Electricity
Electric Charge and Current
All matter contains electric charges. Electric current (I) is the flow of electrical charge β in metal conductors, it is electrons that flow.
Key electrical quantities:
| Quantity | Symbol | Unit | Measured with |
|---|---|---|---|
| Current | I | Amperes (A) | Ammeter (in series) |
| Voltage (potential difference) | V | Volts (V) | Voltmeter (in parallel) |
| Resistance | R | Ohms (Ξ©) | Ohmmeter or calculated |
| Power | P | Watts (W) | Calculated |
Current β the rate of flow of charge. 1 ampere = 1 coulomb of charge flowing per second.
Voltage (potential difference) β the "push" or energy per unit charge that drives current around a circuit. A 9V battery gives 9 joules of energy to each coulomb of charge.
Resistance β anything that opposes the flow of current. Resistance converts electrical energy to heat. It depends on: material, length (longer = more resistance), cross-sectional area (thinner = more resistance), and temperature.
Ohm's Law
For many conductors (ohmic conductors), voltage, current, and resistance are related by Ohm's Law:
V = I x R
Where V = voltage (volts), I = current (amperes), R = resistance (ohms)
Rearrangements:
- I = V / R (current = voltage divided by resistance)
- R = V / I (resistance = voltage divided by current)
Worked examples:
Example 1: A resistor has a voltage of 12V across it and resistance of 6Ξ©. What is the current? I = V / R = 12 / 6 = 2A
Example 2: A lamp has resistance 12Ξ© and current 2A. What is the voltage? V = I x R = 2 x 12 = 24V
Series Circuits
In a series circuit, components are connected in a single continuous loop. There is only one path for current to flow.
Rules for series circuits:
- Current is the same at every point in the circuit
- Voltage is shared between components (voltages add up to the supply voltage)
- Resistance adds up: total resistance = R1 + R2 + R3...
- If one component breaks, the entire circuit stops working
Example: Christmas lights (older style) were series circuits β one broken bulb meant all went out.
Parallel Circuits
In a parallel circuit, components are connected on separate branches. Current can take multiple paths.
Rules for parallel circuits:
- Voltage is the same across each parallel branch (equal to the supply voltage)
- Current splits between branches β each branch carries a fraction of the total current
- Total current = sum of all branch currents
- If one component breaks, others continue to work
Example: Household electrical wiring is parallel β all appliances receive the same mains voltage (230V), and switching off one appliance does not affect others.
Comparing Series and Parallel
| Feature | Series | Parallel |
|---|---|---|
| Current | Same throughout | Splits between branches |
| Voltage | Shared between components | Same across each branch |
| Total resistance | Increases (sum of all) | Decreases (less than smallest) |
| Fault effect | All components fail | Only that branch fails |
| Use | Simple switches, fuses | Household wiring, most appliances |
Electrical Power
Power is the rate at which energy is transferred. In electrical circuits:
P = I x V (power = current x voltage)
Power is measured in watts (W).
Example: A hairdryer drawing 10A from a 230V supply has power: P = 10 x 230 = 2300W (2.3 kW)
Another useful form: P = IΒ² x R (useful when you know current and resistance)
Energy and Cost
Electrical energy used: Energy (J) = Power (W) x time (s)
For bills, energy is measured in kilowatt-hours (kWh): Energy (kWh) = Power (kW) x time (hours)
Electrical Safety
Electrical systems include several safety features:
Fuses β a thin wire that melts if current exceeds a safe level, breaking the circuit. Rated in amperes (e.g., 3A, 5A, 13A). Once blown, must be replaced.
Circuit breakers β electronic switches that automatically trip when current is too high. Can be reset.
Earth wire β connects metal casing of appliances to earth (ground). If a live wire touches the casing, current flows safely to earth rather than through a person.
Insulators β plastic and rubber coatings on wires prevent accidental contact with live conductors.
Double insulation β some appliances have no earth wire because they have two layers of plastic/insulation around all live components.
Components in Circuits
| Component | Symbol role | Function |
|---|---|---|
| Battery/cell | EMF source | Provides voltage to drive current |
| Resistor | Fixed resistance | Controls current; converts energy to heat |
| Lamp/bulb | Resistor + light | Converts electrical energy to light and heat |
| Switch | Open/close circuit | Controls current flow |
| Ammeter | Measures I | Connected in series |
| Voltmeter | Measures V | Connected in parallel |
| Diode | One-way valve | Allows current in one direction only |
| LED | Light-emitting diode | Emits light when current flows |
| Thermistor | Variable resistor | Resistance decreases as temperature rises |
| LDR (light-dependent resistor) | Variable resistor | Resistance decreases as light increases |
Key Terms
| Term | Definition |
|---|---|
| Current (I) | Flow of electrical charge; measured in amperes |
| Voltage (V) | Energy transferred per unit charge; measured in volts |
| Resistance (R) | Opposition to current flow; measured in ohms |
| Ohm's Law | V = I x R |
| Series circuit | Components in a single loop; same current throughout |
| Parallel circuit | Components on separate branches; same voltage across each |
| Power (P) | Rate of energy transfer; P = I x V; measured in watts |
| Fuse | Safety device that melts to break circuit if current is too high |
Common Mistakes
- Connecting an ammeter in parallel β ammeters have very low resistance; connecting in parallel short-circuits the component. Always connect in series.
- Connecting a voltmeter in series β voltmeters have very high resistance; connecting in series blocks current. Always connect in parallel.
- Forgetting that adding resistors in series increases total resistance, but adding them in parallel decreases total resistance.
- Confusing power (W) and energy (J) β power is the rate; energy = power x time.
Tips and Tricks
- Triangle trick for Ohm's Law: draw a triangle with V on top, I and R on the bottom. Cover the one you want to find to see the formula.
- Series: Same current; Split voltage. Parallel: Same voltage; Split current.
- More bulbs in series = dimmer (each gets less voltage). More bulbs in parallel = same brightness (each gets full voltage).
- Household wiring is always parallel β that is why you can switch off the TV without turning off all the lights.