Ch23: Electromagnetism Flashcards
(32 cards)
Which direction do magnetic field lines go?
North to South
How is a uniform field represented in a diagram?
Equally spaced lines in the same direction
Which hand rule is used for a magnetic field around a current carrying wire?
Right hand rule, to determine direction of field
What is fleming’s left hand rule for?
A current perpendicular to a magnetic field, inducing a force.
What is magnetic flux density used as?
A measure of the strength of a magnetic field
What is the definition of Magnetic flux density? What is the symbol used to represent it?
- The force on one metre of wire carrying a current of 1 amp at right angles to the magnetic field
- B
What equation is used to find the force on a wire in a magnetic field?
F = BIL
or
F = BILsinθ
What experiment can be used to find magnetic flux density in the lab?
A wire between two magnets over a scale, the wire experiences a downwards force when current flows and the force can be read on the balance
What is a velocity selector used for?
To separate particles of a certain velocity from a stream of accelerated charged particles moving at a range of speeds.
How does a velocity selector work?
- A charged particle is fired at a collimator.
- It is subject to an electric field and a magnetic field which are applying opposite forces on the particle.
- Unless the two forces cancel out, the particle is deflected.
- By rearranging F=BQv and F=EQ, any particles not deflected have v=E/B
How can a magnetic field induce an E.M.F?
If a conducting rod is moving through a magnetic field, or stationary in a changing magnetic field, the electrons in the rod experience a force and an E.M.F is induced.
What is the symbol for magnetic flux? What is an equation for it?
Φ
- Φ=BA
What is magnetic flux density measured in? What is that unit equal to?
Teslas, T
T=Wb/m^2
What is the difference between magnetic flux density, magnetic flux, and magnetic flux linkage?
Flux: ‘number of field lines’
Flux density: ‘number of field lines per unit area’
Flux linkage: ‘number of field lines for a coil with N turns’
How is flux linkage calculated?
flux linkage= NΦ
What is magnetic flux measured in?
webers, Wb
What is flux linkage measured in?
webers, Wb
How is magnetic flux calculated if the field is not perpendicular to the coil?
Φ=BAcosθ
What is faraday’s law?
The induced e.m.f is directly proportional to the rate of change of flux linkage (flux linkage = BAN(cosθ)
Define a weber (Wb)?
A change in flux linkage of 1 weber per second will induce an emf of 1 volt in a loop of wire
How is faraday’s law represented on graphs?
- On a graph of flux linkage (BAN) against time, the gradient is the e.m.f
- On a graph of e.m.f against time, the area under the graph gives the change in flux linkage (-ΔBAN)
What is Lenz’s law? (How does it affect Faraday’s law?)
The induced e.m.f is always in a direction such as to oppose the change that caused it.
- It is the reason for the minus sign in Faraday’s law
When a changing magnetic field induces an e.m.f in a coil, it produces a current which in turn produces its own magnetic field.
What does Lenz’s law say about this?
- If the original magnetic field is getting stronger, the induced field will oppose it.
- If the original magnetic field is collapsing, the induced field will be in the same direction.
How can Faraday’s law be used in an experiment to find magnetic flux?
By using a search coil in a magnetic field connected to a data logger recording e.m.f.
- On a graph of e.m.f against time, the area under gives change in flux linkage.
- Flux linkage = BAN
- Repeat and take a mean of B
