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A-Level PHYSICS Edexcel > Topic 3 - Electric Circuits > Flashcards

Topic 3 - Electric Circuits Flashcards

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1
Q

current

A

rate of flow of charge

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2
Q

what is the gradient on a charge time graph?

A

Current ( I = Q/t)

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3
Q

how do you work out charge in a current-time graph ?

A

area under the graph (Q = It)

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4
Q

An Ammeter must…

A

have a low resistance and must be placed in series

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5
Q

A voltmeter must…

A

have an infinitely high resistance and must be placed in parallel

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6
Q

another word for voltage is…

A

emf / potential difference

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7
Q

potential difference is…

A

a measure of the amount of energy or unit of charge transferred between 2 points in a circuit

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8
Q

what is Kirchoff’s first law?

A

the total current flowing into a point is equal to the current flowing out of that point. Conservation of Charge

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9
Q

what is Kirchoff’s second law?

A

around any closed loop in a circuit, the sum of the pd across all the components is the pd of the supply. Conservation of energy.

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10
Q

power(watts) is…

A

rate of transferring energy/ rate of doing work.

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11
Q

1 watt of power means…

A

1 joule of energy is used every second

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12
Q

ohms law:

A

V = IR

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13
Q

in a series circuit the current is…

A

the same across all the components

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14
Q

in a parallel circuit the current…

A

total current = is the sum of the current in all the branches

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15
Q

potential difference in a series circuit…

A

total of = sum of pd in all appliances

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16
Q

potential in parallel circuits is…

A

the same across each branch

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17
Q

resistance in a series circuit…

A

total resistance = the sum of the resistance in all the components

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18
Q

resistance in a parallel circuit…

A

reciprocal of Resistance = is the the sum of all the reciprocals of the resistance of all the components

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19
Q

is a resistor a conductor?

A

yes, it’s a conductor so that current can still flow around a circuit however resistors let less current flow than other conductors

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20
Q

what effects the resistance of a resistor?

A

cross sectional area of the wire
length of the wire
material of the wire
temperature of the wire.

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21
Q

what is resistance proportional to?

A

Length

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22
Q

What is resistance inversely proportional to ?

A

Cross-Sectional Area

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23
Q

What is the Resistivity?

A

The resistance of a 1m² 1m long sample (the resistance per unit cube). its a property of a material.

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24
Q

The resistivty is constant providing…

A

the temperature is constant

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25
Q

What is the conductivity?

A

1/ resistivity

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26
Q

On a resistance Length graph what doe the line look like?

A

positive straight gradient

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27
Q

On a resistance length graph what does the gradient of the line tell you?

A

resistivity/ Area

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28
Q

What does a Resistance Area graph look like?

A

A curve getting less steep as it goes down (negative gradient. ITS A 1/X CURVE

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29
Q

What does a resistance 1/cross sectional area graph look like?

A

Positive straight gradient

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30
Q

What does the gradient of a resitance 1/cross sectional area graph mean?

A

Resistivty multiplied by length

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31
Q

Prove 1/R1 + 1/R2 + 1/R3 = 1 / R

A

itotal = i1 + i2 + 13
i = v/r hence:
v/Rtotal = v/R1 + v/R2 + v/R3
v is constant and is the same across all branches therefore 1/R1 + 1/R2 + 1/R3 = 1 / R

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32
Q

What is a potentiometer?

A

A device that provides a potential difference ranging from 0v and Vs (voltage of supply)

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33
Q

Length needed =

A

length (of wire) * (voltage out/ total voltage)

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34
Q

What do the letters stand for in I = nAvq?

A 
I = Curent (A)
v = Drift velocity (ms-1)
A = cross sectional area of the conductor (m2)
n = charge density (m-3)
q = charge on each charge carrier (c)
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35
Q

What is the charge density?

A

the number of charge carriers that can move per m^3

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36
Q

A poor conductor has ___ charge carriers

A

few

37
Q

A GOOD conductor has ___ charge carriers

A

many

38
Q

4v mean 4J per

A

coulomb of charge

39
Q

5ohm resistance means 5v is needed for

A

1A of current

40
Q

Vout/ Vsupply=

A

R2 / (R1+R2)

41
Q

Prove Vout/ Vsupply= R2 / (R1+R2)

A

for the whole circuit Vsupply = I(R1 +R2)
vout = IR2 I= Vout/ R2
Vsupply = vout/ R2 * (R1 *R2)
R2 / (R1+R2)

42
Q

define emf

A

energy supplied per unit of charge by the supply

43
Q

define terminal pd

A

energy transferred per unit of charge to the load

44
Q

define lost volts

A

energy transferred per unit of charge as the electrons flow through the cell

45
Q

What is the difference between R and r?

A

R denotes the load resistance and r denotes the internal resistance

46
Q

on a graph of current against voltage what does the y-intercept and gradient show?

A

y-intercept shows the emf

gradient shows the internal resistance

47
Q

What happens the the graph of current against voltage if there are two cells?

A

the gradient is twice as steep and the intercept is twice as far up

48
Q

Define thermistor

A

resistor whose resistance varies with temperature

49
Q

define LDR

A

a light dependent resistor is a resistor whose resistance varies with light intensity

50
Q

LDR relationship with incident light

A

As more light is incident the resistance decreases

51
Q

What are the two types of thermistors?

A

negative temperature coefficient (NTC) and positive temperature coefficient (PTC)

52
Q

what is an NTC?

A

a thermistor where as the temperature increases the resistance decreases

53
Q

what is a PTC?

A

a thermistor where as the temperature increases the resistance increases

54
Q

What are the two factors which describe how thermistors work (PTC, NTC and LDR the first factor)?

A
  1. increased temperature means increased lattice vibrations which leads to more electron collisions. (this reduces v in I = nAqv and reduces the current i.e. increases resistance
55
Q

What are the two factors which describe how thermistors work (NTC and LDR the second factor)?

A
  1. energy absorbed (light or heat) results in the release of conduction electrons (this increases n in I = nAqv so increased current therefore reduced resistance)
56
Q

how does current flow

A 
free electrons (already in the conductor) are repelled by the negative terminal and attracted by the positive terminal. 
The positive terminal is the long side of a cell. The arrows in a circuit are drawn the opposite way around.
57
Q

insulators

A

have a low number of electrons per m^3 and so not conduct well

58
Q

number of electrons =

A

total charge / charge of an electron

59
Q

1 coulomb

A

the amount of charge that passes a point where a currect of 1 amp flows for 1 second

60
Q

1 volt of p.d.

A

a joule of electrical energy transferred for each coulomb of charge

61
Q

basic points of I=nAvq proof

A 
wire volume = Ad (area*length)
number of electrons in volume of wire = nAd
total charge of all electrons = nAdq
I = Q/t so I = nAdq/t
V = d/t sp I = nAvq
62
Q

resistance

A

the ratio of the p.d. applied across it to the currect passing through it.

63
Q

what effects the resistance of a resistor/wire:

A
  • length (Longer means more resistance)
  • cross sectional area (smaller means more resistance)
  • temperature (in metals a hot wire means more resistance)
  • type of material
64
Q

resistivity ρ

A

the resistance of a 1m^2 1m long sample, measured in ohmmeters

65
Q

filament lamp current voltage graph

A

the resistance increases as the current increases. as the wire gets hotter the current doesn’t increase as much

66
Q

LED/diode current voltage graph

A

semiconductor therefore the reverse diode has very high resistance and the forward diode has very low resistance.
they only conduct when they are forward based

67
Q

as temperature increases resistance increases because (in metals)

A

the ions in the metal lattice vibrate faster, with greater amplitude, this means electrons collide more with ions opposing the flow increasing resistance

68
Q

as temperature increases resistance decreases because (in semiconductors)

A

at rooms temperature there are few free electrons available for conduction. at high temperatures some electrons have enough energy to ‘ escape’ from their atoms and be able to conduct

69
Q

superconductors

A

if a wire is cooled to a low enough temperature it looses all its resistance. Electrons flow through them without any transfer of energy.

70
Q

fuse

A

a thin wired which melts if the current gets too high

71
Q

kilo-watt hour

A

the electrical energy transferred by a 1kw device in 1 hour

72
Q

variable resistor uses

A
  • control current (rheostat)

- to control voltage (potentiometer)

73
Q

Root mean square

A

Square all the values to make them positive. Then find the mean, then square root this mean.

74
Q

Root mean square uses in electricity

A

Finding an average voltage or current when its alternating. (A regular average would just be zero)

75
Q

Potential

A

The amount of energy available at a point in a circuit

76
Q

How does the potential along a wire vary with distance?

A

as distance increases potential decreases

77
Q

what is current?

A

a flow of charged particles which are already in the conductors. free electrons are repelled by the negative terminal and attracted by the positive terminal.

78
Q

which way does current flow

A

short side of the cell to the long side of the cell. (negative to positive, its drawn the other way around as conventional current)

79
Q

precautions to obtain accurate values for temperature when making a thermistor

A
  • stir the water / make sure the thermometer and coil are in the same part of the beaker
  • check for zero error
  • read temperature at eye level to avoid parallax
  • switch on/off between readings to avoid the wire heating up
  • use a small current to avoid the wire heating up
80
Q

explain why resistance increases using the structure of the metal

A
  • As temperature increase the lattice ion vibrations increase therefore electrons will collide more frequently with the vibrating ions
  • More energy dissipated by collisions greater V required
  • Since V increases and R = V/I, R will increase with temperature
81
Q 
Explain, in terms of energy, the difference between potential difference (p.d.) and
electromotive force (e.m.f.).
A
  • p.d. is electrical energy transferred between two points in a circuit
  • emf is the energy supplied to the circuit
82
Q

what happens inside a cell as load resistance increases

A
  • total resistance increases

- if emf remains constant then the current inside the cell must increase

83
Q

why may large wires be used?

A
  • prevent energy dissapation
  • allows large currents
  • lower cable resistance
84
Q

Vout/Vsupply =

for a potentiometer

A

given length/ full length

85
Q

Vout/Vsupply =

for a potential divider

A

R2/R1+R2

R2 is the resistor with the voltmeter in parallel over it

86
Q

If there is a paralell loop over part of a series circuit and a component blows…

A

The resistance of the parallel increases therefore it gets more voltage

87
Q

what does increasing r do?

A

reduces the terminal pd. V due to the lost volts so less energy/power output to load

88
Q

frequency of mains supply

A

50Hz

A-Level PHYSICS Edexcel (16 decks)

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