Electricity pt 2

Question 1

What is a Oscilloscope

Answer 1

Cathode ray oscilloscope (CRO) is a voltmeter

Plug an AC supply in a CRO you get a trace in the screen that’s how the voltage of the supply changes with time

Plug in a DC and the trace is just a straight line.

Input voltage at given point in an AC supply is the vertical height of the trace

Input voltage at given point in a DC supply is the distance from the straight line trace to centre line

Gain dial= controlled how many volts each centimetre division represents in vertical axis

Timebase dial= controlsbhow many milliseconds each division represents in horizontal axis

Question 2

How do you read an AC supply

Answer 2

AC source gives regularly repeating wave.

From this you can work out the period by measuring the horizontal distance between two peaks and to find the frequency:
1
Frequency= ————–
Time period (s)

Question 3

Explain AC and DC

Answer 3

Mains supply is AC (alternating current) which means it is constantly changing direction.
Mains supply=230V
Frequency of mains supply= 50Hz

Cells and batteries supply DC (direct current) which means the surrey is always flowing in the same direction

Question 4

what are the 9 examples of electrical hazards in the home

Answer 4

1) long cables
2) frayed cables
3) cables in contact with something hot or cold
4) water near sockets
5) shoving things into sockets
6) damaged plugs
7) too many plugs in a socket
8) lighting sockets without bulbs in
9) appliances without their covers on

Question 5

How many wires does a standard wire have

Answer 5

most cables have 3 core cables which have a core of copper and coloured plastic coating

Question 6

what are the colours of the core cables in the wire and what do they do

Answer 6

BROWN- live wire
in a main supply alternates between a high +ve and -be voltage

BLUE- neutral wire
is always at 0V. electricity normally flows in and out the live and neutral wires only

GREEN AND YELLOW- earth wire
protecting the wiring and for safety
works together with a fuse to prevent fire and shocks
attacked to metal casing of appliance and carries electricity to earth (away from you) invade something goes wrong

Question 7

what are the features of wires

Answer 7

1) right coloured wire connected to eat pin and screwed in
2) no bare wires inside plug
3) cable grip fasted over the cable outer layer
4) different appliances need different amount of electrical energy. eg thicker wires have less resistance so they carry more current

Question 8

what are the features of three-pin plugs

Answer 8

1) metal parts made of copper or brass because good conductors
2) case, cable grip and cable insulation are made of rubber or plastic because good insulators and flexible
3) keeps electricity flowing where it should

Question 9

how does earthing and fuses prevent electrical overloads

Answer 9

1) if the live wire touches the metal case, then too great a current flows through in the live wire. so goes through the case and out the earth wire
2) this surge in current melts the fuse when the amount of current is greater than the fuse rating
3) this cuts off the live supply and breaks the circuit, thus isolating the whole appliance so it’s imposible to get an electric shock from the case or a fire.
4) fuses and earthing protect the circuits and wiring in your appliance from getting fried in a current surge
5) fuses should be rated just higher than normal operating current
6) larger the current, thicker the cable. so fuse rating increases with cable thickness

Question 10

what is double insulation

Answer 10

if appliance has a plastic casing and no metal parts showing then it is double insulated

appliances with metal cases are earthed to reduce damage of electric shock.

an earthed conductor can’t ever become live.

anything with double insulation doesn’t need an earth wire - just a live and neutral. these are known as two-core cables

Question 11

what does earthing mean

Answer 11

the case must be attached to an earthed wire

Question 12

what are circuit breakers

Answer 12

electrical safety device used in circuits. they protect the circuit from damage is too much current flows (like a fuse)

when a surge in current is detected the circuit breakers break the circuit by opening a switch

Question 13

why are circuit breakers better than fuses

Answer 13

1. circuit breaker can easily be reset by flicking a switch
2. one type of circuit breaker used instead of a fuse and an earth wire is a Risdual Current Circuit Breaker

but

they are more expensive than fuses

Question 14

explain what energy is transferred in speakers, kettles, light bulbs, motors

Answer 14

anything which supplies electricity is also supplying energy

so cells, batteries, generators.. all transferred energy to components in the circuit

Motors= motion
Light= light bulbs
kettle= heat
Speakers= sound

Question 15

what happens to resistors when a current flows through them

Answer 15

1. when current flows through something with electrical resistance then electrical energy is converted into heat energy
2. more current = more heat
   3 bigger voltage= more heat cause pushes more current through
3. filament bulbs work by passing current through a thin wire which heats it up so it flows. waste a lot of energy as heat

Question 16

what does it mean if an appliance is efficient

Answer 16

if an appliance is efficient t wastes less energy

so it transfers not eif their total electrical energy output to useful energy.

eg. power saving lamps like compact fluorescent lamps (CFL) and light emitting diodes are efficient
however
these cost more to buy but over time the money you save on the bills and pays back for the investment

Question 17

how do you calculate energy transferred

Answer 17

energy transferred= power x time

Question 18

what is the formula for electrical power

Answer 18

power= current x potential difference

this can be used to work out the fuse that should be used in an appliance. to work out the size needed you need tow work out the current for the device

Question 19

what does energy being transferred mean

Answer 19

when an electrical charge (Q meandered in coulombs) goes through a change in potential different (V) then energy (E) is transferred

energy is supplied to the charge ag a power source to raise it through a potential

the charge gives up this energy when it fall through an potential drop in components elesewhere in the circuit

Question 20

what’s the for energy transferred per charge passed

Answer 20

energy transformed= potential different x charge

bigger the change in PD, the more energy is transferred for s given amount of charge passing through it

so batter with bigger voltage = supply more energy to circuit for every coloumb of charge that flows round it
because charge is raised up higher at the start, more current will dissipate in the circuit too