Electromagnetism: Fields

Question 1

What is an electric field?

Answer 1

The region that surrounds electrically charged particles where a force is exerted on other electrically charged particles.

Question 2

What is electric field strength?

Answer 2

The electrical force acting on unit positive charge.

Question 3

Sketch an electric field pattern in a single positive point charge.

Answer 3

Hint: Charge = outwards

Question 4

Sketch an electric field pattern in a single negative point charge.

Answer 4

Hint: Charge = inwards

Question 5

Sketch an electric field pattern in two oppositely charged parallel plates

Answer 5

Hint: Parallel, charge towards negative.

Question 6

Sketch an electric field pattern in a positive and negative point charge.

Answer 6

Hint: side to side, energy towards negative.

Question 7

Sketch an electric field pattern in two identical point charges.

Answer 7

Hint: side to side, charges curving away from each other

Question 8

What is electrical potential at a point?

Answer 8

The work done in moving unit positive charge from infinity to that point.

Question 9

What does any charge given to a conductor always reside on?

Answer 9

The outer surface of the conductor

Question 10

What is a consequence of charge given to a conductor residing on the outer surface?

Answer 10

The electric field inside a conductor must be zero.

Question 11

What is electrostatic shielding?

Answer 11

When devices that might be affected by an electric field are enclosed in a conductive material.

Question 12

What is an example of where electrostatic shielding is used?

Answer 12

A Faraday Cage.

Question 13

Sketch a graph of the electric field around a charged sphere or radius r

Answer 13

Hint: straight diagonal line at 0, then vertical line upwards, then long curve down

Question 14

Sketch a graph of the electrostatic potential around a charged conducting sphere of radius r

Answer 14

Hint: Straight line across from top of y axis, then short curve downwards.

Question 15

Summary of Millikan’s Oil Drop Experiment

Answer 15

• Atomiser used to produce spray of tiny oil droplets
• Pulled downwards through a small hole by gravity
• Observing terminal velocity to calculate mass
• Droplets fell between two plates where electric field was applied
• Varied voltage in order to balance downwards gravitational force with upwards electrical force.

Question 16

What is the electronvolt (eV)?

Answer 16

A unit of energy, often used in high-energy particle physics.

Question 17

What is 1eV equal to?

Answer 17

1.6 x 10^-19 J (energy of an electron)

Question 18

What are ferromagnetic materials?

Answer 18

Ones which can become permanently magnetised.

Question 19

What are some examples of ferromagnetic materials?

Answer 19

Iron, steel, cobalt, nickel

Question 20

Magnetic field patterns in an isolated bar magnet

Answer 20

Travelling outwards from north, into south

Question 21

Magnetic field patterns in two opposite poles adjacent.

Answer 21

Travelling outwards from north, into south

Question 22

Magnetic field patterns in two like poles adjacent

Answer 22

Two south ends curving away from each other, north still travelling into south

Question 23

What does the relative spacing of the field lines in a magnet indicate?

Answer 23

The field’s strength. Closer lines = stronger magnetic field

Question 24

What does the direction of the magnetic field around a current carrying wire depend on?

Answer 24

The direction of current flow.

Question 25

Sketch a magnetic field around a solenoid

Answer 25

Hint: shape of magnetic field same as that created around a bar magnet.

Question 26

sketch a magnetic field around the earth

Answer 26

Hint: geographic north pole = magnetic south pole

Question 27

What is magnetic induction, B?

Answer 27

The strength of a magnetic field at a point.

Question 28

Right hand rule for a negative charge:

Answer 28

Thumb - direction of motion (force) First finger - direction of magnetic field (north to south) Second finger - direction of current flow (i.e electron flow)

Question 29

Summarise charge moving perpendicular to a Magnetic field

Answer 29

- force of magnetic field on charged particle is at right angle to motion. - Direction of motion is changed and leads to particle moving in circular path - Force always acts towards centre of curvature and causes centripetal acceleration

Question 30

Derive r = mv/qB

Answer 30

- F = mv^2/r, F = qvB - qvB = mv^2/r - qB = mv / r - r = mv/qB

Question 31

Summarise charge moving at an angle to a magnetic field

Answer 31

- Charge will follow helical path moving forward | - two components of the particle's velocity; perpendicular to magnetic field (vsin0) and parallel to field (vcos0)

Question 32

What is the job of the perpendicular component of the particle's velocity

Answer 32

Creating the circular motion

Question 33

What is the job of the parallel component of the particle's velocity

Answer 33

Creating the pitch in the helilx.

Question 34

What is the pitch of the helix in a magnetic field?

Answer 34

The distance between adjacent loops in the helix after one period.

Question 35

What equation is used to find the pitch in a helix?

Answer 35

p = vcos0 T

Question 36

What equation is used to find the frequency of the rotation in a helix?

Answer 36

f = qB / 2(pi)m

Question 37

What equation is used to find the period of rotation in a helix?

Answer 37

T = 2(pi)m / qB

Question 38

Summarise a bubble chamber

Answer 38

- Charged particle leaves track of tiny bubbles due to ionisation - Tracks spiral inwards due to loss of energy and velocity - particles with opposite charges (eg electron and positron) spiral in opposite directions.

Question 39

What is the force mediating particle of a strong nuclear force?

Answer 39

Gluon

Question 40

What is the force mediating particle of a weak nuclear force?

Answer 40

W(+-) & Z bosons

Question 41

What is the force mediating particle of an electromagnetic force?

Answer 41

Photon

Question 42

What is the force mediating particle of a gravitational force?

Answer 42

Graviton

Question 43

What is the equation used to calculate an absolute uncertainty

Answer 43

(sqrt) (uncertainty X)^2 + (uncertainty Y)^2 + (uncertainty X=Z)^2

Question 44

At what degrees dose maximum force occur in a current?

Answer 44

90 degrees (perpendicular to magnetic field)

Question 45

At what degrees does zero force occur in a current?

Answer 45

0 degrees (parallel with magnetic field)

Question 46

What is the equation used to find time taken in a semicircular path?

Answer 46

t = sv, s = (pi) r t = (pi) r / v = (pi) / v, mv/ qB :. t = (pi) m / qB