Question

3.Explain why if a CD power supply is used, only when the power supply is turned on or of there is an induced current in the second coil (secondary).

4. Explain why when the switch is on, and there is a direct current through the circuit, there is no induced electric current in that coil.

Electromagnetic Induction

I. Objectives

1.   Verify the Faraday-Lenz‘s Law

2. Perform measurements with a set of coils to understand how a transformer works.

II. Theory

In 1819 Hans Christian Oersted discovered that around any current-carrying conductor exists a magnetic field. Then during 13 years Michael Faraday performed a great deal of experiments that lead him to discover the opposite effect:  He obtained electric current from a magnetic field.

We can summarize Faraday’s experiments using the next three examples.

A.  An electric current is induced in a changing magnetic field.

When a magnet is moved toward a loop of wire connected to a sensitive ammeter, the ammeter deflects, indicating that a current is induced in the loop.

B. An electric current is induced in a loop that rotates in a magnetic field.

C. An electric current is induced in a loop when a conducting bar slides on the two fixed conducting rails, in an external magnetic field, changing the area of the loop.

From these experiments we can conclude the following: the electric current and the electromotive force (EMF) are induced, 1) if there is a change in the  magnetic field magnitude, 2) if the orientation of the loops with respect to the magnetic field change,  3) if  the area of the loop changes.

The faraday’s Law state that: The induced electromotive force is directly proportional to the rate of change of the magnetic flux:

                                          Є = - dФ_B/dt      

Where    Ф_B is the magnetic flux:  Ф_B = ∫ B. dA    ; B is the magnetic field vector and dA is a differential vector perpendicular to the surface bounded by the loop.

If B (magnetic field vector) is constant:    Ф_B = B. dA  

                                                                      Ф_B = B dA cos θ

The magnetic flux is proportional to the net number of magnetic field lines that pass through the loop.

Transformer

A transformer consists of two coils, primary and secondary coils, mounted onto an iron core. When the switch in the primary circuit is closed, the ammeter in the secondary circuit deflects momentarily. The EMF (electromotive force) in the secondary circuit is caused by the changing magnetic field through the secondary coil.

Using the Faraday’s law:    Є₁ = - N1 dФ_B/dt;      Є₂ = - N2 dФ_B/dt, because the change in magnetic flux is the same for the primary and secondary coils, we can write:

  Є₁ / N ₁  = Є2 _/ N₂    and consequently_:    Є₂ =   (N₂/ N₁) Є₁

Changing the relation between N₁ and N₂ we can increase or decrease Є₂.

III. Experiments

A. Electromagnetic Induction.

1. Connect a 600 turn’s coil to the galvanometer.

1. Move a magnet slowly toward the coil.
2. Move a magnet quickly toward the coil.
3. Keep the magnet inside the coil
4. Repeat a) and b) moving the magnet away from the coil

Describe what happen with the induced current.

2. Set two coils (200 and 400 turns) onto the iron core. Connect the first to the   multmeter and the second to a CD power supply.

     a) Turn on the power supply (set 5 .0 Volt)

     b) Keep the power supply on

     c) Turn of the power supply

Describe what happen with the induced current.

B. Transformer

1. Set two coils, 400 (primary) and 200 turns (secondary) onto the iron core. Connect the primary to the AC power supply (6.0 Volt) and the secondary to the multmeter.
2. With the same primary and the same voltage change the secondary to 400, 600, 800, 1600, and 3200 turns and measure the voltage in the multmeter for each coil.
3. With 400 – 800 turns combination increase the voltage from 6.0 Volt to 10.0 Volt and measure the voltage in the secondary.
4. Remove the top of the iron core and repeat the last measurement.
5. Using only the straight part of the iron core into the coils, repeat the last measurement.
6. Remove the iron core and set the coils near each other and repeat your measurement again

Answer 1

3. When using a DC source in the primary circuit, there will be no induced current on the secondary circuit, since there is no variation in the magnetic flux.

4. Because there is no variation in the magnetic flux, and for there to be, the current in the primary coil should vary over time, but being a direct current there is no variation in the current and therefore there will be an induced current in the secondary coil, except just at the moment in which the switch is actuated since the current varies from a zero value to a maximum value, but then there will be no more current variation.