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A-Level Physics > 9. Gravitational and Electric Fields > Flashcards

9. Gravitational and Electric Fields Flashcards

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1
Q

What is a force field?

A

A region in which a body experiences a non-contact force

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2
Q

What is a gravitational field?

A

An area where an object with mass experiences a force of attraction

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3
Q

What do field lines tell you?

A

The direction of the force acting on a mass at that point in the field and they indicate the shape of the field

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4
Q

What does the density of fields lines indicate?

A

The strength of the field

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5
Q

What is a radial field?

A

A field where all the field lines meet at the centre of mass, implying the strength of the field weakens the further you are from the object

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6
Q

What type of field does the Earth have?

A

A radial feild

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7
Q

What is a uniform field?

A

A field where all the field lines are parallel and equally spaced, implying the strength of the field is constant

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8
Q

What is the Earth’s field like close to the surface?

A

Uniform (almost)

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9
Q

What does the inverse square law tell us?

A

The strength of the field is inversely proportional to the square of the distance from the centre of mass

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10
Q

What is Newton’s law of gravitation?

A
  • Gravitational force between two objects is proportional to the product of the two masses
  • It is inversely proportional to the distance between their centres squared
  • It is always attractive
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11
Q

What is gravitational field strength?

A

The force on an object per unit mass (vector)

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12
Q

What is gravitational potential?

A

The energy required (work done) to move a 1kg mass from infinity to that point in the field (scalar)

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13
Q

What are equipotential lines/surfaces?

A

Where the gravitational potential is the same value

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14
Q

Where is gravitational potential 0?

A

At infinity

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15
Q

Why is gravitational potential negative?

A

Infinity is where potential is zero so work must be done against the gravitational field to get there, hence the need to start at a negative value

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16
Q

What is the gradient of gravitational potential against distance graph at a point?

A

The gravitational field strength at that point

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17
Q

What needs to be done to move between equipotential lines?

A

Work

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18
Q

Does a satellite in orbit do work?

A

No work is done because it is travelling along an equipotential line, so the gravitational potential difference will be zero

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19
Q

Which direction does the force act on a satellite?

A

The force acts perpendicular to the direction of travel due to circular motion

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20
Q

What is the work being done to move between equipotential lines equal to?

A

The change in potential energy

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21
Q

How can gravitational potential difference be found from a graph of g against r?

A

Area under the graph

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22
Q

State the altitude, time period and uses of a Low Earth Orbit?

A
  • Between 200 - 2000 km altitude
  • Approximately 2 hour time period
  • Used by the space station, weather satellites and spy satellites
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23
Q

State the altitude, time period and use of Geostationary Orbits?

A
  • Only 35,000 km altitude
  • Exactly 24 hour time period
  • In line with the equator
  • Used by communication satellites
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24
Q

Why is an LEO ideal for satellites that take pictures (spy and weather)?

A

The orbit is close to the Earth’s surface, allowing for detailed pictures

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25
Q

Why can LEO satellites be bigger?

A

It requires the least energy to get to so satellites can be bigger

26
Q

What inclination to the equator can an LEO be?

A

Any inclination from 0˚ directly above the equator (equatorial orbit) up to 90˚ (polar orbit)

27
Q

What is a polar orbit? What type of satellites is it ideal for?

A
  • The Earth continues to rotate under the satellite meaning that every time the satellite orbits round, it passes over a new part of the Earth’s surface
  • Ideal for weather satellites
28
Q

What is the main difficulty with satellites in LEO?

A

You need multiple tracking stations, each with a moveable dish to track the satellite as it passes over head

29
Q

What is a geostationary orbit? What type of satellite is it ideal for?

A
  • The satellite has the same time period as the Earth, meaning the satellite remains about the same point on the Earth’s surface
  • Communication satellites
30
Q

What is the main advantage and disadvantage of a geostationary orbit?

A
  • Advantage: tracking dish doesn’t need to move
  • Disadvantage: satellite must be directly above the equator
31
Q

What is a synchronous orbit?

A

Where an orbiting object has an orbital period equal to the rotational period of the body it is orbiting

32
Q

What is a geosynchronous orbit?

A

An orbit with the same time period as the period of the Earth’s rotation (24 hours)

33
Q

What is a geostationary orbit in relation to a geosynchronous orbit?

A

A geosynchronous orbit which is also directly above the equator

34
Q

How does an object horizontal velocity change as a satellite is moved closer to a planet?

A

The closer a satellite is to the planet, the faster it must go because the gravitational force is much stronger so you need greater horizontal velocity

35
Q

Does kinetic and gravitational potential energy change for a satellite in circular orbit? Why?

A

Speed is constant and altitude is constant, meaning the kinetic energy and the gravitational potential energy of the satellite remains constant

36
Q

Does kinetic and gravitational potential energy change for a satellite in elliptical orbit? Why?

A
  • Total energy of the satellite is still the sum of the kinetic and gravitational potential energies and remains constant
  • At closest approach GPE will be lowest, therefore, to conserve energy, KE must increase
  • The opposite happens at the greatest distance
37
Q

What is used to derive Kepler’s 3rd law?

A

Centripetal force (using angular velocity) = gravitational force

38
Q

What is escape velocity?

A

The speed required to escape the gravitational pull of a mass

39
Q

What is used to derive the escape velocity equation?

A

Kinetic energy = gravitational potential energy

40
Q

What is used to derive the orbital velocity equation?

A

Centripetal force (using linear velocity) = gravitational force

41
Q

How is the equation for kinetic energy of an orbit derived?

A

From substituting the orbital velocity equations into the standard kinetic energy equation

42
Q

What is an electric field?

A

An area where an object with charge experiences a force of attraction or repulsion

43
Q

What is the field and electric field strength around a point charge like?

A
  • Radial field
  • All the field lines meet at the middle
  • Electric field strength varies according to the inverse square law
44
Q

What is the field and electric field strength between two charged plates like?

A
  • Uniform field
  • All field lines are parallel
  • Electric field strength is uniform everywhere in the field
45
Q

What do electric field lines tell you?

A

The direction of the force acting on a positive charge at that point in the field

46
Q

Which way do electric field lines point?

A

Away from a positive charge and towards a negative charge

47
Q

Which way do field lines point in a uniform field?

A

From the more ‘positive’ plate to the more ‘negative’ plate

48
Q

Which way do field lines point in a uniform field if both plates are positive?

A

The field lines go towards the least positive plate

49
Q

What is Coulomb’s law?

A
  • Electrical force between two objects is proportional to the product of the charges
  • It is inversely proportional to the distance between their centres squared
  • It can be attractive or repulsive
50
Q

What is electrical field strength?

A

The force on an object per unit of positive charge (vector)

51
Q

What path does a charged particle follow when entering a uniform electric field? Why?

A
  • Parabolic path
  • The charged particle will experience a constant force in one direction
52
Q

Why will a charged particle in a vertical electric field never go completely vertical?

A

The path will never go completely vertical as there is no horizontal force acting so the horizontal component of velocity does not change

53
Q

What is thermionic emission?

A

When a wire gets hot it releases free electrons

54
Q

What is electric potential?

A

The energy required (work done) to move a positive 1C charge from infinity to that point in the field (scalar)

55
Q

What are equipotential lines/surfaces?

A

Where the electric potential is the same value

56
Q

What direction are equipotential lines compared to field lines?

A

Equipotential lines are at right angles to the field lines

57
Q

What is the gradient of a graph of electric potential against distance at a point?

A

Electrical field strength at that point

58
Q

What needs to be done to move between equipotential lines?

A

Work

59
Q

What is the work done to move between equipotential lines equal to?

A

The change in electrical potential energy

60
Q

What is the area under a graph of E against r?

A

Electrical potential difference

A-Level Physics (15 decks)

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