Electrical CircuitsCopy
Cheat Sheet: Electric Circuits (O Level / IGCSE Physics)
4.3.1 Circuit Diagrams and Components
Symbols and Behavior in Circuits
| Component | Symbol Function / Behavior |
|---|---|
| Cell / Battery | Provides d.c. voltage (battery = multiple cells) |
| Power Supply / Generator | Provides energy to circuit (often a.c. generator) |
| Oscilloscope | Displays voltage over time (used for waveforms) |
| Potential Divider | Splits voltage across two or more resistors |
| Switch | Opens or closes circuit |
| Fixed Resistor | Limits current, has constant resistance |
| Variable Resistor | Allows manual change of resistance (used to adjust brightness, volume, etc.) |
| Heater | Converts electrical energy into heat |
| NTC Thermistor | Resistance decreases as temperature increases (used in temperature sensors) |
| LDR (Light-Dependent Resistor) | Resistance decreases as light intensity increases |
| Lamp | Converts electrical energy to light and heat |
| Motor | Converts electrical energy to kinetic energy |
| Ammeter | Measures current (connected in series) |
| Voltmeter | Measures potential difference (connected in parallel) |
| Magnetising Coil | Produces magnetic field when current flows |
| Transformer | Changes a.c. voltage levels (step-up/step-down) |
| Fuse | Safety device; melts when current exceeds safe limit |
| Relay | Electrically operated switch (uses small current to control large current) |
| Diode | Allows current to flow in one direction only |
| LED (Light Emitting Diode) | Diode that emits light when current flows (used in indicators, displays) |
4.3.2 Series and Parallel Circuits
1. Circuit Rules
| Property | Series Circuit | Parallel Circuit |
|---|---|---|
| Current (I) | Same at all points | Splits at junctions: I_total = I₁ + I₂ + … |
| Voltage (V) | V_total = V₁ + V₂ + … | Same across each branch |
| Resistance (R) | R_total = R₁ + R₂ + … | 1/R_total = 1/R₁ + 1/R₂ for two resistors |
2. Combined Resistance
- Series:
R_total = R₁ + R₂ + R₃ + … - Parallel (2 resistors):
R_total = (R₁ × R₂) / (R₁ + R₂)
3. Ohm’s Law Applications
- Use:
V = I × R
to calculate current, voltage, or resistance anywhere in the circuit
4.3.3 Circuit Components – Action and Use
1. NTC Thermistor
- Resistance decreases as temperature increases
- Use: temperature sensor in thermostats, fire alarms
2. LDR (Light-Dependent Resistor)
- Resistance decreases as light intensity increases
- Use: light sensors in streetlights, cameras
3. Variable Potential Divider
- A pair of resistors share voltage (from a power source)
- Changing one resistor changes the output voltage
- Used in volume controls, sensor circuits, etc.
4. Potential Divider Equation (for 2 resistors)
R1/R2 = V1/V2
Where:
R1 and R2 = resistor values
V1 and V2 = voltages across each resistor
Can also find output voltage across R2 using:
Vout = (R2 / (R1 + R2)) * Vtotal