Electromagnetism (DONE)

Question 1

What is a magnetic force and what symbol does the field have?

Answer 1

• A magnetic field is a region where objects experience a magnetic force.
• We call the magnetic field the B field.

Question 2

How does a permanent bar magnet work?

Answer 2

• In a permanent magnet we have a north and south pole.
• If you have a magnet and cut it in half you get 2 smaller magnets meaning you can never separate the north from the south pole.

Question 3

How can you show the field lines of a permanent bar magnet?

Answer 3

• You can show the field lines of the magnetic field of a magnet using iron filings.
• The field lines show the direction that the north end of a magnet would move.

Question 4

How does neodymium work as a magnet?

Answer 4

• The neodymium has a strong magnetic force but if you for example lit a flame and placed it over, the magnetic force would weaken.

Question 5

How do 2 permanent bar magnets interact with each other?

Answer 5

• in a normal permanent magnet the magnetic field lines will move from the North Pole to the South Pole.
• If we had 2 bar magnets pointing at each other with opposite poles facing, we would find we get field lines moving directly from the north to the south pole creating uniform field.
• However if you have the same poles facing each other the field lines will move from the north pole round to the south on the same magnet.
• And there is a region where there is no magnetic field as the poles repel.

Question 6

How can we create a magnet using a wire and current?

Answer 6

• We can make a magnet when we have a piece of wire or something carrying a current.
• Provided you have things with charge moving inside, we then have a magnetic field.

Question 7

How can we investigate the magnetic field around a current carrying conductor?

Answer 7

• We can use a piece of equipment which consists of a wire connected to a Perspex.
• When the power supply is switched on we have electric current flowing through the wire but we cant see the magnetic field.
• In order to look at the magnetic field you can use a compass.
• Without the wire, the compass will always line up with the field lines on earth which go from north to south.
• As you switch on the current and move the compass it will point in different directions.
• This is because it is following the field lines from the wire.

Question 8

How can you draw the field lines around a single point conductor?

Answer 8

• When drawing the field lines around a single point conductor, the field lines will show the direction where a positive charge would move which is anti clockwise.
• when drawing the field lines they are concentric circles that get further and further apart, the density of the field lines shows the strength of the field.
• You can remember the direction of magnetic field around a conductor by using the right hand rule.

Question 9

How can the right hand rule be used to determine the direction of magnetic field lines around a conductor?

Answer 9

• If you do a thumbs up with your right hand, the thumb shows the direction of the current and the curl of your hand shows the direction of the magnetic field.
• If the current is going towards you then the direction is anti clockwise, if the current is going away from you the direction is clockwise.
• The symbol inside the wire on the diagram will tell you which way the current is going, if you imagine an arrow if it is heading away from you, you will see a cross ‘X’ and if it is heading towards you, you will see a single point ‘.’

Question 10

How will 2 magnetic fields around a single point conductor interact?

Answer 10

• When 2 fields are close together the field lines will get closer and closer but will not touch.

Question 11

What are the field lines like around a solenoid?

Answer 11

• If you have a coil of wire (solenoid) in the same setup, when you look at the field lines around an object like this from above, every time the wire comes out of the Perspex the current is travelling towards you, then when it enters the Perspex the current is travelling away from you.
• When you turn the current on you have a strong field which goes from one end out to the other, if you push the compass through the middle of the coil you have a uniform field.
• In a diagram showing the field lines you would have many points where the current goes towards you and away from you.
• If you apply the right hand rule you will draw the field lines around all of the currents heading towards and then all of the current heading away, leaving 2 symmetrical sets of field lines close to each other but not touching.
• This arrangement is similar to that of a bar magnet where the field lines flow from north to south.

Question 12

When do we use flemings left hand rule?

Answer 12

• You have a U shaped magnet with a north and south end and you have a uniform field between the 2.
• If you have a piece of copper and pass a current through it using crocodile clips we can hold this between the poles of the magnets.
• When the current is turned on the wire will either kick up or down depending on what way the current is flowing.
• We have a force on the conductor causing it to move.
• If you have a region with a field that goes from north to south and you have a wire where the current is moving towards you, the direction of the force will be upwards.
• If you had a current going away from you the direction of the force will be in the opposite direction, downwards.
• In order to remember this we use Flemings left hand rule.

Question 13

What is flemings left hand rule?

Answer 13

• Using your left hand and your thumb and first 2 fingers, firstly we have your first finger which counts for the magnetic field which moves from north to south.
• The second finger counts for the current and it is a conventional current.
• Your third finger is your thumb which is at a right angle to the magnetic field and conventional current, the thumb represents the motion of the piece of wire.

Question 14

When a current is passed through a wire in a magnetic field, what does the force experienced by the wire depend on?

Answer 14

• firstly the force is directly proportional to each of the strength of the magnetic field, current through the wire and length of the wire in the field.
• the force is also proportional to sin(theta),
  if the wire is in line with the magnetic field lines this means the current is in the same direction and therefore the force is zero.
• As you rotate the wire through and angle of theta we find that as theta gets to 90 degrees it is when the wire is 90 degrees to the field lines and has the largest force.

Question 15

What equation can be created for the force experienced by a current carrying conductor in a magnetic field?

Answer 15

• We can combine all of the proportional relationships together to say that force is proportional to BILsin(theta).
• Where B = magnetic field strength, I = current, L = length of wire and sin(theta) = angle of wire.
• Provided we measure everything in SI units this gives units for magnetic flux density/magnetic field strength which is B in teslas:
  F = BILsin(theta)

Question 16

What is a Tesla?

Answer 16

• magnetic flux density is 1 tesla when a wire perpendicular to the field with a current of 1 amp experiences a force of 1 newton per unit length.
• This a way we often define lots of units, if we think of the equation B = F/(IL) which has sin(theta) left out as we don’t need it if the wire is perpendicular.
• 1 newton, 1 amp and 1 metre in this equation gives us a tesla.
• We can also say that because F = BIL (when wire is perpendicular to field).

Question 17

What force is experienced by a moving charged particle in a magnetic field?

Answer 17

• We can represent a magnetic field moving towards you by using a series of dot.
• We can look at the motion of a positive unit charge in a magnetic field.
• When the charge enters the field, moving from left to right, if you use Flemings left hand rule the particle will experience a force downwards 90 degrees to the direction it is moving in.
• Therefore this particle starts to move downwards until it reaches a point.
• At this point the magnetic field is still in the same direction however the conventional current is now moving diagonally downwards to the right.
• Therefore the force will be acting perpendicular to the movement which causes the particle to move along a circular parabolic path.
• If you look at the motion over time you can see that the particle follows a curved parabolic path which is different to how particles move in an electric field.

Question 18

How can you derive equations for the force a moving charged particle experiences in a magnetic field?

Answer 18

• Using the equations I = Q/t and L = vt we can substitute this into F = BIL to get F = B(Q/t)vt
• You can simplify to get the force on a charged particle within a magnetic field, F = BQv
• There is a special case of this equation where something has an elementary charge, e.
• When it comes to looking at the motion of electrons and protons we can use a different formula which is F = Bev

Question 19

How can you derive an equation for the radius which a charged particle will move around in a magnetic field?

Answer 19

• Considering the force that acts upon a charged particle at velocity v, with charge Q, Flemings left hand rule tells us that the direction of the force is downwards, and we know that the force F = BQv
• We also know that this force acts 90 degrees to the velocity of the particle and therefore this is the centripetal force which moves with circular motion.
• The centripetal force F = (mv^2)/r can be equated to the equation F = BQv.
• BQv = (mv^2)/r
• If you rearrange to make r the subject r = mv/BQ
• This tells us about the radius the charged particle will move in a magnetic field.
• If you look at an electron in a magnetic field, because it has a smaller mass must also have a smaller radius and it will curve upwards rather than downwards like a proton.

Question 20

What motion do moving charged particles follow in E and B fields (separately and together)?

Answer 20

• If there was just a magnetic field and we placed a positively charged particle in it, we could use Flemings left hand rule to show the curve that the particle takes.
• However if we only had an electric field the particle would repel the positive plate and attract the negative.
• if we combine the fields and have a magnetic field moving towards you and a uniform electric field created by 2 charged plates, the particle within the field will experience 2 forces.
  some particles can have an equal force up and down and these particles will move through the field and out the other side in a straight line.
• If we know that for a particle there is a force B and force E, and the particle moves through the field in a straight line we can equate the 2 equations for force.
• This means BQv = EQ, through simplifying we can say Bv = E or V = E/B
• This equation can be used for a device called a velocity selector.
• It is a filter which means only particles with a certain velocity will pass through, if they are too fast or slow they will move up/down as the E or B values are not equal enough.
• Therefore we know that all the particles that come out of the other side must have the same velocity which is important when we look at particle accelerators and mass spectrometers where we want to control the speeds of particles.

Question 21

What is magnetic flux density?

Answer 21

• The magnetic flux density has symbol B and it tells us the strength of the magnetic field.
• We measure this in the unit Tesla T.

Question 22

What is magnetic flux?

Answer 22

• the magnetic flux is the total number of field lines passing through a surface.
• it has the greek symbol phi
• The magnetic flux = BA.
• Therefore the stronger the field and the bigger the area, the larger the magnetic flux.
• The unit for magnetic flux is Wb which stands for the unit weber.

Question 23

What is magnetic flux linkage?

Answer 23

• Magnetic flux linkage is the magnetic flux running through a coil and is equal to the magnetic flux x N.
• where N is the number of turns on a coil.
• If we look at the total number of field lines moving through a piece of wire it is going to depend on the angle of the wire therefore when it is at 90 degrees which is the min point, the magnetic flux will be zero.
• Therefore the equation for the magnetic flux linkage = (BAcostheta)N
  Or
  Flux linkage = N(phi)

Question 24

What are the differences and similarities between a motor and a generator?

Answer 24

• The difference between a motor and a generator is the direction of energy transfer.
• If you had a motor you would put in electrical energy and create kinetic energy, for generators you put in kinetic energy and get out electrical energy.
• The structure however is the same, you have a magnetic field with a coil of wire rotating in the field.

Question 25

What do AC generators consist of?

Answer 25

• If you take apart the AC generator you will have a permanent magnet with north and south ends creating a magnetic field.
• There is then a rotating coil of wire with many turns in the magnetic field.
• In an AC generator at the end of the coil of wire we have 2 slip rings.
• The first slip ring is attached to one end of the coil and the second slip ring is attached to the other end of the wire.
• At the other end of the wire we have the split ring but this is not used for an AC generator.
• The slip rings will be connected to a circuit including a galvanometer which measures the induced emf in the coil.

Question 26

How does the AC generator create electrical energy?

Answer 26

• The coil of wire along with the slip rings are rotated in the magnetic field between north and south poles.
• This causes a pd across the ends of the wire and when connected to a full circuit it will have an induced emf which then creates a current in the circuit.
• Because the coil rotates within the magnetic field the direction of the induced current changes every half cycle.
• This means that electrical energy is created in the form of an alternating emf which is measured by a galvanometer.

Question 27

How does emf change as the coil rotates in the magnetic field of an AC generator?

Answer 27

• The emf is directly proportional to the rate of change of magnetic flux linkage.
• Therefore we can look at the number of field lines being cut (linkage) by the rectangular coil in order to see how emf changes.
• We have the greatest linkage when the rectangular coil is vertical and perpendicular to the field lines however at this point there is no change in flux linkage meaning the induced emf = 0.
• When the coil is rotated 90 degrees to a horizontal position in line with the field lines the linkage is zero however at this point the rate of change of linkage is maximum meaning induced emf = max.
• When the coil is rotated 180 degrees from starting position the linkage is max but the change in linkage is zero meaning induced emf = 0.
• When the coil is rotated 270 degrees the linkage is zero but the rate of change of linkage is the maximum negative value due to the change in direction of the current (right hand rule) which causes the induced emf to become the maximum value in the negative direction on the galvanometer.
• The change in flux and change in emf can be plotted on a graph against time.

Question 28

How does the graph of magnetic flux against time compare to emf against time for an AC generator?

Answer 28

• As the coil rotates in the magnetic field we will get a sin shaped graph for both the magnetic flux and emf.
• With the starting point of the coil being vertical and perpendicular to the field lines the magnetic flux will be max.
• As the coil rotates through 90 degrees the flux will decrease towards zero and by 180 degrees the flux will be maximum in the negative direction.
• At 270 degrees the flux will decrease in the negative direction (essentially increasing towards positive on the graph) and at 360 degrees the line will be at its original point but 1 time period later.
• The change in emf graph will also be sin shaped however at points of max flux there will be zero emf and at points of min flux there will be max emf induced.
• This is because the rate of change of magnetic flux linkage at max points is zero and maximum at min points.
• The rate of change of magnetic flux linkage can be found by picking 2 points along one of the peaks of the flux graph, the gradient across these 2 points is zero whereas if you picked 2 points on the linear section of the graph you would find the gradient would be max.
• It is essentially shifted to the right by 90 degrees of pie/2 radians.

Question 29

How can the emf induced in a coil in an AC generator be calculated?

Answer 29

• In order to calculate the emf induced in the coil we can look at the rate of change of flux linkage in the coil.
• To find the flux linkage we need to multiply the flux value by the number of turns in the coil, N.
• This means the equation for the emf induced E = - [(change in phi)/(change in time)] N.

Question 30

What can be done to induce a greater emf in an AC generator?

Answer 30

• To induce a greater emf we need to remember that the flux = BA so if we were to increase field strength or increase the area of the coil then emf would increase.
• Also the greater number of turns in the coil and the speed of rotation of the coil will increase emf.
• If the speed of rotation is greater then the time period for 1 rotation will be smaller so the flux cut will be greater per second.

Question 31

What does a transformer consist of?

Answer 31

• The first thing you need for a transformer is a primary coil of wire (solenoid).
• We then add a secondary coil, this coil for example could have a greater number of turns.
• The primary coil needs to be supplied with a current and the secondary coil needs to be connected to a circuit to allow a current to be induced.
• The coils are placed in magnetically soft iron cores which are connected in a ‘U’ shaped structure.
• The transformer is completed with another piece of iron which connects the top of the cores together providing a complete channel for the magnetic field lines.
• Inside this iron piece we have alternating laminations of iron and insulator to make the transformer more effective.

Question 32

How does a transformer allow an emf to be induced across the secondary coil?

Answer 32

• Inside the transformer you have a primary coil and a secondary coil both placed in a magnetically soft ‘U’ shaped iron core which is completed with another iron piece which connects the top of the cores .
• The primary coil is supplied with a current which creates a magnetic field, the north end of the magnetic field can be found using the right hand rule.
• However because it is an alternating current the direction of the current is changing and therefore the north end of the magnetic field will oscillate from one end to another.
• The iron core gives the magnetic field lines a channel to move from the primary coil to the secondary coil.
• The changing magnetic field lines passing through the secondary coil cause an emf to be induced through electromagnetic induction.
• The emf will be induced across the ends of the secondary coil and if this is connected to a circuit a current will be induced.

Question 33

Why are the primary and secondary coils placed in iron cores in a transformer?

Answer 33

• If you place the primary and secondary coils close to each other in air, the magnetic field produced by the alternating current in the primary coil will permeate through the air and some of the magnetic field lines will pass through the secondary coil and create an emf across the secondary coil.
• We can therefore wirelessly transfer electricity however this is not very effective as air is not a good conductor of magnetic field lines.
• This is where we can use an iron core which is magnetically soft (can be magnetised/demagnetised very quickly) meaning magnetic field lines will pass through it easily.
• If you put the 2 coils into the iron cores which keeps the coils close to each other, it gives the magnetic field lines a channel to move from the primary coil to the secondary coil.
• This can be completed with another piece of iron which connects the top of the cores together providing a complete channel for the magnetic field lines.
• It allows any magnetic field lines transmitted by the primary coil to be transmitted through the centre of the secondary coil.

Question 34

Why is the iron piece which completes the iron core in a transformer laminated?

Answer 34

• Inside the iron core we have alternating laminations of iron and insulator.
• This is because eddy currents will be induced in the iron core by the varying magnetic field.
• This isn’t useful as we don’t want current through the iron core we want it through the coils only.
• If you don’t have the laminations you will have high currents in the core which is not good as the high current will cause a loss of energy through heat making the transformer less effective.

Question 35

What symbols do we need to know for the transformer equations?

Answer 35

Vp = pd across primary coil.
Vs = pd across secondary coil.
Ip = current in primary coil.
Is = current in secondary coil.
np = number of turns in primary coil.
ns = number of turns in secondary coil.

Question 36

What do we have to consider when using transformer equations?

Answer 36

• When looking at transformer equations we need to consider the transformer to be 100% efficient meaning there is no energy loss in the coil or iron core.
• This is unrealistic as there will usually be a buzzing noise causing a loss in energy through sound.

Question 37

What are the 2 different types of transformers and what impact do they have on voltage?

Answer 37

• There are 2 different types of transformers, step up and step down.
• Step up transformers will step up the voltage meaning Vs will be larger than Vp.
• Step down transformers will step down the voltage so Vp will be larger than Vs.

Question 38

How can we tell whether a transformer is step up or step down using calculations?

Answer 38

• We can tell whether transformers are step up or step down by looking at the ratio of the number of turns of wire in the primary and secondary coils.
• We can say that np/ns = Vp/Vs.
• This means that if np is greater than ns then Vp will be greater than Vs.
• Therefore we have a step down transformer.
• If np is smaller than ns then Vp is smaller than Vs and therefore we have a step up transformer.

Question 39

How can we derive an equation for the power in and power out in a transformer?

Answer 39

• Provided that the transformer is 100% efficient the power in = power out.
• Electrical power P = IV, so we know that power in = IpVp and power out = IsVs.
• Therefore IpVp = IsVs.
• We can then say that Is/Ip = Vp/Vs = np/ns.

Question 40

How can we calculate what happens to the current in step up and step down transformers?

Answer 40

• We need to use the equation IpVp = IsVs.
• This shows if we had a current and pd in a step up transformer then Vs will be larger meaning Is has to be smaller.
• For a step down transformer the value of Vs will be smaller and therefore the value of Is will be larger.

Question 41

Why are transformers useful in the national grid and what equation proves this?

Answer 41

• Transformers are useful in the national grid as they prevent power loss.
• We can look at the equation P = I^2*R where P is the power lost in order to prove this.
• If you can use a step up transformer to increase the voltage it will in turn reduce the current.
• This is important because if you reduce the current by a factor of 1000, this means the power losses are going to be smaller by a factor of 1000^2.
• This is why we use very high voltages in the national grid because we want a very small current so that there is less of a heating effect and a smaller power loss.