Electromagnetic Induction Flashcards
Magnetic flux density and magnetic flux
It’s useful to think of magnetic flux density as the number of field lines per unit area
And magnetic flux as the total number of field lines in a given area
So that makes magnetic flux the product of the area swept out by a coil in a magnetic field and the magnetic flux density
Magnetic flux = BA the product of magnetic flux density and the area normal to the field
Electromagnetic induction in a rod
If there is motion of a conducting rod relative to a magnetic field, then the electrons will experience a force
This causes them to accumulate at one end
Hence inducing an EMF due to the p.d.
EMF in a solenoid or a flat coil
Move the coil towards or away from the poles of the magnet
Move the magnet towards or away from the coil
EMF is induced by the magnetic field changing as it passes through the magnet
Change in flux linkage and EMF
A change in flux linkage of 1 Weber per second will induce an electromotive force of 1 Volt in a loop of wire
Faraday’s law
The induced e.m.f is directly proportional to the rate of change of flux linkage
Lenz’s law
The induced e.m.f is always in such a direction as to oppose the change that caused it
The direction of induced current is always such as to oppose the change that causes the current
The induced current generates its own magnetic field that opposes the magnetic field that caused it
Use the right hand grip rule to find what direction the current will go in if the direction of the magnetic field is known
Alternator
A generator of alternating current
Slip rings and brushes to connect the coil to an external circuit
The output voltage and current change direction with every half rotation
Transformer
A device that uses electromagnetic induction to change the size of the voltage for an alternating current
Alternating current in the primary/input coil coil produces magnetic flux
Changing magnetic field passes through the iron core to the output coil where it induces an alternating voltage of the same frequency as the input
Increasing induced EMF in a wire
Move the wire faster
Use a stronger magnet
Make the wire into a coil
Root mean square value of alternating current
The value of direct current that would give the same heating effect as the alternating current in the same resistor
Step up transformer
More turns on the secondary coil than on the primary coil
Thus it increases the voltage
Step down transformer
Fewer turns on the secondary coil than on the primary coil
Thus it decreases the voltage
Flux linkage
The product of magnetic flux passing through the coil and the number of turns in the coil
It can be visualised as the volume of the magnetic field passing through the coil multiplied by the strength of the field
It’s a measure of the amount of a magnetic field passing through the coil
= BAN
Back EMF
EMF induced in the spinning coil of an electric motor or in any coil in which the current is changing
It acts against the applied pd
Flemings right hand rule/Dynamo rule
Used to determine the direction of an induced current
Thumb - direction of motion of conductor
First finger - direction of field
Second finger - the direction of the induced current
Eddy currents
Induced currents in transformer cores and other metal parts of AC machines
Increasing transformer efficiency
Low resistance windings (e.g. thick copper wires) to reduce power wasted due to the heating effect of the current
Laminated core which consists of layers of iron and layers of insulation - reduces eddy currents so the magnetic flux is as large as possible, also reduces the heating effect of induced currents in the core as currents cannot flow in a discontinuous conductor or when the resistance is very high
A core of soft iron - easily magnetised and demagnetised, reducing power wasted through repeated de/magnetisation
Design transformers so could are as close as possible - not all magnetic flux is transferred from primary to secondary coil
Why don’t we want eddy currents in the transformer core?
The dissipate power as heat
They create opposing magnetic fields reducing rate of change of flux linkage
Inefficiencies in transformers
Resistance in coils
Heat loss through eddy current
Energy wasted demagnetising and magnetising the core
Not all the magnetic flux is transferred from primary to secondary
Operational controls for oscilloscopes
Focus - making the wave clearer
Brilliance - brightness
Variable - time base e.g. ms per division, change time base to fit more waves on
Volts/division - increases the number of volts per box can be used to make the wave look larger, decreases error
Relationship between induced emf and magnetic flux/flux linkage for a rotating coil in a magnetic field
when induced emf = max, magnetic flux/flux linkage = 0
when induced emf = 0, magnetic flux/flux linkage = max
Conservation of energy in dropping a bar magnet through a copper tube
Induces a magnetic field to oppose the motion of the magnet, the work done against this repulsive force is transferred to electrical energy in the coil
Why does the magnitude of magnetic flux vary as the coil rotates?
The component of the magnetic flux density, B, perpendicular to the area of the coil changes
Flux linkage is greatest when the area of the coil is perpendicular to the magnetic field
It varies depending on cos theta
Magnetic flux is zero when the coil is parallel to B and maximum when coil is perpendicular to B
Key units
Weber (Wb) 1 Wb = 1 Tm^2
Tesla (T) = N/Am NA^-1 m^-1
Draw the following graphs:
(a) a conductor moving at a constant speed perpendicular to a uniform magnetic field (flux linkage against time)
(b) a wire coil rotating at a constant speed in a uniform magnetic field (flux linkage against time and emf against time)
See CGP textbook
What creates the trace on an oscilloscope?
An electron beam moves across the screen to represent a sinusoidal wave produced by an alternating current
Why is root mean square needed?
The voltage of an AC power supply is below the peak value most of the time - so the peak value can’t be used in comparison to a DC power supply
To compare with DC, an average is needed but a normal average won’t work as the positive and negative values will cancel out
So we use the root mean square values of current and voltage
Why does the reading on a top pan balance increase when there is a uniform magnetic field and a wire perpendicular to this field (clamped in place) on the balance?
There is an interaction between the magnetic field and the current-carrying wire
The wire experiences an upwards force due to the magnetic field
Hence the wire exerts an equal and opposite reaction force on the magnets - increasing the balance reading
Equation for a moving conductor in a magnetic field and emf
emf = BLv
Where L is length of the conductor passing through the field and v is velocity at which the conductor moves through the field
