Gravitational and electric fields

Question 1

What is a force field?

Answer 1

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

Question 2

How do force fields arise?

Answer 2

From the interaction of mass, of static (stationary) charge and between moving charges

Question 3

What is gravity?

Answer 3

An attractive force that acts between all masses and has an infinite range

Question 4

What does Newton’s law of gravitation show?

Answer 4

The attractive force due to gravity acting between two masses

Question 5

What is the relationship between the force due to gravity acting between two masses and their separation?

Answer 5

They follow an inverse square law so the force is inversely proportional to the separation squared

Question 6

What is a gravitational field?

Answer 6

A region in space in which objects (masses) experience a force due to gravity

Question 7

What do gravitational field lines show?

Answer 7

The direction of the force acting on a mass in a gravitational field (the direction a stationary mass would move if placed in the field) and the closeness of the lines show the strength of the field at that point

Question 8

What is gravitational field strength?

Answer 8

It is a measure of the intensity of a gravitational field at a particular point, in other words it is the force experienced per unit mass by an object at that point

Question 9

How do the gravitational field lines around the entire Earth differ to those on the surface of the Earth?

Answer 9

The field lines around the Earth as a whole are radial but as you get closer to the field lines near the surface can be considered uniform

Question 10

What is gravitational potential?

Answer 10

The work done per unit mass moving against the field to bring a small test mass from infinity to a specific point in a gravitational field, without changing its kinetic energy

Question 11

Why is gravitational potential always negative?

Answer 11

Because you’re moving against the direction of the field (hence moving in thenegative direction relative to the field)

Question 12

What is the gravitational potential at an infinite distance away from the mass that created the gravitational field?

Answer 12

Zero

Question 13

What is gravitational potential difference?

Answer 13

The change in gravitational potential energy per unit mass between two points in a gravitational field, it is the work being done when moving a unit mass between those two points

Question 14

How can gravitational potential difference be calculated from a gravitational field strength graph?

Answer 14

By working out the area underneath the curve

Question 15

How can gravitational field strength be calculated from a gravitational potential graph?

Answer 15

By calculating the negative gradient of the tangent to the curve at any point

Question 16

What is meant by an equipotential surface (in relation to a gravitational field)?

Answer 16

A line or surface where all of the points along it have the same gravitational potential, meaning there is no gravitational potential difference when a mass is moved along it (hence no work is done)

Question 17

What is the connection between gravitational field lines and equipotential surfaces?

Answer 17

They are perpendicular to eachother at each point and the closer together field lines are the closer togehter the equipotential surfaces are

Question 18

Derive the equation that links the orbital speed of a planet to its radius

Answer 18

Look in booklet for derivation

Question 19

What is a synchronous orbit?

Answer 19

When an orbiting object has an orbital period equal to the rotational speed of the object it is orbiting

Question 20

What is a geostationary orbit?

Answer 20

A specific type of synchronous orbit where a satellite orbits the Earth at the same rate at which the Earth rotates (meaning it completes one orbit in 24 hours), this means the satellite is always directly above the equator and always remains stationary relative to the Earth

Question 21

What is a low polar orbit satellite?

Answer 21

A satellite that orbits close to the Earth

Question 22

Compare the heights of the orbits for a low polar orbit satellite and a geostationary orbit satellite

Answer 22

The low polar orbit satellite orbits much closer to Earth whereas a geostationary orbit satellite orbits further away from the Earth to ensure it orbits at the Earth’s rotational speed

Question 23

Compare the orbital periods of low polar orbit satellites and geostationary orbit satellies

Answer 23

A low polar orbit satellite is closer to Earth so it experiences a greater gravitational pull compared to the geostationary orbit satellite and so the low polar orbit satellite orbits faster and has a shorter orbital period

Question 24

What are 2 common uses of geostationary orbits?

Answer 24

Communication (TV, phone, internet) and weather monitoring

Question 25

Why are geostationary orbits suitable for communication purposes?

Answer 25

It provides a constant signal because the satellite is stationary relative to the Earth so you don't have to angle your reciever to get a signal

Question 26

Why are geostationary orbits suitable for weather monitoring purposes?

Answer 26

You get a continuos view of the same region so it allows real-time weather tracking

Question 27

What are 2 common uses of low polar orbit satellites?

Answer 27

Earth observation (mapping and spying) and weather monitoring

Question 28

Why are low polar orbit satellites suitable for Earth observation?

Answer 28

The satellite is close to the Earth so you can get higher resolution images

Question 29

Why are low polar orbit satellites suitable for weather monitoring?

Answer 29

The satellite has a short orbital period (passes past the same point more frequently) so the weather can be updated every few hours

Question 30

How do you work out the total energy of a satellite?

Answer 30

Add the kinetic and potential energy together as its total energy is always constant

Question 31

Describe the kinetic and potential energy of a satellite in a circular orbit

Answer 31

The satellites speed and distance above the mass it is orbiting are constant so both kinetic and potential energy are also both constant

Question 32

Describe the kinetic and potential energy of a satellite in an elliptical orbit

Answer 32

The satellite speeds up as its orbital radius decreases so the kinetic energy increases and potential energy decreases (and vice versa) , so that total energy remains constant

Question 33

What is meant by escape velocity?

Answer 33

The minimum speed an unpowered object needs in order to leave the gravitational field of a planet without falling back towards it because of gravitational attraction

Question 34

Explain how it is possible for rockets to leave the surface of the Earth travelling at a speed much lower than the escape velocity

Answer 34

The escape velocity tells you the speed needed to escape with a single, unpowered launch whereas rockets use a continuous thrust to overcome gravity, meaning they accelerate gradually and gain enough kinetic energy over time

Question 35

What are the laws of electrostatic attraction and repulsion?

Answer 35

Like charges repel eachother (e.g. 2 negative charges or 2 positive charges) and opposite charges attract (e.g. a negative and a positive charge)

Question 36

What does Coulomb's law show?

Answer 36

The electrostatic force acting between two charges

Question 37

What is the relationship between the electrostatic force between two charges and their separation?

Answer 37

They follow in inverse square law so the force is inversely proportional to the separation squared

Question 38

What is the permittivity of free space?

Answer 38

A measure of how easy/ how difficult it is for an electric field to pass through air or a vacuum

Question 39

What is an electric field?

Answer 39

A region in space in which a charged object experiences an electrostatic force due to the presence of other charges

Question 40

What do electric field lines show?

Answer 40

The direction that a positive test charge would move if placed in the field (hence the direction of the force that would act on the positive charge)

Question 41

What is the definition of electric field strength?

Answer 41

The force per unit positive charge experienced by a charged object placed within an electric field

Question 42

Combine the two equations for electric field strength in a uniform field to obtain an equation for work done

Answer 42

Look in booklet for derivation

Question 43

Describe what happens as a charged particle enters an electric field

Answer 43

It enters the field at right angles to the field and feels a constant force acting parallel to the electric field lines, if the particle is positively charged it will move towards the negative plate and if it is negatively charged it will move towards the positive plate causing the particle to follow a curved path (parabola)

Question 44

What is electric potential?

Answer 44

The work done per unit positive charge moving against the field to bring a positive test charge from infinity to a specific point in an electric field

Question 45

How does the value of electric potential change for attractive and repulsive forces?

Answer 45

Electric potential is positive for repulsive forces and negative for attractive forces

Question 46

What is the electric potential at an infinite distance away from the charge that created the electric field?

Answer 46

Zero

Question 47

What is meant by an equipotential surface (in relation to an electric field)?

Answer 47

A line or surface where all of the points along it have the same electric potential meaninh there is no electric potential difference when a charge is moved along it (hence no work is done)

Question 48

What is the connection between electric field lines and equipotential surfaces?

Answer 48

They are perpendicular to eachother at each point and the closer together the field lines the closer together the equipotential surfaces are

Question 49

How do you calculate the electric potential from an electric field strength graph?

Answer 49

By working out the area underneath the curve

Question 50

How do you calculate the electric field strength from an electric potential graph?

Answer 50

By calculating the gradient of the tangent at any point on the curve

Question 51

Give 4 examples of contact forces

Answer 51

Friction, air resistance, tension and the normal contact force

Question 52

Give 3 examples of non-contact forces

Answer 52

Gravitational, electrostatic and magnetic forces

Question 53

What are the similarities between gravitational and electric fields?

Answer 53

- Non-contact forces acting at a distance - Follow inverse square laws - Have radial field patters around point sources - Use field lines to show strength and directions - Work is done when moving a mass or charge in the field

Question 54

What are the differences between gravitational and electric fields?

Answer 54

- Gravitational fields are caused by masses and electric fields are caused by charges - Gravitational fields are always attractive and electric fields can be attractive or repulsive - Gravitational potential is always negative but electric potential can be positive or negative - Different constants (G and the permittivity of free space)