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BMAT Physics > Electricity > Flashcards

Electricity Flashcards

1
Q

electric current definition

A

flow of charge
rate of movement of charge
will only flow through a component with potential difference across it

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2
Q

cell definition

A

single electrical energy source

supplies circuit with power, provides potential difference

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3
Q

how cell applies potential difference to circuit

A

transfers energy to charges passing through cell

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4
Q

voltage definition

A

potential difference

electrical pressure that causes current to flow in a circuit

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5
Q

potential difference formula

A 
potential difference (volts) = work done (J) / charge (C)
V = W/Q
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6
Q

current formula

A

current (A)= charge (C) / time(s)

I = C/t

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7
Q

resistance definition

A

how difficult for charge to flow through it

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8
Q

resistance formula

A

resistance (ohms, omega)= potential difference (V) / current (A)
R = V/I

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9
Q

how to measure resistance

A

connect ammeter before component

voltmeter across both ends of it

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10
Q

Ohm’s law

A

ratio of voltage to current is constant

R = V/I

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11
Q

ohmic conductor definition

A

conductor that obeys Ohm’s law

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12
Q

current-voltage graph of ohmic conductor

A

straight line as resistance is fixed
resistance is equal to inverse of gradient of line
steeper graph, lower resistance

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13
Q

filament lamp current-voltage graph

A 
more current = more heat = more resistance 
as resistance increases with current, gradient decreases to reflect this
curved graph (looks like S, sigmoid)
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14
Q

power definition

A

rate of energy transfer

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15
Q

power formula

A

P (W) = E (J) / t (s)

power = work done / time taken

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16
Q

power (electricity) formula

A

power (W) = potential difference (V) x current (A)

P = I x V

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17
Q

static electricity

A

insulating materials rubbed together
electrons rubbed off one material and deposited to other material
charges build up
spark/shock happens when enough charges build up and move
material that gains electrons become negative and vice versa

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18
Q

direction of electron transfer in static electricity

A

depends on material
examples:
cloth duster -> polythene
acetate -> cloth duster

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19
Q

static electricity in medicine

A

defibrillators

produced electrical shocks, makes heart contract when placed on patient’s chest

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20
Q

static electricity in dust precipitates

A

dust precipitators remove dust from chimneys
cleans emissions from factories
negatively charged grid in chimneys makes dust particles negative
-ve particles attracted to +ve metal plates, form large particles

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21
Q

static electricity in paint sprayers

A

paint charged so droplets repel each other
gives fine spray
surface being painted given opposite charge to attract paint droplets
gives even coat + less waste

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22
Q

switch function

A

opens/closes circuit

allows current to flow or not flow through it

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23
Q

cell/battery function

A

battery = series of cells
supplies circuit with power
provides potential difference needed to move charge around circuit

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24
Q

lamp function

A

thin wire in inert gas

wire gets hot and flows when current flows through

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25
Q

fuse function

A

stop current from flowing through circuit when current gets very high (melts and opens circuit)

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26
Q

diode function

A

only allows current to flow in one direction

restricts current in parts of circuit

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27
Q

light-emitting diode function (LED)

A

only allows current to flow in one direction

glows when current flows through it

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28
Q

fixed/variable resistor function

A

oppose flow of current

helps set current in circuit to wanted value

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29
Q

thermistor function

A

temperature-dependent resistor

changed resistance according to temperature

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30
Q

voltmeter function

A

measure potential difference in volts across given component
always in parallel with components

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31
Q

ammeter function

A

measure current in amps in component

placed in series with components

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32
Q

light deadens resistor function (LDR)

A

resistor changing resistance depending on amount of light shining on it

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33
Q

current across a series circuit

A

same at any point in the circuit

34
Q

potential difference across series circuit

A

potential difference shared between components

35
Q

energy transferred from battery in circuit

A

total energy transferred from battery to charge = total energy transferred from charge to components

36
Q

resistance across series circuit

A

resistance of circuit = sum of resistance of each component

37
Q

current across parallel circuit

A

current shared between components of circuit

38
Q

potential difference across parallel circuit

A

potential difference is same at any point of circuit

39
Q

resistance in parallel circuits

A

more components = in parallel = lower total resistance p
each individual circuit gives electrons more than one way to get around circuit
also provided higher current

40
Q

efficient versions of filament light bulbs

A

compact fluorescent lamps (CFLs)

light-emitting diodes (LEDs)

41
Q

how to save money with electricity

A

some electricity is converted to waste energy

using more efficient appliances keeps this to a minimum, decreases energy bill in long term

42
Q

magnets definition

A

objects that produce magnetic field

have opposite north and south poles

43
Q

hard magnets definition

A

permanent magnets
generate own magnetic field
able to maintain it constantly

44
Q

examples of hard magnets

A

pure iron
nickel
cobalt
steel

45
Q

soft magnets definition

A

induced magnets
objects that generate magnetic field only if within magnetic field
can only be attracted by other magnets, cannot be repelled
permanent magnets induces only opposite pole on soft magnets

46
Q

soft magnet examples

A

some alloys of iron and other magnetic metals

iron fillings

47
Q

electromagnetic effect definition

A

circular magnetic field generated around wire when wire carries current

48
Q

direction of magnetic field in straight wires

A

anti-clockwise and perpendicular to direction of current

49
Q

direction of magnetic field in solenoids

A

south pole at positive end
north pole at negative end
like a bar magnet

50
Q

how to increase strength of magnetic field around wire

A 
increase current (directly proportional)
increase number of coils in solenoid (each coil has own magnetic field, adding coils increases amount of individual magnetic field)
51
Q

permanent magnets vs electromagnets

A

permanent magnet retains magnetism once magnetised

electromagnet only has magnetism when current passes through it

52
Q

what is produced when wire’s magnetic field interacts with permanent magnetic field

A

force

53
Q

Fleming’s left hand rule

A 
finger = direction of 
thumb = force
index = magnetic field
middle = current

used for motors

54
Q

magnitude of force for straight wire in uniform field when all quantities are perpendicular) equation

A

force (N) = magnetic field stretch (T) x current (A) x length of wire (m)
F = BIL

55
Q

how DC motor works

A

DC current flows through armature on commutator between north and south pole permanent magnets
overall turning motion as 2 opposite forces on two sides of coil

56
Q

how to increase force produced in DC motor

A

more turns of coil
stronger magnetic field
stronger current
longer length of wire

57
Q

electromagnets in speakers

A

variation of input current in soil moved core at specific frequency via motor effect
cone attached vibrates at same frequency
generates pressure waves in air (produces sound)

58
Q

electromagnets in microphones

A

same as speaker except in reverse

generator effect in place of motor effect

59
Q

electromagnets in doors

A

entering correct passcode / using keycard disables electromagnet holding door close

60
Q

electromagnets in toasters

A

magnet holds bread down

roast pops out when time is up and power to electromagnet is cut off

61
Q

electromagnetic induction definition

A

product of a potential difference across an electrical conductor in a changing magnetic field

62
Q

how potential difference can be induced in an electrical conductor

A

magnet moved into coil of wire

electrical conductor moved in a magnetic field not parallel to field lines

63
Q

how direction of induced potential difference/current can be changed

A

changing direction of movement of conductor relative to magnetic field
changing direction of magnetic field (polarity)

64
Q

how AC current is induced

A

magnet moved backwards and forwards in the coil

65
Q

how voltage is increased in induced AC currents

A

moving magnet faster
stronger magnet
more turns of coil
higher area of coil

66
Q

how AC generators work

A

induces current by spinning coil of wire inside magnetic field of magnet or vice versa
current changes every half turn as direction of motion changes due to alternating current

67
Q

how current and voltage increases in AC generator

A

strength of magnetic field
number of turns of coil
rate at which coil/magnet spins (also affects frequency)

68
Q

Fleming’s right hand rule

A 
finger = direction of
thumb = force
index = magnetic field
middle = current

used in generators

69
Q

output voltage of AC generators

A

generator effect only exists when wire is cutting field lines
maximum when 90° or 270° as coil is most perpendicular to magnetic field lines
0 when coil is not cutting magnetic field lines (0°, 180°, 360°)

70
Q

transformer definition

A

device that changes voltage of AC supply

71
Q

step-up transformer

A

changes low voltage supply to high-voltage supply

increases voltage as more turns on secondary coil than first

72
Q

step-down transformer function

A

changes high voltage supply to low voltage supply

increases voltage as less turns on secondary coil than first

73
Q

Structure of transformer

A

primary and secondary coils wound around soft iron core (coils not connected to each other)
primary coil supplies AC to iron core, induces magnetic field which is transferred to secondary coil where current is induced

74
Q

relationship of voltages of primary and secondary coils

A

inversely proportional to each other

75
Q

ratio of voltage across primary and secondary coils

A

depends on number of turns on primary and secondary coil

V1/V2 = N1/N2

76
Q

power in coils formula

A

transformers nearly 100% efficient
power in primary coil = power in secondary coil
power = potential difference x current so
V1 x I1 = V2 x I2

77
Q

transformers in the national grid

A

high power supply = very high voltage or high current
step-up transformers used to produce very high voltage and low current for transmission of electricity (minimises energy loss through heat)
step-down transformers used to reduce voltage to safe value before arriving in homes

78
Q

power loss due to resistance in cables formula

A

power loss (V) = current^2 (A) x resistance (ohms)

79
Q

why high voltage is used

A

high power is made by high voltage or high current

increasing current instead of voltage for high power = more heat loss as power loss is dependent on current

80
Q

switch more transformer

A 
reduced voltage of main supply
converts AC to DC
operates at high frequency
lighter and smaller 
used very little power when no device connected to output terminals
81
Q

isolating transformer

A

same number of turns in primary and secondary coils
same voltage
used to isolate connections from mains

BMAT Physics (7 decks)

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