Magnetic Fields + EM Induction Flashcards
What is a magnetic field?
it is a force field surrounding a magnet or current-carrying wire
What is the motor effect?
a current-carrying wire placed at a non-zero angle to the lines of force of an external magnetic field experiences a force due to the field.
What hand do you use for the motor effect
left hand
When is the force on a current-carrying wire greatest?
when the wire is at right angles to the magnetic field
it is zero when the wire is parallel to the magnetic field.
What is the force on the wire proportional to?
the current, the length of the wire
What is magnetic flux density, B?
the force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines.
What is magnetic flux density measured in?
Tesla (T)
What is the simple electric motor?
a coil of insulated wire which spins between poles of a U-shaped magnet
What is the role of a split ring commutator?
It allows the coil to continue to rotate in one direction by reversing the direction of current.
Why does a current-carrying wire in a magnetic field experience a force?
the electrons moving along the wire are pushed to one side by the force of the field.
What is the hall probe used for?
to measure magnetic flux density
Why is no work done by a magnetic field on a particle?
because it acts at right angles to the velocity of the particle.
Why can a cyclotron not alter the speed of a particle when it is in the field?
Because the force acts perpendicular to the velocity of the particle.
What is electromagnetic induction?
the induction of an electromotive force (i.e. voltage) across an electrical conductor in a changing magnetic field.
How can the induced emf be increased?
moving the wire faster
using a stronger magnet
making the wire into a coil, and pushing the magnet in or out of the coil
What happens to the induced emf when the relative motion between the magnet and coil?
emf is zero
Name two methods of generating an emf
using an electric motor in reverse (as a generator) (falling weight)
using a cycle dynamo
what hand do you use for generator (dynamo) effect?
right hand
What is Lenz’s law?
the direction of the induced emf (and hence current) is always such as to oppose the change that caused it.
What is the unit of magnetic flux?
Weber (Wb)
What is the unit of magnetic flux linkage?
Weberturns
What is Faraday’s Law?
the induced emf in a circuit is equal to the rate of change of flux linkage through the circuit.
What is back emf?
an emf induced in the spinning coil of an electric motor because the flux linkage through the coil changes.
Why is the current high at low motor speed?
the induced emf is small
