Electromagnetism Flashcards
What is magnetic flux density?
- A measure of magnetic field strength
- Magnetic flux per m^2
What is the unit of magnetic flux density?
Tesla or Weber per metre
What is Fleming’s Left Hand Rule?
- First Finger Field
- Second finger current
- Thumb gives motion
How is magnetic flux density calculated?
B = F / Il when the wire is perpendicular to the field lines.
How is the force on a wire calculated?
F = BIlsinθ F = Bqv
When a metal frame is balanced, how can magnetic flux density be calculated?
B = mg/Il
When is there no force acting on a particle moving through a magnetic field?
- When the particle is moving parallel to the field lines.
- When the particle is stationery
How does an electron gun work?
- The heater produces electrons through thermionic emission.
- The electrons are repelled from the heater and attracted towards the anode.
- The electrons are accelerated as they pass through the gun.
What is the electron gun equation?
v = (2eV/m)^(1/2)
How is the radius of the circular path of a particle calculated?
r = mv/Bq
How is specific charge calculated?
e/m = v / Br e/m = 2V/(Br)^2
How does an electron move in a magnetic field?
In a circular path
How does an electron move in an electric field?
In a parabolic curve towards the positive charge since y ∝ x^2
What is a cyclotron?
A particle accelerator
How does a cyclotron work?
- Two dees are placed in a vacuum chamber between the poles of a powerful magnet so the dees are subjected to a perpendicular uniform magnetic field.
- Protons move in a circular path within the dees due to the magnetic field.
- A high frequency alternating voltage is applied between the dees creating an electric field causing the proton to accelerate.
- Once the proton has reached the maximum radius, the proton is deflected out by a charged plate.
How is the time taken to move through a dee calculated?
t = πm/Bq
How is the required frequency of a cyclotron calculated?
f = Bq/2πm
What is a galvanometer?
An ammeter that is designed to measure small currents.
What is the unit of magnetic flux?
Weber (Wb) or Tm^2
How is magnetic flux calculated?
Φ = BA
Why do the electrons in a wire become displaced when moved through a magnetic field?
The electrons experience a magnetic force.
How is the force on an electron in a wire calculated?
F = Bev
How is the electric force on an electron in a wire calculated?
F = eE = e(ε/l)
How is the emf induced in a conductor moving through a magnetic field calculated?
ε = Blv
When is an emf induced but no current?
When there is no external load across the wire.
What is Lenz’s Law?
When an emf is generated by a change in magnetic flux in accordance with Faraday’s Law, the polarity of the induced emf is such that it produces a current to generate a magnetic force that opposes the change which produced the emf.
Give four examples of Lenz’s Law.
- Dynamo
- Spinning copper disc
- Aluminium plate pendulum
- Magnet falling down a copper tube
What happens to a magnetic brake if slits are cut in the disc?
The brake has no effect because no current can flow in the disc.
What is Faraday’s Law of electromagnetic induction?
ε = -N(dΦ/dt)
A wire carrying a current is held mid-way between two poles of a magnet on a top pan balance. If the balance shows X what does the balance show if the current is doubled and reversed?
-2X
Give three causes of energy losses in a transformer.
- Induced currents in the soft iron core
- Reversal of magnetism in the soft iron core
- Resistances in the coils
For a rotating coil, at what value of magnetic flux is the emf greatest?
0
For a rotating coil, when is the rate of change of magnetic flux greatest?
Parallel to the field
For a rotating coil, when does the coil cut flux lines?
Perpendicular to the field
