Electromagnetic Induction occurs when an emf is
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber in a coil due to a
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber magnetic flux.
The English scientist Michael
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber investigated this relationship.
He found that if you moved a magnet in or out of a coil of wire, a
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber was generated (more properly called an emf (electromotive force).
He also realised that the
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber you moved the magnet (or the coil), the greater was the emf generated.
This is now known as Faraday’s Law of Electromagnetic Induction.
Demonstrating Faraday’s Law
1. Move the magnet in and out of the coil
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber and note a slight deflection.
2. Move the magnet quickly and note a
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber deflection.
Later on it was found that the direction of the emf could also be predicted.
This is known as Lenz’s Law.
The two laws together are known as the laws of
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber Induction
The Laws of Electromagnetic Induction.
1. Faraday’s Law states that the
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber of the induced emf is
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber to the
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber of change of flux.
2. Lenz’s Law states that the
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber of the induced emf is always such as to
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber the change producing it.
Magnetic Flux
To introduce the idea of magnetic flux – symbol φ (pronounced “sigh”), consider an area, A in a uniform magnetic field.
When
the magnetic force lines are perpendicular to this area (see diagram)
the total flux (φ) through the area is defined as the product of B by A.
φ = B X A
or
Magnetic Flux
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber (B)
Area A
The magnetic flux, φ, can be visualised as the number of magnetic field lines passing through a given area.
The
number of magnetic field lines per unit area, i.e. B, is then referred
to as the density of the magnetic flux or, more properly, the magnetic
flux density.
The unit of magnetic flux is the
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber and the symbol is
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber Now we are in a position to calculate the induced emf:
Remember Faraday’s Law:
The size of the induced emf is proportional to the rate of change of flux.
So Induced emf = (
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber Flux –
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber Flux) /
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber Taken
In Symbols:
The minus sign is a reference to
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber Law .
The N in the above equation refers to the number of turns in the coil.
Lenz’s Law
Lenz’s Law states that the direction of the induced
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber is always such as to oppose the change producing it.
Explanation
We know that when a magnet and coil move relative to each other, an emf is
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber .
Now if the coil is a conductor the induced emf will drive a current around the coil.
This current has a magnetic field associated with it.
The direction of this magnetic field will always be such as to oppose the change which caused it.
Demonstrating Lenz’s Law :
Magnet and
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber Ring
Apparatus
Aluminium ring, magnet, thread, retort stand.
Procedure
1. Move one end of the bar magnet towards and into the ring. The ring moves
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber from the magnet.
2. Hold the magnet in the
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber and quickly pull it away. The ring
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber the magnet.
Observation
When the magnet moves, the ring responds by moving in the same direction.
Explanation
The
moving magnet induces a current in the ring. This current creates a
magnetic field that exerts a force to oppose the motion of the magnet.
The magnet exerts an equal and opposite force on the ring and so the
ring moves as observed.
To Demonstrate Lenz’s Law (iii): Arago’s Disc (Induction Motor)
Apparatus
Aluminium or copper disc (centre punched), strong magnet, pivot.
Procedure
1. Place the disc on the pivot.
2. Move the magnet quickly in a circular motion above the rim of the disc.
Observation
The disc starts to
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber in the same direction as the magnet.
Explanation
The
moving magnet induces a current in the disc. This current creates a
magnetic field that exerts a force to oppose the motion of the magnet.
The magnet exerts an equal and opposite force and the disc rotates. The
relative motion between the magnet and the disc is reduced.
Applications
Induction motors are used in
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber , tachometers and some electric clocks.
They are also used as large motors in factories as they do not have brushes, commutators etc. to wear out.
Electric Generators
Here mechanical energy is being converted to
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber energy.
An Electric Generator is a device that converts mechanical energy to electrical energy.
The generators in power plants are designed to change direction 50 times a second (
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber = 50 Hertz).
Because the voltage drives the current it follows that the current also changes direction 50 times a second.
Alternating Current (A.C.)
Alternating current is current which changes direction 50 times a second.
Comparing Alternating and Direct Voltage and Curent
If the current (or voltage) is constantly changing, how can we say what its value is?
We can’t take the average value because it’s zero.
We use what’s known as the
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber mean square value (V r.m.s.)
This is obtained by dividing the maximum value (V
o ) by √ 2.
V r.m.s. = (V
o ) / √ 2.
The same works for
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber .
I r.m.s. = (I
o ) / √ 2
We
do this because the magnitude of the rms Alternating Current will have
the same heating effect as a Direct Current of the same magnitude.
e.g. If the rms value of an Alternating Current is 2 Amps, it will the same heating effect as
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber Amps direct current.
Mutual Induction
When the emf field in one coil changes, an
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber is induced in the other.
To Demonstrate Mutual Induction
Procedure
• Set up
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber coils side by side as shown.
• Close the switch – a deflection is seen on the galvanometer.
• Open the switch – a larger deflection is observed.
Observation
Each time the circuit is completed or broken, a
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber is obtained on the galvanometer. The deflection at the break is greater than at the make.
Explanation
At
the make and break of the circuit there is a change in the magnetic
flux linking the coils and so an emf is induced in the secondary coil.
The deflection is greater at the break because the current drops more
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber than it increases.
Mutual Induction and the Transformer
Apparatus
6 V a.c. power supply, coils of wire – 400 turns and 800 turns, soft iron core, two a.c. voltmeters.
Procedure
1. Set up the apparatus as shown below.
2. Switch on the a.c. supply (left hand side)..
ObservationA
continuous reading is obtained on the voltmeter.
Conclusion
The a.c. produces a
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber changing magnetic field.
The size of the induced emf may be increased by
1. Having the coils nearer each other
2. Winding the coils on the
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber soft iron core
This is the principle behind how a transformer works
The relationship between Voltage out and Voltage in for a Transformer
The relationship between Voltage out (Vo) and Voltage in (Vi) is determined by the
2 Aluminium away changing constantly currrent deflection Density direction electrical Electromagnetic emf Faraday Final follows frequency greater induced Initial Lenz’s oppose proportional quicker quickly rate ratio ring root rotate same size slowly speedometers Time two voltage Wb Weber of the number of turns on the primary Coil (Np) to the number of turns on the Secondary Coil (Ns)
Vi = voltage in, Vo = voltage out.
Np = Number of turns in primary,
Ns = Number of turns in secondary
Note
If the voltage is increased, the transformer is called a ‘Step-Up Transformer’
If the voltage is decreased, the transformer is called a ‘Step-Down Transformer’