An electric circuit comprises a cell (or battery), a plug key, electrical component(s), and connecting wires. It is often convenient to draw a schematic diagram, using standardized symbols to represent different components.
| Component | Symbol | Description |
|---|---|---|
| Electric Cell | ⊕━⊖ | Single power source |
| Battery | ⊕━⊖⊕━⊖ | Multiple cells in series |
| Switch (Open) | ━ ○ ━ | Circuit breaker (off) |
| Switch (Closed) | ━•━ | Circuit breaker (on) |
| Bulb | ⊗ | Light source |
| Resistor | ━▭━ | Resistance element |
| Ammeter | ━(A)━ | Current measurement |
| Voltmeter | ━(V)━ | Voltage measurement |
In 1827, Georg Simon Ohm discovered the relationship between current and potential difference. The potential difference (V) across the ends of a metallic wire is directly proportional to the current (I) flowing through it, provided temperature remains constant.
Where:
- V = Potential Difference (Volts, V)
- I = Current (Amperes, A)
- R = Resistance (Ohms, Ω)
Key Facts:
- 1 Ohm = 1 Volt / 1 Ampere
- R = V/I (resistance calculation)
- I = V/R (current calculation)
- Resistance opposes current flow
- Named after Georg Simon Ohm (1787-1854)
Activity 11.1 - Exploring Ohm's Law
Setup: Circuit with nichrome wire (0.5m), ammeter, voltmeter, and 1-4 cells (1.5V each)
Observations:
| No. of Cells | Current (A) | Voltage (V) | V/I (Ω) |
|---|---|---|---|
| 1 | 0.15 | 1.5 | 10.0 |
| 2 | 0.30 | 3.0 | 10.0 |
| 3 | 0.45 | 4.5 | 10.0 |
| 4 | 0.60 | 6.0 | 10.0 |
💡 The V/I ratio remains constant! This demonstrates Ohm's Law - the linear relationship between voltage and current.
Activity 11.2 - Comparing Resistances
Setup: Test circuit with different components: nichrome wire, torch bulb, 10W bulb
Results:
💡 Different components offer different resistances to current flow. A rheostat can be used to vary resistance in a circuit.
Practice Questions
- What is the purpose of using circuit symbols?
- State Ohm's Law and write its mathematical expression.
- If V = 12V and I = 2A, calculate the resistance.
- Why does current differ for different components in the same circuit?
Answers:
1. To represent circuit components in a standardized, simple way
2. V ∝ I or V = IR (at constant temperature)
3. R = V/I = 12V/2A = 6Ω
4. Different materials offer different resistance to electron flow