Electromagnetism Flashcards by Cristian Beard

What is a magnetic field .

A Magnetic Field is a Region Where a Force is Exerted on Magnetic Materials

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What can be used to represent a magnetic field on a diagram

Field lines

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What is the direction of travel of field lines

North to south

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What does the distance between the field lines say about the magnetic field?

The closer the lines, the stronger the field.
If the field lines are equally spaced and in the same direction the field is uniform (i.e. the same everywhere).

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What is caused by a wire carrying current

A magnetic field

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What can be said about the be said about the magnetic field around a current carrying conductor?
(Specifically a long straight wire in this case)

1) The field lines are concentric circles centred on the wire.
2) The direction of the field can be worked out with the right-hand rule.

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How does the right hand rule work?

Stick your right thumb up, like you’re hitching a lift.

Your thumb points in the direction of conventional current and your curled fingers point in the direction of the field.

(This also works for a flat coil)

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What does the magnetic field of a flat coil and solenoid look like?

See CGP - page 150

Also look at “learn these coil types”

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Learn these coil types

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What is induced by a current following perpendicular to a uniform magnetic field

1) The field around the wire and the external magnetic field will interact causing a force on the wire.
2) The direction of the force is always perpendicular to both the current direction and the magnetic field
- it’s given by Fleming’s left hand rule…

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Hoŵ does flemings left hand rule work?

The (F)irst finger points in the direction of the external uniform magnetic (F)ield, the se(C)ond finger points in the direction of the conventional (C)urrent. Then your thu(M)b points in the direction of the force (in which (M)otion takes place).

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What is the size of the force on a current carrying wire at a right angle to an external magnetic field proportional to

The magnetic flux density

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What is magnetic flux density used for

Magnetic flux density is used as a measure of the strength of a magnetic field.

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What the definition of magnetic flux density

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

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What is the equation for force (on a current carry conductor) when the current is at 90° to the magnetic field

F= BIl

When current is at 90° to the magnetic field, the size of the force, F is proportional to the current, 1, the length of wire in the field, I, as well as the flux density, B. This gives the equation:

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Is magnetic flux density vector or scalar vector of

Vector

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What is magnetic flux density measured in

Teslas

1 Tesla = Wb/ m^2 ?

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What component of the magnetic field (parallel or perpendicular to the wire) causes a force

The force on a current-carrying wire in a magnetic field is caused by the component of the magnetic field which is perpendicular to the wire -
B x sin (theta)

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If a wire is at an angle of theta to a magnetic field what is the equation for force

F = BIlsin(theta)

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What happens when the direction of the current and magnetic field are parallel

No force

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Practical on flux density

How do you find the force acting on a single charged particle moving through a magnetic field?

Note this is given in the formula sheet: so the ability to derive it is no necessary:

F= BQv

Since I=Q/t

t=l/v where l is the distance a particle moves and v is the particles velocity

Combined: I = Qv / l

Since F=BIl = BQV

What is the path of a charged particle moving in a magnetic field

By Fleming’s left hand rule the force on a moving charge in a magnetic field is always perpendicular to its direction of travel.
Mathematically, that is the condition for circular motion

What are example of the uses of the motion of charged particles in a magnetic field

This effect is used in particle accelerators such as cyclotrons and synchrotrons, which use magnetic fields to accelerate particles to very high energies along circular paths.

It’s also used in mass spectrometers to analyse chemical samples. lons (charged particles) with the same velocity are made to enter a magnetic field which deflects them in a curved path towards a detector. The radius of curvature depends on the charge and mass of the particles (see equation below). The identity of the ions reaching the detector can be deduced from their mass to charge ratio.

If a charged particle is traveling along a circular path while in a magnetic field, what can be said about the force acting on said particle

The centripetal force and the force due to the magnetic field are equivalent for a charged particle travelling along a circular path.

How could you calculate the radius of a charged particle moving in a circular path

For uniform circular motion Newton’s second law gives: F = mv^2/ r

So, for a charged particle following a circular path in a magnetic field (where F= BQv): BQv = mv^2/r
3) Rearranging gives: r = mv/BQ

Where: m is the mass of the particle, v is its speed and r is the radius of the circular path. B is flux density and Q is charge

What is the purpose of velocity selectors and how do they preform it

Velocity selectors are used to separate out particles of a certain velocity from a stream of accelerated charged particles moving at a range of speeds. They do this by applying both a magnetic and an electric field at the same time perpendicular to each other, while a stream of particles is fired perpendicularly to both fields at a device with a narrow gap called a collimator.

Explain the forces a charged particle will experience in a velocity selector (in the example on the CGP pg-153)

The magnetic field tries to deflect particles upwards
- check this with Fleming’s left hand rule. The force
on each particle is F = BQv (see previous page).
• The electric field tries to deflect particles downwards (opposite charges attract, like charges repel).
The force on the particle is F = EQ (see p.146).

What an expression for the velocity a charged particle must have in order to make it through the collimator in a velocity selector

Particles will be deflected unless the forces balance
(i.e. BQv = EQ). Cancelling Qs and rearranging gives:

V = E/B

Only particle with that specific velocity will travel in a straight line to pass through the gap in the collimator.

How can you changer the velocity selected in a velocity selector

You can select and vary the velocity of the particles that get through the collimator by changing the strength of the magnetic or electric fields.

Use of velocity selectors

Velocity selectors are often used in mass spectrometers to ensure that the accelerated particles entering the magnetic field have the same velocity.

How can an EMF be induced in a conducting rod by using magnetic field

If there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force (see p.150), which causes them to accumulate at one end of the rod.

This induces an electromotive force (e.m.f.) across the
resultant positive. charge accumulated electrons
ends of the rod exactly as connecting a battery to it would — this is called electromagnetic induction.

the rod is part of a complete circuit, then an induced current will flow through it.

How can EMF be induced in a coil or solenoid

1) You can induce an e.m.f. in a flat coil or solenoid by:
• moving the coil towards or away from the poles of a magnet.
• moving a magnet towards or away from the coil.
2) In either case, the e.m.f. is caused by the magnetic field (or ‘magnetic flux’) that passes through the coil changing.
3) As above, if the coil is part of a complete circuit, an induced current will flow through it.

Understand don’t learn - what is the reason that the movement of a conductive rod in a magnetic field induced a EMF

The motion of the coil (and the electrons in it) relative to the magnetic field makes the electrons move because they experience a magnetic force given by Bev, where B is the magnetic flux density, e is the elementary charge, and v is the relative speed between the coil and magnet. The moving electrons constitute an electrical current within the coil,

How can you calculate total magnetic flux

Magnetic flux density, B, is a measure of the strength of the magnetic field (you can think of it as the number of field lines per unit area).
So the total magnetic flux, ф, passing through an area, A, perpendicular to a magnetic field, B, is defined as:

ф = ВА

where ф is magnetic flux in webers (Wb), B is magnetic flux density (T) and A is area (m^2).

How can magnetic flux be thought of

You can think of flux as the number of field lines. But remember that flux is continuous - field lines are just a way of drawing it.

How does magnetic flux change when you have a coil N turns

If you have a coil of N turns, rather than a single loop, you need to talk about flux linkage instead which is just flux multiplied by N:

flux linkage = N ф

(Unit is also Wb

How do you calculate magnetic flux when the direction of the field lines (and therefor magnetic flux) is not perpendicular the the area of the coil

1) When the magnetic flux isn’t perpendicular to the area of the coil you’re interested in, you need to use trigonometry to find the component of the flux that is perpendicular to the area
2) If 0 is the angle between the magnetic flux and the normal to the plane of the coil

ф = BAcos 0

And flux linkage
flux linkage = N ф = BAN cos 0

See examples of CGP

What is faradays Law

FARADAY’S LAW: The induced e.m.f. is directly proportional to the rate of change of flux linkage.

How can faradays Law be expressed as an equation

Induced E.M.F = - flux linkage change/ time taken = - N Δ ф/ Δt

Look at the CGP of faradays law on a graph

What is lenz’s law

LENZ’S LAW: The induced e.m.f. is always in such a direction as to oppose the change that caused it.

Explain how Lenzs law works in application

A changing magnetic field induces an e.m.f. in a coil
If the coil is part of a complete circuit, a current is induced in the same direction as the induced e.m.f.
The induced current then produces its own magnetic field.
Lenz’s law says:

If the original magnetic field is getting stronger, the induced magnetic field will be in the opposite direction to try to weaken it.

• If the original magnetic field is getting weaker (collapsing), the induced magnetic field will be in the same direction to try to maintain it.

The area of a flat coil is perpendicular to a magnetic field as it collapses by 50% as shown below. What will be the direction of the current induced in the loop? See CGP example

A collapsing field means the field is getting weaker and the field lines are getting further apart.
So by Lenz’s law the current induced in the coil will induce a magnetic field in the same
direction as the collapsing field to try to maintain the original field.

Use the right-hand rule to find the direction of the induced current. The induced field is to the left, so the induced current is clockwise when viewed from the right.

How does Lenz’s law work when applied to a rod

A conductor moving through a magnetic field induces a current if it is connected to a circuit.
• Lenz’s law says that the induced current will produce a force to oppose the motion of the conductor (a resistance).

You know the directions of the magnetic field and the induced resistance force, so you can use Fleming’s left hand rule to find the direction of the induced current and so the direction of the induced e.m.f.

How can you use Faraday’s Laws to investigate magnetic Flux

This set up shows how you can find the magnetic flux density, B, using a search coil and a data logger…
1) Place two bar magnets a small distance apart with opposite poles facing each other — they should be far enough apart not to snap together, but otherwise as close as possible to give a uniform field.
2)
Get a search coil — this is a small coil of wire with a known
search coil~
number of turns (N) and a known area (A). Connect it to a data recorder and set the recorder to measure the induced e.m.f. with a very small time interval between readings.
3) Place the search coil in the middle of the magnetic field so that the area (A) of the coil is parallel to the surface of the magnets.
bar magnets
Start the data recorder. Keeping the coil in the same orientation, immediately move the coil out of the field.
4) An e.m.f. will be induced due to the magnetic flux density through the coil changing from maximum to zero as you remove the coil from the field.
5) Use your data or the data recorder to plot a graph of induced e.m.f. against time.
6) Using Faraday’s Law, estimating the area under the graph of e.m.f. against time gives you an estimate for the total flux linkage change (p.155).
7) Flux linkage = N = BAN (p.154), so to find B, divide the total
flux linkage change by coil area (A) and number of turns (N).
8)
Repeat this experiment several times and find the mean of your values for B.

What is lenz’s law the conservation of?

Conservation of energy

What are useful equations to know for charged particles traveling in a circular path

How do you find the emf of a conducting rod of length l moving through a perpendicular uniform to a magnetic at constant velocity

Difference between magnetic flux and magnetic flux density?

Magnetic flux density is measured in Teslas

Magnetic flux is measured in Wb

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Electromagnetism Flashcards

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