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A2 Physics > Topic 12 > Flashcards

Topic 12 Flashcards

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1
Q

Definition: Gravitational field

A

A region in space where mass experiences a force.

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2
Q

Equation: Gravitational field strength (uniform field) *

A

g = F / m

g = gravitational field strength (N kg⁻¹)

F = force (N)

m = mass (kg)

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3
Q

Definition: Gravitational field strength

A

The gravitational force per unit mass at a point in the field.

(N kg⁻¹)

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4
Q

What is an approximation of a uniform gravitational field?

A

The field near the surface of a planet or star.

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5
Q

Definition: Gravitational potential Vgrav

A

The gravitational potential energy per unit mass.

(J kg⁻¹)

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6
Q

Equation: Gravitational potential (uniform field)

A

ΔVgrav = gΔh

ΔVgrav = change in gravitational potential (J kg⁻¹)

g = gravitational field strength (N kg⁻¹)

Δh = change of height (m)

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7
Q

Equation: Newton’s Law of gravitation *

A

Fgrav = Gm1m2 / r2

Fgrav = gravitational force between two objects (N)

G = gravitational constant (6.67x10⁻¹¹ Nm²kg⁻²)

m1 = mass of first object (kg)

m2 = mass of second object (kg)

r = distance between two masses (m)

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8
Q

What is the connection between the gravitational force exerted by ‘the moon on the earth’ and ‘the earth on the moon’?

A

The gravitational force is equal regardless of differences in size or mass.

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9
Q

Equation: Gravitational field strength (around a point mass) *

A

g = Gm / r2

g = gravitational field strength (N kg⁻¹)

G = gravitational constant (6.67x10⁻¹¹ Nm²kg⁻²)

m = mass of point mass (kg)

r = distance between the two mass (m)

NOTE: r = radius of point mass if working out field strength at surface.

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10
Q

What are the assumptions made using ‘g=Gm/r2’?

A

The mass being acted upon by gravity is negligible compared to the mass of the point mass.

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11
Q

What is the variation of gravitational field strength due to the Earth?

A
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12
Q

Equation: Gravitational potential (radial field) *

A

Vgrav = -GM / r

Vgrav = gravitational potential (J kg-1)

G = gravitational constant (6.67x10⁻¹¹ Nm²kg⁻²)

M = mass of point mass (kg)

r = distance between masses (m)

NOTE: r = radius of point mass if working out field strength at surface.

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13
Q

What assumptions are made using gravitation equations?

A

That the value is being measured outside of the surface of the mass.

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14
Q

Equation: Gravitational potential energy (radial field)

A

GPE = -GMm / r

GPE = gravitational potential energy (J)

G = gravitational constant (6.67x10⁻¹¹ Nm²kg⁻²)

M = mass of point mass (kg)

m = mass of orbiting mass (kg)

r = distance between masses (m)

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15
Q

Why do you have to use different equations for uniform and radial fields?

A

Because gravitational field strength isn’t constant in radial fields.

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16
Q

What happens as a mass moves towards a planet?

A

It’s GPE decreases and work is done against it by the gravitational field.

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17
Q

What is the relationship between ΔGPE and ΔVgrav?

A

ΔGPE = ΔVgrav x mass

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18
Q

How does energy change as the radius of an orbit decreases?

A
  • Ek increases, GPE decreases
  • Overall energy decreases due to losses in heat.
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19
Q

Equation: Orbits

A

m2v2 / r = m2rω2 = Gm1m2 / r2

m2 = mass of object in orbit (kg)

v = orbital velocity (ms-1)

r = radius of orbit (m)

ω = angular velocity (rad s-1)

G = gravitational constant (6.67x10⁻¹¹ Nm²kg⁻²)

m1 = mass of planet (kg)

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20
Q

What is the relationship between Fgrav and Fcentripetal which allows us to derive the orbital law?

A

Fgrav = Fcentripetal

⇒ m2rω2 = Gm1m2 / r2

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21
Q

How is an object kept in orbit?

A

It experiences a gravitational force which provides a centripetal force.

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22
Q

Equation: Mass-energy *

A

ΔE = c2Δm

ΔE = change in energy (J)

c = speed of light (3.00x108 ms-1)

Δm = change in mass (kg)

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23
Q

Equation: Atomic mass units *

A

u = 1.66 x 10-27kg

This is approximately equal tot he mass of a proton/neutron.

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24
Q

Definition: Binding energy

A

The energy needed to split the nucleus into individual nucleons and move them apart.

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25
Q

How is binding energy affected by nuclei size?

A

As the size of the nuclei increases, the total binding energy of a nucleus also increases.

26
Q

What are the sources of background radiation?

A
  • Radon.
  • Medical radiotherapy.
  • Gamma rays from ground / buildings.
  • Food and drink.
  • Cosmic.
27
Q

Equation: Count rate of source

A

Count rate (of source) = Measured count rate - background count rate

28
Q

Definition: Ionising power

A

How many ions are produced per unit distance in a particular material.

29
Q

Definition: Penetrating power

A

How far radiation can travel through various materials and what thickness of a particular material is needed to absorb them.

30
Q

Features: Alpha radiation

A

Low penetrating power (Paper / skin)

High ionising power

Small range in air (0.02-0.03m)

31
Q

Features: Beta radiation

A

Medium penetrating power (thin aluminium)

Medium ionising power

Medium range in air (1-2m)

32
Q

Features: Gamma radiation

A

High penetrating power (lead)

Low ionising power

Large range in air (barely absorbed)

33
Q

Alpha particles

A

A helium nucleus.

4He24α2

Charge = +2e

34
Q

Beta particles

A

A high energy electron.

<span>0 </span>β -1

Charge = -1e

35
Q

Gamma radiation

A

An EM photon (of very short wavelength).

<span>o</span>γo

Charge = 0

Mass = 0

36
Q

Conservation laws of nuclear transformations

A
  • Conservation of baryon number.
  • Conservation of charge.
  • Conservation of leptop number.
37
Q

What is a feature of gamma decay?

A

The structure of the nucleus is not changed, it just removes energy.

38
Q

Definition: Random decay

A

We cannot predict which nucelus will be next to decay.

39
Q

Definition: spontaneous decay

A

WE cannot influence the decay of radioactive nuclei.

40
Q

Definition: Half life

A

The time taken for half of the unstable nuclei to decay.

The time taken for the activity of a sample to half.

41
Q

Definition: Activity

A

The number of decays per second in an isotope.

(Bq)

42
Q

Equation: Activity *

A

A = dN / dt = -λN

A = activity (Bq)

N = number of nuclei

λ = decay constant

43
Q

Equation: Half life *

A

t1/2 = <span>ln2</span>/λ

t1/2 = half life (s)

λ = decay constant

44
Q

Equation: Radioactive decay *

A

N = Noe-λt

A = Aoe-λt

N = number of nuclei

No = initial number of nuclei

λ = decay constant

t = time

45
Q

What do you need to take in to account when discussing dangers of radiation to body?

A
  • Will activity change over the time you are in contact with source?
  • Is this type of radiation able to penetrate the body or does it have to be inhaled?
  • How ionising is this type of radiation?
  • How long is the half life?
  • If it does get in the body it could damage your cells.
46
Q

When is alpha radiation dangerous?

A

Alpha particles are unable to penetrate the body so are only dangerous if they are inhaled. Once inhaled, they may damage your cells.

47
Q

Why do nuclei recoil when they emit particles?

A

Because momentum must be conserved.

48
Q

What is a radioactive atom?

A

An atom which has an unstable nucleus and emits alpha, beta or gamma radiation.

49
Q

<span>23</span>Fb18

A

23 = proton + neutron number

18 = proton number

neutron number = 23 - 18 = 5

50
Q

What is a feature of positron emission?

A

It increases the number of neutrons in nucleus.

51
Q

How do you increase the accuracy of count rates?

A

Record the count for a longer time as decay is random.

52
Q

What are the features of fusion?

A

The binding energy per nucleon increases.

Total mass decreases.

53
Q

What are the features of fission?

A

The binding energy per nucleon increases.

Total mass decreases.

Number of free neutrons increases

54
Q

How does fusion release energy?

A

Small nuclei fuse together to produce a larger nucleus. During this process the mass decreases and so according to ΔE = c2Δm energy is released.

55
Q

What are the conditions needed for fusion?

A

Very high temperatures to overcome the repulsion between nuclei.

Very high density to maintain a high collision rate.

56
Q

Why can fusion only produce elements up to iron?

A

Because the binding energy per nucleon decreases for larger elements and so would require a net input of energy to produce them.

57
Q

What is the process of nuclear fission?

A

A large nucleus collides with a neutron and becomes unstable, splitting into two smaller nuclei. Some neutrons are also emitted which go on to cause further fissions in a chain reaction.

58
Q

How does fission release energy?

A

Large nucleus splits into smaller nuclei, with an overall decrease in mass. Also, the binding energy per nucleon increases. Therefore, due to ΔE = c2Δm, energy is released.

59
Q

Why is fusion safer than fission?

A

Fission reactors produce more radioactive waste and are harder to control.

60
Q

Why is fusion more sustainable than fission?

A

Fuel for fission is a limited resource, whereas fuel for fusion is virtually unlimited.

61
Q

Why is it hard to sustain a contolled fission reaction?

A

Extremely large temperatures and densities need to be maintained.

A2 Physics (14 decks)

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