Magnetic Fields

In alot more detail, what exactly happens to the ammeter reading AND why, as well as the acceleration.

1. As the Magnet Approaches and Begins to Enter the Coil

• Initially, no magnetic flux is being cut since the magnet is far from the coil.

• As the magnet enters, more magnetic field lines begin to cut across the coil.

• At this moment, the rate of change of magnetic flux is at its maximum (since it has just gone from zero to some nonzero value).

• This induces an emf, which produces eddy currents in the coil.

• According to Lenz’s Law, the eddy currents produce their own magnetic field, opposing the motion of the magnet.

• As a result, the magnet’s acceleration decreases (but it still continues to fall).

• The ammeter needle deflects to the right as the induced current reaches a maximum value in one direction.

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2. At the Exact Center of the Coil

• At this point, the magnet is fully inside the coil, and the flux linkage (total flux passing through the coil) is at its maximum.

• However, because the amount of flux being cut remains constant for a very brief moment, the rate of change of flux becomes zero (dΦ/dt = 0).

• Since emf is proportional to dΦ/dt, this means that for this short moment, no emf is induced, and thus no current is induced.

• The ammeter needle moves back to zero, since there is no current at this instant.

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3. As the Magnet Moves Past the Center and Exits the Coil

• After passing the center, the rate of change of flux is no longer zero, but now it decreases in the opposite direction (becomes negative).

• Since emf = - dΦ/dt, and dΦ/dt is now negative, the induced emf switches direction.

• This means the induced current also reverses direction.

• The ammeter needle now deflects to the left as the current reaches its maximum value again, but in the opposite direction.

• The eddy currents also reverse, producing a magnetic field in the opposite direction compared to before.

• Instead of repelling the magnet’s north pole (as before), the new magnetic field attracts the north pole, continuing to oppose the magnet’s motion.

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4. When the Magnet Fully Leaves the Coil

• As the magnet moves further away, fewer field lines are cut by the coil.

• The rate of change of flux decreases, reducing the induced emf and current.

• Eventually, when the magnet is far enough away, no more flux is cut, and dΦ/dt returns to zero.

• This causes the ammeter needle to return to zero, as no current is flowing anymore.

Figure 1 is a PLAN VIEW (SO above). Figure 2 is a Side view! Using FLHR, can see that B acting INTO PAGE on figure 1. Now do a 90 degree rotation to figure 2, and it is now vertically downwards!

The deflection of the beam due to the electric field can be cancelled by applying a suitable uniform magnetic field in the same region as the electric field. (i) What direction should the magnetic field be in to do this?

MAGNETIC FIELD NOT FORCE! Electric force acting downwards, so magnetic force acts upwards. Since v to the right, and F up, using FLHR acts into the page

How can you improve efficiencies in a transformer (In great detail, 4 different ways)

When a transformer is in operation, there are ac currents in the primary and secondary coils. The coils have some resistance and the currents cause heating of the coils, causing some energy to be lost. This loss may be reduced by using low resistance wire for the coils. This is most important for the high current winding (the secondary coil of a step-down transformer). Thick copper wire is used for this winding, because thick wire of low resistivity has a low resistance.

2 The ac current in the primary coil magnetises, demagnetises and re-magnetises the core continuously in opposite directions. Energy is required both to magnetise and to demagnetise the core and this energy is wasted because it simply heats the core. The energy wasted may be reduced by choosing a material for the core which is easily magnetised and demagnetised, ie a magnetically soft material such as iron, or a special alloy, rather than steel.

3 The magnetic flux passing through the core is changing continuously. The metallic core is being cut by this flux and the continuous change of flux induces emfs in the core. In a continuous core these induced emfs cause currents known as eddy currents, which heat the core and cause energy to be wasted. The eddy current effect may be reduced by laminating the core instead of having a continuous solid core; the laminations are separated by very thin layers of insulator. Currents cannot flow in a conductor which is discontinuous (or which has a very high resistance).

4 If a transformer is to be efficient, as much as possible of the magnetic flux created by the primary current must pass through the secondary coil. This will not happen if these coils are widely separated from each other on the core. Magnetic losses may be reduced by adopting a design which has the two coils close together, eg by better core design, such as winding them on top of each other around the same part of a common core which also surrounds them.

Equations for working out the power loss in the transformers and the cables?

Cables: P= I²R (since we dont know the voltage across them)

Transformers P = VI

Out of page!

BA = Magnetic flux in ENTIRE COIL

B

Remember, the way the force acts is the opposite to force felt on balance. If pointing up, will feel positive force, if down, feel negative force (N3L) Also, F = BIL

A, electric fields accelerate charged particles in straight lines, not circular!

What 2 things do eddy currents do in a core?

• Their magnetic field acts against the field that induced it, reducing the fields strength. Since the primary coil's flux is partially cancelled out, less magnetic flux reaches the secondary coil. This leads to inefficiencies and energy losses by generating heat.

State Lenz’s law (2 marks)

• Direction of induced EMF

• Will act to oppose the change (in magnetic flux) that produced it.

With a coil of wire and a conducting tube, if both are not connected to a circuit, which one(s) will allow eddy currents to be produced and why?

Eddy currents are loops of electrical current induced in a conductor when exposed to a changing magnetic flux. The key requirement for eddy currents to form is that the material must be a continuous conductor where free electrons can move in loops.

A coil of wire is made of individual loops of insulated wire. Although the wire itself is a conductor, the insulation between the loops prevents free electron movement across the coil.

• Since eddy currents require a continuous conductive path, the breaks in conductivity between turns of the coil prevent currents from circulating freely within the material.

• However, if the ends of the coil were connected to form a complete loop, then a current would be induced, but it would no longer be classified as an eddy current—it would be a normal induced current in a circuit.

Why can’t RADIAL magnetic fields occur?

Radial magnetic fields do not exist because magnetic fields are generated by moving charges. A stationary point charge produces a radial electric field but does not produce a magnetic field. When a point charge is in motion, it creates a circular magnetic field around its path, not a radial one. The nature of magnetic fields means they always form loops or concentric circles around moving charges, rather than radiating outward from a point like electric fields do. (Also F = BQV, where if V is 0, then F is also 0)

F = BQv = BIL may be used to calculate magnetic forces. What condition is needed for these equations to apply?.

BQv when magnetic field is perpendicular to velocity

BIL when magnetic field is perpendicular to current.

Into page, since the electric field is exerting an electric force downwards on the ion (E = F/Q) the magnetic field needs to produce a force vertically upwards. Since ion travelling right, using FLRH you can see the field will be into the page.

An alternating current is passed through a wire which is in between 2 magnets. Explain why the wire will vibrate vertically. (3 marks)

force F is perpendicular to both B and I [or equivalent correct explanation using Fleming LHR] (1)

magnitude of F changes as size of current changes (1)

force acts in opposite direction when current reverses [or ac gives alternating force] (1)

continual reversal of ac means process is repeated (1)

Force points towards centre due to circular motion

Velocity of electron is OPPOSITE to what second finger in FLHR does (since its conventional current)

Therefore (-z)

Explain why the electron is accelerating even though it is travelling at constant speed in a circle. (2 marks)

• Acceleration is the rate of change of velocity and velocity has both magnitude (speed) and direction.

• The direction of the electron is constantly changing, even though its speed remains constant, the velocity is changing so acceleration is occurring.

Outline the essential features of a step-down transformer when in operation. (2 marks)

primary coil with more turns than secondary coil (1) (wound around) a core or input is ac (1)

Describe two causes of the energy losses in a transformer and discuss how these energy losses may be reduced by suitable design and choice of materials. (6 marks)

TRANSFORMER NOT CABLES!

Thumb is force remember!!! First finger points from north to south, Second from P to Q so therefore downwards.

How can you reduce eddy currents in a core?

Use a laminated core

How are eddy currents produced in transformers?

• Changing magnetic field in the transformer core, caused by AC current in the primary coil, induces an emf in the conductive material.

• The induced emf causes circulating currents, known as eddy currents.

• Eddy currents generate energy losses in the form of heat.

• To reduce these losses, transformer cores are made from laminated sheets to limit eddy current flow.

Why is flux linkage minimum when coil is parallel to magnetic field?

Flux linkage is the product of magnetic flux and the number of turns in the coil. Magnetic flux is the amount of magnetic field passing through an area. When the coil is parallel to the magnetic field lines, no field lines pass through the coil, so the flux is zero, and thus the flux linkage is also zero.

Why is iron used for the core?

Since it is easily magnetised and demagnetised which is useful for AC.

Why does having more turns result in a higher voltage?

• More turns (NNN) directly results in higher induced voltage because the voltage from each turn adds up.

• Having more turns also increases the area through which the magnetic flux passes, causing a greater change in flux linkage, which contributes to the higher induced voltage.

What is Faraday’s law?

Induced emf is proportional to (NOT EQUAL TO) the rate of change of flux linkage

What equation links emf to B, L and v?

E = BLv

Are magnetic and electric fields always perpendicular to each other when acting on a charged particle?

Yes

Why is no force produced when magnetic field and current carrying wire are parallel to each other?

F = BIL sin (theta) so when theta is 0, no force

Is the motion a charged object experiences in a magnetic field circular or parabolic?

Circular, since there is a resultant force.

What must be remembered about L in F = BIL?

L is the length of wire in the magnetic field!!! Not the entire length.

Also, if there is a loop of wire, you must give the TOTAL LENGTH if it were stretched out.

Which way on FLHR do the magnetic and current fingers point?

Positive to negative (North to south)

Difference between Motor and Generator effect?

The motor effect produces motion from electricity, while the generator effect produces electricity from motion.

For motor effect, use FLHR

For generator, FRHR

How does generator and motor effect work?

• Motor Effect: A current passing through a conductor in a magnetic field causes a force to act on the conductor, leading to motion.

• Generator Effect: A conductor with a current passing through it, when moved in a magnetic field, induces an emf (electromotive force).

What are the 2 equations for peak emf?

2BLv and BANw

What is significant about f = BQ/2 pi m

Regardless of the radius and velocity, particles in a cyclotron have the same time period for their motion. This means the frequency of the alternating voltage (NOT CURRENT) applied between the dees can be matched to the time period, allowing all particles to be accelerated efficiently.

In terms of electromagnetic induction, how does a transformer step down voltage? (4 marks)

• AC is applied to the primary coil. ✅

• This creates a changing magnetic flux in the core (The primary coil’s magnetic field interacts with the core, creating a magnetic flux in the core.

• Since the current in the primary coil is alternating, the magnetic flux is constantly changing.). ✅

• The changing flux links to the secondary coil and inducing a voltage (emf) in it. ✅

• If the secondary coil has fewer turns, the output voltage is lower than the input voltage (step-down transformer). ✅

2 reasons why resistance in cables should be kept low?

• Minimises power loss

• Avoids overheating/fires

Definition & units of Tesla

1 Tesla is 1 newton per amp meter (T or weber/m²)

Reasons why transformers may not be 100% efficient?

flux leakage / copper losses / iron losses / hysterysis losses not just “heating” or “heat loss”

Way to remember of X or O means going into or out of paper?

X (straight lines and so is I so its INTO)

O (Out)

What is the relationship between the frequency and

What causes hysterysis loss?

Hysteresis loss refers to the energy lost as heat in a material when its magnetization changes in response to an alternating magnetic field.

How do the magnetic and electric fields act on the dees?

Magnetic field: Acts perpendicular to the dees, exerting a force that keeps the particles in circular motion.
Electric field: Exists in the gap between the dees, alternating in polarity to accelerate the particles each time they pass through.

Why do the particles spend the same amount of time in the dees?

Since frequency of circular motion doesn’t depend on the radius

Why does the p.d between the dees have to be alternating?

The particle would experience electrostatic repulsion after leaving the second dee, which would slow it down instead of accelerating it.

What is the equation for the largest (peak) EMF induced in a magnetic field?

E0 = BAN w (where w is omega = theta/time)

What does a graph plotting EMF induced and flux linkage against time show you?

When EMF is maximum, flux linkage is a minimum (They are 90 degrees out of phase with eachother so plot a sin and cosine graph for them)

When is induced EMF largest for a coil in a magnetic field?

When most field lines are cut (so when parallel, they are all cut)

What is 1 Tesla?

1 Tesla = 1 N/Am (One newton per amp meter)

What is the unit for magnetic flux density?

Tesla

When is flux linkage at a maximum?

When cos(theta) = 1(theta = 0)

A (Flux linkage = Epsilon/w) since Epsilon = BANw

Why does a neutron not feel a force when travelling through a magnetic field?

Since F= BQv, and neutrons have no charge, that means there is no force experienced.

Why is the flux greatest when a coil is perpendicular to magnetic field lines?

Because the maximum number of field lines pass through it.

Why does flux linkage change?

Flux linkage changes when field lines are cut (motion through a field) or when a coil rotates (changing flux through it).

Why does flux linkage change as a coil is rotated through a magnetic field?

When coil perpendicular to field lines, maximum flux enters, as the coil is rotated, less field lines can enter through the coil, so the field lines reduce (they are cut). This reduces the flux linkage.

When is a current induced?

If a conductor is part of a complete circuit.

What are the units of flux linkage?

weber turns (or just wb)

Why does an induced emf usually occur on plane wings?

Planes wings act as a conductor and the plane is cutting through the earths magnetic field, therefore an emf is induced.

What does the emf time graph look like for a coil in a magnetic field perpendicular to each other? (where are min and max?)

What does Lenz’s law state?

An induced EMF will always act in such a direction to oppose the change that caused it.

What direction does ANY object travel if a force acts perpendicular to its velocity?

Circular motion

What are the 2 units for magnetic flux density?

Tesla (T)

Weber (Wb)

What is an equation linki

What is the definition of magnetic flux density & what letter is assigned to it?

The force on 1 meter of wire carrying a current of 1 amp at right angles to the magnetic field.

With the equation F= BIL, what is special about the length L?

It is the length of the wire INSIDE THE FIELD ONLY! Not the entire length of the wire!

Does F = BIL give you the maximum force that could be experienced by the wire?

Yes!

What 2 things must you do when using Flemings Left Hand Rule?

Point first finger from north pole to south pole

Point second finger in direction of current!

What is a Cyclotron and what is it made up off?

A particle accelerator.

Two hollow semi-circular electrodes (dees) with a uniform magnetic field applied perpendicular to the plane of the electrodes, and an alternating potential difference applied between the electrodes.

How could you make a dc supplied wire in a magnetic field vibrate?

Change from dc to ac.

Why does changing to AC cause vibration of a wire in a magnetic field?

Since the direction of force acting on the wire is perpendicular to direction of current, as the current reverses, the direction of the force also changes.

Using FLHR, what direction is the force acting on this electron?

Since its negative, use the second finger to find the direction of flow BUT REMEMBER that the second finger is from positive to negative, so in this case the direction is THE OPPOSITE way to which the second finger points.

How do you represent a magnetic field going into AND out of the page?

Into = cross

Out = dot

In circular motion, is the force always perpendicular to the direction of travel?

Yes

What is a solenoid?

A solenoid is a long coil of wire consisting of multiple turns, which produces a uniform magnetic field inside when an electric current flows through it.

Why when the switch is closed, is a current induced in 2? (3 marks)

• Coil 1 is now a current carrying wire, meaning a magnetic field is generated.

• Coil 2 now experiences a change in magnetic flux (a change in magnetic field)

• This change in magnetic flux (due to Faradays law) causes an EMF to be induced, meaning an electric field is induced and therefore a current is also induced.

What must you have if you have an EMF?

An electric field.

Another phrase for change in flux?

Change in magnetic field (experienced by an object)

What is the basis of Faradays Law?

A wire experiencing an change in flux will induce an EMF which will induce a current (which will produce its own magnetic field).

What is the basis of Lenz’s law?

Conservation on energy! (e.g GPE of magnet in a conducting tube is converted into the EMF). The induced EMF/current always acts in the opposite direction to the change that causes it (e.g the magnets motion in this case downwards.

How is a current induced when a magnet is placed inside a conducting tube?

• When the magnet (which has a magnetic field) enters the conducting tube, a change in magnetic flux (change in magnetic field) occurs.

• This change due to Faraday’s law induces an EMF. This also induces a current that flows in a circular shape around the tube.

Why does a magnet in a conducting tube fall slower than in a regular tube?

• The current induced due to an emf being induced due to the flux change (see above) produces a magnetic field.

• This magnetic field produced opposes the magnets own magnetic field. (The like poles oppose each other). This causes an opposing force that slows the fall due to gravity down.

Where does the GPE go as a magnet is slowed due the opposing force of the magnetic fields of both the magnet and the conducting material?

The GPE that is lost due to the repulsion is converted into EMF and making the electric field! This shows that energy is conserved!

What is meant by oscilloscope trace?

The waveform displayed on an oscilloscope screen

What is the trace on an oscilloscope created by?

An electron beam moving across the screen

How can you calculate the average voltage in an alternating supply?

Find the root mean squared voltage

What causes energy loss in transmission cables?

The resistance of the cables

What must the current be in a transformer for it to work?

Alternating current!!

What is a soft magnet?

A magnet which can change its magnetic field easily.

Why is iron used in a transformer? (2 marks)

Iron is a soft magnet, meaning it can change its magnetic field easily

This means it can produce a rapidly changing magnetic flux

How does a transformer work (in detail?)

What voltages are used in the transformer equations?

Vp/Vs = Np/Ns (where the voltages are the ROOT MEAN SQUARE VOLTAGES!)