2 - Electricity

Electrical conductors

Electrical conductors

Materials that conduct charge easily - current can flow through

Usually metals e.g. copper + silver

Electrical insulators

Don’t conduct charge very well - current can’t flow

e.g. plastic + rubber

Static charge

Charge which builds up in one place and is not free to move
More common on insulators, where current can’t flow

Electrostatic charge

• Common cause of static electricity is friction
  When two insulating materials are rubbed together, electrons are scraped off one and dumped on the other

• Leaves a +ve electrostatic charge on one and -ve electrostatic charge on the other

• Which way electrons are transferred depends on two materials involved

Static charge on conductors

• Static charges can occur on conductors too - cars often get static charge on outside because they gain/lose electrons from air rushing past them

• Charged conductor can be discharged safely by connecting to earth with metal strap
  Electrons flow down strap to ground if charge is negative and flow up the strap from ground if charge is positive

Experiment: charging insulating materials by friction

• Static charges can be caused by friction

• e.g. polythene + acetate rods being rubbed with cloth duster

• When polythene rod is rubbed with duster, electrons move from duster to rod
  Rod becomes negatively charged + duster is left with equal positive charge

• When acetate rod is rubbed, electrons move from rod to duster
  Duster becomes negatively charged + rod is left with equal positive charge

Laws of attraction

Unlike charges attract

Like charges repel

Sparks

• As electric charge builds on isolated object, voltage between object and earth (0 volts) increases

• If voltage gets large enough, electrons can jump across gap - spark

• Can also jump to any earthed conductor nearby - which is why you can get static shocks from clothes, or getting out of a car

• Usually happens when gap is small

Lightning

• Rain drops + ice bump together inside storm clouds, knocking off electrons → top of cloud = positively charged, bottom of cloud = negative

• Creates huge voltage + big spark

Fuelling

• As fuel flows out of filler pipe, static can build up

• Can easily lead to spark

• Solution: make nozzles out of metal so charge is conducted away instead of building up

• Have earthing straps between fuel tank and fuel pipe

Inkjet printer

• Tiny ink droplets are forced out of fine nozzle, making them electrically charged

• Droplets are deflected as they pass between two oppositely charged metal plates

• Droplets are attracted to plate of opposite charge + repelled from plate with same charge

• Size + direction of voltage across each plate changes so each droplet is deflected to hit different place on paper

• Lots of tiny dots make up printout

Photocopier

• Image plate is positively charged
  Image of what you’re copying is projected onto it

• Whiter bits of what you’re copying make light fall on plate and charge leaks away in those places

• Charged bits attract negatively charged black powder, transferred onto positively charged paper

• Paper heated so powder sticks

Series circuits

• Components connected in a line, end to end, between +ve and -ve of power supply

• If you remove/disconnect one component, circuit is broken + all stop working

• Not very useful, only a few things are practically connected in series e.g. fairy lights

Voltage in series circuit

• There’s a bigger supply p.d. when more cells are connected in series

• Total p.d. of supply is shared between components

• p.d. of each component depends on its resistance

Current in series circuit

• Current same everywhere, I₁ = I₂

• Size of current depends on total p.d. and total resistance of circuit

Resistance in series circuit

• Total resistance of circuit depends on number + type of components

• Total resistance is sum of resistance of each component in circuit

Parallel circuits

• Each component is separately connected to +ve and -ve of supply

• If you remove/disconnect one component, others are hardly affected

• This is how most things are connected
  e.g. cars and household electrics
  Each light switch in your house is part of branch of parallel circuit (turns one light on/off)

• Everyday circuits often contain mixture of series and parallel parts - rules of series circuits apply to components on same branch

Current in parallel circuit

• Current shared between branches

• Total current flowing around circuit = total of all currents through separate components

• There are junctions where current splits or rejoins

• Total current going into junction = total current leaving it

• Current through branch depends on resistance, higher resistance = lower current

Voltage in parallel circuit

• P.d. is same across all branches, V₁ = V₂

Resistance in parallel circuit

• Total resistance of circuit decreases if you add second resistor in parallel

Wire voltage-current characteristics

Current through wire is proportional to voltage

Resistor voltage-current characteristics

Current through resistor is proportional to voltage

Different resistors have different resistances

Metal filament lamp voltage-current characteristics

As temp of metal filament increases, resistance increases

Diode voltage-current characteristics

Current only flows through diode in one direction

Ammeter

• Measures current (in amps) through component

• Must be placed in series anywhere in main circuit

Voltmeter

• Measures voltage (in volts) across component

• Must be placed in parallel around component

Investigating V-I characteristics

• Component, ammeter and variable resistor are in series, so can be put in any order

• Voltmeter must be placed in parallel around component under test

• As you vary variable resistor, it alters current flowing through circuit

• Allowing you to take pairs of readings from ammeter and voltmeter

Effect of changing resistance on current

More resistance = less current

LDR

• Changes resistance depending on how much light it receives

• In bright light, resistance falls

• In darkness, resistance is highest

• Useful for various electronic circuits e.g. burglar detectors

Thermistor

• Temperature-dependent resistor

• Hot = resistance drops

• Cool = resistance increases

• Useful for temperature detectors e.g. car engine temp sensors, thermostats

LED

• Light-emitting diodes emit light when current flows through them in forward direction

• Used for numbers on digital clocks and traffic lights

• Unlike light bulb, don’t have filament that can burn out

LEDs and lamps can be used to…

indicate presence of current in circuit

Often used in appliances to show they’re switched on

Equation: Voltage, Current and Resistance

V = IR

Voltage = Current x Resistance

[V] = [A] x [Ω]

Current

Rate of flow of charge around a circuit

Equation: Charge, Current and Time

Q = It

Charge = Current x Time

[C] = [I] x [s]

In solid metal conductors, current is…

a flow of negatively charged electrons

Plugs

• Have three wires - live, neutral, earth

• Only live and neutral wires usually needed, but earth wire stops you getting hurt if something goes wrong

• LIVE WIRE alternates between HIGH +VE AND -VE VOLTAGE of 230V

• NEUTRAL WIRE always at 0V

• Electricity normally flows in through live and neutral wire

• EARTH WIRE and fuse are just for safety

Double insulation

If appliance has plastic casing and no metal parts showing, it’s said to be double insulated

Plastic is insulator, so stops current flowing - meaning you can’t get a shock
Anything with double insulation doesn’t need earth wire

Earthing and fuses

• If fault develops in which live touches metal case, then because case is earthed, big current flows through live wire, case and earth wire

• Surge in current melts the fuse, cutting off the live supply

• This isolates the whole appliance, making it impossible to get electric shock from case
  Also prevents risk of fire caused by heating effect of large current

Circuit breakers

• Protect circuit from damage if too much current flows

• When circuit breakers detect surge in current, they break circuit by opening a switch

• Circuit breaker can easily be reset by flicking a switch on the device
  This makes them more convenient than fuses (have to be replaced once melted)

Heating effect in resistors

When there is electrical current in resistor, there is energy transfer which heats resistor

Because electrons collide with ions in lattice that make up resistor as they flow through it
This gives ions energy, causing them to vibrate and heat up

Heating effect increases resistor’s resistance - so less current flows

Heating effect can cause components in circuit to melt - so circuit stops working

Use of heating effect in resistors

Toasters contain coil of wire with very high resistance

When current passes through coil, temp increases so much that it glows and emits IR (heat) radiation which cooks bread

Equation: Power, Current and Voltage

P = IV

Power = Current x Voltage

[W] = [A] x [V]

Fuse ratings

• Fuses have current ratings and should be rated as near as possible but just higher than the normal operating current

• To work out the fuse needed, you need to work out the current that item normally uses

Equation: Energy, Current, Voltage and Time

E = IVt

Energy transferred = Current x Voltage x Time

[J] = [A] x [V] x [s]

Alternating current (a.c.)

Current is constantly changing direction

Used for mains supply, e.g. UK mains supply is approx. 230V

Direct current (d.c.)

Current keeps flowing in same direction

Used in cells and batteries

Voltage

Energy transferred per unit charge passed

One volt = one joule per coulomb

Equation: Energy, Charge and Voltage

E = QV

Energy transferred = Charge x Voltage

[J] = [C] x [V]