Electromagnetic Induction Flashcards
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
A search coil outside of an electromagnet travels at a constant speed to and through an electromagnet solenoid
What happens to the induced emf as the search coil moves (3)
As the coil moves it’s cuts magnetic flux
The induced emf is proportional to the rate of change of flux linkage
As the coil moves through an area of higher magnetic flux density more emf is induced
Magnitude of the emf will increase until a peak value then it will decrease
Faradays Law
How does a bar magnet entering a coil connected to an ammeter demonstrate lenz’s law (4)
The polarity of the pole entering is the same as the polarity of the coil
Shown by current rotating around the coil clockwise or anticlockwise
this opposes the motion of the bar magnet
without
Otherwise the conservation laws are broken (energy)
What is the purpose of the iron core in a transformer?
Ensures more of the flux from the primary coil links with the secondary coil
By strengthening the magnetic field
Why do transformers work continuously when supplied with alternating current?
Alternating current produces a ever changing flux within the primary coil .
That induced an ever changing emf in the secondary coil
How does a transformer work
Ac supply is applied across the primary coil
This induced a magnetic field across the primary coil
The magnetic field magnetised the iron core
The alternating currrent & field causes the ever changing magnetic flux linkage in the primary coil that links with the secondary coil
this induces an alternating emf and therefore alternating current in the secondary coil
For a transformer equation what type of voltage is used in the calculation?
What is the equation?
Vrms
If a transformer is 100% efficient then ____________
If a transformer is x% efficient then ___________
(x/100)* Input power = Output power
(with rms current and voltages)
How do you calculate number of turns
Nф/BA = N
as ф=BA
Where Nф =Flux linkage
What is observed when S is closed?
Meter will show a current initially then return to zero
Current within coil p produced magnetic flux for an instant which links with coil q
As in q a change in flux is experienced an emf is induced
The induced emf causes a current through coil q
Returns to zero as there isn’t a constant rate of change of magnetic flux
How do you calculate Peak Ac current
or
I0
PeakVoltage / Resistance
What is the voltage and frequency of the mains AC
230v
50Hz
How day you work out the average power in an AC
Irms* Vrms
Define root mean square voltage
estimate of the value of non-oscillating voltage that would produce the same power dissipation in a resistive load as the alternating voltage
the square root of the time average of the voltage squared.
Ammeter defects in the opposite direction then goes to zero
While s is still closed the immediate reduction in resistance reduces the current
causing a negative change in magnetic flux in coil p
that links with coil q
inducing an opposite emf and opposite current flow to section A
