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Physics > Mains Electricity > Flashcards

Mains Electricity Flashcards

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1
Q

what is the equation linking energy transfered, charge flow and potential difference?

A

e=qv

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2
Q

whats the equation that links power potential difference and current?

A

p=iv

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3
Q

what is the equation that links power, resistance and current?

A

p=i^2 x r

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4
Q

what is the deffinition of power?

A

“The rate of energy transfer”

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5
Q

what kind of appliances use the most power?

A

Appliances that need to create heat usually use the most power. Eg. hairdryer, washing machine, kettles, cooker

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6
Q

what is electrical power?

A

“The rate at which an electrical appliance transfers energy by electrical working”

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7
Q

what are the 3 types of fuses?

A

3A, 5A or 13A

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8
Q

Why is it dangerous to use a fuse with too high a

current rating?

A

fuses would only melt when the current exceeded that amount and the appliance may be damaged / start a fire

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9
Q

what is electrical current?

A

the flow of electric charge round the circiut. current will only flow around a complete circiut if theres a potential difference.

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10
Q

what is the equation that links charge flow, current and time

A

q= i xt

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11
Q

what is the electrical work done by the battery equal to?

A

the energy transferred to the resistor.

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12
Q

where is energy transfered from the chemical store of the battery?

A

to the thermal energy store of the resistor.

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13
Q

when does a resistor become hot?

A

When charge flows through a resistor, energy is transferred to the resistor, so the resistor becomes hotter.

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14
Q

why does the resisor become hotter?

A

When electrons flow through a resistor they repeatedly collide with the vibrating metal ions of the resistor, transferring energy to the ions.
The ions on the resistor gain kinetic energy and vibrate even more. This causes the resistor to become hotter.

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15
Q

what is the equation that links energy power and time?

A

e = p xt

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16
Q

what is the equation that links energy potential differnece, curretn and time

A

e= ivt

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17
Q

what carries electriciy to the appliance?

A

cable that is attached to the plug

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18
Q

why are the 3 pins made of brass?

A

Because brass is a good conductor and, as an alloy, it is hard and won’t bend so it will work over and over again

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19
Q

An electrical cable contains three wires, all made of copper.
Why is copper used?

A

Because it is an excellent conductor and in malleable (bendy) so the cable can flex without breaking

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20
Q

Each wire is encased in plastic.

Why?

A

Because it is an insulator this stops short circuits

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21
Q

The whole cable is encased in another layer of plastic.

Why?

A

Because it is an insulator and protects the inner cables from damage

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22
Q

what colour is the live wire?

A

brown

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23
Q

what colour is the earth wire?

A

yellow and green

24
Q

what colour is the neutral wire?

A

blue

25
Q

what is the function of the live wire?

A

The live wire carries a current that alternates between a negative and positive potential difference.

26
Q

what is the function of the earth wire?

A

The earth wire is a safety wire that is needed to earth appliances with a metal case.
This makes it safer to touch the appliance if it develops a fault.

27
Q

what is the function of the neutral wire?

A

The neutral wire completes the circuit. It is kept at a zero voltage by the electricity company.

28
Q

why is the one longer pin in a plug?

A

The earth pin is longest so it is always connected before the live and neutral wires, this way the device is always earthed before any current flows.

29
Q

what is earthing?

A

the metal case of an appliance is automatically connected to earth through the earth wire.

This stops the metal casing becoming live if the wire breaks and touches the case.
If the case did become live and you touch it, you would be electrocuted.

30
Q

what is another way to improve the saftey of elecricity useage?

A

make appliances double insulated.

31
Q

why does double insulated mean that the case cannot become live?

A

These have plastic cases, without any wires connected to the case, therefore, the case cannot become live because plastic does not conduct electricity.

32
Q

what does it mean if one layer of plastic is damaged?

A

the appliance is still safe for the user.

This means they don’t have to have an Earth wire

33
Q

what is a short circuit?

A

Because it allows a large current to flow it will cause the wire in the fuse to melt and break
This will cut off the electricity

34
Q

what happens if the live wire touches the neutal wire?

A

a very large current passes at the point of contact.

35
Q

what is direct current?

A

when the current only flows in one direction: a stright line

36
Q

what usually supply dc electricity, and how many volts?

A

Batteries and cells supply DC electricity, with a typical cell supplying around 1.5 V

37
Q

what is alternating current?

A

if the current constantly changes direction, it is called alternating current (AC).

38
Q

what type of electricity is an ac supply?

A

Mains electricity

39
Q

what is the voltage of the uk main supply?

A

230 V

40
Q

what is the frequency of the uk mains supply?

A

50 hertz

41
Q

why do we use alternating current?

A

for transformers

42
Q

how do you calculate frequency?

A

f = 1/T

43
Q

what do most devices contain that reduce the voltage?

A

Most devices (laptops, computers, phone chargers, etc) contain step down transformers to reduce the voltage from 230V to 12V or 6V.

44
Q

what do most devices use to rectify the current from ac to dc?

A

diodes

45
Q

what is Rectification?

A

Rectification means making the current flow in one direction. We use diodes to rectify AC for devices that need DC

46
Q

what is the national grid?

A

The national grid is a system of cables and transformers that links electricity users (home, factories, etc) to electricity producers (power stations).

47
Q

what do step up transformers do?

A

increase the PD

48
Q

what do step down transformers do?

A

decrease the PD

49
Q

Why do we use high PD when transmitting electricity over the national grid?

A

By increasing the potential difference we can reduce the current that needs to flow to transmit the same amount of energy. (P = IV)

50
Q

why is reducing the current important?

A
  • Lower current means less heating of the wires
  • Less heating of the wires means less energy is wasted
  • This means the system is more efficient.
51
Q

why dont the birds get electrocuted?

A

Both feet are touching the wire and are at about 400,000V but there is no potential difference between them so no current flows.

52
Q

what are the energy transfer methods?

A
  • Mechanical working (Forces, W = Fs)
  • Electrical working (Circuits, E = IVt)
  • Conduction (temperature changes in solids)
  • Convection (currents in liquids / gasses)
  • Radiation (eg heat light from the sun to us)
53
Q

what is the equation that links power energy and time?

A

Power (W) = Energy (J) / time (s)

54
Q

what is efficiency?

A

Efficiency is how good a machine is at transforming the input energy into the energy form(s) we want it in.

55
Q

how do you work out the efficiency percentage?

A 
Efficiency(%) = useful energy out x 100
				    total energy in
or
Efficiency(%) = useful power out x 100
			           total power in

Physics (12 decks)

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