MODULE 6: Electromagnetism Flashcards
Explain how a transformer works
Alternating current supplied to the primary coils causes a change in flux which is transferred to the secondary coil through flux linkage in the ferromagnetic core. The amount of induced current is then proportional to Np/Ns= Is/Ip
Describe how back EMF is induced to ensure that the motor does not continue to spin faster
Back emf is induced in a motor as it begins to spin. The change in magnetic flux produced by the spin induces a back emf that opposes the motion of the motor, hence counteracting the speeding up of the motor (Lenz’s law) and ensures the conservation of energy
Identify and explain the role of the most important component of an AC generator
The most important component of the AC generator is the slip rings. They provide electrical contacts while allowing the rotation of the armature, this in turn allows an alternating current to flow through the generator thus generating the AC output emf/voltage
Describe the back emf of a motor in before, after and during operation
Before the motor is turned on, back emf and emf is 0, as the motor is turned on the EMF induced is proportional to its voltage, and initialy the back EMF is large and short lived but increases to a constant value (a little bit less than the voltage) that varies periodically with the angular rotation of the motor. If a load is placed the back emf decrease significantly aswell as the angular rotation of the motor. Motor speed also decreases and reduces back emf. When it is turned off back emf and emf is back to 0.
Explain why a charged particles have a curved trajectory in a magnetic field
the magnetic force acts perpendicular to the intial velocity of the particle, as such it is a centripetal force which produces a circular path.
NOTE: circular paths are uniform, as they experience a constant centripetal force with no resistance to their motion (travelling at a vacuum, moving in at constant speed)
Explain how an AC induction motor operates
An AC induction motor is made up of a squirrel cage rotor surrounded by pairs of electromagnets, usually 3 pairs.
AC is fed into the electromagnets out of phase with each other, 120° out of phase in the case of the 3-phase induction motor. The AC in the electromagnets produces an alternating magnetic field from each pair of electromagnets.
Since they are out of phase, they effectively produce a rotating magnetic field at the rotor. The rotating magnetic field induces a current in the bars of the squirrel cage which then experience a force due to the motor effect. This causes a net torque on the rotor in the same direction as the rotation of the magnetic field
Explain how an AC induction motor generates torque
An induction coil consists of two main components: a conducting rotor within a stator, around which are wrapped several electromagnets. By turning the electromagnets on and off in sequence, the rotor experiences a rotating magnetic flux.
By Faraday’s Law, this generates a voltage and thus eddy currents within the rotor.
By Lenz’s Law, these eddy currents produce a magnetic field that opposes the changing flux.
As a result,the rotor experiences a force, and begins to rotate, in the same direction as the magnetic field.”
Compare and contrast the structure and purpose of simple motors and generators
Have similar structures with a rotating coil in a magnetic field connected to an external circuit via a slip ring/split ring commutator
motor -> electrical to mechanical energy using motor effect
generator-> mechanical to electrical energy using electromagnetic induction
Explain how electromagnetic induction is applied in magnetic braking
An electromagnetic brake uses electromagnetic induction and Lenz’s law to produce a braking force. As the rotating disc passes through the magnetic field produced by the electromagnets, eddy currents are produced in the disc. These eddy currents flow in a direction according to Lenz’s law, in order to oppose the change in magnetic flux. The eddy currents interact with the magnetic field from the electromagnets to produce a force on the disc in the opposite direction to its motion
