Ch23: Electromagnetism Flashcards
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
How does a generator use magnetic fields? What does it produce?
Generators convert kinetic energy to electric, by spinning a coil in a magnetic field and producing an alternating current.
What is a transformer?
Devices that change the size of a voltage for alternating current.
In a transformer, what happens when there are more turns on the secondary coil?
It is a step up transformer, and and voltage is increased.
What can be done to a transformer to increase it’s efficiency?
Using a laminated core.
Why is electricity in the national grid transported at a low current?
To reduce power loss because P=I²R
What is a standard mains voltage?
230V
What is a standard voltage for national grid transmittion?
400,000V
Why are transformers important for the national grid?
To reduce current and increase voltage for transmission around the national grid, this low current reduces power loss.
