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AA - Physics A Level Y12 > Topic 3 - Electric Circuits > Flashcards

Topic 3 - Electric Circuits Flashcards

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1
Q

Define electric current

A

‘The rate of flow of charged particles’
(Spec point 31)

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2
Q

Give the equation for current

A

I = delta Q/delta t

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3
Q

A)What is the charge of elementary charge
B) what is its unit

A

1.6 x 10^-19
Coulombs

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4
Q

What is the charge of an electron in terms of elementary charge?

A

-e

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5
Q

What is the symbol for elementary charge?

A

e

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6
Q

What is the charge of a proton?

A

e

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7
Q

What is 1 mole of something?

A

6.02 x 10 ^23 of that thing

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8
Q

What is avogadros constant (give the number)

A

6.02 x 10^23

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9
Q

Current is the same/different across a series circuit

A

Same

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10
Q

Electrons flow -/+ to -/+

A

Negative to positive always

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11
Q

Conventional current flows from the -/+ terminal to the -/+ terminal and why?

A

Positive to negative
Current is defined as flow of positive charge (you can see this from the equation) and as electrons carry negative charge, the direction of current flow is the opposite to the direction of the flow of electrons

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12
Q

True or false: only electrons can cause current to flow

A

False - protons and other ions can as well

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13
Q

What is the equation for potential difference

A

V = W/Q
Potential difference = work done/charge

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14
Q

What is EMF?

A

Electromotive force - the work done per unit charge by the power supply or cell converting energy into electrical energy of the charges

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15
Q

Define potential difference

A

The ENERGY TRANSFERRED between two points in a circuit PER UNIT CHARGE

(Things in capitals are important marking points)

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16
Q

Net electric charge flow is the sum of what

A

Negative and positive charge flow - do not take the magnitude of the negative flow, as the negative cancels out the positive as net charge flow measures the total imbalance in charge, not the movement of electrons

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17
Q

Explain why a proton in a space between a positively charged surface and a negatively charged one (where it is further from the negative surface than the positive), accelerates towards the negatively charged surface. Explain the energy transfers in this process.

A
  1. The proton has a high electrical potential energy at first when it is far from the negative surface and There is a downward force on the proton
    a. Because like charges repel and opposite charges attract, so the charge is repelled from the top and attracted to the bottom
    1. The speed of the proton increases as it accelerates and so the kinetic energy of the proton increases
      The electrical potential energy decreases and it reaches the negative surface
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18
Q

A proton has a higher electrical potential energy far/close to a negatively charged surface

A

Far - as there is a large potential for work to be done

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19
Q

True or false: potential is the same as electrical potential energy

A

False

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20
Q

Positive charges move from a high/low potential to a high/low potential

A

High to low

As think of all positive things as a high potential as they have a high potential to move to the negative thing as their point charge is pointing outwards. They all want to go to the negative (low potential)

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21
Q

Potential is defined between two points/at a point

A

At a point

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22
Q

What are the units for potential

A

Volts

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23
Q

What is the mass of an electron

A

9.1 x 10 ^-31 kg

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24
Q

What is the radius and what is the nucleus of an atom?

A

Radius = 0.1nm = 1 x 10^-10
Nucleus = 10fm = 1 x 10^-14

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25
Q

[l] = ?

A

Metres - m
(The question means what is the unit of length)

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26
Q

[ T ] = ?

A

Kelvin - K

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27
Q

Symbol for time

A

t

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28
Q

Moles = [?]

A

n

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29
Q

What is the symbol for the amount of a substance?

A

n

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30
Q

How many degrees Kelvin is 100 degrees Celsius?

A

373

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31
Q

How many degrees Kelvin is 0 degrees Celsius?

A

273

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32
Q

What is 0 degrees Kelvin in Celsius?

A

-273

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33
Q

What happens at O degrees Kelvin

A

All particles stop moving, they have 0 kinetic energy

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34
Q

Give the 7 base units and their symbols

A

Length - l -metres - m
Time - t - seconds - s
Mass - m - kg
Temperature - T - Kelvin - K
Current - I - Amperes - A
Amount of a substance - n - moles - mol
Luminous intensity - Lv - Candela - Cd

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35
Q

How many derived units are there?

A

Anything that isn’t a base unit is a derived one - and you can always write a derived unit in terms of base units

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36
Q

What is Joules expressed in base units using the equation for kinetic energy?

A

Kg (m/s)^2
REMEMBER THE BRACKETS - kgm^2/s^2

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37
Q

Describe in words what V = IR means

A

Potential difference across the resistor = current through the resistor x the resistance of the resistor

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38
Q

What is kirchoff’s current law?

A

Sum of currents into a junction equals sum of currents out

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39
Q

Explain the derivation of kirchoff’s current law using the conservation of charge

A
  • net charge in a closed system doesn’t change
  • therefore sum of charge into a junction = sum of charge out of a junction
  • Divide the charge on each side by time (ensure you still use sigma to denote sum of in this line of working!)
  • therefore sum of current in = sum of current out
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40
Q

Is current vector or scalar and why?

A

Scalar - as it can only be positive or negative, so it is one dimensional
It is also defined as change in current over change in time, and both change in current and change in time are scalar quantities, and you can’t get a vector quantity from a scalar one

41
Q

What is kirchoff’s voltage law?

A

The sum of the potential difference is equal to the sum of the EMFs around a closed loop in the circuit

42
Q

How is kirchoff’s voltage law derived?

A

The law of the conservation of energy states energy cannot be created or destroyed
Sum of Energy released by EMFs = sum of energy dissipated by components
Sum of VQ = sum of VQ
Therefore sum of potential difference released by EMF = sum of potential difference across component

43
Q

What is the symbol for femto and what order of magnitude is it?

A

f - 10^-15

44
Q

What is the symbol for femto and what order of magnitude is it?

A

f - 10^-15

45
Q

If you move a charge across a voltage of 1V then the charge gains how much energy

A

An energy whcih is equal to its charge
(If you moved it across more than 1V then it gains energy that is its charge x the voltage )

46
Q

Explain why potential difference across branches of a parallel circuit (with the same component on each) is the same

A
  • Kirchhoff’s voltage law says voltage in a closed loop is equal to voltage of EMF in that loop
  • therefore applying this law to the two loops of the two branches shows that the voltage on one branch = voltage on other branch = voltage of EMF
47
Q

Explain why potential difference splits across components connected in series

A
  • kirchoff’s voltage law states that the sum of the voltages = the sum of EMF in a closed loop
  • therefore the voltage of the EMF is split across the components connected in series, as they only form one closed loop
48
Q

Describe how current splits across branches in parallel

A

It splits according to the ratio of the resistances, favouring lower resistances (more current flows in the lower resistance branch)

49
Q

How do you find total resistance in series?

A

The sum of the resistances

50
Q

What are the only 3 laws you need to know about circuits in a level physics?

A

Kirchoffs voltage law, his current law, and v = IR

51
Q

Derive the equation for total resistance in a series circuit

A

KVL says total voltage = sum of voltages
As V = IR, IR total = sum of IR for each component
KCL says I total = sum of currents
Therefore we can cancel the total current to get
R total = sum of individual resistances

52
Q

Derive the equation for total resistance in a parallel circuit

A

Total current = sum of currents due to KCL
As V=IR, total current = V/R = the sum of V/R for each component
Due to KVL, voltage of EMF is the same as voltage of each component, so that can be cancelled
Therefore 1/Rtotal = sum of 1/R for each component

53
Q

What is the key part of KVL which you need to pay attention to to get questions about KVL correct?

A

Sum of voltage = sum of EMF IN A CLOSED LOOP (a closed loop means a loop with no breaks (just a loop in series which links the EMF to the component(s))

54
Q

Are Resistor 1 and resistor 3 connected in series or parallel and why?

A

They are not connected in series, and they are also not connected in parallel!!
They can’t be in series as they have different currents, but they can’t be in parallel because they are not

55
Q

Are Resistor 1 and resistor 3 connected in series or parallel and why?

A

They are not connected in series, and they are also not connected in parallel!!
They can’t be in series as they have different currents, but they can’t be in parallel because they are not

56
Q

What relationship do R2 and R3 have and what relationship do R1 and Requivalent have?

A

R2 and R3 are in parallel and R1 and R equivalent are in series

57
Q

What is the relationship between R1 and R2 in this circuit?
How about R2 and R3?
What about R2 and R5?
What about Requivalent (R2+R3) and Ra?

A

Nothing
Series
Nothing
Parallel

58
Q

What is the potential difference across the resistor?

A

1.5V

59
Q

How do you calculate the total EMF of multiple EMFs connected in series with each other

A

Sum of individual EMFs

60
Q

Why would you connect two cells in parallel with each other (given this doesn’t increase total voltage supplied)?

A

It lengthens the lifetime for each cell, as each cell is providing a lower current (and so the rate of loss of charge for each cell is lower)
This can still make a circuit run as the total current in the other branches is the sum of each current provided by the cells

61
Q

Ammeters have a zero/nonzero resistance in real life and what does this mean for the recorded value of current?

A

Non zero
It makes the recorded value lower than the true value as V=IR and the resistance increases so current decreases

62
Q

What is the resistance of an ideal ammeter?

A

0 ohms

63
Q

With a normal resistor (with non infinite resistance), is the measured potential difference higher or lower than the true value or why

A

Lower, as the equivalent resistance decreases and so the potential difference decreases

64
Q

What is the density of water in g/cm cubed an in kg per metre cubed

A

1g/cm cubed and 1000kg per metre cubed

65
Q

what is the potential difference across A1 and B1 compared to A2 and B2?

A

A1 and B1 resistance is 10 Volts as think of the height of the full resistance as 15V, which splits proportionally to resistance
A2 and B2 is 7.5 Volts, as the equivalent resistance across A2 and B2 is 5 ohms, and so the potential difference splits half and half across the 5 ohm resistor and the 5 ohms of equivalent reisstance

66
Q

a battery has an internal ____

A

resistance

67
Q

the EMF Of a battery is the voltage measured across a) the cell part of the battery OR b) the cell and the internal resistor

A

a)the cell part of the battery (without internal resistor)

68
Q

what is lost volts

A

EMF - terminal potential difference (on other words the voltage of the internal resistance of the cell)

69
Q

what is terminal potential difference

A

potential difference across two ends of a battery (this is lower than EMF as some energy is dissipated by heating up the internal resistor)

70
Q

what is EMF? (in terms of a battery)

A

EMF of an ideal battery with no internal resistance

71
Q

Explain how you would measure the resistance of a resistor of 1600 ohms using a multimeter

A

Connect the resistor across the port labelled COM and the one labelled ‘VOhmsmA’( this may vary depending on the current in the circuit)
Then switch to. The 2000ohms setting as the resolution is highest.

72
Q

Explain how a standing wave is formed 6 marker:

A
  1. Two waves must be travelling in opposite directions (usually one wave and its reflection) with the same frequency, amplitude and speed
  2. The waves meet and undergo superposition
  3. Constructive interference occurs when the waves meet in phase
  4. This results in antinodes which are positions of maximum amplitude
  5. Destructive interference occurs when the waves meet in antiphase
  6. This results in nodes which are positions of zero amplitude
73
Q
A
74
Q

Give an equation linking the current and resistance of an internal resistor, EMF, and terminal pd

A

Terminal pd = EMF - current through internal resistor x resistance of internal resistor

75
Q

When does the terminal potential difference in a circuit equal the EMF (give 2 circumstances)

A

when internal resistance is 0, or current is 0

76
Q

Both of these circuits have 0 internal resistance - Is there a difference in brightness between the bulbs in the left circuit and the right one and why/why not?

A

The brightness would be the same - as there is no internal resistance in the battery, the ost volts in both cases is the same (0), and doesn’t increase when the current increases (when new bulbs are added). Therefore the potential difference across each bulb is constant (due to KVL), and as power = v^2 / R (and R and V are hence constant for each individual bulb) then the power for each bulb is the same.
The only things whcih change are the resistance (which decreases as you add more branches and therefore give more places for current to flow) and current (which increases as resistance has decreased)

77
Q

Will the bulbs in one circuit be brighter than the other - if so which one and why?

A

The bulb in the leftmost circuit will be brightest as the lost volts is lower than the other circuit, as the lost volts is current x resistance, and as current is higher in the right circuit (as there are more branches which lowers resistance and hence increases current), the lost volts is higher which means the pd for each branch is lower (as the EMF is a finite source of potential difference)

They therefore have a lower power (as power is V^2 / R) and therefore a lower brightness

78
Q

Give the minimum answer (not necessarily a good answer) for why the headlights of a car are really bright at the start when they are turned on and then quickly get dim

A
  • When the engine is started, the motor is drawing a large current.
    • The terminal potential difference across the cell decreases
    • the potential difference across each light bulb is smaller
    • So the light is dimmer
79
Q

In the practical about finding the EMF and internal resistance of an electrical cell, what do you plot on a graph

A

terminal pd against current (as current is the independent variable) and terminal pd can be directly measured.
The gradient of that graph is therefore the negative of resistance and the y intercept is the EMF of the cell
This can be derived from the equation terminal pd = EMF - (current x resistance)

80
Q

Describe the circuit set up for the practical on determining the EMF and internal resistance of an electrical cell

A
81
Q

When writing definitions you must write ___as well

A

equations

82
Q

___is related to the brightness of a bulb

A

power

83
Q

What is the definition of power

A

rate of transfer of energy

84
Q

What is V2/V1 for a transformer

A

N2/N1

85
Q

What is I2/I1 for a transformer

A

n1/n2

86
Q

What are the charge carriers in a metal

A

electrons

87
Q

Only the ___electrons in a metal are conducting electricity

A

outer

88
Q

Define the number density of free charge carriers

A

number of charge carriers in 1m cubed of a material

89
Q

What order is the number density of free charge carriers typically

A

10^28

90
Q

What is the unit of number density of free charge carriers

A

m^-3

91
Q

What is a free electron in an atom

A

The electron in the outer shell

92
Q

Define drift velocity

A

The average speed of the free charge carriers in a conductor under the influence of a potential difference

93
Q

Resistance is ___proportional to drift velocity

A

Inversely

94
Q

As collision frequency increases, drift velocity ___

A

Decreases

95
Q

Collision frequency define

A

How many times per second charge carriers and positive lattice ions collide

96
Q

Why does the light turn on instantly when you turn on the switch

A

The electromagnetic field propagates at the speed of light, even though the drift velocity is much lower than the speed of light

97
Q

What is the equation for I which includes drift velocity and some other things

A

I = n q A v, n = number density of free charge carriers, A = cross sectional area of conductor, v = drift velocity, q = charge on each charge carrier

98
Q

Explain, in terms of particle behaviour and drift velocity, why the resistance of a metal wire changes as temperature increases 4 marks

A
  • As temperature increases, positive lattice ions vibrate with a larger amplitude
    • As a result, the collision frequency between the free electrons in the wire and the positive lattice ions increases
    • Therefore, the mean drift velocity of the electrons decreases
    • Since the resistance of a material is inversely proportional to the drift velocity of the charge carriers in the material, so the resistance of the wire increases
99
Q
A

AA - Physics A Level Y12 (17 decks)

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