Electromagnetic Induction Flashcards
Define magnetic flux
The total number of magnetic field lines passing normally through a given area is known as magnetic flux through that area.
Define electromagnetic induction
The phenomenon of producing induced emf or current in a conductor or coil due to change in magnetic flux or field is called electromagnetic induction.
Expression for magnetic flux through a given area
d(phi) = Bcos(theta).ds
SI unit of magnetic flux
Wb
SI unit of magnetic induction
Wb/(m*m) or T
State Faraday’s first law of electromagnetic induction
Faraday’s first law states that whenever there is a change in flux associated with the coil, an emf is induced in the coil.
State Faraday’s second law of electromagnetic induction
According to Faraday’s second law of electromagnetic induction, the magnitude of induced emf is directly proportional to the rate of change of magnetic flux through the coil.
State Lenz law of electromagnetic induction
The direction of induced emf or current in the coil or conductor is such as to oppose the change in magnetic flux which produces it.
Explain Faraday’s coil and magnet experiment
Explanation of Lenz law
Pg 257
Tries to oppose change which is responsible for it.
Mechanical energy converted into electrical energy.
In accordance with law of conservation of energy.
Give the theoretical proof of e= -(d(phi)/dt)
Pg 257-258 d(phi)/dt = d(Blx)/dt = Blv f= Bil dW= -Bil.dx dW/dt= ei dw= eidt = -Bildx e=-Blv
State Flemings right hand rule
Stretch the thumb, first finger and middle finger of the right hand so that they are mutually perpendicular to each other. If the first finger represents the direction of magnetic field and the thumb represent the direction of motion of the conductor, then the middle finger represents it direction of induced emf or current in the conductor.
What are Eddy currents?
Every metal consists of a large number of free electrons which are randomly moving within the metal. When the metal is subjected to changing magnetic field or flux, they experience forces and move in circular path of different radii depending in their speeds. There circular paths produce currents which resemble whirlpools or eddies in liquid. Hence, they are called Eddy currents.
How can Eddy currents be demonstrated?
Copper plate suspended by means of thread made to oscillate in its own plane like a pendulum.
Amplitude of oscillations decreases slowly.
If made to oscillate between poles of magnets, oscillations decrease fast and then it stops.
If slots are made amplitude decreases less rapidly.
Why do the oscillations do a copper plate suspended by a long thread, made to oscillate in its own plane like a pendulum stop when it is made to oscillate between the poles of magnet?
Magnetic flux linked with the plate goes on decreasing. Induced currents set up in the copper plate in the form of closed loops. They oppose oscillatory motion and are called Eddy currents.
Why do the oscillations do a copper plate suspended by a long thread, made to oscillate in its own plane like a pendulum stop less slowly when slots are made, when it is made to oscillate between the poles of magnet?
Electrons have to cover larger path to form loops (Eddy currents).
Why are thick blocks of metals never used as core in the transformer? What is used instead?
Eddy currents are set-up. So, generation of heat takes place.
To avoid: cores are made up of thin metal strips insulated from each other known as laminated cores.
Applications of Eddy currents
Deadbeat galvanometer
Induction furnace
Electric brakes
Speedometer
Explain application of Eddy currents in deadbeat galvanometer
Opposes oscillation of coil of insulated wire and brings it quickly to rest position.
Explain application of Eddy currents in Induction furnace
Small piece of metal in rapidly changing electric field. Strong Eddy currents are produced in the block of metal? Large mount of heat is produced and block melts.
Explain application of Eddy currents in electric (magnetic induction) brakes
To stop train, driver cuts off electric power supply to motor.
At the same time magnetic field is applied to the rotating drum fixed to the axle. Strong Eddy currents in rotating drum oppose the motion and train stops almost immediately.
Explain application of Eddy currents in speedometer.
Strong magnet, kept rotating at the speed of the vehicle, in an aluminum drum pivoted by means of a spring.eddy currents produced in drum. Drum turns in direction of magnet. Pointer attached to drum indicated speed on a calibrated scale.
Define self-induction
The phenomenon of production of an induced emf in a coil due to change in current in the same coil is known as self-induction.
What does the self inductance depend on?
Number of turns, shape, area of the coil and also on the material of the core.
What metals increase the value of self-inductance?
Iron, cobalt, nickel