Module 6.3 Flashcards
Faraday’s Law
The magnitude of the induced EMF is directly proportional to the
rate of change of magnetic flux linkage
Field Lines
Lines that show the direction in which a magnetic North monopole
would experience a force if placed at that point in a field. Magnetic field lines point
from North to South.
Fleming’s Left-Hand Rule
The relative direction of motion, field direction and
current direction in the motor effect can be represented by the thumb, first finger
and second finger of the left hand respectively. For the motion of a charged
particle in a magnetic field, its direction replaces the current direction
Force on a Charge Particle
A charged particle moving through a magnetic field
will experience force equal to the product of the charge, its velocity and the
magnetic flux density
Force on a Current-Carrying Conductor
A current-carrying conductor will
experience a force when placed in a magnetic field. The direction of the force can
be determined using Fleming’s left-hand rule
Lenz’s Law
The direction of an induced current is such that it opposes the
current that created it.
Magnetic Field
A region of space in which magnetic materials and moving
electric charges feel a force
Magnetic Flux Density
The force per unit current per unit length on a
current-carrying wire placed at 90º to the field lines. Sometimes also referred to as
the magnetic field strength
Magnetic Flux Linkage
The magnetic flux multiplied by the number of turns, N,
of the coil.
Magnetic Flux
A value which describes the magnetic field or field lines passing
through an area. It is the product of magnetic flux density and the perpendicular
area it passes through.
Tesla
The unit of magnetic flux density
Transformer
A device used to increase or decrease the voltage with two sets of
coils with different numbers of turns wrapped around a magnetic core. The
transformer is step-up if the number of coils on the secondary coil is greater than
the number on the primary coil. The transformer is step-down if the number of coils
on the secondary coil is fewer than the number on the primary coil.
Velocity Selector
A combination of a magnetic field and an electric field, which
results in charges passing through and leaving with a specific velocity
Weber
The unit of magnetic flux
How can you map field lines around a magnet
State 2 ways
- You can place iron filings on a piece of paper and the put the magnet on the paper and the filings will align to the field
- You can also use a plotting compass and place it in various positions around the magnet, mark the direction of the needle at each point and connect them.
Why does a compass point to the North pole of the earth
The Earth’s geographic north pole is actually the magnetic south pole, so the north pole of the compass magnet (the needle) lines up with the Earth’s field and points to the magnetic south (field lines go from north to south), which is what we call the geographic north
How do you work out the shape of the field around a solenoid
Where the current is going anticlockwise around the coil is the north pole. At the south pole the current goes clockwise. The shape of the field is then similar to the bar magnet.
Define the motor effect
When a current carrying conductor is placed within a Magnetic Field, it experiences a force perpendicular to the flow of current and the field lines which pushes it out of the field
Describe an experiment to measure flux density
1) Place a horseshoe magnet on aa digital balance and zero it,
2) Connect rigid piece of straight wire to DC supply, variable resistor and ammeter (in series)
3) Align the wire so the force on it acts upwards (so there will be a downward force on the magnet - Newton’s third law)
4) Measure the length of the wire in the field
5) Record extra mass on the balance and use F = mg
6) Plot a graph of current against mass - gradient gives BL/g. Since L and g are known, B can be calculated
Why do charged particles move in a circular orbit in a Magnetic Field
Force is always perpendicular to the velocity of the particle, so they end up being forced in a circular orbit. The particles undergo centripetal acceleration, with the centripetal force being the magnetic force
What is the purpose of a velocity selector
They isolate particles of a specific velocity. This is useful for things like mass spectrometry
How does a velocity selector work
There are electric plates above and below so the electric force acts upwards, and there is a Magnetic Field passing through sideways so the magnetic force acts downwards. In order for the particles to pass through undeflected, the electric and Magnetic fields must be balanced so BQv = EQ
From this, you can derive v = E/B. If velocity is too fast or too slow the particles will be deflected and not pass through
Explain Lenz’s law in terms of energy
Lenz’s law follows the principle of the conservation of energy. If the induced emf was in a direction that aided the change which caused it, it would be creating electrical energy from nowhere.
What is a search coil
A flat coil of insulated wire connected to a galvanometer (a sensitive ammeter). It can be used to determine magnetic flux density from the current induced in the coil when it is withdrawn from a Magnetic Field.
How can you measure magnetic flux using a search coil
1) Place the coil in a Magnetic Field of known strength and pull it out again. Since the max current is proportional to magnetic field density, you can calculate the constant of proportionality, k (therefore calibrating the coil)
2) Place the coil in the field that is to be measured and withdraw it. Use the value of k and the current to calculate the flux density
3) Use formula:
magnetic flux = BAcos(x)
How does a simple A.C generator work
The flux linkage in the coil changes continuously inducing an alternating current in the voil
List 4 ways the peak emf of an A.C generator be increased
1) Increase the speed of rotation
2) Increase the magnetic flux density of the field
3) Increase the cross-sectional area of the coil
4) Increase the number of turn on the coil
What is the purpose of a transformer
Transformers change the peak value of an alternating PD to a different value. Step-up transformers increase it, step-down transformers decrease it
Describe the structure of a simple transformers
Two coil - primary and secondary coil - wrapped around the two sides of a laminated iron ring. For a step-up, there are more turns on the secondary coil. For a step-down, there are more turns on the primary coil then secondary coil
How does a transformer work
An alternating current is run through the primary coil, which induces an alternating Magnetic Field in the iron core. This, in turn, induces an alternating emf in the secondary coil
What role do transformers play in the National grid
Step-up transformers are used to increase the voltage (and decrease current) before the electricity travels long distances. This is to reduce energy lost as heat due to resistance in the wires as the electricity passes through
