23 magnetic fields

Question 1

define magnetic field?

Answer 1

a region of space in which a moving charged particles are subject to a magnetic force.

Question 2

define magnetic field line?

Answer 2

the path which a north pole would take when placed in a magnetic field. field lines go from north to south

Question 3

how can you map field lines around a magnet?

Answer 3

you can place iron fillings on a piece of paper and then put a magnet on the paper and the fillings will align to the field.

Question 4

how do you represent the strength of a magnetic field on a diagram?

Answer 4

it is represented by how close together the field lines are- the closer they are the stronger the field ie the density of the field lines

Question 5

define magnetic flux density

Answer 5

the force per unit current per unit length on a current carrying conductor placed in a magnetic field perpendicular to the field lines ( magnetic flux per unit area

Question 6

what is the unit of magnetic flux density?

Answer 6

Tesla (T) = N x m^-1 x A^-1

Question 7

why does a compass point to the north pole of the earth?

Answer 7

the earths geographic north pole is actually the magnetic south pole so the north pole of the compass magnet ( the needle ) lines up with the earths field and points to the magnetic south (field lines go from north to south) which is what we call the geographic north

Question 8

how do you work out the shape of the field around a current carrying wire?

Answer 8

the right hand thumb rule. thumb is in the direction of the conventional current and the field goes around the wire in the direction of the fingers

Question 9

how do you work out the shape of the field around a solenoid?

Answer 9

where the current is going anticlockwise around the coil is the north pole. at the south pole, the current goes clockwise. the shape of the field is then similar to a bar magnet.

Question 10

define the motor effect?

Answer 10

when a current carrying conductor is placed within a magnetic field, it experiences a force perpendicular to the flow of the current and the field lines which pushes it out of the field

Question 11

how can you predict which direction the force will push the conductor?

Answer 11

using Flemings left hand rule: thumb is motion, first finger if field and the second finger is the current

Question 12

give the formula relating magnetic force, flux density, current, length and angle between the field and conductor?

Answer 12

F=BIL sin(x)

Question 13

describe an experiment to measure flux density/

Answer 13

1-place a horseshoe magnet on a digital balance and zero it 2- connect rigid piece of straight wire to DC supply, variable resistor and ammeter (series) 3- align the wire so the force on it acts upwards ( so there is downward force on magnet- newtons 3rd law) 4- measure the length of wire in the field 5- record the extra mass on the balance and use this to calculate force. 6- plot graph of current against mass- gradient gives BL/g. since L and g are both known, B can be calculated

Question 14

what is the formula for the magnetic force on a moving charge at 90 degrees to the field lines?

Answer 14

F=BQv where v is velocity , b is magnetic flux density, Q is charge of particles and F is magnetic force

Question 15

why do charged particles move in a circular orbit in a magnetic field?

Answer 15

force is always perpendicular to the velocity of the particle, so they end up being forced in a circular orbit. the particles undergo centripetal acceleration, with the centripetal force being the magnetic force

Question 16

how can you derive the formula for the radius of the circular orbit?

Answer 16

equating the formula for the centripetal force and the formula for magnetic force ( since they are the same thing in this context), you get mv^2/r = BQv , rearranged is r =mv/BQ

Question 17

using r= mv/BQ, explain how increasing the mass, velocity, flux density and charge affects the radius of the orbit

Answer 17

increasing mass or velocity will increase the radius. increasing flux density or charge will decrease the radius.

Question 18

what is the purpose of a velocity selector?

Answer 18

they isolate particles of a specific velocity. this is useful for things like mass spectrometry

Question 19

how does a velocity selector work?

Answer 19

there are electric plates above and below so the electric force acts upwards, and there is a magnetic field passing through sideways so the magnetic force acts downwards. in order for the particles to pass through undeflected, the electric and magnetic fields must be balanced so BQv=EQ which is rearranged to v=E/B. if velocity is too fast or too slow the particles will be deflected and not pass through

Question 20

define magnetic flux?

Answer 20

the product of magnetic flux density and the area perpendicular to the field lines.

Question 21

what is the unit for magnetic flux?

Answer 21

Weber (Wb) where 1Wb= 1 Tm^2

Question 22

what is the formula for magnetic flux?

Answer 22

magnetic flux = BA cos (x)`where B is magnetic flux density, A is area perpendicular to the field, x is the angle between the normal to the coil and the field lines

Question 23

define magnetic flux linkage?

Answer 23

the magnetic flux of an entire coil of wire. this is the product of the magnetic flux and the number of turns on the coil. flux linkage is also measured in Wb is represented by formula N x magnetic flux where n is number of coil

Question 24

state lenz law?

Answer 24

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

Question 25

explain lenzs law in terms of energy?

Answer 25

lenz law follows the principle of the conservation of energy. if the induced emf was in a direction that aided the change which caused it, it would be creating electrical energy from nowhere. for example, if the north pole of a bar magnet was pushed into a solenoid and that end became a south pole, it would then pull the magnet into the coil faster and the field would get stronger pulling the magnet in faster still

Question 26

state faradays law?

Answer 26

the induced emf in a circuit is proportional to the rate of change of flux linkage throughout the circuit

Question 27

what is the formula that links faradays law and lenz law

Answer 27

induced emf is equal to the -(change in flux linkage)/change in time

Question 28

what is a search coil?

Answer 28

a flat coil of insulated wire connected to a galvanometer (sensitive ammeter). it can be used to determine magnetic flux density from the current induced in the coil when it is withdrawn from a magnetic field

Question 29

how can you measure magnetic flux using a search coil?

Answer 29

1- place the coil in a magnetic field of known strength and pull it out again. since current max is proportional to B, you can calculate the constant of proportionality (therefore calibrating coil) 2- place the coil in the field that is to be measured and withdraw it. use the value for k and the current to calculate the flux density. 3- calculate magnetic flux using BAcos(x)

Question 30

what is the structure of a simple AC generator?

Answer 30

a rectangular coil which spins in a uniform magnetic field

Question 31

how does a simple AC generator work?

Answer 31

the flux linkage in the coil changes continuously, inducing an alternative current in the coil , flux linkage = B A N cos(x) and x and A are constantly changing when coil turns

Question 32

how do emf and flux linkage vary in a simple AC generator?

Answer 32

EMF is maximum when the coil is parallel to the field lines whereas flux linkage is maximum when the coil is 90 degrees to the field lines

Question 33

how can the peak emf of an AC generator be increased?

Answer 33

1) increase the speed of rotation 2) increase the magnetic flux density of the field 3) increase the cross sectional area of the coil 4) increase the number of turns on the coil

Question 34

what is the purpose of a transformer?

Answer 34

transformers change the peak value of an alternating PD to a different value. step-up transformers increase it, step-down transformers decrease it

Question 35

describe the structure of a simple transformer?

Answer 35

two coils - primary coil and secondary coil - wrapped around the two sides of a laminated iron ring. for a step up, there are more turns on the secondary coil. for a step down, there are more turns on the primary coil.

Question 36

how does a transformer work?

Answer 36

an alternating current is run through the primary coil, which induces an alternating magnetic field in the iron core. this, in turn, induces an alternating emf in the secondary coil

Question 37

give the formula that relates the number of turns with the potential difference of each coil?

Answer 37

Vs/Vp=Ns/Np where V is potential difference and N is number of turns

Question 38

for an ideal transformer( 100% efficent), give the formula relating potential difference and current in both coils?

Answer 38

if efficency = 100%, power in primary coil= power in secondary coil. which means Ip x Vp = Is x Vs . rearranged is Ip/Is=Vs/Vp

Question 39

what role do transformers play in the national grid?

Answer 39

step-up transformers are used to increase the voltage (and decrease current) before the electricity travels long distances. this is to reduce energy lost as less current means less energy emitted heat in the wires as the electricity passes through

Question 40

describe an experiment to investigate the relationship Vs/Vp=Ns/Np

Answer 40

vary the number of turns on each of the coils and measure the peak potential difference of each one every time. then use the values to prove the relationship between potential difference and number of turns. it is better to use an oscilloscope than a voltmeter for an alternating pd since it is easier to see the peak value

Question 41

how can you investigate the efficiency of a transformers?

Answer 41

measure the current in each coil with an ammeter and a variable resistor. the variable resistor is used to vary the current at a constant pd. use the formula Pout/Pin x 100 to calculate the efficiency of the transformer