Magnetic fields Flashcards
What is the function of the core of a transformer?
Increases linkage of magnetic flux
What is the function of the secondary coil in a transformer?
Has a varying magnetic flux passing through it, which induces an emf that is determined by the number of turns.
Explain how the efficiency of a transformer is increased by constructing the core out of thin iron sheets.
The sheets of laminations are made from insulator/ high resistivity material. This reduces eddy currents/ charge flowing in the core, which means that smaller emfs are induced in the core.
Why shouldn’t the voltage in a long-distance transmission be too low?
If the voltage is too low then power is transmitted at a high current = energy loss due to heating.
Why shouldn’t the voltage in a long-distance transmission be too high?
If the voltage is too high, it creates major insulation difficulties - electrical breakdown of insulation
What is Lenz’s law?
The direction of the induced current is such that it will oppose the change of flux that is producing it.
What shape are the magnetic field lines induced by a current in a wire?
Cocentric rings around the wire
What is the definition of a Tesla?
A force of 1N on 1 metre of wire carrying 1A of current perpendicular to a magnetic field.
What do the fingers represent in Fleming’s left hand rule?
Force (or motion)
Field
Current (conventional current = opposite to direction flow)
What is the condition for using Fleming’s left hand rule for a charged particle moving in a magnetic field?
The charge must be positive. If the charge is negative, the force is reversed.
What direction is the force on a moving charge in a magnetic field compared to its velocity?
Perpendicular
How do you find the formula for radius of a charged particle’s circular path and what is it?
F = BQv and F = (mv^2)/r
r = (mv)/(BQ)
What are some of the uses of a cyclotron?
Producing ion beams for radiotherapy, radioactive tracers
In what direction does the magnetic field in a cyclotron act?
Perpendicular to the plane of the electrodes
What type of voltage is used in a cyclotron?
AC
Why does changing the strength of a magnetic field not impact the speed of a particle?
Because the force exerted by the magnetic field is always perpendicular to the direction of travel.
What happens when particles reach the edge of the ‘dee’?
The move across the gap, where they are accelerated by the electric field, meaning that the radius of their circular path will increase as they move through the second electrode. When the particle reaches the gap again, the alternating electric field changes direction, allowing the particles to be accelerated again.
What are the two laws of electromagnetic induction?
Faraday’s law and Lenz’s law
What is Faraday’s law?
The magnitude of induced emf is equal to the rate of change of flux linkage
What is Lenz’s law?
The direction of induced current is such as to oppose the motion causing it.
What is the formula for flux that contains velocity?
B x l x v x delta t
What is the formula for emf that contains v?
emf = Blv
What’s the word for a graph shaped like a sine curve?
Sinusoidal
What is the peak voltage on an oscilliscope?
V 0 (subscript)
The distance from the equilibrium to the highest / lowest point
