mains electricity

Question 1

current (A)

Answer 1

rate of flow of charge q = i t

Question 2

pd (V)

Answer 2

energy transferred per unit of charge e = q v

Question 3

resistance (O)

Answer 3

ratio of pd to current
v = ir

Question 4

series circuit

Answer 4

total resistance = sum of individuals, current stays same, pd shared

Question 5

parallel circuit

Answer 5

resistance decreases as more branches added, less than smallest individual resistor
current shared pd same

Question 6

iv characteristics resistor

Answer 6

current directly proportional to pd
r is constant as long as temp is constant (obeys ohms law)

Question 7

iv characteristics filament bulb

Answer 7

i not directly proportional to v
as i increases temp of filament increases
leads to increased vibrations of positive metal ions
harder for electrons to pass so more resistance

Question 8

iv characteristics diode

Answer 8

i only in one direction
infinite resistance in other direction

Question 9

direct current or dc from a cell/battery

Answer 9

current only flows in one direction, flows from positive to negative, flows from a battery around a circuit and back again

Question 10

alternating current or ac via plugs and sockets

Answer 10

current constantly changes direction, usually oscillates from a max in one direction to a max in the other direction
type of current used in domestic circuits

Question 11

uk domestic supply

Answer 11

230 v dc equivalent and 50 hz

Question 12

oscilloscope

Answer 12

essentially voltmeter on y axis and time base on x axis

Question 13

calculate frequency from oscilloscope

Answer 13

find time scale, see how many grids for 1 wave, convert to s then do 1/time period

Question 14

live wire plugs

Answer 14

brown wire, alternates between + and - 325 V

Question 15

neutral wire

Answer 15

blue wire grounded at power station, close to 0V

Question 16

earth wire

Answer 16

green and yellow, for safety, prevents shocks at 0V, longer than other pins to open live and neutral sockets

Question 17

fuse

Answer 17

melts if too much current flows, prevents oveheating

Question 18

cable grip

Answer 18

holds cable securely

Question 19

how does earth wire work

Answer 19

many appliances have metal cases, earth wire creates a safe route for current to flow thru if live wire were to touch the casing

earth terminal connected to metal casing so current goes thru earth wire instead. strong current surges thru bc it has low resistance, this breaks fuse and disconnects appliance

Question 20

shocks - why?

Answer 20

bodies at earth potential 0V, touching live wire creates a big pd between wire and earth
causes current to flow through our body, resulting in an electric shock

Question 21

double insulation

Answer 21

some appliances eg straighteners do not have an earth wire bc they have plastic casings, or have been designed so live wire cannot touch the casing

Question 22

power equation current pd

Answer 22

p = vi

Question 23

power equation current resistance

Answer 23

p = i squared r

Question 24

kilowatt hrs

Answer 24

amount of energy transferred when a 1kw device is used for an hour
equal to 3.6M J or 3600000 joules
often called a unit

Question 25

national grid

Answer 25

network of cables and transformers carrying electricity generated by power stations and by renewable energy generators to our homes

(power station) - step up transformer - transmission cables - step down transformer - (homes)

Question 26

step up transformer

Answer 26

increases voltage and decreases current so less energy is wasted as heat

Question 27

step down transformer

Answer 27

decreases voltage and increases current so it is at a safe level