magnetic fields

Question 1

where do field lines go in a magnetic field and whats another name for them?

Answer 1

they go from north to south
also called flux lines

Question 2

when will a magnetic field be induced around a wire?

Answer 2

when current flows in a wire or any other long straight conductor

Question 3

how can you work out the direction of a magnetic field around a current-carrying wire?

Answer 3

use the right hand rule, thumbs up

Question 4

what’s a solenoid?

Answer 4

a coil with length

Question 5

what’s Flemings left hand rule?

Answer 5

thumb= direction of force
first finger= magnetic field
second finger= current
all perpendicular

Question 6

how could you make a wire through a magnetic field vibrate?

Answer 6

pass an ac current through it

Question 7

magnetic flux density definition

Answer 7

the force on 1 metre of wire carrying a current of one amp at right angles to the magnetic field

Question 8

describe an experiment to investigate the effect of current on the force exerted on a current-carrying wire in a magnetic field (RP10)

Answer 8

1. set up a top pan balance with a square loop of wire fixed to it, so that it is standing up and that the top of the loop passes through a magnetic field, perpendicular to it
2. connect the wire in a circuit with a variable resistor, ammeter and dc power supply. zero the top balance when no current is flowing
3. vary the current using the variable resistor. at each current value, record the current and mass. repeat three times and average
4. convert into force using F=mg
5. plot a graph of force-current and draw line of best fit
6. gradient is B x length

Question 9

what happens to the radius of a charged particle moving in a magnetic field if the mass is increased?

Answer 9

increases

Question 10

what happens to the radius of a charged particle moving in a magnetic field if the velocity is increased?

Answer 10

increases

Question 11

what happens to the radius of a charged particle moving in a magnetic field if the magnetic flux density is increased?

Answer 11

decreases

Question 12

what happens to the radius of a charged particle moving in a magnetic field if the charge is increased?

Answer 12

decreases

Question 13

what are some uses of cyclotrons?

Answer 13

producing radioactive tracers
producing high-energy beams for radiotherapy

Question 14

describe the structure of a cyclotron

Answer 14

two hollow semi-circular electrodes with alternating pd
they have a slight gap between them
a uniform magnetic field is applied perpendicular to the plane of the electrodes

Question 15

how does a cyclotron work?

Answer 15

-charged particles are fired to one electrode
-the magnetic field causes it to flow a semi-circular path and return to the gap between electrodes
-the pd between them creates an electric field which accelerates them across the gap
-the velocity is now higher so the particle takes a path with a larger radius
-particle continues spiralling out until it exits

Question 16

what is magnetic flux density?

Answer 16

the measure of the strength of a magnetic field
B- symbol
T- units

Question 17

what happens when a conductor is moved in a magnetic field?

Answer 17

if it cuts through field lines, an emf is induced in the conductor
the electrons in the rod will experience a force which causes them to accumulate at one end of the rod- this induces an emf across the ends of the rod= electromagnetic induction

Question 18

what is electromagnetic induction?

Answer 18

when an emf is induced in a conductor that cuts through magnetic field lines

Question 19

how can you induce an emf in a flat coil or solenoid?

Answer 19

moving the coil towards or away from the poles of the magnet
moving a magnet towards or away from the coil

Question 20

how can you investigate flux linkage with a search coil? RP11

Answer 20

1. the stretched metal spring acts as a solenoid when connected to an alternating power supply
2. the alternating supply means the magnetic field of the solenoid is constantly changing- meaning the flux through the search coil is changing which can induce an emf
3. make sure the peak of the ac voltage from the ac power supply stays the same throughout the experiment
4. the search coil should have a known area and a set number of loops of fine wire. it’s connected to the oscilloscope to record induced emf in the coil
5. set up the oscillator so it only shows the amplitude of the emf as a vertical line (turn off base time)
6. a protractor is used to measure the orientation of the normal to the area of the search coil as an angle from the line of the magnetic field
7. position the search coil so its about halfway along the solenoid and within the inside of the solenoid but not touching it
8. then record induced emf in the search coil from the amplitude of the oscilloscope trace
9. rotate the search coil so its angle to the solenoid and the magnetic flux lines changes by 10 degrees
10. record the induced emf and repeat until rotated 90 degrees

Question 21

where’s the magnetic field of the solenoid the strongest?

Answer 21

inside the solenoid

Question 22

what should happen to the emf as the search coil is turned and why? RP11

Answer 22

as you turn the search coil, emf should decrease
because the search coil is cutting fewer flux lines as the component of the magnetic field perpendicular to the area of the coil gets lower so magnetic flux linkage experienced by the coil is lower
emf should be maximum at 0 degrees
and 0 at 90 degrees

Question 23

what is faraday’s law?

Answer 23

induced emf is directly proportional to the rate of change of flux linkage

Question 24

what does the symbol ø represent in magnetic fields?

Answer 24

magnetic flux (Wb)

Question 25

what is Lenz's law?

Answer 25

the induced emf is always in such a direction as to oppose the change that caused it

Question 26

what produces an induced current?

Answer 26

the energy used to pull a conductor through a magnetic field, against the resistance caused by the magnetic attraction

Question 27

what is flemings left hand rule?

Answer 27

thumb= force of resistance first finger= field direction second finger= induced emf direction

Question 28

how is emf induced in a rotating coil?

Answer 28

when a coil rotates uniformly in a magnetic field, the coil cuts the flux and an alternating emf is induced

Question 29

how can the shape of the graph of induced emf be altered?

Answer 29

changing speed of rotation or the size of the magnetic field

Question 30

how does increasing speed of rotation effect frequency and emf

Answer 30

increased frequency and increase the maximum emf

Question 31

how does increasing magnetic flux density effect frequency and emf

Answer 31

frequency stays the same, increase the maximum emf

Question 32

how do generators work?

Answer 32

they convert kinetic energy into electrical energy- they induce an electric current by rotating a coil in a magnetic field

Question 33

what is alternating current?

Answer 33

one that changes direction with time the voltage across a resistance gap goes up and down in a regular pattern

Question 34

how can you measure an alternating current with an oscilloscope?

Answer 34

-you can use an oscilloscope to display the voltage of an alternating current -the trace you see is made by an electron beam moving across the screen -the vertical height of the trace at any point shows the input voltage at that time -you can select how many volts per division you want -an alternating current source gives a regularly repeating sinusoidal waveform

Question 35

what are the three basic pieces of information you can get from an ac oscilloscope trace?

Answer 35

the time period the peak voltage the peak-to-peak voltage

Question 36

symbol for peak voltage

Answer 36

V zero

Question 37

how do you work out average power for an ac supply?

Answer 37

average power= I rms x V rms

Question 38

what is the UK mains electricity voltage?

Answer 38

230V

Question 39

what are transformers?

Answer 39

devices that make use of electromagnetic induction to change the size of the voltage for an alternating current

Question 40

what are some important uses of transformers?

Answer 40

transfer of electricity from power stations to the nation and in many electrical equipment

Question 41

how do transformers work?

Answer 41

-an alternating current flows in the primary coil causing the core to magnetise, demagnetise and remagnetise continuously in opposite directions -this produces a rapidly changing magnetic flux across the core so a magnetically soft material is needed eg. iron -the rapidly changing magnetic flux in the iron core passes through the secondary coil where it induces an alternating voltage of the same frequency

Question 42

what are step-up transformers?

Answer 42

increase the voltage by having more turns on the secondary coil then the primary

Question 43

what are step-down transformers?

Answer 43

reduce the voltage by having fewer turns on the secondary coil

Question 44

why is a transformer not 100% efficient?

Answer 44

1. the metallic core is being cut by the continuously changing flux 2. heat generated by resistance in coils 3. energy needed to magnetise and demagnetise the core is wasted as it heats the core

Question 45

how does the metallic core being continuously cut by changing flux cause energy losses? transformers

Answer 45

induces an emf in the core which causes currents called eddy currents. these cause it to heat up and energy to be lost

Question 46

what are eddy currents?

Answer 46

looping currents induced by the changing magnetic flux in the core they create a magnetic field that acts against the field that induced them, reducing the magnetic field also dissipate energy by generating heat

Question 47

how can effects of eddy currents be reduced?

Answer 47

by laminating the core- having layers of the core separated out by thin layers of insulator so a current can't flow

Question 48

how to reduce heat generated by resistance in the coils of transformers?

Answer 48

use wires with low resistance eg. thick copper wires

Question 49

how to reduce the energy needed to magnetise and demagnetise the core for transformers?

Answer 49

use a magnetically soft material that magnetises and demagnetises easily eg. iron

Question 50

how can we reduce magnetic losses in transformers?

Answer 50

a core design in which the coils are as close as possible can be used

Question 51

how do you work out the energy lost in a transformer?

Answer 51

E=Pt

Question 52

is the national grid high or low current and why?

Answer 52

lowest possible current to reduce energy losses due to less heating in the cables

Question 53

how can you investigate the relationship between the number of turns and the voltage across the coils of a transformer? what would you need to add to investigate the relationship between current and voltage for a given number of turns?

Answer 53

put two C-cores together and wrap wire around each to make the coils begin with 5 turns in primary and 10 in secondary coil turn on the ac supply to the primary coil. use a low voltage as transformers increase voltage record the voltage across each cell keeping primary voltage the same, repeat with different number of turns add a variable resistor to the primary coil and an ammeter to both circuits

Question 54

Define the tesla

Answer 54

The tesla is the (strength) of the magnetic field/flux density that produces a force of 1 newton in a wire of length 1m with 1 ampere (flowing perpendicular to the field).