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Flashcards in Electromagnetism Deck (34)
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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?