Electricity

1. Electric Charge and Current

Electricity is the flow of electric charge, typically carried by electrons in a circuit. The size of the current depends on the rate of flow of charge:

• Q is measured in coulombs (C)

• I is measured in amperes (A)

• t is measured in seconds (s)

A current will only flow through a circuit if there is a potential difference (voltage) across it and a complete closed loop.

2. Potential Difference and Resistance

• Potential Difference (V) is the energy transferred per coulomb of charge:

  

• Measured in volts (V)

• A cell provides potential difference to push charges around the circuit.

Resistance (R) opposes the flow of current. The higher the resistance, the lower the current for a given voltage:

• Resistance is measured in ohms (Ω)

3. Components and Circuit Symbols

Key components include:

• Resistor: limits the current

• Variable resistor: changes the resistance

• Lamp: lights up when current flows

• Diode: allows current in one direction only

• Thermistor: resistance decreases as temperature increases

• LDR (Light Dependent Resistor): resistance decreases as light intensity increases

• Ammeter: measures current (in series)

• Voltmeter: measures potential difference (in parallel)

Understanding how to draw and interpret circuit diagrams is essential.

4. Series and Parallel Circuits

Series Circuits

• Components connected end-to-end

• Current is the same throughout the circuit

• Total potential difference is shared between components

• Total resistance is the sum of individual resistances:

  

Parallel Circuits

• Components connected across the same potential difference

• Potential difference is the same across each branch

• Current is shared between branches

• Adding resistors in parallel decreases the total resistance

This is why household appliances are connected in parallel—to ensure they all get the same voltage.

5. Current–Voltage Characteristics

Not all components obey Ohm’s Law (where current is directly proportional to voltage):

• Ohmic conductors (like resistors) have a linear I-V graph

• Filament lamps have a curve because resistance increases with temperature

• Diodes have very low current until a certain voltage is reached, then current increases rapidly

Understanding these graphs is vital for recognizing component behavior.

6. Electricity in the Home

UK mains electricity is:

• AC (Alternating Current) at 230 V, 50 Hz

• AC means the current changes direction

• DC (Direct Current) flows in one direction (e.g., from a battery)

Three-core cables have:

• Live wire (brown): carries current to appliance (at 230 V)

• Neutral wire (blue): completes the circuit (at 0 V)

• Earth wire (green and yellow): safety wire to stop appliances becoming live

Fuses and circuit breakers protect the wiring and users. The earth wire prevents electric shocks by providing a path to ground.

7. Electrical Power and Energy Transfer

Appliances convert electrical energy into other forms (e.g., thermal, light, kinetic). The power of an appliance is the energy it transfers per second:

• Power is measured in watts (W)

To calculate energy transferred:

• Energy is measured in joules (J)

In practical situations, this is used to calculate how much energy an appliance uses, which helps determine cost.

8. The National Grid

The National Grid is the network of cables and transformers that distributes electricity across the UK.

• Power stations generate electricity

• Step-up transformers increase voltage and reduce current to reduce energy loss during transmission

• Step-down transformers decrease voltage for safe use in homes and businesses

High-voltage transmission is more efficient because less energy is lost as heat in the wires.

9. Static Electricity

Static electricity occurs when insulating materials are rubbed together, causing electrons to be transferred.

• One object becomes positively charged, the other negatively charged

• The charges are static—they build up and can discharge suddenly (like lightning or a shock from a doorknob)

Like charges repel, and opposite charges attract.

The force between charged objects is non-contact—it acts at a distance and gets stronger as the distance decreases.

10. Dangers and Uses of Static Electricity

Dangers:

• Sparks can ignite flammable gases or vapors

• Risk of electric shock

Precautions:

• Earthing: connecting objects to the ground with a conductor to safely discharge excess charge

• Insulation and conductive flooring can reduce risk

Uses:

• Paint sprayers: charge the paint and object so paint spreads evenly

• Photocopiers and air purifiers use static electricity to attract particles