4 Electricity and magnetism

Question 1

equation for resistance

Answer 1

R = V/I

Question 2

What is the relationship of the resistance of a metallic wire to its length?

Answer 2

Restistance is directly proportional to its length.
- as length increases the resistance increases.

Question 3

What is the relationship of the resistance of a metallic wire to its cross-sectional area

Answer 3

resistance is inversely proportional to its cross-sectional area.
- As area increases the resistance decreases

Question 4

equation for electrical power

Answer 4

P = Iv

Question 5

equation for electrical energy

Answer 5

E = Ivt

Question 6

potential divider what

Answer 6

splits p.d between two resistors in proportion to the resistances

Question 7

current in series circuit

Answer 7

current at every point in a series circuit is the same

Question 8

resistors in parallel circuits

Answer 8

the combined resistance of two resistors in parallel is less than that of either resistor by itself

1/Rt = 1/R1 + 1/R2 + 1/R3…

Question 9

advantages of connecting lamps in parallel in a lighting circuit

Answer 9

the bulbs in the parallel circuit will be brighter than those in the series circuit

Question 10

total resistance in series

Answer 10

Rt = R1 + R2 + R3…

Question 11

the sum of currents entering a junction in a parallel circuit is…

Answer 11

equal to the sum of currents that leave the junction

Question 12

Voltage in a parallel circuit

Answer 12

Voltage is the same in every branch

Question 13

describe the action of a variable potential divider.

Answer 13

the resistor with the greatest resistance will have a larger potential difference than the others.

Question 14

equation for two resistors used as a potential divider

Answer 14

R1/R2 = V1/V2

Question 15

hazards of damaged insulation

Answer 15

if someone touches the exposed piece of wire, they could be lethally shocked

Question 16

hazards of overheated cables

Answer 16

Passing too much current through too small of a wire can cause it to overheat. Leading to electrical fires.

Question 17

hazards of damp conditions

Answer 17

if moisture comes into contact with live wires, the moisture could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk

Question 18

what does a mains circuit consist of

Answer 18

live wire, a neutral wire and an earth wire

Question 19

why must a switch be connected to the live wire for the circuit to be switched off safetly

Answer 19

so that the appliance is always at zero potential difference

Question 20

live wire

Answer 20

copper wire coated with brown plastic along which the current enters the device. It’s carry the power from the source to a switch or appliance.

way to remember color
(live = earth = dirt = brown)

Question 21

fuse

Answer 21

a glass or ceramic canister containing a thin wire that melts if the current gets too high

Question 22

neutral wire

Answer 22

copper wire coated with blue plastic that also connects to the cable in the wall and completes the circuit. It carries the circuit back to the original power source

(neutral - feeling slightly blue)

Question 23

earth wire (not needed)

Answer 23

copper wire coated in striped plastic that provides a path for current to flow from the case of the device to the ground if there is a fault

(earth - sun and earth - yellow and green)

Question 24

use and operation of trip switches

Answer 24

Basically fuses: when too much current passes through a given circuit the switch is ‘tripped’ and the circuit is broken

Question 25

How to choose appropriate fuse ratings and trip switch settings

Answer 25

Find the amps of the circuit then multiply that by 125%

Question 26

why must the outer casing of an electrical appliance must be either non-conducting (double insulated) or earthed?

Answer 26

Double insulation protects the user of the appliance from an electrical shock by preventing any possibility of the external casing becoming live

Question 27

what protects the circuit and cabling for a double-insulated appliance?

Answer 27

a fuse without an earth wire protects the circuit and the cabling for a double insulated appliance