Safety / Intro

• The mains supply and batteries are sources of Electrical Energy • Electrical Appliances are Energy Changers

Power

• Heat producing appliances have a power rating greater than 1000 Watts • Power Rating is the amount of energy used per second • Appliances greater than 700 W are protected by a 13 A fuse

Fuses

• This protects the flex from overheating, melting and causing a fire.

• Fuses are connected to the LIVE pin.

• When the fuse blows it isolates the appliance from the ‘high voltage’ supply

• Mains Plugs

Plugs

Human Conductivity

• We conduct electricity very well • When wet the body’s conductivity increases • Bathroom light switches, shower switches are either outside the bathroom or on a cord

Earth Wire

• Connected to casing of metal appliances • If live wire touches the casing ‘ current flows through earth wire to ground ( often via the water pipes ) • A large current flows , blowing the fuse and isolating the appliance from the high voltage mains.

Double Insulation

• Appliances with casings made of plastic DO NOT need an earth wire • The casing is an insulator

Electric Current

• This is a Flow of Charged Particles

»Q = I x t

• Charge Current Time • Coulombs ( C ) Amps ( A ) Seconds ( s )

Voltage

• Energy Given to each coulomb of charge • Units are Volts ( V ) • Measured by Voltmeter , • Connected in parallel

Current

• This is a flow of Charged Particles • Units are Amps, ( A ) • Measured using an Ammeter • Connected in Series

Series Circuits

• Current is the SAME at ALL points • Voltage across Supply equals the voltage across Resistor 1 + voltage across resistor 2 • V s = V 1 + V 2

Parallel Circuits

• Voltage across each is the same • Current through resistor 1 plus current through resistor 2 equals the supply current • I t = I 1 + I 2 branch

Resistance

• Ohm’s Law V = I x R at constant Temp • Resistance : measure of how easy it is for charges to flow.

» V = I x R » Volts ( V ) Amps ( A ) Ohms ( Ω

R s

=

Series Resistors

R 1

+

R 2

+

R 3

1 R p

Parallel Resistors

=

1 R 1 1

+

R 2 1

+

R 3

Variable Resistors

• A length of wire is wrapped into a cylinder • Different lengths can be tapped into • The longer the length the bigger the resistance • The bigger the resistance the lower the current

Variable Resistors

• Uses • Light Dimmer Switches • Volume controls • Speed Controls • Temperature Controls for grills/ ovens … • Computer Joysticks • Fuel gauges

Used to detect ‘Open Circuits’ or Breaks in Circuits such as blown bulbs.

Component is blown if there is NO reading on the meter.

Continuity Tester

Power

This is the amount of energy transferred per second. Units are Watts ( W ). Heat producing appliances have big power ratings

Energy Tranferred Power

=

P Time One Watt is one joule per

sec

ond

(

1 W

=

E

=

1 J t

/

s

)

Power

• Power can also be calculated from

Power

=

Voltage x Current P

=

V x I Power

=

Current squared x resis

tan

ce P

=

I 2 x R Power

=

Voltage squared

÷

resis

tan

ce P

=

V 2 R

Lamps

• Discharge Lamps ( Fluorescent Tubes ) • An electric current passes through the gas • Virtually NO heat Energy is produced • MUCH more efficient than • Filament lamps • An electric current is passed through a piece of resistance wire which heats up • Produces more heat energy than light energy

Direct Current

• Current flows in one direction ONLY • Electrons flow from Negative to Positive • Size of Voltage remains constant • Batteries supply D.C.

Alternating Current

• Mains electricity is A.C.

• The current constantly changes direction • Mains electricity is 50 Hz , 50 cycles in 1 second • The size of the voltage is constantly changing. Quoted value ( 230 V mains )is smaller than the peak value Quoted value, 230 V Peak value, 325 V for mains Current flows when below 0V line 0V Line

Behind the Wall

• Household appliances are connected in parallel : same voltage ( 230 V ) and independent switching • Sockets are wired in a RING circuit: • Current flows via 2 paths therefore smaller currents flow and thinner cable can be used • Less cable needed than conventional parallel • Easy to add extra sockets

Behind the Wall

• Lighting circuit draws less current than Ring circuit for sockets • Lighting circuit conventional parallel • Lighting circuit uses a 10A circuit breaker as opposed to a 30A on the ring circuit

Fuses / Circuit Breakers

• Mains fuses protect the mains wiring • Circuit Breaker is an automatic switch that switches off when current exceeds stated value • Circuit breakers can easily be reset and used again but fuses must be replaced

Kilowatt Hours

• I unit of electricity is 1kWh • I.e. A 1kW appliance switched on for 1 hour • total units = Power (kW) x time ( hours ) • 1 kWh = Power x time » 1000 W x 60 x 60 seconds » = 3 600 000 Joules

Electromagnetism

Field around a wire • If a wire is placed inside a magnetic field it experiences a force Current flows out of board N S S S N N Current flows into board

Electromagnetism

• Relays • Motors • Bells • Loudspeakers • Tape Recorders • All use Electromagnetic effect

8

The d.c. motor

d • Each side of the coil experiences forces acting in opposite directions the coil rotates

N

Rotation is produced

+VE S -VE

9

d.c. motor

a • If the flow of current WAS NOT reversed • a then the coil would flip back • A SPLIT RING COMMUTATOR reverses the flow of current

+VE N S -VE

+VE N S -VE

Commercial motors

• More coils : more powerful motor, smoother turning • Electromagnets instead of permanent magnets : increased magnetic field, switched on / off • Brushes as contacts : mould themselves to shape of commutator

•

Commercial motors