Electricity

Electric current (in metals) definition

An electrical current in a metal is the flow of charged particles called electrons.

Electric current unit and measurement

Current is measured in amperes (A) using an ammeter connected in series.

Electric circuit definition

An electric circuit is a system that has a power supply providing electrical energy, a load that transforms the energy into another form, and a complete conducting path for electrons to flow around the circuit.

Voltage definition and measurement

Voltage is measured in volts (V) using a voltmeter connected in parallel in the circuit. Voltage measures the amount of energy with which electrons are pushed and only changes if energy is given to or taken from the electrons.

Resistance definition

Resistance is a measure of the opposition to the flow of current through a material.

Resistance unit

The unit of resistance is the ohm (Ω).

Circuit symbol: battery / power supply

Represented by long and short parallel lines. The long line indicates the positive terminal.

Circuit symbol: connecting wires

Lines connecting components to form a complete circuit path.

Circuit symbol: variable resistor

A resistor symbol with an arrow through it, showing resistance can be adjusted.

Circuit symbol: switch

Switch symbols show open (circuit broken) or closed (circuit complete).

Circuit symbol: light globe

A circle with a cross inside representing a lamp.

Circuit symbol: voltmeter

A circle with V inside; connected in parallel.

Circuit symbol: ammeter

A circle with A inside; connected in series.

Circuit diagrams

Circuit diagrams represent electrical circuits using standard symbols.

Series circuit rules

In a series circuit the current is the same everywhere and the sum of the voltages across components equals the battery voltage.

Parallel circuit rules

In a parallel circuit the total current equals the sum of currents in each branch and the voltage across each branch equals the battery voltage.

Ohm’s law

V = IR (Voltage = Current × Resistance).

Series resistance formula

Total resistance in series is R = R₁ + R₂ + R₃ …

Parallel resistance formula

Total resistance in parallel is 1/R = 1/R₁ + 1/R₂ + 1/R₃ …

Circuit analysis

Simple circuits can be analysed to determine current, voltage and resistance around the circuit.

Constructing circuits

Circuits can be constructed using ammeters in series and voltmeters in parallel to measure current and voltage.

Experiment: voltage, current and resistance

The relationship between voltage, current and resistance can be investigated using a resistor, light bulb, or combinations of resistors.

Series experiment observation

For two resistors or bulbs in series: current is the same everywhere and voltages add to the battery voltage.

Parallel experiment observation

For two resistors or bulbs in parallel: currents add to equal the battery current and voltage is the same across each branch.

Voltage–current graphs

The current flowing through a conductor is directly proportional to the voltage across it.

Resistance from graph gradient

Resistance can be determined from the gradient of a V–I graph using R = rise/run = V/I.

Choosing graph points

Any two points on the straight line give the same gradient, so choose points that can be read accurately.

Brightness comparison: series vs parallel

Two identical bulbs in parallel are brighter than two in series because in series they share the potential difference but in parallel each receives the full battery voltage.

Uses of parallel circuits

Parallel circuits allow current to still flow if one component fails and allow components to be controlled individually.

Series vs parallel advantages

Parallel circuits are more reliable and allow independent switching; series circuits are simpler but one break stops the whole circuit.

Law of conservation of energy

Energy cannot be created or destroyed, only converted from one form to another.

Power definition

Power (P) is the rate at which work is done or energy is converted.

Power unit

The unit of power is the watt (W).

Power formula using energy

Power = Electrical energy transformed ÷ time.

Power formula using voltage and current

P = VI.

Brightness and power of lightbulbs

The brightness of a light bulb relates to the power it consumes.

Brightness difference explanation

Bulbs in parallel can be about four times brighter than in series because they receive twice the voltage and current.

kWh definition

A kilowatt-hour (kWh) is a unit of energy used in domestic and commercial electricity consumption.

Energy formula

Energy = Power × Time.

Energy star rating criteria

Efficiency = (useful output energy ÷ input energy) × 100%.

Meaning of energy star ratings

More stars indicate greater efficiency and less electricity needed for the same performance.

Efficiency and energy conservation

Efficiency compares useful output energy to input energy using the law of conservation of energy.

Energy efficient lighting: incandescent

Incandescent bulbs produce light by heating a tungsten filament above 2000°C.

Energy efficient lighting: compact fluorescent lamp (CFL)

In a CFL current flows through a gas tube producing UV light which hits a fluorescent coating that emits visible light.

Energy efficient lighting: LED

LEDs produce light when electrons move through a semiconductor microchip and release energy as visible light.

LED lifespan advantage

On average an LED lasts about 25 times longer than a filament bulb.

Practical: energy comparison

Experiments can compare the energy transformed over time in circuits or appliances.

Ohm’s law units relationship

Volts = Amps × Ohms.

Voltage definition in energy terms

Voltage = energy per charge (1 volt = 1 joule per coulomb).

Ampere definition

Current = charge per second (amps = coulombs per second).

Series resistance reminder

R = R₁ + R₂ + R₃ … Current is the same everywhere and voltages add to battery voltage.

Parallel resistance reminder

1/R = 1/R₁ + 1/R₂ + 1/R₃ … Current splits between branches and adds to total.

Resistance comparison tip

Two equal resistors in parallel have half the resistance of the same resistors in series.

Brightness problem solving tip

To determine brightness of bulbs, find the current through them.

Circuit drawing tip

Trace all possible paths when drawing circuit diagrams.

Voltage water analogy

Voltage is like water pressure or height difference pushing water through pipes.

Current water analogy

Current is like the flow rate of water through a pipe.

Resistance water analogy

Resistance is like a narrow pipe restricting water flow.

Sankey diagrams

Sankey diagrams represent energy transfers with arrow widths showing energy amounts; forward arrows show useful energy and downward arrows show wasted energy.

Purpose of a variable resistor

A variable resistor is used for safety to limit current and to fine-tune voltage or current.

Power equations additional forms

P = VI, P = I²R, and P = V²/R.

Energy equation reminder

Energy = Power × Time.

kWh conversion

1 kWh = 3.6 MJ (3.6 × 10⁶ J).

Example battery calculation

A 1.5 V, 2000 mAh battery supplies about 2 A. P = VI = 1.5 × 2 = 3 W. Energy in 1 hour = 3 × 3600 = 10800 J.

Voltmeter property

A voltmeter ideally has infinite resistance so no current flows through it.

Circuit layout tip

A light bulb should not be placed directly on a corner in a circuit diagram.