Magnetic Fields: Flashcards
Magnetic field:
Force field around a magnet or current wire which acts on any other magnet or current wire in the field. Goes from north to south by aligning itself with earths magnetic field.
Motor effect:
Current wire placed at a non zero angle to magnetic field experiences a force.
When motor effect is greatest:
At 90 degrees to the magnetic field.
Magnetic flux density:
Force per unit length per unit current on a current carrying conductor at a right angle to magnetic field. In N per metre per amp.
F=BILsin theta:
Component of magnetic field is perp. to current so this equation exists.
Couple on coil in a mag. field:
Each side experiences a force in opposite directions due to the mag field so moments applied where distance is the per distance between the line of action of the forces.
Circular motion of electron beam in mag. field:
Experiences force perpendicular to direction of motion due to mag. field so circular motion happens.
Prove F = BQv
I =Q/t and L =vt so F =BIL becomes F=BQv or F=Bev (also becomes F=BQvsin theta if it is not perp to the field).
Why magnetic field doesn’t do work on a moving particle:
Force is perpendicular to the motion so it only changes the direction and not the speed and Ek stays the same.
Why must equipment for moving charge particles be evacuated first:
To prevent loss of speed of charged particles by collisions with air molecules / particles.
What cyclotron is used for:
Produce high energy beams for radiation therapy.
How cyclotron works:
2 Dees with magnetic field running through them ( perp to the planes).
High energy alternating voltage across them in vacuum chamber.
1)Charged particles directed into a dee near the centre of the CT. They are forced on circular path by the magnetic field.
2)When it crosses into another dee the voltage reverses and they accelerate into the other dee.
3) This process is repeated.
They accelerate because the time taken for the particle to move around does not depend on its speed.
4) Particles radius increases each rotation and when it is equal to the radius of the dee then it is ejected.
Electromagnetic induction and how increase it:
EMF induced in a wire when it cuts across the lines of a magnetic field (if it part of a complete circuit then the induced EMF forces electrons around the circuit.
Increase : move wire faster, use stronger magnet make wire into a coil and pushing the magnet in and out of the coil.
Other ways to induce an EMF:
Use electric motor in reverse - falling weight makes coil turn and then an emf is induced and forces a current around the circuit.
Using a cycle dynamo - EMF is induced in the coil as the dynamo spins (magnet on a stick that spins???)
Work done in an induced emf:
E.g. in a cycle dynamo, the work done to keep it spinning is equal to the energy of induced EMF. Rate of energy transfer from the source of induced EMF is equal to EMF supplied to components
Energy transferred per second in induced EMF equation:
E * I = energy transferred per unit charge. Therefore: E * I * Q = Energy transferred from the source per second.
Explain why EMF is induced:
When a metal rod or wire etc is moved across a magnetic field or vice versa then all the free electrons are forced to one end of the rod. Hence one end is negative and the other positive so emf is induced.
If the relative motion between rod and field stops then the EMF stops as the field stops exerting a force on the free electrons
Which way current travels in north pole of a solenoid:
North pole - aNticlockwise
Lenz’s Law:
Direction of induced emf always is such that it poososes the change that caused the current.
Explanation of Lenz’s Law:
Energy must be conserved so the induced current can’t be in a direction to help the change that caused it as this would create electrical energy out of thin air.
Prove faradays law:
Work done by applied force to conductor is W = Fs = BIls
Charge transferred along the conductor in the same time is Q = It
Therefore induced E = W/Q = BILs/It
I cancels out and L change in s is the area swept out by the conductor.
Therefore E = BA/change in time
Weber:
Unit of magnetic flux = BA
1 Wb = 1Tm^2
it only acts at right angles.
Faradays law:
Induced EMF in a circuit is equal to the rate of change of flux linkage through the circuit.
Why does faradays law include a minus sign:
Indicates that the induced emf acts in a direction as to oppose the change that caused it.
Alternate EMF equation:
E = Blv as A = l*change in s
How does fixed coil in changing magnetic field create induced emf:
Current in the solenoid changes so magnetic field changes so an emf is induced in the coil
Peak value of AC current:
Peak pd or current in either direction.
Peak to peak value:
Difference between the peak value one way and the peak value in the opposite direction.
Waveform:
Variation with time on an oscilloscope.
How lamp varies connected to oscilloscope of ac source:
Lamp lights up at peak value in either direction - fades between these peak values.
Sinusoidal current:
Mean power over a full cycle is half the peak power - 1/2I0^2R where I0 is peak power.
Root mean square of AC:
Direct current that would give the same power and heating effect as AC in the same resistor - root mean square.
Therefore, (Irms)^2R = 0.5I0^2R
Describe transformer:
Primary coil and secondary coil with he same iron core.
When primary is connected to ac pd an alternating magnetic field is produced in the core which passes through the secondary coil so an alternating emf is induced in the 2nd coil.
Transformer rule:
Vs/Vp = Ns/Np
Difference between step up and step down transformer:
Step up - more turns on secondary coil.
Step down - Fewer turns on secondary coil so voltage is decreased.
How efficiency of transformers is increased:
- Low resistance windings to reduce power wasted through current heating’s.
- Laminated core (layers of iron separated by insulator) which reduced induced currents in the actual core / eddy currents so magnetic flux is high as possible.
-core is soft iron so is easily magnetised and demagnetised which reduced wasted power through repeated magnetisation and demagnetisation.
Efficiency of transformer equation:
Power to secondary coil/power ot primary coil
Back emf in the transformer:
Changing flux creates back emf in both coils. Acts against primary voltage which makes the primary current very small when the secondary current is ‘off’.
When secondary is on, it opposes primary coils magnetic field so back emf is reduced in primary coil.
Primary current is larger than when when the secondary current is off.
Why is transmission of power over long distance more efficient at high volts than lower volts:
Current needed to deliver a certain amount of power is reduced if voltage is increased so power wasted due to heating is reduced.
