3. Magnetic Fields and Electromagnetic induction

Question 1

B = magnetic flux density
Definition:

Answer 1

The strength of the magnetic field

Question 2

What quantity is B

Answer 2

A vector

Question 3

u⁰ définition

Answer 3

The permeability of free space
4(pi) x 10‐⁷

Question 4

Units of B

Answer 4

Teslas (T)

Question 5

What is needed to use the right hand rule?

Answer 5

The distance from the wire is perpendicular

Question 6

What represents the field INTO the page?

Answer 6

Cross

Question 7

What represents the field OUT of the page?

Answer 7

Circle

Question 8

How can you increase the B of a solenoid?

Answer 8

The number of coils
Add an iron core
Increase current

Question 9

B equation with long straight wire

Answer 9

B = u⁰ x I / 2(pi) x a

Question 10

B equation with a solenoid

Answer 10

B = u⁰ x n x I

Question 11

What does n represent?

Answer 11

The number of turns or coils per metre
Number of turns / Length

Question 12

Force on a current carrying wire

Answer 12

F = BIL sinø

Question 13

What are the conditions for sin ø

Answer 13

It is the angle between the magnetic field line

Question 14

If F = BIL and mv²/r then what can you figure out

Answer 14

Centripetal force
Omega
Frequency
Time period

Question 15

What rule is used for figuring out directions of current

Answer 15

Fleming left hand rule

Question 16

What does the 1st finger represent

Answer 16

Magnetic field

Question 17

What does the 2nd finger represent

Answer 17

Current

Question 18

What does the thumb represent

Answer 18

Force

Question 19

Force on moving charge equation

Answer 19

F = Bqv Sinø

Question 20

What are the three types of particle accelerators

Answer 20

Linear accelerator
Cyclotron
Synchrotron

Question 21

What current does a linear accelerator use

Answer 21

constant AC

Question 22

Why do the tubes get longer in a linear accelerator

Answer 22

The particle needs to spend the same time period in each tube as the speed increases
The frequency of AC is constant

Question 23

Where is the particle accelerated in a cyclotron or synchrotron

Answer 23

In between the gaps and Dees

Question 24

What is the difference between a cyclotron and a synchrotron

Answer 24

Synchrotron = circular (2 Dees) = constant radius= increasing field
Cyclotron = spiral = constant field= path with increasing radius

Question 25

Why do the protons move in a spiral

Answer 25

The electric field accelerates protons across the gaps
The force is towards the centre
As speed increases so does the radius due to mv²/r
The force is centripetal keeping it in circular motion

Question 26

how can the acceleration of a proton increase

Answer 26

increase the potential difference

Question 27

what direction do field lines travel in

Answer 27

north to south

Question 28

what conditions must there be for f=bvq

Answer 28

the magnetic field must be perpendicular to the velocity

Question 29

what are the situations where the magnetic field = 0`

Answer 29

when the angle is 0 to moving parallel to the field
when the particle is at rest / velocity = 0

Question 30

if a particle entered a uniform magnetic field and the field then increased what would happen to the path of the particles?

Answer 30

they would become more curved and the radius of both particles would decrease

Question 31

What is the hall effect

Answer 31

A semiconductor is placed within a magnetic field and a potential difference is created due to the forces on the moving charge carriers

Question 32

how to increase the hall effect

Answer 32

use a semi conductor to provide a better voltage
which decreases the uncertainty
also a semiconductor has a smaller n in I=nave so will decrease Vhall

Question 33

Why can devices use the hall effect to measure magnetic fields

Answer 33

Vhall is directly proportional to B for a constant current

Question 34

What is the cutting flux

Answer 34

when the field lines of a magnetic field are ‘cut’ by a metal

Question 35

what happens when a magnet falls through a metal pipe

Answer 35

the magnet slows down and so takes longer to travel through the tube

Question 36

what is lenzs law

Answer 36

the direction of any current resulting from an induced emf changes to the oppposite direction to the change in flux linkage

Question 37

what is faradays law

Answer 37

the emf induced is proportional to the rate of change in flux linkage
also proportional to the rate of flux cut

Question 38

What equations are used for a velocity selector?

Answer 38

F = eq = bvq
Rearrange to solve for a variable

Question 39

how do particles travel in a straight line in a velocity selector

Answer 39

the two forces have to be equal to each other

Question 40

Why does increasing the pd between plates increase the velocity

Answer 40

E = Bv
E = v / d
bigger pd means bigger electric field and bigger E means bigger velocity

Question 41

How to use a hall probe

Answer 41

Place the probe within the magnetic field at right angles

Question 42

How to derive the hall voltage

Answer 42

F= Eq
F= Bvq
Set them equal
E = V/d
Substitute that in and rearrage
Vhall = Bvd

Question 43

What is the relationship between B and Vhall

Answer 43

Directly proportional

Question 44

How do you induce an EMF

Answer 44

change the area over time
change the flux density over time

Question 45

What are the four factors that effect the instantaneous EMF

Answer 45

Flux density
Area of the coil
Angular velocity of the rotation
Position of the coil (eg angle it is in the field)