1.2 Electricity

Electrical current

The flow of electrical charge; the greater the rate of flow of charge, the greater the current

Conditions for charge to flow

The circuit must be closed (no open switches) and there must be a source of potential difference (battery/cell)

Current in a single closed loop

The current has the same value at any point in the loop

Effect of resistance on current

The greater the resistance of a component, the smaller the current for a given potential difference

Ohmic conductor

A component with constant resistance; current is directly proportional to potential difference — the I-V graph is linear

Filament lamp behaviour

Non-ohmic; resistance increases as temperature increases, so the I-V graph is non-linear (curves)

Diode behaviour

Allows current to flow freely in one direction only; has very high resistance in the reverse direction

Thermistor behaviour

Resistance decreases as temperature increases; used in temperature detectors and thermostats

LDR (Light Dependent Resistor)

Resistance decreases as light intensity increases; resistance is greatest in the dark; used in automatic night lights

How current increases resistance in a resistor

Electrons collide with atoms, transferring energy and causing them to vibrate more, making it harder for electrons to flow

Effect of wire length on resistance

Greater length = greater resistance and lower current, as electrons must pass through more atoms

Series circuit — current

Current is the same at every point in the circuit

Series circuit — potential difference

PD of the power supply equals the sum of PD across each component

Series circuit — resistance

Total resistance = sum of all individual resistances (R_total = R1 + R2 + …)

Parallel circuit — current

Current from the source equals the sum of current through each branch

Parallel circuit — potential difference

PD is the same across every branch, equal to the PD of the power supply

Parallel circuit — resistance

Total resistance is less than the smallest individual branch resistance

AC (Alternating Current)

Current that continuously varies, switching between positive and negative; supplied by the mains

DC (Direct Current)

Movement of charge in one direction only; supplied by cells and batteries

UK mains electricity

AC supply at approximately 230 V and a frequency of 50 Hz

Live wire

Brown wire at 230 V; carries the alternating potential difference from the supply; dangerous even when the mains switch is off

Neutral wire

Blue wire at 0 V; completes the circuit

Earth wire

Green and yellow striped wire at 0 V; a safety wire that only carries current if there is a fault, preventing the appliance casing from becoming live

Power rating

Shows the power an appliance uses in watts; a higher power rating means more energy used

National Grid

A system of cables and transformers linking power stations to consumers across the UK

Step-up transformer

Increases potential difference from the power station to the National Grid; since P = IV is constant, current decreases and less energy is lost

Step-down transformer

Decreases potential difference from the National Grid to consumers for safety

Like and opposite charges

Like charges repel; opposite charges attract

Insulator

Does not conduct electricity because electrons are fixed and cannot flow through the material

Conductor

Can conduct electricity because electrons are delocalised and free to flow

Static electricity

Produced when two insulators are rubbed together; electrons transfer from one to the other, leaving one positively charged and one negatively charged

Sparking

Occurs when enough charge builds up; charge jumps through the air from the negative object to the positive object to balance the charges

Electrostatic force

Force experienced by charged objects; greater charge or closer distance = greater force; it is a non-contact force

Electric field lines direction

Point in the direction a positive charge would move — away from positive charges and towards negative charges, at right angles to the surface

Electric field strength

Stronger closer to the charge and where field lines are more densely packed