Electricity

Electric Charge and Current

All matter contains electric charges. Electric current (I) is the flow of electrical charge — 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 (Ω)	Ohmmeter or calculated
Power	P	Watts (W)	Calculated

Current — the rate of flow of charge. 1 ampere = 1 coulomb of charge flowing per second.

Voltage (potential difference) — 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 — 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 6Ω. What is the current? I = V / R = 12 / 6 = 2A

Example 2: A lamp has resistance 12Ω 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 — 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 — 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 — 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 = I² 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 — 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 — electronic switches that automatically trip when current is too high. Can be reset.

Earth wire — 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 — plastic and rubber coatings on wires prevent accidental contact with live conductors.

Double insulation — 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 — ammeters have very low resistance; connecting in parallel short-circuits the component. Always connect in series.
• Connecting a voltmeter in series — 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) — 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 — that is why you can switch off the TV without turning off all the lights.