Section 9 - Gravitational and Electric Fields

Question 1

Is the force in a field contact or non-contanct?

Answer 1

Non-contact

Question 2

Is force a vector or scalar quantity?

Answer 2

Vector

Question 3

What are the similarities between electrostatic and gravitational forces?

Answer 3

• Inverse square force laws
• Potential concept
• Equipotential surfaces
• Use of field lines

Question 4

What are some differences between electrostatic and gravitational forces?

Answer 4

The gravitational forces from masses always attract, whilst charges may repel or attract

Question 5

What is gravity?

Answer 5

The universal attractive force which acts between all matter

Question 6

What is G?

Answer 6

The universal gravitational constant

Question 7

What is Newton’s law of gravitation?

Answer 7

Force between 2 objects is proportional to the product of their masses/charges and inverseley proportional to the square of their seperation

Question 8

What can field lines tell you about a field?

Answer 8

The direction of the field and the strength of the field depending on the density of the field lines

Question 9

What is g?

Answer 9

• The force per unit mass in a uniform field
• In a radial field the magnitude of g is the proportionality constant at that point between force and mass

Question 10

What is gravitational potential?

Answer 10

The work done/energy required in bringing 1 kg/unit mass from infinity to the point

Question 11

What is the gravitational potential difference?

Answer 11

Gravitational potential difference is the difference in the gravitational potentials of 2 points in a gravitational field

Question 12

What is an equipotential surface?

Answer 12

A surface in which every point on the surface has the same potential

Question 13

How much work is done when you move 1km in any direction on an equipotential?

Answer 13

No work is done when moving across equipotentials, as the potential at each point is the same

Question 14

Why is gravitational potential a negative value?

Answer 14

• Work needs to be done to move an object from the inside of the field to the outside of the field
• SInce outside the field’s potential is defined as 0 then the potential inside the field must be negative

Question 15

How is the orbital period related to the radius of a circular orbit?

Answer 15

T^2 ∝ R^3

Question 16

What equations could you use to find the speed of an orbiting satellite?

Answer 16

The orbiting object (mass m) is in circular motion, so we would use F = ma with F = GMm/r^2 rearranged to a=v^2 r=ω^2 x r.
This can be sold to find the speed (V0, angular speed (ω), the radius of the orbit or using T=2π/ω its period

Question 17

Compare the PE and KE of a lower orbit to a higher one

Answer 17

A lower orbit has less potential energy and more kinetic energy than a higher orbit

Question 18

What is the period of a geosynchronous/geostationary orbit?

Answer 18

24 hours

Question 19

When calculating the force between two particles, what can air be treated as?

Answer 19

A vacuum

Question 20

For a charge sphere the charge can be assumed to be at what part of the sphere?

Answer 20

The centre

Question 21

Which is stronger? The gravitational force of subatomic particles or the electrostatic force

Answer 21

The electrostatic force

Question 22

Electric field line always go from ________ to ________.

Answer 22

Electric field lines go away from positive charge to negative charge

Question 23

What is electric field strength?

Answer 23

The force per unit charge acting on a positive charge

Question 24

What is electric potential energy?

Answer 24

The work done in bringng a unit charge from infinity to that point

Question 25

What is the equation for the magnitude of E (electric field strength) in a uniform electric field?

Answer 25

Potential difference between plates (V)/ distance between plates

Question 26

What is the trajectory of a particle entering a uniform field at right angles?

Answer 26

It is parabolic

Question 27

What are the 2 relationships from Coulomb's law for electric fields?

Answer 27

E ∝ 1/r² V ∝ 1/r

Question 28

What is the proportionality law for gravitational fields?

Answer 28

g ∝ 1/r² g ∝ M

Question 29

What is the equation for escape velocity?

Answer 29

V = (2GM/r)^1/2

Question 30

What does a r against g graph look like?

Answer 30

Diectly proportional until r=R (maximum point that's a sharp point), then a decreasing curve which curves back towards the x axis

Question 31

Show that the radius of a charged particle in a magnetic field is r=mv/Bq

Answer 31

1. F=mv^2/r 2. F=Bqv 3. mv^2/r=Bqv 4. r=mv/Bq

Question 32

How do you determine the speed of an ion as it passes through an electric field?

Answer 32

1. Ke=Work done 2. 1/2mv^2 = qV 3. v=(2qV/m)^1/2

Question 33

Discuss the energy transfers that take place as an ion passes through an electric field

Answer 33

1. Potential energy of ion is transferred to kinetic energy of ion 2. Power supply transfers energy to the ion 3. Decrease in energy stored in supply = increase in kinetic energy stored by the ion

Question 34

Where should you launch a satellite from to minimise the amount of fuel used?

Answer 34

1. The equator 2. Satellite has the largest initial kinetic energy from the Earth's rotation

Question 35

What is the equation for the gravitational potential energy of a satellite in orbit about a mass?

Answer 35

-GMm/r The minus is important!!

Question 36

How does linear speed change in a higher orbit?

Answer 36

1. Smaller 2. v=√GM/r 3. v∝1/√r

Question 37

Where on equipotentials is the gravitational field strength the strongest?

Answer 37

Where the field lines are closest together

Question 38

When comparing how high a mechanism can fire an object into the air on different planets/moons what should you do?

Answer 38

1. Find g for both 2. Set mgh=mgh to the other 3. m cancels out 4. Sub g values and given h value in

Question 39

How do you find the work done against gravity in a uniform gravtitational field?

Answer 39

1. Find ΔV across the 2 plates 2. Multiply by the ratio of distance travelled vertically 3. Multiply by mass

Question 40

How do you show that a gravitational field is not uniform?

Answer 40

ΔV/Δr is changing

Question 41

What does the gradient of V against d represent?

Answer 41

The magnitude of the gravitational field strength

Question 42

What can be assumed as g if no other information is given?

Answer 42

The acceleration of a small mass released from rest just above the surface of a planet/moon

Question 43

What is the first thing you should do when there's a satellite question?

Answer 43

Set centripetal force equal to force between 2 masses

Question 44

How do you find the point between 2 masses where the resultant gravitational field is 0?

Answer 44

1. g1= g2 2. GM1/r1^2=GM2/r2^2 3. M1/r1^2=M2/(d-r1)^2 4. d-r/r=√M2/M1 5. d/r-r/r=√M2/M1 6. d/r = √M2/M1 +1

Question 45

State what is represented by gravitational field lines

Answer 45

The **direction** of **force** on a **mass**

Question 46

When field lines converge what is the change in speed of an object?

Answer 46

* It will speed up * Because the potential is lower at the point where the lines converge

Question 47

What is the work done on a moving charged particle?

Answer 47

W=Fs W=Eqd

Question 48

Potentials are scalar what does this mean in terms of combining them?

Answer 48

You can just add them together

Question 49

How do you find the acceleration of a charged particle?

Answer 49

F=EQ ma=EQ a=EQ/m

Question 50

Why is the time taken to fall vertically between metal plates independent of the mass of the particle?

Answer 50

F=mg F=ma ma=mg a=g So the time to fall depends only on distance and acceleration

Question 51

Why is the horizontal acceleration of a particle different for each particle falling between 2 plates?

Answer 51

* Mass is not constant since particle mass will vary * Charge on a particle is not constant * Acceleration = Eq / m * E constant but charge and mass are ‘random’ variables so q / m will vary