9. Gravitational and Electric Fields

Question 1

What is a force field?

Answer 1

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

Question 2

What are uniform and radial fields?

Answer 2

Radial: field lines point towards a sphere or point mass from all angles.
Uniform: field lines are parallel to each other on a flat surface.

Question 3

What is the equation for Newton’s law of gravitation?

Answer 3

F = (G m(1) m(2)) / r^2
Where G is the gravitational constant, m 1 and 2 are the masses of the two objects, and r is the distance between the centre of the objects.

Question 4

How does the inverse square law apply to gravitational fields?

Answer 4

F is inversely proportional to r^2.

Question 5

What is the formula for gravitational field strength for radial fields?

Answer 5

g = GM / r^2

Question 6

What is the formula for gravitational potential?

Answer 6

V = -GM / r

Question 7

What is gravitational potential?

Answer 7

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

Question 8

What is gravitational potential energy?

Answer 8

The energy that an object holds in a gravitational field.

Question 9

What is gravitational potential difference?

Answer 9

The energy needed to move a unit mass to a different distance from the mass creating the field that it is in.

Question 10

What is an equipotential line?

Answer 10

A line that you can travel along in a field with no change in potential energy.

Question 11

Which force keeps satellites in orbit?

Answer 11

Centripetal force (the gravitational force).

Question 12

How is the time period of an orbit related to the radius, r?

Answer 12

T^2 is directly proportional to r^3
or
t is directly proportional to √r^3

Question 13

What is escape velocity?

Answer 13

The minimum speed that an object needs to completely leave the gravitational field of an object.

Question 14

What is the equation for escape velocity?

Answer 14

v = √((2GM) / r)

NOTE: if at the surface, r is the radius of the planet, as r is the distance to the centre of the planet.

Question 15

What is a synchronous orbit?

Answer 15

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

Question 16

What is a geostationary satellite?

Answer 16

A satellite in synchronous orbit, but also is over the exact same place on the Earth. This means that the orbit must be in the plane of the equator.

Question 17

What are low orbiting satellites?

Answer 17

Satellites which orbit between 180km and 2000km from the surface of the earth.

Question 18

What is an electric field?

Answer 18

A field around a charged object that can attract or repel other charges objects.

Question 19

What is Coulomb’s law?

Answer 19

F = (1 / 4πε(0)) * (Q(1) Q(2) /r^2)

Question 20

Does coulomb’s law follow the inverse square law?

Answer 20

Yes.

Question 21

What direction does the vector E (electric field strength) point?

Answer 21

The direction that a positively charged point mass would move.

Question 22

What direction do field lines point between two plates that are both positively charged?

Answer 22

From the most to the least positively charged plate.

Question 23

What is the formula for electric field strength in a uniform field?

Answer 23

E = V / d

Where V is the potential difference between the plates.

Question 24

What is a synchronous orbit?

Answer 24

When a satellite orbits with an orbital period equal to the rotational period of the object that it is orbiting.

Question 25

What is a geostationary satellite?

Answer 25

A satellite with a synchronous orbit that orbits in the same plane as the equator so that it is always above the same place on the Earth.

Question 26

What is a low orbiting satellite?

Answer 26

Satellites that orbit between 180km and 2000km above the Earth.

Question 27

What is an electric field?

Answer 27

A region in which charged objects experience a force.

Question 28

What is Coulomb’s law?

Answer 28

- It gives the force of attraction or repulsion between two charged objects in a vacuum (air can be treated as a vacuum). F = (1 / 4πε(0)) * (Q(1)Q(2) / r^2)

Question 29

Is Coulombs law an inverse square law?

Answer 29

Yes, F is inversely proportional to r^2.

Question 30

What is the definition of electric field strength?

Answer 30

The force per unit positive charge.

Question 31

What is the equation for electric field strength that links E, F and Q?

Answer 31

E = F / Q

Question 32

For a point charge with a positive charge placed in an electric field, which directions would the field lines point?

Answer 32

Away from it.

Question 33

What directions do the field lines point between two positively charged parallel plates?

Answer 33

From the one with the most to the least positive voltage.

Question 34

What is the formula for electric field strength?

Answer 34

E = (1 / 4π ε(0)) * (Q / r^2) (for radial fields)

Question 35

What is the equation for electric field strength in uniform fields?

Answer 35

E = V / d **NOTE: the electric field strength is the same at all points in the field.**

Question 36

What is absolute electric potential?

Answer 36

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

Question 37

What does absolute electric potential depend on?

Answer 37

- How far the point charge is from the charge creating the field. - The size of that charge.

Question 38

What is the formula for absolute electric potential?

Answer 38

V = (1 / 4π ε(0)) * (Q / r) If V is positive then Q is positive and the force is repulsive, and vice versa.

Question 39

What does it mean when two points in an electric field have a different absolute electric potential?

Answer 39

There is an electric potential difference between them, and so energy is needed to move a unit charge between those points.

Question 40

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

Answer 40

The electric potential difference between the two points.

Question 41

When moving a charge across a potential difference, what affects the energy needed to do so?

Answer 41

- The size of the charge that you are moving. - The size of the potential difference that you want to move it across.

Question 42

What is the formula for work done in moving a charge?

Answer 42

∆W = Q∆V