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IB Physics HL Core > Topic 5: Electric potential difference, current and resistance > Flashcards

Topic 5: Electric potential difference, current and resistance Flashcards

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1
Q

Define electric potential difference.

A

The energy that a charge has as a result of its position in an electric field.

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2
Q

What happens when a charge is placed in an electric field?

A

The charge experiences a force (either in direction of field, is positive, or against field lines, if negative)

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3
Q

What happens when a charge moves around in an electric field?

A

Work is done and the charge will either gain or lose electric potential energy.

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4
Q

Determine the change in potential energy when a charge moves between two points at different potentials.

A
  1. When a charge moves between two points at different potentials, there is an increase in electric potential energy
  2. If the electric potential energy at B is greater than the electric potential at A, work has to be done to push the charge from A to B
  3. However, the charge at B would be pushed by the electric field to A
  4. This push would accelerate its movement so that the loss in electrical potential energy would be the same as the gain in kinetic energy
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5
Q

What is the equation used to determine the change in potential energy when a charge moves between two points at different potentials that involves:

  1. electric potential energy change
  2. force
  3. distance?
A

electric potential energy change = F d

Where:

electric potential energy change in J

F is force in N

d is distance in m

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6
Q

What is the equation used to determine the change in potential energy when a charge moves between two points at different potentials that involves:

  1. electric potential energy change
  2. energy of the charge
  3. charge
  4. distance?
A

electric potential energy change = E q d

Where:

electric potential energy change in J

E is difference in potential energy in J

q is charge in C

d is distance in m

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7
Q

Derive an equation for the velocity of a charge

A
  1. kinetic energy gained = potential energy lost
  2. 1/2 m v^2 = E q d
  3. m v^2 = 2 E q d
  4. v^2 = (2 E q d) / m
  5. v = sqrt ( (2 E q d) / (m) )

Where:

m is the mass of the charge in kg

v is the velocity of the charge in m s^-1

E is difference in potential energy in J

q is the charge in C

d is the distance in m

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8
Q

Define the electronvolt.

A

The energy that would be gained by an electron moving through a potential difference of 1 volt.

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9
Q

What is the charge of an electron?

A

1.6 X 10^-19 C

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10
Q

Derive an equation for the charge of an electronvolt.

A

energy gained = potential difference X charge

1 electronvolt = 1 volt X 1.6 X 10^-19 C

1 electronvolt = 1.6 X 10^-19 C

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11
Q

What does the actual energy difference between two points, A and B, depend on?

A
  1. The charge that is moved
  2. If the charge is doubled, then the energy distance would also double
  3. The quantity that remains fixed between A and B is the energy difference per unit charge
  4. This is called the potential energy difference between the points
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12
Q

What is the equation used to determine the potential difference between two points that involves:

  1. energy difference
  2. charge?
A

V = E / q

Where:

V is potential difference between two points in V

E is energy difference in J

q is charge in C

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13
Q

Define electric current.

A

The force per unit length between parallel current-carrying conductors. It is the rate of flow of electrical charge.

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14
Q

What is the equation used to determine current that involves:

  1. charge
  2. time?
A

I = q / t

Where:

I is current in A

q is charge flowed in C

t is time taken in s

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15
Q

When is there a direct current?

A

If a current flows in one direction.

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16
Q

When is there an alternating current?

A

If a current constantly changes direction.

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17
Q

When does a current occur?

A

When charges move.

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18
Q

What is a circuit?

A

The path that currents follow.

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19
Q

Why does a current move across a body?

A

As a result of a potential difference across a body.

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20
Q

What creates a potential difference?

A

A power supply.

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21
Q

Why does work occur when charges move?

A

As a result of interactions between the conduction electrons and the lattice ions.

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22
Q

Define drift velocity.

A

The speed of the electrons due to the current.

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23
Q

Define resistance.

A

The mathematical ratio between potential difference and current.

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24
Q

What is the equation that defines resistance?

A

R = V / I

Where:

R is resistance in Ω

V is potential difference in V

I is current in A

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25
Q

What must happen for current to flow if something has a high resistance?

A

If something has a high resistance, it means that a large potential difference across it is needed in order for current to flow.

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26
Q

Define resistor.

A

A device with constant resistance; an ohmic device.

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27
Q

What is the equation used to determine resistance that involves:

  1. resistivity
  2. length
  3. cross-sectional area?
A

Where:

R is resistance in Ω

ρ is resistivity in Ω m

L is length in m

A is cross-sectional area in m s^-2

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28
Q

State Ohm’s Law.

A

The current flowing through a piece of metal is proportional to the potential difference across it, providing that the temperature remains constant.

V ∝ R (providing temperature is constant)

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29
Q

When is a device said to be ohmic?

A

If current and potential difference are proportional.

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30
Q

What is an example of an ohmic device?

A

Metal at constant temperature.

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31
Q

When is a device said to be non-ohmic?

A

If current and potential difference are not proportional.

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32
Q

What is an example of a non-ohmic device?

A
  1. Filament lamp
  2. Diode
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33
Q

Draw the I-V characteristics of an ohmic resistor.

A
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34
Q

Draw the I-V characteristics of a non-ohmic resistor.

A
35
Q

Derive an expression for electrical power dissipation in resistors.

A
  1. potential difference = (energy difference) / (charge flowed)
  2. current = (charge flowed) / (time taken)
  3. P = VI
  4. power dissipation = ( (energy difference) / (charge flowed) ) X ( (charge flowed) / (time taken) )
  5. P = (energy difference) / (time taken)
36
Q

Define electromotive force (emf).

A

The total energy difference per unit charge around a circuit. It is not a force measured in Newtons, but an energy difference per charge, measured in volts. It is the same as potential difference if no current flows.

37
Q

Describe the concept of internal resistance.

A

Energy is used up inside the battery itself as a result of the battery’s internal resistance.

38
Q

Apply the equations for resistors in series.

A

Rtotal = R1 + R2 + …

39
Q

Apply the equations for resistors in parallel.

A

1/Rtotal = 1/R1 + 1/R2 + …

40
Q

What happens to current and potential difference in a series circuit?

A

Current is constant

Potential difference is split up

41
Q

What happens to current and potential difference in a parallel circuit?

A

Current is split up

Potential difference is constant

42
Q

Describe the use of ideal ammeters.

A

An ideal ammeter has zero resistance.

43
Q

Describe the use of ideal voltmeters.

A

An ideal voltmeter has infinite resistance.

44
Q

Describe a potential divider.

A

Potential dividers divide up the potential difference within a circuit, so that parts of a circuit only receive the potential difference that they require. Potential dividers usually consist of two or more resistors arranged in series across a power supply.

45
Q

Explain the use of light dependent resistors in potential divider circuits.

A

A device whose resistance depends on the amount of light shining on its surface. An increase in light causes a decrease in resistance.

46
Q

Explain the use of thermistors in potential divider circuits.

A

A resistor whose value of resistance depends on its temperature. Most are semi-conducting devices that have a negative temperature coefficient (NTC). This means that an increase in temperature causes a decrease in resistance.

47
Q

Explain the use of strain gauges in potential divider circuits.

A

A sensor whose output potential difference depends on any a small extension or compression that occurs, which results in a change of length.

48
Q

What are sensors used for?

A

These devices can be used in potential divider circuits to create sensor circuits. The output potential difference of a sensor circuit depends on an external factor.

49
Q

What is an electric field?

A

A charge (or combination of charges) is said to produce an electric field around it.

An electric field is the force experienced per unit positive test charge - (so really it is an effect)

E = F/q2

where q2 is the charge placed in the field

50
Q

If you place a charge in a field, what will the magnitude of the force depend on?

A

It will depend solely on the value of the charge placed in the field.

51
Q

What is the elecronvolt?

A

The energy that would be gained by an electron moving through a potential difference of 1 volt.

52
Q

What is 1 volt?

A

1 volt = 1JC-1

53
Q

How many volts is an electronvolt?

A

1.6 x 10-19 V

54
Q

In what direction is the charge moving if work is done?

A
  • against the direction of the electric field
  • against field lines
  • towards the positive charge

[assuming positive test charge]

55
Q

In what direction is the charge moving if pushed (gain KE but losing EPE)

A
  • charge is ‘let go’
  • along the field lines
  • in direction of the electric field
  • away from positive (towards negative)

[assuming positive test charge]

56
Q

What is electrostatic (electric) potential energy?

A

It is the energy that a charge has as a result of its position in an electric field.

(Works like GPE in that the ‘higher’ it is, the more energy it has. Here it is that the further against the field direction it is, the higher the energy.)

57
Q

Equation for EPE

A

ΔEPE = E x q x d

where E is the electric field strength, q is the charge of the particle, d is the distance moved against the field.

58
Q

Why can charged objects be created by rubbing?

A

Friction causes electrons to be transferred from one object to another

When one loses electrons, the other gains.

One becomes positively charged, the other negatively.

They attract eachother.

59
Q

Explain Coulomb’s law

A

If the charges are the same, the particles repel.

If the charges are opposite, the particles attract.

Each particles experiences a force of the same size (but in opposite directions).

The force is proportional to the size of both charges (the product of them) and inversely proportional to the square of the distance between them.

60
Q

Coulomb’s law as an equation

A

F = k (q1q2)/r2

where k = 1/4πε0

so

F= (q1q2)/4πε0r2

(sometimes q1 and q2 are written as Q and q)

61
Q

How should you approach a question about Coulomb’s law if there are two or more charges near another charge?

A

The overall force can be worked out using vector addition.

62
Q

What do field lines represent?

A

At any point in the field:

  • direction: closest field lines to that point
  • magnitude: number of field lines near that point i.e. the distance between lines.
63
Q

Field line rules

(3)

A
  • cannot cross: they are the resultant and a particle cannot go in two directions at once.
  • begin at the positive charge, end at the negative charge.
  • the distance between them indicate the field strength.
64
Q

Conventional current vs. “real” current

A

Conventional current:

  • shows/represents the flow of negative charge
  • flows from positive to negative

“Real” current/electron flow:

  • negative (conduction) electrons flow
  • opposite movement to conventional current
65
Q

Describe what happens when electrons flow through a wire.

A

As they move, they interact with latice ions, meaning work needs to be done. As current flows, the heating effect warms the wire.

The speed is called the drift velocity.

66
Q

Electric potential difference equation involving energy

A

V = W/q

where W is work done, q is charge

67
Q

Derive V when a charge is moving in the direction of the electric field.

A

gain in kinetic energy = loss in electric potential energy

1/2mv2 = Eqd

mv2 = 2Eqd

v2 = 2Eqd/m

v = √(2Eqd/m)

68
Q

What type of quantity is potential difference and what does this mean?

A
  • potential difference is in JC-1
  • potential difference is a single, fixed, scalar quantity for a given electric field

This means that the work done between two points does not depend on the path.

The electric field is therefore conservative.

69
Q

What two conservation laws are essential in circuits, particularly in Kirchoff’s laws?

  • define them
  • which of Kirchoff’s laws do they relate to?
A

Conservation of charge:

sum of charges before = sum of charges after

(in a closed system)

relates to Kirchoff’s 1st law

Conservation of energy:

the total energy is a closed system is constant. Energy cannot be created nor destroyed, only transferred into another form

relates to Kirchoff’s 2nd law

70
Q

Kirchoff’s 1st law written mathematically

A

Σ I = 0

(junction)

71
Q

Kirchoff’s 2nd law written mathematically

A

Σ V = 0

(loop)

72
Q

Define: Kirchoff’s 1st law

A

The sum of the currents going into any point in the circuit is equal to the sum of the currents flowing out of that point

73
Q

Define: Kirchoff’s 2nd law

A

In any closed loop in a circuit, the sum of the emf in a loop is equal to the sum of the potential difference around a loop.

74
Q

How can you estimate the drift velocity of electrons?

A

Given:

n = number density of charge-carriers (electrons)

q = their charge

v = their average speed

in time Δt

therefore, average distance = v x Δt

so volume of charge moved at point = A (cross-sectional area) x vΔt

so number of charge carriers = n x AvΔt

so charge moved past a point ΔQ = q x nAvΔt

current = I = ΔQ/Δt

I = nAvq

75
Q

What is internal resistance?

What does it look like on a circuit diagram?

A

The electrical resistance inside a cell due to the materials from which the cell is made, measured in ohms

76
Q

How can you find internal resistance mathematically?

A

emf = I x Rtotal

emf = I x (R + r)

emf = IR + Ir

emf - IR = Ir

emf - V = Ir

r = (emf - V) / I

77
Q

A device with constant resistance is said to be…

+

IR graph

A

an ohmic device

e,g, a resistor

78
Q

Define: primary cell

A

A cell that cannot be charged, the chemic reaction is irreversible.

Once it cannot provide electrical energy, it is thrown away

79
Q

Define: secondary cell

A

A cell that is designed to be charged, the chemical reaction is reversible.

A reverse electrical current charges the cell to be reused many times until worn out.

80
Q

Define: charge capacity

A

How much charge can flow through a cell until it stops working.

Normally is amp-hours (Ah)

1 Ah = charge that flows when a current of 1 Amp flows for 1 hour. (1Ah = 3600C)

81
Q

What are discharge characteristics?

(3)

A

The terminal potential difference of a typical cell:

  • loses its initial value quickly
  • has a stable and reasonably constant valve for most of its lifetime
  • dips/curves towards the end of its lifetime.
82
Q

Summarise what will happen when the slider moves up

A

The total current in the circuit is constant (Kirchoff’s first law). As resistance R1 increases (slides up), Vout will increase. (V=IR in that small loop)

As R1 increases, R2 decreases. Using Kirchoff’s second law we know that, the p.d. across it (V2) will decrease.

83
Q

Summarise what will happen when the slider goes down

A

The total current in the circuit is constant (Kirchoff’s first law). As resistance R1 decreases (slides down), Vout will decrease. (V=IR in that small loop)

As R1 decreases, R2 increases. Using Kirchoff’s second law we know that, the p.d. across it (V2) will increase.

IB Physics HL Core (13 decks)

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