C23 (Magentic Fields) Flashcards
How can you create an electromagnetic field
Moving wire in a magnetic field
Movement of a magnetic
Unit of magnetic flux
Weber (Wb)
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
Magnetic flux linkage definition
As the product of the number of turns in the coil (N) times by the magnetic flux
Equation for flux linkage
No. Turns x magnetic flux
Step-up transformers
Step-up transformers has more turns on secondary than on primary coil, Vs > Vp (vise versa for Step-down transformers).
What do transformers not work on
DC supplies (there’s no alternating current)
Transformers symbol equation
Ns / Np = Vs / Vp
In a transformer the more rapid the change in flux the…
Larger the induced e.m.f
What are transformers used for
increase / decrease e.m.f. Transformers which increase, called step-up transformers, decrease called set-down.
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
Faraday’s law of electromagnetic induction
The magnitude of the induced emf is directly proportional to rate of change of magnetic flux linkage
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
Lenz’s law
The direction of any induced emf or current is always in a direction that opposes the change that’s producing it
Which 2 laws do you combine
Lenz’s law
Faraday’s law
For an alternating current generator, the induced emf is at maximum when…
the flux linkage is zero
What are the 2 types generators can you have, to produce induced emf
Can have magnet or coil rotating
In an alternating current generator, when is flux linkage at its maximum
When the emf is zero
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.
