Electrical quantities

What are the two types of electric charge?

Positive and negative.

Electric Charge

Property of matter causing attractive or repulsive electrical forces.

What subatomic particles carry electric charge?

Electrons (negative) and protons (positive).

Neutral Atom

An atom with equal numbers of protons and electrons so overall charge is zero.

What happens when opposite charges meet?

They attract.

What happens when like charges meet?

They repel.

Coulomb (C)

Unit of electric charge.

What is charging by friction?

Charging an insulator by rubbing it with another material.

Charging by Friction

Process where electrons transfer from one insulator to another.

Why does a polythene rod become negatively charged?

It gains electrons from the cloth.

Why does an acetate rod become positively charged?

It loses electrons to the cloth.

Electrostatic Force

Force of attraction or repulsion between charged bodies.

What does a gold-leaf electroscope detect?

Electric charge.

Gold-Leaf Electroscope

Instrument used to detect electric charge via leaf movement.

Why does the leaf rise when a charged rod approaches?

Electrons are repelled or attracted causing leaf to gain same charge as plate.

What happens when a charged rod touches the electroscope?

Charge is transferred and leaf stays up.

What happens when you touch the plate with a finger?

Charge goes to earth and the leaf falls.

What is an electric field?

A region where an electric charge experiences a force.

Electric Field Direction

Direction of force on a positive test charge.

Electric Field Lines

Lines showing direction and strength of an electric field.

Electric Field Around Point Charge

Radially outward for positive

Uniform Electric Field

Electric field between two parallel charged plates with parallel lines.

Conductor

A material allowing charge (electrons) to flow easily.

Insulator

A material that resists charge flow and has no free electrons.

Why do metals conduct electricity?

They have delocalised electrons that move freely.

Why do insulators not conduct electricity?

They have no free electrons.

What is electric current?

Rate of flow of electric charge.

Electric Current

Flow of charge from positive to negative terminal (conventional).

Ammeter

Device used to measure current in series.

Digital Ammeter

Accurate meter displaying current digitally.

Analogue Ammeter

Meter with needle; affected by parallax error.

Current Equation

Current = Voltage/Resistance (I = V/R)

Charge (Q)

Measured in coulombs (C).

Time (t)

Measured in seconds (s).

Conventional Current

Flow of positive charge from positive to negative terminal.

Electron Flow

Actual electron movement from negative to positive terminal.

Direct Current (DC)

Steady current flowing in one direction.

Alternating Current (AC)

Current that reverses direction repeatedly.

Mains Electricity

AC supply at about 230 V and 50 Hz in the UK.

DC vs AC

DC constant direction; AC changes direction continuously.

Electromotive Force (EMF)

Energy supplied per coulomb of charge by a source.

EMF Equation

Voltage + Current x Resistance ( V + IR)

What is EMF measured in?

Volts (V).

Potential Difference

Work done per coulomb of charge between two points.

Potential Difference Equation

V = W/Q (Work done/charge)

Voltmeter

Device that measures potential difference in parallel.

Resistance

The opposition to current.

Ohm’s Law formula

Voltage = Current x Resistance (V = IR)

What happens when resistance increases?

Current decreases.

Ohm (Ω)

Unit of resistance.

I-V Graph (Ohmic)

Straight line through origin showing constant resistance.

I-V Graph (Filament Lamp)

Curved graph due to increasing resistance with temperature.

I-V Graph (Diode)

Current flows only in forward direction.

Resistance of a Wire

Opposition caused by eletron-ion collisions.

How does wire length affect resistance?

Longer wire means more collisions, which leads to higher resistance.

How does wire thickness affect resistance?

Thicker wire = lower resistance.

Resistance Proportionalities

R is directly proportional to length L and R is inversely proportional to A.

Electrical Energy Equation

Electrical energy = Voltage x Current x time

Energy Transfer

Energy passed from power source to components via moving charges.

Why do appliances heat up?

Energy transferred to thermal store in components.

Electrical Power

Rate of energy transfer.

Power Equation (Energy) (Watts)

Power = Energy/time (P = E/t)

Power Equation (Electrical)

Power = Voltage x Current (P = IV)

Watt (W)

Unit of power equal to J/s.

Kilowatt-Hour (kWh)

Unit of energy for household electricity.

1 kWh in joules

3.6 x 10^6 J.

Energy Cost Calculation

Cost = energy (kWh) x price per kWh.

Why do appliances use kWh instead of joules?

Joules are too small for practical household use.