 (series). Resistors of 3 ohms and 5 ohms are joined in series to an 8 V cell. Total R = 3 + 5 = 8 ohms. I = 8 / 8 = 1 A.
Household sockets and lamps in a house are wired in parallel so that each works on its own.
6. Safety Rules
- Never touch bare wires or sockets with wet hands.
- Do not overload a socket with many appliances.
- Use fuses and circuit breakers.
- Switch off and unplug before repairing an appliance.
- Keep children away from electric poles and fallen wires.
Remember: V = I x R. Cover the quantity you want in the triangle and the other two show the formula.
Ready to practise?
Education Always (education.inspiringalways.com). Original notes written to follow the NaCCA Basic 8 Integrated Science curriculum.