Electromagnetic Induction Flashcards
(38 cards)
Describe the process of electromagnetic induction
An emf is induced when a magnetic flux linkage changes,
when conductor cuts across magnetic fields lines magnetic flux linkage changes
electrons experience a force pushing them to a side of the conductor, producing a potential difference (emf)
Describe how to find the direction of an induced current
Flemings left hand rule
Thumb = Motion of conductor (force)
First finger = field
Second finger = current
State the direction of current at each of the pole of a solenoid
Right hand circle
North-Anti-clockwise
South-Clockwise
Define magnetic flux
What is its unit?
ф = BA
The product of magnetic flux density and area
Weber
1 Wb = 1 tesla per unit area
Define magnetic flux linkage?
What is the unit?
NФ=BAN
The product of magnetic flux with the number of turns (n) on the coil
Weber turns
State Faradays Law?
The induced emf in a conductor is proportional to the rate of change of flux linkage through the circuit
Emf = -N* Change in ф * 1/t
State Lenz’s Law
The direction of induced emf always opposes the change that is causing it
The south pole of a magnet is moved into a coil of wire so that it induced a current
What direction does the current pass around the coil as the magnet moves in?
South pole presented so south pole induced
to conserve energy a south pole is induced to counteract the change that is inducing the current
South = clockwise
The south pole of a magnet is moved into a coil of wire so that it induced a current
What direction does the current pass around the coil as the magnet moves out ?
bar magnet passes through
to conserve energy a north pole is induced to counteract the change that is inducing the current
Direction = Anti-clockwise
Explain why Lenz’s Law exists
Energy is always conserved; it cannot be created
The induced current cannot be in a direction to aid the change causing it; as this would create electric and kinetic energy.
What is the flux linkage of a coil, when the normal to the plane of the coil is at an angle to the magnetic field.
Nф=BAN*cos(x)
Sketch a graph to show how flux linkage changes a coil is rotated in a magnetic field.
What is the significance of the gradient of the flux linkage-time graph
The emf is the negative of the gradient of the flux linkage against time graph
What a coil is rotated in a magnetic field, at which points is the rate of change of flux linkage
. Maximum
.Minimum
A) when the coil is parallel to the field (and when flux linkage = 0)
B) When the coil is perpendicular to the field (and the flux linkage is at a maxium)
Define the root mean square value of alternating current
An estimate value of direct current that would give the same heating effect as the alternating current in the same resistor.
Describe the construction of a transformer
Two insulated coils, the primary coil and the secondary coil wound around a soft iron laminated core at opposing sides
State 3 Design features of transformers that improve their efficiency
1) Low resistance windings - To reduce wasted energy die to the heating effect of the current
2) Soft iron core which is easily magnetised and demagnetised ( This reduced the power loss in repeated magnetisation and demagnetisation
3) A laminated core (layers of iron separated by an insulator) to reduce eddy currents within the core. This ensures the magnetic flux is as high as possible
State four ways of increasing the peak emf produced by a generator
1) Increasing the frequency of the rotation
2) Stronger magnet (larger b)
3). Bigger coil (larger cross sectional area )
4) More turns (Larger N)
What happens to currents and potential difference in a step-up transformer.
The potential difference in the secondary coil is greater then the P.D in the primary coil
The current in the secondary coil is less then the current in the primary coil
as power in both coils of identical resistivity is the same
If the transformer is completely efficient then
IsVs = IpVp
What happens to currents and potential difference in a step-down transformer.
The potential difference in the Primary coil is greater then the P.D in the secondary coil
The current in the primary coil is less then the current in the secondary coil
as power in both coils of identical resistivity is the same
If the transformer is completely efficient then
IsVs = IpVp
What kind of current is produced and used by a transformer
Why?
An Alternating current
An emf is induced by an ever-changing magnetic flux hence the emf induced is alternating producing alternating current
What happens when you move a straight conductor through a magnetic field
The electrons experiences a force pushing them to an end doc a conductor creating an emf across it.
As the rod obeys faradays law, it is changing flux as it moves through the field hence an emf is induced.
What is an eddy current
As the primary coils ever changing magnetic flux induces emf in the secondary coil, it also induces emf and hence mini currents within the magnetized iron core
What would be the emf produced when rotating a coil at a constant rate within a magnetic field
