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AQA A Level Physics(Option A: Astrophysics) > Gravitational and Electric Fields > Flashcards

Gravitational and Electric Fields Flashcards

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1
Q

What is a gravitational field?

A

A force field where any object with mass will experience an attractive force

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2
Q

What is a point mass?

A

A mass with negligible volume, or a uniform sphere whose mass acts as if it’s concentrated at the centre

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3
Q

What is the law of gravitation?

A
  • An inverse square law
  • Force is proportional to the inverse of distance between two point masses squared
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4
Q

Define gravitational field strength

A

The force per unit mass, in N/Kg

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5
Q

What is the approximate value of g on Earth?

A

9.81N/kg

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6
Q

Describe the graph of g against r for Earth

A
  • Exponential when x = Re, where Re is the surface of the Earth
  • Exponential where g = 9.81N/kg
  • L shaped
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7
Q

Define gravitational potential

A

The gravitational potential energy that a unit mass would have at a specific point

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8
Q

Why is gravitational potential negative?

A

As work is being done against the gravitational field to move an object out of it

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9
Q

Describe the graph of V against r

A
  • x axis(r) at the top, y axis(V) negative to zero
  • Exponential when x = The radius of the sphere
  • r shaped curve
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10
Q

What is gravitational potential difference?

A

The energy needed to move a unit mass

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11
Q

Derive the equation for gravitational potential energy

A
  • ∆W = m ∆V
  • ∆Potential energy = m ∆V
  • Potential energy = m(-GM/r) = -GMm/r
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12
Q

What are equipotentials?

A

Lines(in 2D) or surface(in 3D) that join all the points with the same gravitational potential
- This means ΔV = 0 and as ΔW = mΔV, work done is also 0

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13
Q

What is a satellite?

A

Any smaller mass which orbits a much larger mass

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14
Q

How are satellites kept in orbit?

A

By the gravitational pull of the mass they’re orbiting

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15
Q

Derive the equation for the orbital speed of a satellite

A
  • F = mv²/r and F = GMm/r
  • mv²/r = GMm/r
  • v² = GM/r
  • v = √GM/r
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16
Q

What is the relationship between orbital speed and orbital radius of a satellite?

A
  • Orbital speed is inversely proportional to the square root of orbital radius
  • v = √GM/r
  • v = √1/r
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17
Q

What is the orbital period?

A

The time taken for a satellite to complete one orbit

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18
Q

Derive the equation for the orbital period of a satellite

A
  • s = d/t , so v = 2πr/T
  • T = 2πr/v = 2πr/√GM/r
  • T = 2πr√r/√GM
  • T² = 4π²r³/GM
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19
Q

What is the relationship between orbital period and orbital radius of a satellite?

A

Orbital period squared is proportional to the orbital radius cubed

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20
Q

What is the on the y-axis and x-axis of a graph on orbital period?

A
  • y axis: Period, in years
  • x axis: orbital radius, in
    x10⁶ m
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21
Q

How would kinetic and potential energies change for a satellite in an elliptical orbit?

A
  • As the radius decreases, satellite speeds up, increasing kinetic energy and decreasing potential energy
  • Total of kinetic and potential energy is always constant
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22
Q

Define escape velocity

A

The minimum speed an unpowered object to leave the gravitational field of a planet and not be pulled back due to gravitational attraction

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23
Q

What principle is used to derive the escape velocity equation, stating the formula based on the principle?

A
  • Conservation of energy
  • Kinetic energy lost = Gravitational potential energy gained
24
Q

Derive the escape velocity equation

A
  • KE = 1/2mv² and GPE = GMm/r
  • 1/2mv² = GMm/r
  • 1/2v² = GM/r
  • v² = 2GM/r
  • v = √2GM/r
25
Q

What is a synchronous orbit?

A

An object has a orbital period equal to the rotational period of the object it’s orbiting

26
Q

What are geostationary satellites?

A

An Earth satellite that orbits directly over the equator and is always above the same point of Earth

27
Q

Define low orbiting satellites

A

Satellites which orbit between 180km-2000km above Earth

28
Q

State the advantages and a disadvantage of low orbiting satellites

A

Advantages:
- Cheaper to launch
- Require less powerful transmitters as closer

Disadvantage:
- Proximity to Earth and high orbital speeds mean multiple satellites working together are needed for constant coverage

29
Q

State 3 applications of low orbiting satellites

A
  • Communications
  • Mapping
  • Monitoring weather
30
Q

What are point charges?

A

A charge with negligible volume, or a uniform sphere whose charge acts as if it’s concentrated at the centre

31
Q

State Coulomb’s Law

A

F = kQq/r² , where k = 1/4πε0 , ε0 = permittivity of free space in F/m

32
Q

State the relationship the force on the object has with the distance between charges?

A
  • Inverse square law
  • Force is proportional to the inverse of the distance between charges squared
33
Q

Define electric field strength, E

A

Force per unit positive charge

34
Q

What are electric field lines?

A

Lines that show the direction of the force that would act on a charge

35
Q

Which way do electric field lines point for positive and negative point charges?

A
  • Lines point outward for a positive point charge
  • Lines point inward for a negative point charge
36
Q

Which way do the lines point between two parallel plates?

A

From the higher voltage plate to lower voltage plate

37
Q

What surrounds a point charge?

A

A radial field

38
Q

What is the relationship between electric field strength and distance from the point charge?

A
  • Inverse square law
  • Electric field strength is proportion to the inverse of the distance from the point charge squared
39
Q

If a point charge is no longer used, does the inverse square law still apply to the radial field?

A

No, electric field strength will not be proportional to the inverse of the distance from the charge

40
Q

Describe the graph of Electric field strength against r for a point charge

A
  • E decreases in a curve as r increases
  • For a point charge, E becomes infinitely large as r tends to 0
41
Q

Describe the graph of Electric field strength against r for a charged sphere

A
  • Exponential on the x-axis at Rs, where Rs = sphere radius
  • Exponential on the y-axis at Es where Es = Electric field strength at surface of sphere
  • E decreases from the exponentials as r increases
42
Q

How can a uniform field be produced?

A

By connecting two parallel plates to the opposite poles of a battery

43
Q

How can you determine if a particle has charge?

A
  • By putting the particle in a uniform field
  • A positive particle that enters at right angles to the field lines will have its force in the same direction as the field lines
  • A negative particle that enters at right angles to the field lines will have its force in the opposite direction of the field lines
  • Particle accelerates at right angles and follows a curved path in either direction
44
Q

Define absolute electric potential

A

The electric potential energy a unit positive charge would have at that point

45
Q

State the equation for absolute electric potential

A

V = kQ/r, where k = 1/4πε0

46
Q

Describe the graph for V against r for a positive charge

A
  • Graph curving in an l shape
  • As r increases, V decreases
47
Q

Describe the graph for V against r for a negative charge

A
  • Max value of V = 0
  • x axis on the top
  • Graph curving in an r shape
48
Q

What does the gradient of a tangent for a V against r graph for a positive or negative charge give?

A

The electric field strength, E = ΔV/Δr

49
Q

What is electric potential difference?

A

The difference between the absolute electric potential of two points within an electric field

50
Q

What does the area under an E against r graph give?

A

Change in electric potential

51
Q

How is the area under a E against r graph calculated?

A

By counting the squares of the area to be found

52
Q

What is the equation for energy needed to move a charge across a potential difference?

A

ΔW = QΔV

53
Q

Derive the equation for the energy needed to move charge across a potential difference

A

E = ΔV/d = F/Q
- ΔV/d = F/Q
- QΔV = Fd and as W = Fd, for a moving charge:
- QΔV = ΔW

54
Q

What is the main difference between gravitational fields and electric fields?

A

Gravitational fields can only be attractive whereas electric fields can be attractive and repulsive

55
Q

Which one of these forces can be ignored at a subatomic level and why?

A

Gravitational force as despite being close together, all the particles have incredibly small mass

56
Q

Where is the gravitational potential energy stored?

A

In the gravitational field, between the masses

57
Q

Where is electrical potential energy stored?

A

In the electric field, between the charges

AQA A Level Physics(Option A: Astrophysics) (15 decks)

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