electricity

Question 1

condition for electrical charge to flow

Answer 1

for electrical charge to flow through a closed circuit, the circuit must include a source of potential difference

Question 2

define the size of the electric current

Answer 2

the rate of flow of electrical charge

Question 3

does current have different values at different points in a single closed loop?

Answer 3

no, a current has the same value at any point in a single closed loop

Question 4

what does the current through a component depend on and how

Answer 4

the current through a component depends on both the resistance of the component and the potential difference across the component. the greater the resistance of the component, the smaller the current for a given potential difference across the component

Question 5

function of a thermistor

Answer 5

a component whose resistance is related to temperature

Question 6

function of an LDR

Answer 6

a component whose resistance is related to to the amount of light falling in it

Question 7

function of a diode

Answer 7

they only allow current to flow through them in one direction

Question 8

function of an LED

Answer 8

LEDs emit light when they conduct electricity; current can only flow through them in one direction.

Question 9

function of an on/off switch

Answer 9

an on-off switch allows current to flow when closed

Question 10

function of a cell

Answer 10

a cell supplies electrical energy

Question 11

function of a battery

Answer 11

batteries supply electrical energy and are made up of multiple cells

Question 12

function of a resistor

Answer 12

it restricts the flow of current in a circuit

Question 13

function of a variable resistor

Answer 13

a component used to vary and control the current in a circuit

Question 14

function of a lamp

Answer 14

a component that transforms electrical energy into light

Question 15

function of a fuse

Answer 15

a safety device that will break the circuit and stop current flowing

Question 16

function of a voltmeter

Answer 16

used to measure potential difference between two points in a circuit. connected in parallel

Question 17

function of an ammeter

Answer 17

used to measure current in a circuit. connected in series

Question 18

relationship between resistance and ohmic conductors; what would this look like on a graph?

Answer 18

the current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. this means that the resistance remains constant as the current changes

on a graph plot potential difference against current; the resistance is a straight line through the origin (extends on both negative and positive values)

Question 19

relationship between resistance and components; what would this look like on a graph?

Answer 19

the resistance of components, such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component. the resistance of a filament lamp increases as the temperature of the filament increases

on a graph plot potential difference against current; the resistance is a curved line from the negative values that goes through the origin and curves in the positive values

Question 20

relationship between resistance and diodes; what would this look like on a graph?

Answer 20

the current through a dioxide flows in one direction only; the diode has a very high resistance in the reverse direction

Question 21

how does resistance change in
a) a thermistor
b) an LDR

Answer 21

a) the resistance of a thermistor decreases as the temperature increases
b) the resistance of an LDR decreases as light intensity increases

Question 22

rules for components connected in series

Answer 22

• there is the same current through each component
• the total p.d. of the power supply is shared between the components
• the total resistance of two components is the sum of the resistance of each component e.g. Rₜₒₜₐₗ = R₁ + R₂

Question 23

rules for components connected in parallel

Answer 23

• the p.d. across each component is the same
• the total current through the whole circuit is the sum of the currents through the separate components
• the total resistance of two resistors is less than the resistance of the smallest individual resistor
• total resistance is the reciprocal of all of the resistances added together

Question 24

why does adding resistors in parallel decrease total resistance

Answer 24

in a parallel circuit, the net resistance decreases as more components are added, because there are more paths for the current to pass through

Question 25

why does adding resistors in series increase total resistance

Answer 25

in a series circuit, the net resistance increases as more components are added; the current is the same in every single component in the circuit, so the more resistors we add, the harder it is for current to flow

Question 26

why are parallel circuits more advantageous than series circuits

Answer 26

if a wire were to be faulty in the series circuit, the whole circuit would be broken, whereas if a wire were to be faulty in a parallel circuit, only the junction(s) affected would be broken, and the other lamps would still function

Question 27

what is the mains electricity

Answer 27

an ac supply, which in the UK, the domestic electricity supply has a frequency of 50Hz and is about 230V

Question 28

difference between direct and alternating current

Answer 28

direct current is the movement of charge in only one direction, whereas alternating current is the backward and forward movement of charge

Question 29

describe alternating current

Answer 29

the p.d. constantly changes, reaching a peak of 230V in one direction, reverting to 0, then reaching a peak of 230V in the opposite direction and repeating. there are 50 of these cycles per second, so the frequency is 50Hz

Question 30

function and colour of the live wire

Answer 30

brown; the live wire carries the alternating potential difference from the supply. this wire has a p.d. of 230V

Question 31

function and colour of the neutral wire

Answer 31

blue; the neutral wire completes the circuit. this wire has a p.d. of 0V

Question 32

function and colour of the earth wire

Answer 32

green and yellow stripes; the earth wire is a safety wire to stop the appliance becoming live. this wire would be at 0V

Question 33

why is the insulation covering each wire colour coded

Answer 33

for easy identification

Question 34

how does a fuse work

Answer 34

fuses have different capacities for how much current can pass through; if it surpasses the amount, then the fuse overheats and melts which breaks the circuit, to ensure that other components which could be dangerous with excessive amounts of current do not receive it

Question 35

what happens when charge flows

Answer 35

energy is transferred as it does work against the resistance of the circuit (work is done)

Question 36

what do all domestic appliances do

Answer 36

transfer electrical energy into other forms of energy, typically kinetic and thermal energy; hence we can divide appliances into two main categories: those that transfer electrical energy to the kinetic energy of motors, or those that transfer electrical energy to the thermal energy of heating devices

Question 37

energy transfer to the mains in a blender

Answer 37

they are powered by electrical energy, which is then transferred to the kinetic energy of electric motors to allow for movement (some energy will be dissipated as thermal energy due to friction)

Question 38

energy transfer to the mains in a kettle

Answer 38

they are powered by electrical energy, which is transferred to thermal energy in the heating element

Question 39

examples of appliances that fit under transferring electrical energy into both kinetic and thermal energy categories

Answer 39

hair dryers, washing machines

Question 40

what are everyday appliances designed to do

Answer 40

bring about energy transfers

Question 41

define power rating

Answer 41

the maximum power that appliances can safely operate at / the maximum rate of energy transfer

Question 42

describe the difference in power rating between appliances designed to transfer electrical energy to thermal energy vs kinetic energy

Answer 42

usually, appliances designed to transfer electrical energy to thermal energy have a much higher power rating than those designed to transfer to kinetic energy, as it takes much more energy per second to heat than to move

Question 43

misconception about power

Answer 43

a higher power does not necessarily mean that an appliance transfers more energy usefully i.e. appliance 1 might have a very high power but a very low efficiency

Question 44

define the national grid

Answer 44

a system of cables and transformers linking power stations to consumers

Question 45

what is transferred using the national grid

Answer 45

electrical power

Question 46

what is the issue with getting electricity to homes? how can this issue be reduced; what is the problem with this resolution?

Answer 46

energy is lost in power cables due to the resistance of the wires; we can reduce energy loss by building power stations near to homes, but this isn't feasible, as people don't want to live near power stations

Question 47

how are unwanted energy transfers reduced in the national grid

Answer 47

by using step-up and step-down transformers; step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use

Question 48

why is electrical power transferred at a very high p.d. across step-up transformers

Answer 48

in our P=IV equation, if we increase the potential difference keep the power (rate of energy transfer) the same, then we have a lower current. a lower current results in less energy being lost

Question 49

define insulators; give examples

Answer 49

insulators are materials that do not conduct electricity, as electrons cannot move through them e.g. cloths and plastic rods

Question 50

why are metals good conductors of electricity

Answer 50

electrons can easily flow through metals

Question 51

what happens when you rub certain insulating materials together

Answer 51

when they are rubbed against each other (e.g. a cloth and a plastic rod), they become electrically charged. negatively charged electrons are rubbed off one material and on to the other. the material that gains electrons becomes negatively charged and the material that loses electrons becomes positively charged. these positive and negative charges are equal and opposite

Question 52

why is static electricity called that

Answer 52

because the two insulators now become electrostatically charged

Question 53

what happens when two electrically charged objects are brought close together

Answer 53

they exert a force on eachother; two objects that carry the same type of charge repel, two objects that carry different types of charge attract. attraction and repulsion between two charged objects are examples of non-contact force

Question 54

how can static electricity cause a spark under certain conditions

Answer 54

when we rub two insulators and electrons are transferred from insulator A to insulator B, insulator A becomes positively charged and B becomes negatively charged. the electrons from insulator B are therefore strongly attracted to insulator A's positive charge, so sometimes, these electrons can jump across the gap when they are brought close to each other, causing a spark

Question 55

how do you prevent sparks leading to fires e.g. on aircraft fuelling

Answer 55

the refuelling truck and the aeroplane are earthed to prevent any charged building up and leading to a spark

Question 56

how do sparks commonly take place

Answer 56

between an object with a static charge and an earthed object

Question 57

describe how sparks take place between static charged objects and earthed objects - what assumptions do you make

Answer 57

as electric charge builds up on an object, the p.d. between the charged object and the earthed object increases. with a high enough p.d., the electrons can jump across the gap and be transferred from the charged object to the earthed object. you must assume that the uncharged object is earthed, not positive

Question 58

when is an electric field created

Answer 58

a charged object creates an electric field around itself

Question 59

determine the strength of an electric field

Answer 59

the electric field is strongest close to the charged object; the further away from the charged object, the weaker the field

Question 60

what happens when another charged object is placed in the electric field

Answer 60

it experiences a non-contact force; the force gets stronger as the distance between the objects decreases

Question 61

how can we draw electric field lines

Answer 61

draw lines perpendicular to the surface with arrows that point in the direction of a positive particle that is placed within the field i.e. field lines for a positive object will go away from the object (if you place a positive object in the field it will be repelled) and field lines for a negative object go towards the object (if you place a negative object in the field it will be attracted)