C23 (Magentic Fields) Flashcards by Peter Abdul-Ahad

How can you create an electromagnetic field

Moving wire in a magnetic field
Movement of a magnetic

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Unit of magnetic flux

Weber (Wb)

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Definition of magnetic flux

Defined as the product of the component of the magnetic flux density is perpendicular to the area at the cross-sectional area

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Magnetic flux linkage definition

As the product of the number of turns in the coil (N) times by the magnetic flux

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Equation for flux linkage

No. Turns x magnetic flux

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Step-up transformers

Step-up transformers has more turns on secondary than on primary coil, Vs > Vp (vise versa for Step-down transformers).

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What do transformers not work on

DC supplies (there’s no alternating current)

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Transformers symbol equation

Ns / Np = Vs / Vp

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In a transformer the more rapid the change in flux the…

Larger the induced e.m.f

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What are transformers used for

increase / decrease e.m.f. Transformers which increase, called step-up transformers, decrease called set-down.

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What do AC generators obey

Faraday’s law and Lenz’s law, as the induced current is in the opposite direction to the change in flux

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Faraday’s law of electromagnetic induction

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

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How can you change the direction of the induced emf and hence the current

When a magnet is pulled away from a coil instead of being pushed into the coil

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Lenz’s law

The direction of any induced emf or current is always in a direction that opposes the change that’s producing it

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Which 2 laws do you combine

Lenz’s law
Faraday’s law

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For an alternating current generator, the induced emf is at maximum when…

the flux linkage is zero

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What are the 2 types generators can you have, to produce induced emf

Can have magnet or coil rotating

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In an alternating current generator, when is flux linkage at its maximum

When the emf is zero

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What condition must be met for 100% efficient transformers

the output power from secondary is equal to output power from primary coil.

Equation for 100% efficient transformers

Vs x Is = Vp x Ip

Reasons why transformers don’t have 100% efficiency

Due to heating of primary and secondary coil (increasing resistance)

Due to eddy currents (current induced in core, set up in the core of a transformer).

Reduce energy losses in the transformer by:

Increase inefficiency by using low-resistance windings (ensure coils are thick as possible), reduce power loss due to heating effect of the current.

Making a laminated core with layers of iron separated by an insulator helps minimise currents induced in the core itself (eddy currents). This too minimises lose due to heating.

What does the fact that core of transformers are made of soft iron mean

Very easy to magnetise and demagnetise, helps improve overall efficiency of transformer.

Transformers are commonly used in the national grid to…

step-up and down e.m.f.

Electricity is transmitted across the national grid at a high voltage and low current to reduce energy lost as heat in the wires.

What are the 3 natural magnetic elements?

Iron Nickel Cobalt

Magnetic materials can only be...

Attracted

How can you magnetise a material?

By passing it through a magnetic field

Earth has a magnetic field (which direction)?

From south towards north (Magnetic field due to the molten iron core).

Magnetic field definition:

The region around a permanent magnet or a current carrying conductor, in which other magnetic objects will experience a force.

In what direction are the lines of a magnetic field drawn in?

In the direction a ‘free’ north pole would move (north to south).

Compare between the 3 types of fields what the (field is produced by):

Grav, mass of an object Electric, charged objects Magnetic, moving charges

Compare between the 3 types of fields what the (Field acts on):

Grav, test mass (other masses) Electric, charges Magnetic, moving charges

Compare between the 3 types of fields what’s the (field strength defines as):

Grav, F = -GM / r or g = F / m Electric, E = F / Q Magnetic, B = F / I x L x sin(theta)

What does equally spaced and parallel magnetic field lines represent?

A uniform field (the strength of the magnetic field doesn’t vary).

Magnetic field is stronger when...

Lines are closer, I.e for bar magnetic, filed strongest at its north and south poles.

When a charged particle moves, what does it create?

A magnetic field, and therefore when a current oases through a wire, a magnetic field is produced.

How do you explain the magnetic field in a bar magnet?

Created by electrons whizzing around the iron nuclei.

(Magnetic field of a current carrying conductor) How can you work out the direction of the current and magnetic field?

RIGHT hand grip rule: current is direction of thumb, other fingers show direction of the magnetic field.

How can you represent current (in a wire) coming out of the page?

Circle with DOT

How can you represent current (in a wire) going into the page?

Circle with CROSS

If the magnetic field and current are parallel then that’s the magnitude of the force?

There is no force (that will be experienced).

Fleming’s Left Hand Rule (definition):

A current-carrying conductor is surrounded by its own magnetic field. When the conductor is placed in an external magnetic field, the 2 fields interact. These 2 magnets experience equal and opposite forces.

Fleming’s Left-hand rule: (What does each finger represent)?

Thumb, direction of motion (force) on the wire Index finger, direction of external magnetic field Middle finger, direction of the conventional current ALL PERPENDICULAR TO EACH OTHER

What’s the SI unit for magnetic flux density (B)

Tesla (T)

What’s is one Tesla (1T)?

The magnetic field density when a wire of length 1m carrying current 1A at right angles to the field, experiences a force of 1N.

Magentic flux density equation: (What does the size of the force on the current-carrying wire depend on)?

F = B x I x L x sin(theta) Depends on: - Strength magnetic field, B (magnetic flux density) - Current in wire , I - Length of wire in the field, L - Sin(theta), where theta is the angle between the magnetic field and current direction.