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Electromagnetic induction..

posted Oct 27, 2015, 1:48 PM by Upali Salpadoru   [ updated Oct 14, 2017, 11:54 AM ]

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Electromagnetic Induction.

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Fig.1. The pointer fluctuates as long as the wire moves up and down. Courtsey U tube

As the motion would be up and down the direction of the current will alternate.

This is a method discovered by Michael Faraday to convert kinetic energy to Electrical energy.

The method involves the movement of a conductor in a magnetic field cutting the magnetic lines of force.

As long as you can keep the conductor and the lines of force interfering with each other, there will be a current this way or that way. The moment you stop the movement the current will also stop.


The direction of the Generated Current

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Fig.2. Fleming’s Right hand rule to get the Current.


Position the three fingers shown at right angles to each other.

If the Daddy finger, (thumb) can show the movement, Mother finger, (fore finger) shows the magnetic field the Brother finger, (centre finger) will indicate the current.


 Magnetic field convention.
This is a way to show the direction of magnetic force of lines when they are perpendicular to the printed page.
 X - Going into the page.
 o - Coming out of the paper.



Fig.3. A loop entering a magnetic field.


As the length BC crosses the lines of force it develops a current from C to B according to the Right hand rule. AB and DC will not produce an EMF as they do not cross the lines.

When the entire lopp has entered and passing through there will be no current. Do You know why?

As the currents in BC and AD will be in opposing directions they will cancel each other. When the BC has passed through and AD is cutting the lines it will get a current  from D to A.


When a conducting loop is entering a magnetic field, the induced electromotive force depends on the perpendicular length of the wire (or wires) crossing the field, the magnetic flux and the velocity of motion.


The formula:-  Emf. = Field strength x Length of conductor x velocity.

                            V = Bxlxv. V = Blv.

 Example 1.
A loop of copper wire 0.25 m. long passing through a magnetic field 0.5 Tesla at a velocity of 15 ms-1 . What will be the voltage of the current induced
 a. When front side is entering.
 b. When the loop is completely inside .
c,  When the rare side only moving inside..



 Solution.

 a. Using the formula:-      V = Bxlxv.

          We get              V  = 0.5 x 0.25 x 15.

                                    V = 1.88 Volts.

b.   V = 0 Volts.

 c.   V =  1.88 Volts with the plus and minus changed .

 



Fig. A rotating solenoid in a magnetic field.

Data.

Area of solenoid = 0.14 x 0.20  =  0.028 m2

Magnetic field     = 0.25 Tesla.

Time for a quarter turn  = 0.5 seconds.


Problem.

  1. Calculate the magnetic flux for a single loop and the induced voltage for a 90° turn .

                   



 Solution

  1. Magnetic flux = Field strength x Area =  B x A     =  0.25 T x 0.028 m2  =0.007 Wb.

  2. Induced voltage  =  Change in flux / time taken  =

                                     Ɛ ( V)=   Δ,φ / Δ,t

                   =0.007 / 0.5   =   0.014 V

       If the solenoid had 100 loops the voltage will be  0.014 x 100 =  1.4 V.


Lenzes Law

If an induced current flows, its direction is always such that it will oppose the change which produced it.

If a south pole of a magnet enters a coil the current induced will form a south pole opposing the movement. This necessitates the using of mechanical energy to get electrical energy. This upholds the law that “ Energy can neither be created nor destroyed”.


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Fig.4  Inserting a South pole of a magnet creates a South pole by the induced current.



\Motion due to a current


Just as a current is induced when a conductor crosses a magnetic field, a current in a conductor should produce a force.   


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Fig.5. Movement of a current carrying conductor in a magnetic field.


This is a method to keep a conductor in a magnetic field. As it is hanging it can move to left or right. According to the given conditions which way will it swing?









Fig.6  Fleming's left hand rule to get the  movement.  







Faraday's Law of electromagnetic induction.


Units on electromagnetism

Magnetic Field

Magnetic flux

A magnetic field is the region in space where magnets and moving charges experience a force.

The SI unit for measuring magnetic field strength is the tesla (T).


Symbol:-  B


If the magnetic field is perpendicular to the direction of particle’s motion, then we have,

F=qvB


This is a measurement to show how much magnetic field passes through a given area.

The unit is tesla x meter2.

This is the same as 1 weber. Wb.


Symbol:- phi=Φ, ( φ or ϕ)



Magnetic flux =  magnetic field x Area.

           \Phi=BA



 Faraday's Law
Voltage induced in a conductor is equal to the rate of changing magnetic flux.
 
V = φ / t



Mutual Induction

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Fig.7. A change in EMF in coil A results in a change in a nearby coil.


The two insulated coils shown here are independent of each other. There is no way a current passing in coil A to get to coil B; there is a strange phenomenon that occurs.

When the switch is closed there is deflection in the meter but soon comes back to zero. When the current is stopped again there is a deflection.


What does that show?


When a current passes in A the soft iron core becomes magnetic. This creates a change in the magnetic flux which induces an emf in the coil B. When a current is flowing in A, as there is no change in magnetism, coil B does not develop a current. If you open the swith now a change in the magnetic flux will occur and a current will be induced in B.  This phenomenon is called mutual induction. The two insulated coils shown here are independent of each other. There is no way a current passing in coil A to get to coil B; there is a strange phenomenon that occurs.

When the switch is closed there is deflection in the meter but soon comes back to zero. When the current is stopped again there is a deflection.

What does that show?

When a current passes in A the soft iron core becomes magnetic. This creates a change in the magnetic flux which induces an emf in the coil B. When a current is flowing in A, as there is no change in magnetism, coil B does not develop a current. If you open the swith now a change in the magnetic flux will occur and a current will be induced in B.  This phenomenon is called mutual induction


This is the principle behind a Transformer.

Transformer



Fig.8 Step down transformer.


A transformer has two separate insulated coils wound on a core of iron.

When a fluctuating current such as an alternating current as fed into one coil, a similar current is produced in the other coil due to mutual induction. The strength will depend on the number of windings in the two coils.

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      Example 2.

Find the number of turns required in the secondary coil to obtain 12 V current using 220V main domestic supply. The primary coil has 1000 turns.

Vs/Vp = Ns/Np

12/ 220 = N/ 1000

N= 12x1000 / 220

N= 54.


Efficiency of a transformer


When converting one form of energy into another we can never obtain 100% results. Some energy will be converted into various other un-required types. In other words some energy will be wasted.  The ratio of energy supplied and the useful energy obtained is the efficiency. This is usually given as a percentage.


 Efficiency % =   Power supplied / Power obtained.

Electrical Power  = Volts x Amperes





Generating Electricity 


Electrical induction is used to generate electrical power for domestic and inductrial use. Principle behind alternating current production is explained here. But the actual generators are much more complex than this.


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Fig.9. AC Generator.

The coill shown in red lines is made to rotate in a magnetic field. As the turns marked B go up it develops a current , the direction of which can be obtained by the Left hand rule.EMF produced by the turns in A will supplement the induced current. The wires on right and left will not give an EMF as they do not cross the magnetic lines.
As the two ends rotate with the coil they cannot be fixed to any wires out side. The device used here is to connect the to two rings, which will also rotate with the coil and the brushes to be in contact with rings.
As the side B after a half turn will change the direction, the current will change. So the current produced here will be an alternating current. If you use one coil as shown even that current will not be uniform. When the coil is in a vertical position there will be no current. 
Do you know why?
  
Getting a Direct Current.
In order to get DC, only a minor modification is necessary. Can you think of a method to do this?
What is necessary is to get the turns moving up always to touch the same brush. This has been accomplished by using a split ring instead of two rings. The diagram is given below.


















Always the side that is going down will touch the red brush and the side coming up will touch the blue brush.
Yet the current will not be uniform, as when the turns are moving in a horizontal plane will not induce a current.

If you wind another coil in the armature the power produced can be doubled. Then you also will have to have a ring with 4 sections.  Number brushes can remain the same.

DC Motor

DC generator can also be used as a motor. If you supply a current through the brushes the coil can rotate. If you use the Flemings Right hand rule yoy can know which side will go up.


           

1.0 Explain what the mini violtmeter will indicate when:-

1.1. When a copper coil is slowly lowered into N pole.

1.2 When it is slowly lifted out.

1.3 when  it is lowered into the S pole

1.4  When the coil is lifted out of the S pole.   (5x4=20 marks)


2.0 A bar magnet freely falls through a copper loop.


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2.1  What will be direction of the induced current on entering the loop ?

Will it be clockwise or counter clockwise?

2.2 What will be the direction when the magnet emerges from the loop?.

                                                                                   (5x2 = 10 marks)




3.0

Q 3.jpgA metal rod AB is moved in different directions inside a horse shoe magnet. Describe the induced current between a and B.for the following movements.

3.1  Left;  that is from N to S.

3.2 Right from S to N.

3.3 AB moved towards the cavity of the horse shoe magnet.

( 5x3= 15 marks)


Q. 4.0

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AB is a 0.25 m. long conductor moving in a magnetic field of strength 5.0 Tesla. The velocity perpendicular to the magnetic lines is 5 ms-1.Q 4.0.jpg

Find the following:-

4.1 The direction of the magnetic lines.

4.2 The direction of the current in AB.

4.3 The EMF induced using the formula

V = B lv.

4.4 If the circuit has a resistance of 0.5 Ohms what would be the reading on the Ameter?

4.5 If the voltmeter is replaced by a battery, so that positive comes to A , what is likely to happen?

                                                                (5x5 = 25 marks)

5.0  

Wires leading to primary circuit are not shown


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5.1  What is shown by the picture and the diagram?

5.2 230 V , AC is given to the primary circuit what will be the voltage to the lamp shown? Neglect energy losses.

5.3  What is the expected voltage between E and A?

5.4 If you double the turns in the primary, what effect will be there on resulting voltages?                              (5x4= 20 )


       

6.0 The diagram shows a DC Motor.

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6.1 Which side will go up due to current in the coil, A or B ?

6.2 What happensafter half aturn?                         (5x2=10)


      For the Answers click  Physics- answer page A - K




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