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A Level Physics > 03 - Electric Circuits > Flashcards

03 - Electric Circuits Flashcards

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1
Q

Define electrical current.

A

Electrical current is the rate of flow of charge.

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2
Q

State the defining equation for current.

A

I = change in charge / change in time

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3
Q

What is the unit of current?

A

Ampere (Amps), A

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4
Q

Define voltage.

A

Voltage is the work done per unit charge.

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5
Q

State the defining equation for voltage.

A

V = W / Q

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6
Q

What equation links current, voltage and resistance?

A

Voltage (V) = Current (A) × Resistance
V = IR

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7
Q

What is Ohm’s Law?

A

If Ohm’s Law is obeyed, the current and voltage passing through a component are directly proportional, when at a constant temperature.

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8
Q

Describe the distribution of current in a series circuit.

A

In a series circuit, the current is the same at all positions in the circuit.

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9
Q

What law is the distribution of current in a series circuit a consequence of?

A

The law of the conservation of charge.
Charge cannot be created or destroyed and so in a closed loop, the flow of charge must be the same throughout.

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10
Q

Describe the distribution of current in a parallel circuit.

A

In a parallel circuit, the current is split between the different branches. The current entering each branch must be the same as the current leaving it.

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11
Q

Explain how the conservation of charge applies to parallel circuits.

A

Charge cannot be created or destroyed meaning the total current leaving the source must equal the sum of the currents in all the individual branches.

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12
Q

What law is the distribution of potential differences in a circuit a consequence of?

A

The law of the conservation of energy.

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13
Q

Describe the distribution of potential differences in a series circuit.

A

The total potential difference is split across all the components in a series circuit, in the ratio of their resistances.

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14
Q

Describe the distribution of potential differences in a parallel circuit.

A

The sum of the potential differences in each branch of a parallel circuit, is the same and will be equal to the potential difference of the source.

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15
Q

What equation relates power, current and potential difference?

A

P = IV

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16
Q

How can you calculate the work done over a given time period, by a component with a known potential difference and current?

A

Combining P = WIt and P = IV gives:
W = IVt

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17
Q

Explain the IV characteristic for a filament bulb.

A

• The higher the current, the higher the temperature of the metal filament
• The higher the temperature, the higher the KE of the metal ions and so the more they vibrate
• This makes it harder for current to flow and so the resistance of the bulb increases

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18
Q

What is Kirchhoff’s first law?

A

The total current into injunction is the same as the current out of the junction. This is because of the conservation of charge.

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19
Q

What is Kirchhoff’s Second law?

A

In a closed loop the sum of the P.D’s across all components is zero. Any charge that starts and ends in the same point must’ve gained as much energy as it’s lost. This is due to the conservation of energy.

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20
Q

What does the gradient of an IV graph show?

A

It shows the rate of change of current with p.d

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21
Q

What three physical factors affect the resistance of a wire?

A
  1. Length
  2. Cross-Sectional Area
  3. Resistivity
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22
Q

How does increasing the length of a conductor affect its resistance?

A

The longer the conductor is, the higher its resistance, assuming all other factors remain the same.

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23
Q

How does increasing the cross-sectional area of a conductor affect its resistance?

A

The larger the cross-sectional area of a conductor, the lower its resistance, assuming all other factors remain the same. This is because there is a greater area for the ions to collide with the lattice.

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24
Q

What is resistivity?

A

A characteristic property of a material that determines its resistance to current
flow.

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25
Q

Compare the resistance of two conductors of the same dimensions, but with different resistivities.

A

The conductor with the higher resistivity, will have the higher resistance.

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26
Q

How is resistance shared across components in a series circuit?

A

Total resistance of a component widen series, is the sum of the individual resistances. This is because in series circuits, there is one fixed current. And the sum of potential drops equals the sum of the potential rises. V(total) = V1 + V2 +V3…. Because of the conservation of energy.

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27
Q

What is the rule for resistance of the resistors wired in parallel?

A

The inverse of the total resistance for components wired in parallel is this some of the inverses of each individual resistance

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28
Q

What is the relationship between the amount of resistors you have in parallel to the amount of resistance

A

The more resistors you have in parallel the lower the resistance

The more resistors we add in parallel, the more ‘pathways’ the current has to go through, so it is easier for current to flow through the circuit

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29
Q

What are the two ways of measuring resistance?

A
  1. Collect current and p.d readings, then do R = V / I
  2. Use a multimeter.
30
Q

What is the best method of measuring resistance?

A

The multimeter is the best way of measuring resistance, because there is less current being used, so the wire wouldn’t heat up as much as if you used the other method
Use a multimeter, so it doesn’t create a high temperature which would affect the resistivity

31
Q

Describe and explain the change in the resistance of the bulb as the potential difference across it is increased

A

As the potential difference increases the resistance will also increase this because the temperature increases due to the increase in current from the increase in p.d which increases the amplitude of the oscillation of the lattice ions leading to more frequent collisions of electrons with lattice ions.

32
Q

State the equation linking the factors affecting the resistance of a conductor.

A

R= resistivity x length / area

33
Q

How do you get the resistivity of a material from a resistance - length graph with resistance on the y-axis and length on the X axis

A

r (resistivity) = (Resistance x area) / length

34
Q

What equation links the number of charge carriers in a metal, along with their drift velocity, to the current flow?

A

I = nqVA

A = area
Q = charge
N = number density
V = drift velocity
Or if in an electrical circuit:
I = Anev
Where e = the charge of an electron

35
Q

What is drift velocity?

A

the average velocity with which electrons ‘drift’ in the presence of an electric field

36
Q

What actually happens inside a wire when an electric current flows

A

Free electrons move down the wire. They collide with ions in the wire lattice - this inhibits flow, and is the cause of electrical resistance. The electrons do not move in a straight line but on average progress down the wire.

37
Q

What are the physical factors affecting the size of the current measured in a wire?

A
  • The size of the charge on the carrier
  • Number of free electrons per meter cubed able to flow “ number density”
  • The cross-sectional area of the wire
  • How fast on average the electrons are moving “ drift velocity”
38
Q

How do you calculate the resistance of a component from an IV graph?

A

You determine the value of the and read of the corresponding current. Then do R = V / I

39
Q

How do you get the electromotive force from an IV graph with terminal PD and load current?

A

Draw a line of best fit and the Y intercept is the electromotive force

40
Q

How does the potential along a uniform current-carrying wire vary as you move along it?

A

The potential will increase as you move along the wire. Because resistance is proportional to potential difference

41
Q

What is the electromotive force of a power supply?

A

The electromotive force is the amount of energy that is transferred to each
Coulomb of charge that passes through it.

42
Q

What is the terminal potential difference of a power supply?

A

The terminal potential difference of a power supply is the actual potential difference that is produced across the terminals of the supply and is provided to the circuit.

43
Q

Why does the electromotive force of a supply differ from its terminal potential difference?

A

Some of the energy that is transferred to the charges is used to overcome the internal resistance of the power supply.

44
Q

Define internal resistance.

A

The internal resistance of a power supply is the electrical resistance, when a current flows, due to the materials within the supply itself.

45
Q

Describe a method to determine the terminal potential difference of a battery.

A

Attach a voltmeter directly across the terminals of the battery when it is isolated.
Since no current flows, no potential is lost due to internal resistance and so the voltmeter will read the value of the terminal p.d.

46
Q

2 equations for EMF

A

E = IR + Ir
Or
E = V + Ir

47
Q

Is p.d or current responsible for the heating effect

A

Current

48
Q

What is semiconductor?

A

A semiconductor is a component that can change its resistance based on external conditions.

49
Q

Give two examples of semiconductors and state what they are sensitive to.

A
  1. Thermistors are sensitive to temperature
  2. LDRs are sensitive to light
50
Q

Describe how the resistance of a thermistor changes as temperature decreases.

A

As temperature decreases, the resistance of a thermistor will increase.

51
Q

Explain why the resistance of a thermistor will decrease with an increase of temperature.

A

• Increased temperature causes more energy transfer to lattice ions / atoms
• More charge carriers released / available
• I= nAvq, so relative increase in I
• R = VI so as I increases R decreases as they are inversely proportional to each other.

52
Q

Describe how the resistance of a LDR changes as light intensity increases.

A

As light intensity increases, the resistance of a LDR will decrease.

53
Q

What is a potential divider?

A

A potential divider is a pair of resistors used to split the potential difference over them in a given ratio.

54
Q

What is the link between the resistances and the potential differences in a potential divider?

A

The potential difference of the circuit is split in the ratio of the resistances of the resistors.

55
Q

How would the potential difference over R2 change as the light intensity increases in the following circuit? This is a potential divider circuit with R2 over the Vout

A

As light intensity increases, the resistance over the LDR will decrease. This means that the fixed resistance R2 will take a larger ratio of the circuit potential and the potential difference across it will increase.

56
Q

What are the 2 Potential divider equations

A

Vout = Vin ( R1 / (R1 + R2))
Vout = Vin ( l(little L) / L)

57
Q

Describe the motion of the particles within a wire with a p.d and without a p.d

A

Without a p.d: the motion of the electrons is in all directions and colliding with the metal ions. The net movement of electrons is 0.
With a p.d: the electrons are still moving in all directions but are generally being pushed along the wire (drift velocity) from negative terminal to the positive terminal

58
Q

Describe electrical resistance when temperature increases

A

The energy of the ions increases causing them to vibrate more. The electrons therefore transfer more energy to the ions in trying to oppose them. This causes even more energy which excites the outer electrons which causes heat and light to be produced.

59
Q

Why is the filament lamp IV graph an S shape

A

As you increase the current, the temperature increases, and therefore the resistance increases so it curves

60
Q

In the I = nqVA equation what does the q stand for and what is its value

A

It stands for the charge on charge carriers: electrons/protons.
If it’s an electron it’s value is -1.6x10^-19 C, if it’s a proton, this value is positive

61
Q

Why do we use potential divider circuits

A

To change the potential difference across a component. This is done by reducing the resistance over the component by adding another resistor or something else which changes resistance, such as a thermistor or a variable resistor

62
Q

what happens to current and p.d and resistance in parallel when a component is removed and why

A

The resistance will increase due to there being less “pathways” for the current to go down so the resistance us more concentrated in the components in series which will decrease the current. As resistance increases p.d also increases

63
Q

How do you calculate the number of electrons passing a point

A

Work out the charge
Then divide it by the electron charge (-1.6 x 10 ^-19) —> this is on the equation sheet

64
Q

Describe how you would get a value for the EMF, and internal resistance from a terminal PD and current graph

A

Current on x axis, terminal p.d on y axis
EMF is the Y intercept and the internal resistance is the negative gradient

65
Q

When the circuit is assembled, using the correctly calculated resistance value and a battery of e.m.f. 12 V, it is found that the maximum output from the potentiometer is slightly less than 5.0 V.
Explain why the maximum output is slightly less than predicted.

A

Due to there being internal resistance, there is a loss of volts so the terminal potential difference is less than the EMF.

66
Q

How do solar cells generate a current from sunlight

A

When the semiconductor is exposed to light, it absorbs the lights energy and transfers it to electrons. This extra energy allows the electrons to flow through the material as an electrical current. Solar cells actually cause a current to flow.

67
Q

How do LDR’s decrease resistance when light intensity increases

A

Light energy excites electrons into the conduction band of the semiconductor material.
The more intense the light, the more charge carriers are in the conduction band, so the higher the current for the same p.d.

68
Q

Describe how thermistors work with conduction bands and charge carriers

A

Excited electrons can move into a higher band (conduction band). They are then free to move between atoms, and therefore these are the ones that act as charge carriers, n, for electrical current.

69
Q

What happens to power and energy output as current increases

A

When currents in a cable are higher, more energy is dissipated to the surroundings through heating

70
Q

Explain why the resistance of a metal sample increases with an increase of temperature.

A

• A rise in temperature causes the amplitude of the vibrating ions to increase (1)
• This causes the number of collisions per second between the ions and the moving electrons to increase (1)
• So the rate of flow of electrons decreases (causing the resistance of the metal to increase
(1)

71
Q

Explain the relationship between p.d and resistance

A

• (As V increases) acceleration/velocity/energy of electrons increases
• greater energy transfer in collisions with lattice / ions
• increasing the temperature of metal/filament/ions
• amplitude of vibrations of lattice/ ions increases
• collision (rate) between lattice /ions and electrons increases
Or shorter distance between collisions
Or greater chance of collision
• due to the increase in resistance, current doesn’t increase in proportion to potential difference Or due to the increase in resistance the ratio of I/V decreases

A Level Physics (6 decks)

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