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A2 Physics > Topic 3 > Flashcards

Topic 3 Flashcards

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1
Q

Definition: Current

A

The rate of flow of charge.

(A)

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2
Q

Equation: Current *

A

I = ΔQ / Δt

I = current (A)

ΔQ = charge (C)

Δt = time (s)

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3
Q

Definition: Power

A

The rate of energy transfer.

(W)

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4
Q

Equation: Power *

A

P = VI

P = power (W)

V = potential difference (V)

I = current (A)

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5
Q

Definition: Potential difference

A

A measure of the work done / energy transferred per unit charge passing through a conducting element.

(V)

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6
Q

Equation: Potential difference *

A

V = W / Q

V = potential difference (V)

W = work done (J)

Q = charge (C)

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7
Q

Definition: Ohm’s Law

A

At a constant temperature, the current through a conductor is proportional to the pd applied across it.

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8
Q

What is a assumption made when using Ohm’s Law?

A

The temperature is constant.

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9
Q

What does an Ohm’s Law graph look like?

A

A graph of V against I produces a straight line passing through the origin.

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10
Q

What are non-ohmic components?

A

Ones which do not follow Ohm’s Law.

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11
Q

What are the properties of a filament lamp?

A

As the temperature increases, it’s resistance increases, because the amplitude of oscillations of the lattice ions increase. Therefore, there are more collisions between electrons and lattice ions. This means the rate of increase of current decreases as temperature is increased.

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12
Q

What does the graph of a filament lamp look like?

A

y axis = current

x axis = pd

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13
Q

What are the properties of a diode?

A

It only allows current to flow in one direction.

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14
Q

What does the graph of a diode look like?

A

y axis = current

x axis = pd

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15
Q

What are the properties of a NTC thermistor?

A

As temperature increases, resistance decreases.

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16
Q

What does the graph of an NTC resistor look like?

A

y axis = current

x axis = pd

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17
Q

What are the features of a series circuit?

A

Conservation of charge states:

• Current is equal.

Conservation of energy states:

  • pd is shared.
  • Resistance is shared.
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18
Q

What are the features of a parallel circuit?

A

Conservation of charge states:

• Current is shared.

Conservation of energy states:

  • pd is equal.
  • 1/R = 1/R₁ + 1/R₂ + 1/R₃
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19
Q

If a lamp glows brighter than another, what does that suggest?

A

There must be a larger resistance across it, meaning it has a greater pd as more energy is transferred per unit charge.

20
Q

How do thermistors work?

A

As the temperature increases, the resistance decreases and therefore so does the pd. This causes the heater to be turned off.

21
Q

Equation: Current in a conductive metal *

A

I = nqvA

I = current (A)

n = carrier density

q = charge per carrier (1.6x10⁻¹⁹C)

v = mean drift velocity of charge carriers (ms-1)

A = cross-sectional area (m2)

22
Q

Definition: Carrier density

A

The number of charge carriers per cubic metre.

23
Q

What are the features of an insulator?

A

Thy have no mobile charge carriers and therefore a very high resistivity.

24
Q

What are the features of semiconductors?

A

Have low carrier densities and therefore a high resistivity.

25
Q

What are the features of metals?

A

They have a very large carrier density and therefore a very low resistivity making them good conductors.

26
Q

What is the effect of temperature on the resistivity of metals?

A

As temperature increases, the lattice vibrations decrease, meaning there are less collisions between charge carriers and these lattices. Therefore the mean drift velocity of the carriers increases and so the resistivity of the metal increases.

27
Q

How does the resistivity of NTC thermistors change with temperature?

A

As temperature increases, so do the lattice vibrations, meaning there are more collisions between charge carriers and these lattices. Therefore the mean drift velocity of the carriers decreases and so the resistivity of the thermistor decreases.

28
Q

How do light-dependent resistors work?

A

When energy falls on an LDR in the form of light, it frees more charge carriers, therefore increasing the carrier density and lowering the resistivity.

29
Q

Definition: Resistivity *

A

R = ρl / A

R = resistance (Ω)

ρ = resistivity (Ωm)

l = length (m)

A = cross-sectional area (m2)

30
Q

What is the unit of resistivity?

A

Ωm

31
Q

What is a feature of resistivity?

A

It is the same for a material in any form as long as the temperature is constant. Changing the temperature will change the resistivity.

32
Q

What precaution needs to be taken when finding the resistance of a wire in the lab?

A

The temperature must be kept constant so that resistance doesn’t increase. If you increase the current, the temperature will increase and therefore so will the resistance.

33
Q

Definition: Drift velocity

A

The mean velocity of charge carriers inside the metal conductor.

34
Q

How does resistance affect pd?

A

The greater the resistance, the greater the Ek that the charge carriers gain during collisions with lattice ions. Therefore, a greater pd is required to produce the same current.

35
Q

Definition: emf

A

The energy gained per unit charge by charges passing through the supply.

(V)

36
Q

What is caused by internal resistance?

A

A loss of energy through dissipation.

37
Q

How do you find the emf and internal resistance from a VI graph?

A

ε = y intercept

-r = gradient

38
Q

Equation: emf

A

V = ε - Ir

V = (terminal pd) pd across the cell (V)

ε = emf (V)

I = current (A)

r = internal resistance (Ω)

Ir = ‘lost volts’

39
Q

How would you set up a potential divider circuit?

A
40
Q

Equation: Potential divider

A

V₀ = Vᵢ x R₁

R₁ R₂

V₀ = output voltage across (V)

Vᵢ = input voltage across cell (V)

R₁ = ‘variable’ resistor which V₀ is measured across (Ω)

R₂ = second resistor (Ω)

41
Q

How do you identify emf from a circuit diagram?

A

The pd given inside the battery/cell.

42
Q

What effect does the internal gradually increasing during use have on the power output?

A

The total resistance of the circuit will increase, causing the current to decrease. Therefore, power will decrease as P=VI.

43
Q

Why must a voltmeter have infinite resistance?

A

So that current isn’t drawn through it, because the measured current would not be the true current going through the component.

44
Q

Why should an ammeter have negligible resistance?

A

So that the current through it isn’t decreased.

45
Q

What circuit should be used to determine emf?

A
  • Ammeter in series.
  • Variable resistor in series.
  • Cell (ε and r) in series.
  • Voltmeter parallel to cell.
46
Q

How does the p.d across a resistor change as the current changes in a EMF circuit?

A

As current decreases, the p.d across the resistor must increase as V = ε - Ir and ε and internal resistance can both be considered as constant.

A2 Physics (14 decks)

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