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OCR Physics B Unit 4 > 4.1 Creating Models > Flashcards

4.1 Creating Models Flashcards

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1
Q

What is a model?

A

A set of assumptions that simplifies and idealises a particular problem.

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2
Q

Give an advantage of creating models.

A

They can be extended to describe other unrelated processes such as capacitors and radioactive decay.

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3
Q

What could make an atom unstable?

A

Too many or too few neutrons or too much energy in the nucleus.

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4
Q

What is radioactive decay?

A

Unstable atoms breaking down by realising energy or particles until they reach a stable form.

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5
Q

What type of model can you use to predict the behaviour of a radioactive sample decaying randomly?

A

Exponential decay.

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6
Q

What is the activity of a radioactive sample?

A

The number of unstable atoms that decay each second.

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7
Q

How does activity of a radioactive sample relate to the size of the sample?

A

It is proportional.

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8
Q

What is the decay constant, λ?

A

The probability of a given nucleus decaying in a certain time.

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9
Q

What are the units of the decay constant?

A

s^-1

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10
Q

How do you calculate activity of a sample, A, in terms of the number of unstable nuclei and the decay constant?

A

A = λ x N

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11
Q

What are the units for activity?

A

Becquerels (Bq)

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12
Q

How do you calculate the rate of change of the number of unstable nuclei?

A

dN/dt = -λ x N

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13
Q

What is the half-life of an isotope?

A

The average time it takes for the number of undecayed atoms to halve.

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14
Q

How can you calculate half life of an isotope from its decay constant?

A

t½ = ln2 / λ

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15
Q

How can you calculate the number of unstable nuclei remaining from the original number and the decay constant?

A

N = N0 x e^(-λ x t) where t is measured in seconds.

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16
Q

What are capacitors?

A

Devices that store electrical charge.

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17
Q

What is capacitance defined as?

A

The amount of charge stored per volt.

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18
Q

How do you calculate capacitance?

A

C = Q / V

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19
Q

What are the units of capacitance?

A

Farads (F).

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20
Q

What is the symbol for and order of a picofarad?

A

pF, x 10^-12

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21
Q

What is current defined as?

A

Rate of flow of charge.

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22
Q

Give three uses for capacitors.

A

Flash photography, defibrillators and back-up power supplies.

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23
Q

Why are capacitors useful in camera flashes?

A

Because a very short, bright flash is required.

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24
Q

Why are capacitors useful in back-up power supplies?

A

Large capacitors discharge over a number of hours, maintaining a steady flow of charge during a power cut.

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25
Q

Describe a test circuit used to measure the capacitance of a capacitor.

A

Batteries, variable resistor ammeter and capacitor in series with a voltmeter across the capacitor.

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26
Q

How would you measure capacitance of a capacitor using a test circuit?

A

Constantly adjust variable resistor to give a constant current. Find voltage and charge at regular intervals (area under I vs t graph) and plot a Q against V graph. The gradient is the capacitance.

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27
Q

Where is work done when charging a capacitor?

A

Removing charge from one plate and depositing it onto eh other.

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28
Q

How can you find energy stored by a capacitor graphically?

A

Find the area under a V against Q graph.

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29
Q

Give the equations used to calculate energy stored by a capacitor.

A 
E = 1/2 x Q x V
E = 1/2 x C x V^2
30
Q

What is the name of the insulting material between the two plates of a capacitor?

A

Dielectric.

31
Q

Describe the flow of electrons in a capacitor once it has been connected to a battery.

A

Electrons flow onto plated connected to negative battery terminal, building up negative charge. This charge repels the electrons off the other plate. This causes a p.d. between the plates.

32
Q

Why does current decrease as the capacitor charges?

A

Electrostatic repulsion from electrons on the plate make it increasing harder for more electrons to be deposited. When p.d. across the capacitor equals p.d. across the battery, the current falls to zero.

33
Q

Which two factors affect the time it takes to discharge a capacitor?

A

The capacitance of the capacitor and the resistant of the discharging circuit.

34
Q

Why does capacitance affect discharge time?

A

It governs the amount of charge that can be stored at a given voltage.

35
Q

Why does resistance in the discharging circuit affect the time it takes to discharge a capacitor?

A

It determine the current in the circuit.

36
Q

Describe how time affects the rate of discharge of a capacitor.

A

It is initially high but the rate falls as charge on the capacitor decreases.

37
Q

Give the equation for rate of change charge on a discharging capacitor.

A

dQ/dt = -Q / (R x C)

38
Q

Give the equation for charge remaining on a capacitor.

A

Q = Q0 x e^(-t / (R x C))

39
Q

What is the time constant?

A

The time taken for charge on a discharging capacitor to fall to 37% of Q0. τ = RC

40
Q

How would you calculate the time taken for charge left to decay to half?

A

t½ = ln2 x RC

41
Q

Express N = N0 x e^(-λ x t) in a way that allows you to find λ from a straight line graph.

A

ln(N) = -λt + ln(N0)

42
Q

What is simple harmonic motion, SHM?

A

An oscillation in which acceleration of an object is directly proportional to its displacement from the midpoint, and is directed towards the midpoint.

43
Q

Give an example of a system that exhibits SHM.

A

A simple pendulum.

44
Q

Explain how potential energy (PE) and kinetic energy (KE) varies with displacement.

A

As the object moves toward the midpoint, the restoring force does work, transferring PE to KE. When the displacement is zero, PE is zero and KE is maximum. At the amplitudes, KE is zero and PE is maximum.

45
Q

How does the sum of the potential and kinetic energy vary, if at all, with displacement?

A

It is constant provided the system is not damped.

46
Q

If the sine or cosine graph for displacement has amplitude A express the maximum velocity and acceleration in terms of A.

A

Maximum velocity is (2πf)A and maximum acceleration is (2πf)^2 x A

47
Q

What is the cycle of an oscillation?

A

Moving from maximum displacement to maximum negative displacement and back again.

48
Q

What is frequency of an SHM?

A

The number of cycles completed per second.

49
Q

What is the time period of an SHM?

A

The time taken, in seconds, to complete 1 full cycle.

50
Q

SHM is isochronous, what does this mean?

A

The period and frequency are independent of the amplitude and the initial phase of the motion.

51
Q

Give the equation for SHM in terms of x, t and f.

A

d^2 x/d t^2 = -(2πf)^2 x

52
Q

Give the equation for displacement when x starts at a maximum.

A

x = A cos(2πf t)

53
Q

Give the equation for displacement when x starts at the equilibrium position.

A

x = A sin(2πf t)

54
Q

Give the equation for force on a spring.

A

F = K x

55
Q

What is the formula for the period of a mass oscillating on a spring?

A

T = 2π √(m/k)

56
Q

How would you use a graph to find the energy stored in a spring?

A

Find the area under an extension against force graph.

57
Q

What is the equation for energy stored in a spring?

A

E = 1/2 k x^2

58
Q

What is the formula for the period of a pendulum?

A

T = 2π √(l/g)

59
Q

What is a free vibration?

A

A vibration where no energy is transferred to or from the surroundings.

60
Q

What is a natural frequency?

A

The frequency that a mass will oscillate at after having been displaced from its equilibrium position.

61
Q

How can you calculate total energy of a freely oscillating mass on a spring?

A

E = 1/2 m v^2 + 1/2 k x^2

62
Q

What are forced vibrations?

A

When a system is forced to vibrate by an external force.

63
Q

What is the name of the frequency of the external force causing a system to oscillate?

A

The driving frequency.

64
Q

What is resonance?

A

When the driving frequency is equal to the natural frequency the system gains more and more energy from the driving force causing oscillations with a very large amplitude.

65
Q

Give four examples of resonance.

A

Organ pipes, pushing someone on a swing, sound causing glass to smash and radio.

66
Q

What is damping?

A

When an oscillating system loses energy to the surroundings.

67
Q

Give one example of a damping force.

A

Air resistance.

68
Q

Why might systems be deliberately damped?

A

To stop them oscillating or minimise the effect of resonance.

69
Q

What is critical damping?

A

Damping that reduces the amplitude in the shortest possible time.

70
Q

What is overdamping?

A

Damped systems that take longer to return to equilibrium than a critically damped system.

71
Q

How does damping affect resonance?

A

It reduces the resonance peak and therefore the amplitude of oscillation at resonant frequencies.

72
Q

Give the SHM equation for a mass oscillating on a spring.

A

d^2 x/d t^2 = -(k/m) x

OCR Physics B Unit 4 (5 decks)

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