Topic 5 - Electricity

Question 1

what is a voltmeter?

Answer 1

measures p.d and connected in parallel to the component

Question 2

what is a ammeter?

Answer 2

measures current and connected in series to the component

Question 3

what is current?

Answer 3

the rate of flow of electric charge ( A )
which flows from positive to negative terminal

Question 4

charge equation

Answer 4

Q = I t
charge = current x time

Question 5

potential difference

Answer 5

a source of potential difference allows current to flow through a circuit ( driving force ). Meausred in V

Question 6

How does changing the resistance affect the size of the current?

Answer 6

the greater the resistance of the component the smaller the current ( for a given p.d )

Question 7

equation linking current, p.d , resistance

Answer 7

V = IR
p.d = current x resistance

Question 8

I-V characteristic of ohmic conductor

Answer 8

the current through ohmic conductor is directly proportional to the p.d across it.
- it has a constant resistance ( for given temp )

Question 9

I-V characteristic of filament lamp

Answer 9

• as the current increases the resistance also increases
• less current can flow per unit of p.d and so the graph gets shallower

Question 10

why does the resistance of a filament lamp increase with current

Answer 10

• when an electrical charge flows through it transfers energy to thermal energy store of filament
• this heats up and glows
• resistance increases with temperature
• as more current flows the lamp heats up more, increasing resistance

Question 11

I-V characteristic of diode

Answer 11

• only lets current pass through one direction
• in the opposite direction R initially high but drops drastically as p.d increases letting current flow

Question 12

safety precaution with diode

Answer 12

• use a protective resistor to stop the current becoming too high and damaging the diode

Question 13

p.d in series circuits

Answer 13

total V = V₁ +V₂ + …..

Question 14

current in series circuits

Answer 14

total I = I₁ = I₂ = …etc

Question 15

resistance in series circuits

Answer 15

total R = R₁ +R₂ + …..

Question 16

p.d in parallel circuits

Answer 16

total V = V₁ = V₂ = …etc

Question 17

current in parallel circuits

Answer 17

total I = I₁ +I₂ + …..

Question 18

resistance in parallel circuits

Answer 18

adding more resistors in parallel decreases the overall resistance of the circuit
total resistance less than smallest individual resistance

Question 19

resistance of LDRs

Answer 19

• in bright light the resistance falls
• in darkness the resistance is highest

Question 20

resistance of thermistors

Answer 20

• in hot conditions the resistance falls
• in cold conditions resistance is highest

Question 21

uses of LDRs

Answer 21

automatic night lights, burglar detectors

Question 22

uses of thermistors

Answer 22

thermostats

Question 23

sensor circuits

Answer 23

• if it gets hot, the resistance of the thermistor decreases so takes a lesser share of the p.d
• this means the p.d across the output ( e.g : fan ). will increase, meaning it will get switched on ( faster )

Question 24

what is a static charge?

Answer 24

• an electric charge that can’t move
• found on electrical insulators where charge can’t flow freely or isolated conductors

Question 25

what happens when two insulators are rubbed together?

Answer 25

- creates friction which gives the electrons energy - negatively charged electrons transferred from one materila to the other - gain electron = N , lose electron = P

Question 26

how do sparks form?

Answer 26

- when charge builds up on an object the p.d between it and the earth ( 0V ) increases. - if the p.d is large enough, electrons can jump across the gap between charged object and earth/ earthed conductor

Question 27

what is an electric field ?

Answer 27

- a region around an electrically charged object in which forces can act on an object, causing it to attract or repel

Question 28

what happens when two electrically charged object are brought close?

Answer 28

they exert a force on each other called electrostatic attraction / repulsion

Question 29

electric field strength

Answer 29

- further away the weaker the force of attraction/repulsion - field lines further apart

Question 30

how do sparks work in terms of electric fields

Answer 30

- the object has a large static charge and so there is a large p.d between it and earth - causes a strong electric field - electric field ionises air particles so the air can conduct electricity - spark jumps from object to earthed conductor

Question 31

alternating current ( A.C )

Answer 31

current is constantly changing direction as the positive and negative terminals alternate

Question 32

direct current ( D.C )

Answer 32

current always flows in the same direction ( uni - directional )

Question 33

why is A.C. used for mains electricity

Answer 33

- easier to generate - easy to increase/decrease p.d of transformer

Question 34

voltage of mains electricIty in UK

Answer 34

230V

Question 35

frequency of mains electricIty in UK

Answer 35

50 Hz

Question 36

why is each wire in a three core cable colour coded?

Answer 36

so that it is easy to tell different wires apart

Question 37

live wire colour

Answer 37

brown

Question 38

neutral wire colour

Answer 38

blue

Question 39

earth wire colour

Answer 39

green and yellow

Question 40

purpose of live wire

Answer 40

carries the A.C from supply to appliance at 230V

Question 41

purpose of neutral wire

Answer 41

carries the A.C away from appliance and completes the circuit

Question 42

purpose of earth wire

Answer 42

for protection and safety - provides an alternate low-resistance path for the high current to flow in case of a fault - this prevents the current from going to the person

Question 43

purpose of fuse

Answer 43

fuse connected to the live wire if a fault occurs high current will cause fuse to heat up and melt - this will break the circuit and stop dangerously current from flowing

Question 44

power equation

Answer 44

P = VI

Question 45

energy equation

Answer 45

E = Pt or VIt

Question 46

step up transformer

Answer 46

- increases p.d and - decrease current

Question 47

step down transformer

Answer 47

- decreases p.d to safe domestic levels - and increases current

Question 48

why is the p.d increased first

Answer 48

- for a given power, if you increase p.d you decrease current - less current means wires heat up less and less thermal energy is dissipated - this makes National grid more efficient