electricity

Question 1

which way does current flow

Answer 1

negative to positive, but drawn on diagrams oppositely (conventional current)

Question 2

energy changes in circuit

Answer 2

chemical -> electrical -> other eg light, thermal, sound’

Question 3

how to investigate how length of wire affects resistance and result

Answer 3

• put a battery, ammeter, voltmeter in parallel to wire, and length of wire in a circuit
• tape wire to metre ruler
• connect circuit to wire using crocodile clips at 0cm and 10cm
• close switch, record current & pd
• move clip 10cm up, repeat
• calculate resistance using V/I
• plot resistance against length on graph and draw best fit
• resistance of wire is proportional to its length

Question 4

issues with investigating how length of wire effects resistance and how to resolve

Answer 4

• if line doesn’t go through origin, zero/systematic error as first clip is not at 0m. subtract the zero error from all readings
• if temp increases, resistance increases. use low PD to keep current low and reduce heating. turn off current between readings

Question 5

what is a variable resistor

Answer 5

moving the position of the slider changes length of wire, changing resistance
as resistance of wire is proportional to its length

Question 6

how to investigate how current through a component depends on PD across component

Answer 6

• in circuit put component, battery, ammeter, variable resistor, voltmeter in parallel
• record PD across resistor and current through resistor in table
• adjust variable resistor and record new readings several times
• switch direction of battery to reverse direction of PD
• continue recording readings
• dont leave circuit connected for too long as temperature will affect results
• plot current against PD graph

Question 7

how is the I-V characteristics practical different for diodes

Answer 7

• diodes very easily damaged by high current, extra resistor to keep current low
• low current needs sensitive ammeter - milliammeter

Question 8

What is charge measured in?

Answer 8

coulombs

Question 9

What is potential difference?

Answer 9

the energy transferred for each coulomb of charge

Question 10

what is ohms law?

Answer 10

voltage = current x resistance
v = i r

Question 11

what will ammeters and volmeters read in a series circuit?

Answer 11

ammeters: the same
voltmeters: shared between components

Question 12

what will ammeters and voltmeters read in parallel circuit

Answer 12

ammeters: shared between branch
voltmeters: same in each branch, split within branch

Question 13

how to find PD of a battery

Answer 13

voltage of cell x number of cells, only if both cells are in same direction

Question 14

why is resistance needed and what is it

Answer 14

for circuit to work, electrons must travel through wire and components
resistance is the pd (energy) needed for current to travel through component

Question 15

What makes a wire hot?

Answer 15

Electrons collide with ions in the wires, ions vibrate, heats up

Question 16

how to tell if something has low/high resistance

Answer 16

voltage/current (ohms law)
- a higher pd to current means more energy for current to pass component

Question 17

what is an ohmic conductor and describe resistance

Answer 17

resistance is constant: current through resistor is directly proportional to potential difference, ONLY AT CONSTANT TEMP

Question 18

describe resistance in a filament lamp

Answer 18

not directly proportionate, as when used, temp increases, causing resistance to increase.
because ions vibrate more, so collide with electrons more, so more energy needed for current to pass (resistance)

Question 19

what is a diode and describe resistance

Answer 19

current flows in one direction only, as resistance is very high in reverse direction
means current can flow (resistance) in forward direction, but cannot in reverse direction

Question 20

what does graph of pd and current look like for ohmic conductor

Answer 20

Question 21

what does graph of pd and current look like for filament lamp

Answer 21

Question 22

what does graph of pd and current look like for diode

Answer 22

Question 23

symbol for diode

Answer 23

Question 24

symbol for led and why is it used

Answer 24

allows current to flow in only one direction (like diode) but gives off light when current flows. this means it is energy efficient

Question 25

resistor in series vs parallel

Answer 25

- add together - total of two in parallel is less than resistance of smallest, as there are two pathways for current so more total current in circuit. larger current with unchanged PD means resistance decreases ass current = pd/resistance

Question 26

ldr symbol and purpose

Answer 26

changes resistance depending on light dark = high resistance light = low resistance

Question 27

uses of ldr

Answer 27

switching off phone when put to ear - when in light, resistance is low, meaning pd is low - pd is shared in series, so pd across lamp is high, lamp lights up - when against ear, resistance is high, meaning pd is high -pd is shared in series, so pd across lamp is low,

Question 28

thermistor symbol and purpose

Answer 28

changes resistance depending on temperature colder = high resistance warmer = low resistance

Question 29

use of thermistor

Answer 29

thermostat in computer (fan turns on when temperature is high) - when temp is low, resistance is high, meaning pd is high - pd is shared in series, so pd across cooling fan is low, fan at slow speed - when temp is high, resistance is low, meaning pd is low - pd is shared in series, so pd across cooling fan is high, fan at high speed incubators for premature babies - thermistor detects fall and causes lamp to turn on and alarm to sound

Question 30

what is resistance measured in

Answer 30

ohms

Question 31

components in a plug and what are they made out of

Answer 31

earth wire neutral wire live wire copeper with plastic coating

Question 32

colour, function, and danger of live wire

Answer 32

brown carries alternating PD from supply (230v) and is connected to fuse - extremely dangerous, fatal if touched as current flows through person to earth

Question 33

function and symbol for the fuse

Answer 33

contais thin wire that melts if the current gets too high

Question 34

colour and function of the neutral wire

Answer 34

blue completes circuit (0v)

Question 35

colour and function of earth wire

Answer 35

green and yellow stripes safety wire to stop appliance becoming live (0v) - attaches to metal casing and to ground

Question 36

risk of electrical appliances and how do three-core cables stop this

Answer 36

if live wire touches case, case becomes live (230v), becomes dangerous - earth wire provides alternate path to earth for the current - fuse melts

Question 37

how do different appliances transfer energy

Answer 37

movement eg blender: electrical energy-> kinetic energy of electric motors heat eg kettle: electrical energy -> thermal energy both eg washing machine: electrical energy -> kinetic energy of electric motors + thermal energy

Question 38

What is power and how is it measured

Answer 38

rate of energy transfer 1 watt - one joule per second

Question 39

compare power rating in appliances creating kinetic energy vs thermal

Answer 39

thermal has much higher power rating

Question 40

current from cell vs mains electricity

Answer 40

cell: one direction, DC mains: changing direction, AC

Question 41

benefits of AC

Answer 41

easy to use a transformer to increase or decrease the PD

Question 42

AC vs DC on oscilloscope

Answer 42

AC: PD rises and falls as current moves back and forth DC: PD does not change

Question 43

what is a potential difference and frequency of mains electricity?

Answer 43

230v 50Hz

Question 44

what are the five stages in the National Grid?

Answer 44

Power station, step up transformers, high-voltage transmission lines, step down transformers, consumers

Question 45

what are the transformers used for and why

Answer 45

- to change voltage and current in transmission lines - high current means energy lost to thermal energy stores of surroundings as wire heats up - increasing voltage lowers current as v x c = power

Question 46

what does step up transformer do

Answer 46

increases PD to 400 000v to lower current, reducing energy loss

Question 47

what does step down transformer do

Answer 47

decreases voltage to 230v and passes it into homes

Question 48

how can you determine whether a transformer is a step up or step down?

Answer 48

The number of coils

Question 49

As power transferred stays the same, what happens as the voltage increases on the National Grid?

Answer 49

Current decreases and vice versa

Question 50

what happens when two insulating materials are rubbed together?

Answer 50

Electrons will transfer from one to the other, leaving one positively charged and one negatively charged

Question 51

how does a Vandergriff generator work

Answer 51

- electrons pass from top plastic roller onto moving belt the pass of onto earth - top becomes positive, spreads across dome - person touches, becomes positively charged, hairs are positive and repel eachother

Question 52

How are electric fields drawn?

Answer 52

Lines stick out perpendicularly from surface, arrows from positive to negative. Closer together lines equals stronger field

Question 53

Why do sparks occur?

Answer 53

Potential difference between the object and the earth increases, causing electrons to jump between