Magnetic Fields

Question 1

What is a Magnetic field?

Answer 1

a region where a force is exerted on a magnetic material

Question 2

What is the direction of the magnetic field?

Answer 2

North to South

Question 3

What do the field lines look like on a current carrying wire?

Answer 3

the field lines are concentric circles centred around the wire

Question 4

How can we find the direction of the magnetic field for a current carrying wire?

Answer 4

right hand rule

Question 5

What is a solenoid?

Answer 5

consists of a wire coiled up into a spiral shape

Question 6

What happens when you place a current carrying wire in a magnetic field?

Answer 6

the field around the wire and the field from the magnets interact. the field lines from the magnet contract to form a “stretched catapult” effect where the flux lines are closer together. i.e causes a force on the wire

Question 7

what happens if the current is parallel to the magnetic field lines?
- what condition must be kept?

Answer 7

no force acts
- the current has to be perpendicular

Question 8

How do you find the direction of the force?
- what does each finger represent?

Answer 8

• Flemings Left Hand rule
• thumb: force
  first finger: magnetic field
  middle finger: current

Question 9

What is the equation for the Force on a current-carrying wire in a magnetic field?

Answer 9

F= BIL

Question 10

What is the magnetic flux density?
Units?

Answer 10

• the force per unit length per unit current on a current carrying conductor at right angles to the magnetic field lines.
• Tesla, T

Question 11

what is magnetic flux density a measure of?

Answer 11

the number of field lines passing through the region of space per unit cross-sectional area

Question 12

What is the equation for the Force on charged particles moving in a magnetic field?

Answer 12

F= Bqv

Question 13

How are charged particles deflected in a magnetic field?

Answer 13

charged particles are deflected in a circular path

Question 14

what is an application for the circular deflection of charges particles in a magnetic field?

Answer 14

a cyclotron, used for radiotherapy

Question 15

What is the function of a cyclotron?
what do they produce?

Answer 15

uses a magnetic field to keep charged particles moving in a circle and electric field to accelerate
-produces X-rays

Question 16

what is the equation for the radius of curvature of a charged particle?

Answer 16

r= mv/BQ

Question 17

How does a cyclotron work? D

Answer 17

There are two Dees, which are electric plates that cause charged particles to move in a circle and increase in speed between the plates (due to the magnetic force) This increases their radial displacement. the polarity of the plates are reversed to produce a spiral motion.

Question 18

what is the equation for the time period of a particle in an orbit in a cyclotron?

Answer 18

t= 2πm/BQ

Question 19

what is a velocity selector?

Answer 19

a device that uses perpendicular electric and magnetic fields to select charged particles with a specific velocity

Question 20

What feature does the ion need to have to go through a velocity selector?

Answer 20

equal top charge

Question 21

what is magnetic flux?

Answer 21

the magnetic field lines passing through a given area

Question 22

what is magnetic flux linkage?

Answer 22

the product of the magnetic flux and the number of turns on a coil through which the field passes

Question 23

what is the equation for magnetic flux?

Answer 23

φ= BAcosθ

Question 24

what is the equation for magnetic flux linkage?

Answer 24

Nφ= BANcosθ

Question 25

what is the equation for Faraday's law?

Answer 25

emf= N Δφ/Δt

Question 26

What is Faraday's law?

Answer 26

the magnitude of induced emf is equal to the rate of change of flux linkage

Question 27

What is Lenz's law?

Answer 27

the direction of induced current is such as to oppose the motion causing it

Question 28

what is the equation for Lenz's law?

Answer 28

emf= - (N Δφ/Δt)

Question 29

explain Lenz's law with the magnet approaching a coil analogy

Answer 29

1. As the magnet approaches the coil, there is a change of flux through the coil so an emf and a current is induced. 2. The direction of induced current is opposite to the motion of the magnet. (with repulsion) 3. As the magnet passes through the coil there is no change in flux 4. As the magnet leave the coil, there is a change in flux, so current is induced which opposes the motion of the magnet (with attraction)

Question 30

what is the equation for maximum emf?

Answer 30

ε= ωBAN

Question 31

what is the equation for minimum emf?

Answer 31

ε= -ωBAN

Question 32

What is the equation for emf?

Answer 32

ε= ωBANsin(ωt)

Question 33

what is an alternating current?

Answer 33

current that changes direction

Question 34

what is an oscilloscope?

Answer 34

device which shows the variation of voltage with time

Question 35

-what would an oscilloscope show for a direct current? -what about when the time base is switched off?

Answer 35

-trace will show a straight line parallel to the axis -only a dot will be seen on the screen

Question 36

-what would an oscilloscope show for an alternating current? -what about when the time base is switched off?

Answer 36

-a sinusoidal waveform -a straight vertical line

Question 37

what is peak voltage on an oscilloscope?

Answer 37

distance from the equilibrium to the highest point

Question 38

what is peak to peak voltage on an oscilloscope?

Answer 38

distance from the minimum point to the maximum point

Question 39

what is the root mean square (rms) voltage on an oscilloscope?

Answer 39

the average of all the squares of the possible voltages

Question 40

What is the equation for rms current and rms voltage?

Answer 40

Iᵣₘₛ= I₀/√2 Vᵣₘₛ=V₀/√2

Question 41

what is the electricity supplies in homes in the UK in volts?

Answer 41

230Vᵣₘₛ

Question 42

what is the equation for power with peak current?

Answer 42

P=1/2 x I₀²R

Question 43

how does a transformer work?

Answer 43

1. an alternating current flows in the primary coil 2. this causes an alternating magnetic field in the soft iron core which means the flux linkage in the secondary coil is constantly changing therefore and alternating emf is induced in the secondary coil

Question 44

-what is an ideal transformer? -what is the equation for an ideal transformer?

Answer 44

-all the flux linkage produced by the primary coil passes through the secondary coil I₁V₁=I₂V₂

Question 45

what is the transformer equation?

Answer 45

V₂/V₁=N₂/N₁

Question 46

what is the efficiency equation for a transformer?

Answer 46

e(I₁V₁)=I₂V₂

Question 47

what is a step up transformer?

Answer 47

increases the a.c/p.d as the secondary has more turns

Question 48

what is a step down transformer?

Answer 48

decreases a.c/p.d as the secondary has less turns

Question 49

what are eddy currents?

Answer 49

when there is a continuous change in flux, an emf is induced, causing currents known as eddy currents to occur

Question 50

what the main causes of energy loss in a transformer and their solutions?

Answer 50

-eddy currents (laminated core) -resistance in the coils (solved by using thick wire) -if the core is not easily magnetised (magnetically soft iron core is then used)

Question 51

how can the effect of eddy currents be reduced?

Answer 51

-using a laminated iron core, meaning that the core is made using layers of iron between layers of an insulator so amplitude is reduced -core made of a high resistivity metal