Gravitational and electric fields

Question 1

What do gravitational field lines represent?

Answer 1

The direction of the force that would affect a mass placed at that point in a gravitational field

Question 2

Newtons law of gravitation

Answer 2

-The force is directly proportional to the product of the masses
-The force is inversely proportional to the distance between the centres squared
-The force is always attractive

Question 3

Gravitational force equation

Answer 3

F=GMm/r^2

Question 4

Define gravitational field strength + units

Answer 4

The gravitational force per unit of mass.
Nkg^-1 or ms^-2

Question 5

Is gravitational field strength scaled or vector?

Answer 5

Vector

Question 6

Gravitational field strength equation

Answer 6

g=GM/r^2
g=F/m

Question 7

Define gravitational potential + units

Answer 7

The work done to move a 1kg mass from infinity to that point in the field.
Jkg^-1

Question 8

Why is gravitational potential negative?

Answer 8

Infinity is where potential is 0, but work has to be done to get there starting from negative.

Question 9

What is an equipotential surface?

Answer 9

Where gravitational potential is all the same

Question 10

Gravitational potential equation

Answer 10

V=-GM/r
Remember negative

Question 11

Why is no work done when a mass moves along an equipotential in a gravitational field?

Answer 11

The mass is moving perpendicular
W=Fd and d=0 so no work done

Question 12

What is Keplers third law?

Answer 12

r^3=GMT^2/4π^2

Question 13

What is the escape velocity?

Answer 13

The minimum speed an unpowered object needs to leave a gravitational field

Question 14

Low Earth Orbit examples

Answer 14

Space station
Weather satellites
Spy satellites

Because detail pictures can be taken
Also bigger satellites available as less energy required

Question 15

Low earth orbit time period

Answer 15

2 hours

Question 16

Low Earth Orbit altitude

Answer 16

200-2000km

Question 17

Geostationary orbit example

Answer 17

Communication satellites

Question 18

Geostationary orbit time period

Answer 18

24 hours same as earth

Question 19

Geostationary orbit altitude

Answer 19

35000km
In line with equator

Question 20

Escape velocity equation and how to derive

Answer 20

1/2mv^2=GMm/r

V=sqrt(2GM/r)

Question 21

Velocity of orbit equation

Answer 21

V=sqrt(GM/r)

Question 22

Polar orbit

Answer 22

Earth rotates under satellite
So every time the satellite orbits round it passes over a new part of earths surface
This allows the satellite to scan the whole earths surface
Ideal for weather satellites available

Question 23

What is the main difficulty with a low Earth orbit?

Answer 23

Multiple tracking stations needed
Each with a moveable dish
To track the satellite as it passes over

Question 24

What do electric fields lines represent?

Answer 24

The direction and magnitude of a force acting on a positive charge in an electric field

Question 25

State Coulomb’s law

Answer 25

For a force between 2 point charges
The force is proportional to the product of the charges
The force is inversely proportional to the distance between the centres squared

Question 26

Electric force equation

Answer 26

F=1/4πϵ x Qq/r^2

Question 27

What direction to electric field lines go in?

Answer 27

From positive to negative
Positive to less positive
Etc

Question 28

Define electric field strength + units

Answer 28

The electrostatic force per unit positive charge
NC^-1 or Vm^-1

Question 29

Electric field strength equation

Answer 29

General -> E=F/Q
Radial field -> 1/4πϵ x Q/r^2
Uniform field -> E=V/d

Question 30

Define electric potential and state its units

Answer 30

The work done per unit charge in moving the positive charge from infinity to that point in the field

JC^-1

Question 31

Why is escape velocity higher in practice?

Answer 31

Air resistance means extra energy is required

Question 32

How to derive Keplers law?

Answer 32

GMm/R^2 = mw^2R

W=2pi/T