Electromagnetic induction Electromagnetic induction In this section you will learn: The concept of electromagnetic induction What effects the size and direction of the induced current/emf Experiment to demonstrate the electromagnetic induction. Concept of magnetic flux and unit Understand and use Faraday's laws of electromagnetic induction. Understand and use Lenz's law How electricity is generated using generators. Experiment to demonstrate Lenz's law Understand a.c. current and voltage. Calculate r.m.s I and V values for a.c. Understand mutual and self induction Effect of inductors ón a.c. Uses of inductors.
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Electromagnetic induction In this section you will learn: The concept of electromagnetic induction What effects the size and direction of the induced current/emf.
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In this section you will learn:The concept of electromagnetic inductionWhat effects the size and direction of the induced current/emfExperiment to demonstrate the electromagnetic induction.Concept of magnetic flux and unitUnderstand and use Faraday's laws of electromagnetic induction.Understand and use Lenz's lawHow electricity is generated using generators.Experiment to demonstrate Lenz's lawUnderstand a.c. current and voltage.Calculate r.m.s I and V values for a.c.Understand mutual and self inductionEffect of inductors ón a.c.Uses of inductors.a.c. And capacitanceTransformers, step up and step down and uses.
We have seen that if you have a current flowing in a magnetic field, a force will act on it to try to move it.
A current is induced in a conductor which moves inside a magnetic field.
What might happen if you move a wire in a magnetic field?
As long as there is a changing magnetic field an emf is induced.
Whenever the magnetic field passing through a coil changes and emf is induced. This is called electromagnetic inductionelectromagnetic induction.Show using datastudio with a coil, voltage and current sensor.Show using datastudio with a coil, voltage and current sensor.
The size of the emf depends on:The strength of the magnetic fieldThe number of loops (turns) in the coilThe speed of the coil or magnet.
The direction of the emf depends ón:If the magnet is approaching the coil or moving away from it.If a north or south pole goes in first.
Notice the S pole in the coil to oppose the motion of the magnet towards it. (How?)
Notice the N pole of the coil to oppose the motion of the magnetic away from the coil. (How?)
Mains electricity is high voltage and its direction changes 100 times every second (alternating current). This makes its behaviour very different to the electricity that we get from a cell or battery.
R.M.S values of a.c.When we give a value of say 5A to an a.c current we mean it has the same heating effect as 5A d.c.
It has a maximum value of greater 5A in each direction. This 5A value is called the rms value.(Note we take rms (root meán square) values as way averaging the the a.c I and V as an ordinary average would give zero.)The following are the rms equations for I, V and P
Irms
= I0 / 2 or I
o = I
rms x 2
Vrms
= V0 / 2 or V
o = V
rms x 2
P = Irms
X Vrms
or P = Irms
R2
I0 and V
0 are the max values of I and V respectively
Getting the electricity to your home is not an easy business. A great deal of energy could be lost in the cables that go from the power station to the local distribution point.
We know that V = IR and that P=IV
so we can see that P = I2R - this is an important result.
Suppose a long wire has a resistance of 2 and we pass a current of 10A through it. P = I2R = 102 x 2 = 200W
that means that 200W of power are wasted in the wireWhat if we put 10 times the current through it: 100A?
P = I2R = 1002 x 2 = 20000W
that means that 20kW of power are wasted in the wire10 times the current and 100 times the wasted power. For this very reason electricity companies must transmit their high powers at low currents.
Fortunately it is possible to do this by first stepping up the voltage (this lowers the current) and then transmitting it. It can finally be stepped down when it is close to the place that it will be used in.
The size of the induced emf is increased byHaving the coils closer togetherWinding the coils on the same soft iron core.Increasing the number of turns of both or either of the coils.
Demostrate mutual inductance using two coils.
Induction Induction
Self-inductance.Self-inductance.When an a.c current passes through a coil it induces an emf due to the changing magnetic field of the current. This induced emf oposes the varying emf that causes it. (called back emf)
The size of the back emf can be increased by winding the coil on a soft iron core.
Induction Induction
A.C. and inductorsA.C. and inductorsA coil has a greater resistance to a.c than d.c.
When a d.c flows through a coil its resistance is simply its ohmic resistance using V = I R
When a.c flows through a coil because of the constantly changing magnetic field the resistance is the ohmic resistance plus the resistance due to the back emf which opposes the current causing it.
The greater the self inductance the more resistance a coil offers to a.c.
Induction Induction
Uses of Inductors.Uses of Inductors. Smooth out variations in d.c power supply units. Tuning circuits for radios. Dimmer switches used in stage lighting.
Capacitors and a.cCapacitors and a.cCapacitors do not allow d.c to flow. (once charged)Capacitors do allow a.c to flowThis is because the capacitor charges and discharges as the current changes direction
Induction Induction
Induction Induction
If a changing magnetic field in one coil induces an emf in a nearby coil this is called mutual induction.
The voltages across the two coils vary in direct proportion to the ratio of the number turns (N) on the coils. I.e.
However, two coils just placed next to each other would make a very inefficient transformer as much of the magnetic field from the primary windings would not pass through the secondary windings.
The field can be concentrated by placing both coils onto a continuous, laminated, soft iron core.
However, two coils just placed next to each other would make a very inefficient transformer as much of the magnetic field from the primary windings would not pass through the secondary windings.
The field can be concentrated by placing both coils onto a continuous, laminated, soft iron core.
Primary
The input
Secondary
The output
The continuous nature of the core makes sure that as much of the field from the primary, passes through the secondary.
However, two coils just placed next to each other would make a very inefficient transformer as much of the magnetic field from the primary windings would not pass through the secondary windings.
The field can be concentrated by placing both coils onto a continuous, laminated, soft iron core.
Primary
The input
Secondary
The output
Soft iron is used as it has no residual magnetism.
Work is not wasted by overcoming residual magnetism in the soft iron.
However, two coils just placed next to each other would make a very inefficient transformer as much of the magnetic field from the primary windings would not pass through the secondary windings.
The field can be concentrated by placing both coils onto a continuous, laminated, soft iron core.
Primary
The input
Secondary
The output
The laminations of the core mean that it is made of thin sheets of soft iron, insulated from each other.
This stops eddy currents flowing in the core which would be a waste of energy, producing unwanted heat in the core.
Electricity in power stations is generated a high voltages 20 -30 kVand then stepped up to220- 400 kV to reduce loses in transmission of the power in the grid. This has to be reduced down to 220V for the domestic use.
Computers, televisions and all manor of electronic devices need transformers to supply the necessary voltage for the device.