Fields

Question 1

Direction of the field

Answer 1

Gravitational- direction of the force on a small test mass at that point

Electric- direction of a force on a positive charge at that point

Magnetic- direction of force in a magnetic North Pole

Question 2

Radial field

Answer 2

Directed towards centre
Diverges
Strength of field decreases as density of lines decreases

Question 3

Uniform field

Answer 3

Lines are equally spaced and parallel
Same magnitude and direction throughout
Near field limit

Question 4

Newton’s law of gravitation

Answer 4

F= GMm/ r^2

Question 5

Gravitational constant

Answer 5

6.67 x 10^-11 Nm^2kg^-2

Question 6

Relationship T and r

Kepler’s law

Answer 6

T^2 proportional to r^3

T^2 = 4pi^2/ GM r^3

Question 7

As r increases …

Answer 7

Speed of satellite decreases (v = GM/r)

Period of satellite increases

Question 8

Geosynchronous orbit

Answer 8

Period of 24 hours
Or it’s above the equator
Or it’s in the same direction as the Earth’s rotation

Question 9

Geosynchronous satellites in communication

Answer 9

Dish can be pointed at a fixed point (does not have to track)

Dishes needed as the weak signal can be collected across the area of the disc and focussed on the receiving antenna

Question 10

Gravitational field strength

Answer 10

g = GM/r^2

Question 11

Gravitational potential

Answer 11

V =-GM/r

Scalar

Units = J/kg

Question 12

V at infinity

Answer 12

0

Question 13

Work (gravitational)

Answer 13

W = mdV

Away = positive
Towards =negative

Question 14

Gravitational potential energy

Answer 14

U= -GMm/r

Question 15

Kinetic energy

Answer 15

Ek= GMm/2r

Question 16

Total energy (gravitational)

Answer 16

Et = -GMm/2r

Negative

Question 17

Increasing Et

Answer 17

Gains energy, less negative, r increases

Question 18

Escape velocity

Answer 18

Root (2gR)

Question 19

Coulomb’s law

Answer 19

F = Qq/4pi epsilon r^2

Question 20

Positive

Answer 20

Repulsive

Question 21

Epsilon 0

Answer 21

Permittivity of free space
Ease of setting up an electric field
8.85 x10^-12

Question 22

Similarities between gravitational and electric fields

Answer 22

Inverse square law for forces
Non contact forces
Infinite range

Question 23

Difference gravitational and electric fields

Answer 23

Electric acts on changes, gravitational on masses

Gravitational is attractive whereas electric can be attractive or repulsive

Question 24

Electric field strength

Answer 24

E= Q/ 4 pi epsilon r^2

Question 25

Electric potential

Answer 25

V = Q/ 4pi epsilon r Scalar

Question 26

Sign of W

Answer 26

+/- negative work | +/+ or -/- positive work

Question 27

Potential gradient (electric)

Answer 27

E = dV/ dr

Question 28

Work (electric)

Answer 28

W =qdV

Question 29

Equipotential lines

Answer 29

Lines of equal potential | At right angles to the field

Question 30

Electrical potential energy

Answer 30

U = Qq/4 pi epsilon r Energy of whole system

Question 31

Distance of closest approach

Answer 31

R min = Qq/ 2pi epsilon m v^2

Question 32

Capacitance

Answer 32

C=Q/V | Farads F

Question 33

Work done by a capacitor

Answer 33

E = 1/2 QV E = 1/2 C V^2 E = 1/2Q^2/C

Question 34

Time constant

Answer 34

RC Seconds

Question 35

Charging a capacitor Charge

Answer 35

Q = Qmax (1-e^-t/RC)

Question 36

Charging a capacitor Voltage

Answer 36

V = Vmax (1-e^-t/RC)

Question 37

Charging a capacitor Current

Answer 37

I = Imax e^-t/RC

Question 38

Discharging a capacitor Charge

Answer 38

Q = Qo e^-t/RC

Question 39

Discharging a capacitor Voltage

Answer 39

V = Vo e^-t/RC

Question 40

Discharging a capacitor Current

Answer 40

I = Io e^-t/RC

Question 41

Relative permittivity

Answer 41

Epsilon r = C/Co ``` C = capacitance with dielectric Co = capacitance with vacuum ```

Question 42

Capacitance equation (dielectric)

Answer 42

C = epsilon o epsilon r A/ d

Question 43

Current at right angles to the field feels force

Answer 43

F = BIl

Question 44

Fleming’s left hand rule

Answer 44

``` Thumb= force 1st = field 2nd = current ```

Question 45

Force on a moving charge perpendicular to the field

Answer 45

F = Bqv

Question 46

Charged particle moving parallel to a magnetic field

Answer 46

No force

Question 47

Radius of a circular path

Answer 47

r= mv/Bq

Question 48

Time period for 1 full circle

Answer 48

T = 2 pi m/ Bq

Question 49

Acceleration in a magnetic field

Answer 49

Changes direction so changes v so accelerated Circular path F is perpendicular to v and B No work done so Ek does not change, so no increase in speed

Question 50

Why a cyclotron needs to be evacuated

Answer 50

Collisions, loss of Ek, deceleration, changes v which changes r

Question 51

Alternating charge of the dees of a cyclotron

Answer 51

So proton accelerated across the gap Needs to alternate and reverse electric field Polarity can switch at regular intervals Frequency = Bq/ 2 pi m

Question 52

The feature of the ions that allows them to be separated

Answer 52

In ion separator v B and q are constant r= mv/Bq Ions departed due to their mass

Question 53

Velocity selected in a velocity separator

Answer 53

Only 1 velocity where Bqv= Eq and there will be no deflection V=E/B

Question 54

Why ions originally have a range of speeds

Answer 54

Gases Random continuous motion of gases Brownian motion Move with varying speeds and directions

Question 55

Magnetic flux

Answer 55

Phi= BA cos theta Theta= angle between field lines and normal to plane of the area

Question 56

How to reduce magnetic flux

Answer 56

Reduce B Reduce A Rotate- as the angle changes as will the area perpendicular to the field

Question 57

Faraday’s law induced EMF

Answer 57

EMF = N dphi/ dt

Question 58

To increase the induced EMF

Answer 58

Increase N Increase v and increase l (which increases A) Increase B

Question 59

For a rod length l moving at constant speed v

Answer 59

EMF = BvL