Electromagnetic Induction Flashcards
How is magnetic flux(Wb) calculated? And what is it?
Magnetic flux density x Perpendicular area. It is the measure of the amount of field lines passing through a given area
How do you calculate the effective flux through this coil?
Must take component of field perpendicular to surface
(Or component parallel to normal of surface)
How does the number of turns in a coil affect its flux?
Flux doesn’t change
But flux linkage increases
How do you find the flux linkage through this coil?
Must take component of field perpendicular to coil
(Or component parallel to normal of coil)
How do you induce an emf in this coil?
Moving the magnet in and out of the coil
Causes a change in flux linkage
(Faraday’s Law) which causes an emf which causes a current to flow
If the magnet is stationary why is there no emf induced?
Emf only induced if the flux linkage is changing
(Faraday’s Law)
Which way does the current act in this coil and why?
Current acts to create a magnetic field opposing the increase in flux linkage
(Tries to keep magnet from entering)
Due to Lenz’s law
Which way does the current act in this coil and why?
Current acts to create a magnetic field opposing the decrease in flux linkage
(Tries to keep magnet from leaving)
Due to Lenz’ law
Why does moving the magnet faster induce a larger emf?
The rate of change of flux linkage is greater
(Faraday’s law)
What is Faraday’s Law of electromagnetic induction?
The magnitude of the emf is equal to the negative of the rate of change of flux linkage
What is Lenz’ law?
The direction of an induced emf tends to produce a current which opposes the
change causing it
(The direction of an induced emf, due too faradays law, tends to produce a current which creates a counter magnetic field that opposes the
first agnetic field)
Why can’t the current act in this direction?
The magnetic field produced by lenz’s law attracts the magnet (in the same direction as the first magnetic field)
Increasing its kinetic energy
Energy has been created out of nothing!!!
(Against Lenz’ law)
What is the right hand grip rule?
A current in a coil produces a magnetic field (this is how lenz’s law works) as shown the same way a magnet moving into a coil induces an emf in the coil
What is wrong here?
Flux linkage becomes negative when coil flips!!!
How is the induced emf related to the graph of flux linkage?
(From Faraday’s law)
What is the corresponding graph of induced emf?
emf = -ve gradient
If the coil moves across the field at a constant speed what do the flux linkage and emf graphs looks like?
emf = -ve gradient
How do you calculate the emf induced between the ends of this conducting bar moving across a magnetic field?
When do you use Fleming’s Right Hand Rule
For generators!!!
F - the electrostatic force
B - magnetic flux density (north to south)
I - the direction of current (+ve terminal to -ve terminal)
For the AC generator when is the max emf induced?
When change in flux linkage is max
(When flux linkage = 0)
For the AC generator when is 0 emf induced?
When change in flux linkage is 0
(When flux linkage = max)
What is the corresponding emf graph for the AC generator?
How is the max induced emf calculated for the AC generator?
How do you increase the max induced emf of the AC generator?
Increase any of the terms in the equation:
How is the AC generator different to the DC motor?
Generator: Kinetic energy -> Electrical energy
(Motor: Electrical energy -> Kinetic energy)
Generator: Slip rings keep each side of coil connected to same side of circuit
(Motor: Commutator ring switch polarity every half cycle)
What is the equation for flux linkage?
N Φ = NBA
Make sure you use the perpendicular components of flux and area
What is the definition of flux linkage?
The total magnetic field (flux) traveling through a coil with multiple turns
How does moving a magnet into a coil induce an emf?
As the magnet moves into the coil the electrons in the coil experience a force from Fleming’s LHR. This causes the electrons to travel around the coil, building up opposite charges on either side of the coil. The difference in charges causes a potential difference across the coil inducing an emf. When the magnet is moved out of the coil the electrons experience an opposite force so the charge build up is reversed, meaning the emf induced is reversed and the current if connected up to a circuit would flow the other way. The magnet must be moving as magnets only effect charged particles if either one of them is moving
What are the two applications of Lenz’ law?
- Magnet in a pipe
- Interacting coils (magnet in a coil)
Describe the ‘magnet in pipe’ scenario
As a magnet falls through a pipe, there is a change in flux linkage above and below the magnet. Therefore, due to Faraday’s law of EM induction, there is an emf induced inside the pipe (coil) on either side of the magnet. Due to Lenz’ law, the currents act to oppose the change in flux linkage by creating a counter magnetic field that repels the magnet from below and attracts the magnet from above.
Describe the ‘magnet in half pipe’ scenario
As a magnet falls through a pipe, there is a change in flux linkage above and below the magnet. Therefore, due to Faraday’s law of EM induction, there is an emf induced above and below the magnet. However, as the pipe isn’t closed (doesn’t act like a coil), currents can’t flow so no counter magnetic fields are formed by Lenz’s law. Therefore, there is no force on the magnet falling and travel time is unaffected.
Describe the ‘interacting coils’ scenario
When you have two coils, and you connect one to a battery, the current flows through the first coil creating a magnetic field (Faraday’s law) **. There will be a positive change in flux linkage which causes an induced emf to oppose the change in flux linkage in the other coil (Lenz’s law) which causes current to flow in the other coil . This all happens in reverse when the battery is disconnected from the first coil.
What is the formula for the emf of a conducting bar moving through a magnetic field?
E = BLv
E - induced emf
B - magnetic flux density
L - length of conductor
v - average velocity of conductor
Why is an emf induced when a conducting bar travels through a magnetic field?
As the bar covers an area there is a force acting on that bar and from FLHR the electrons in the bar all move to one side. This difference in charges across the bar causes an induced emf. The area the bar has covered while moving can be imagined as the area of the coil and we can therefore work out Flux Linkage
How does Lenz’ law work?
As a magnet goes through a coil, the change in flux linkage in the coil induces an emf which causes a current to travel around the coil, producing a magnetic field which opposes the change in flux linkage (repels the magnet entering and attracts the magnet leaving the coil)
