EC-7d  Induction - Faraday Discovery

INTRODUCTION:

Michael Faraday (1791-1907) was a bookbinder journeyman (English for apprentice). He read some of the books he was supposed to bind, and became interested in `natural philosophy' (the term science did not exist in the XVIII$^{th}$ century).
In 1813 he applied for a job as a technician in the laboratories of the Royal Laboratories to professor Davy, he got the job. Some people say that this was the greatest discovery Davy ever made! He worked all his life in the laboratories, starting as a technician, became Director of the Laboratories, was elected to The Royal Society of London, and became a Professor at the Royal Institution. Some of you will be pleased to know that he knew no math, and never used math in his researches.

Faraday performed many kinds of experiments, in chemistry, optics, and metallurgy, but perhaps the most important experiments were on electricity and magnetism. Having learned of Oersted's experiments on the magnetic fields produced by an electric current, he wondered if the reverse could be true: perhaps magnetic fields could in turn produce an electric current. His first experiments were unsuccessful, until he realized that it was the change in magnetic field that produced a momentary current.

His first experiments were similar to the one you will be doing today: two separate coils were wound on an iron ring, the current in the primary coil was interrupted, and the momentary current in the secondary was observed.

The explanation of this effect is, as you know, that the change in current in the primary circuit produces a change in the magnetic field that exists inside the secondary coil, the electromotive force in the induced secondary is then

$${\cal E} = -AN \cdot(\Delta B/\Delta t)$$

where $N$ is the number of turns, and $A$ is the cross-sectional area of the secondary coil.

OBJECTIVES:

To observe the induced momentary emf that appears in a secondary circuit when the current in the primary circuit is turned on, or interrupted. This is the original Faraday discovery: induction of a current by a varying magnetic field.

Figure 7: Faraday's experiment

In procedure II you will observe that induced momentary emf appears also in the primary circuit itself when the current is turned on, or interrupted; you will be observing the self induction of a coil. Faraday used a switch to interrupt or start the current as shown in Fig 1; you will use the computer to do the same job: the computer will produce a square wave, the voltage will change abruptly from 0 volts (current off = switch open) to a value of about 4 Volts (current on = switch closed) as shown below.

Figure 8: The square wave

EQUIPMENT:

$\Rightarrow$

PC and interface, with the power amplifier plugged in port A.

$\Rightarrow$

Two voltage sensors are plugged in the interface. The voltage sensor plugged in the B port measures the voltage across the external resistor; the voltage sensor plugged in the C port measures the voltage across the coil.

$\Rightarrow$

Computer Controlled Power Amplifier.

$\Rightarrow$

A pair of nested coils. The outer one is the `primary'. It has about 2600 turns of fine wire and has a resistance of $\sim 95 \Omega$. The primary coil is insulated from the inner secondary coil, an iron bar fits inside the inner coil.

$\Rightarrow$

A plug board and a $100 \Omega$ resistor.

PROCEDURE I:

1.

Click on the Launch EC-7D icon below (web version) to initiate the PASCO software window.

2.

Check that the power amplifier is on, (the yellow pilot light labeled `Power' should be on and the red pilot light light labeled `Distorting' on the power amplifier should off).

3.

Check that the amplitude is $\sim$ 4 Volts, and the frequency is $\sim$ 10 Hz. CLK on the ON button of the Signal Generator, then DCLK on RECORD. DCLK on STOP after a second.

Figure 9: The schematic setup

Figure 10: The physical setup

4.

The Monitor now displays two curves:
- one curve (curve A) looks like a square wave, but not quite; it shows the voltage across the external $100 \Omega$ resistor, and therefore tells you the current flowing in the coil.
-the other curve (curve B) shows the induced voltage across the secondary coil

5.

CLK on GRAPH SETUP (8) then print the graph of one of the peaks [instructions in PASCO Interface and Computer Primer]

QUESTIONS:

Q1

The shape of curve A is not quite a `square wave'. Why not? HINT: this is an LR circuit with a time constant $\tau = L/R$

Q2

Are the peaks of voltage across the secondary coil, (curve B) that correspond to current on vs. current off about the same size? Explain.

Q3

Are the peaks of voltage across the secondary coil, that correspond to current on vs. current off of the same polarity? Explain.

PROCEDURE II: (15 min)

1.

Remove the voltage sensor banana pins from the terminals R and S of the secondary coil and insert them into the terminals P and Q of the primary coil, without disconnecting these terminals from the power supply.

2.

CLK on the ON button of the Signal Generator, then DCLK on RECORD, DCLK on STOP after a second.

3.

The Monitor now displays two curves:
- one curve (curve A) looks like a square wave, but not quite; it shows the voltage across the external $100 \Omega$ resistor, and therefore tells you how the current flowing through the primary coil varies with time.
-the other curve (curve B) shows the voltage across the primary coil.
The two curves look very similar: they should have about the same height, and should rise and fall at the same times.

If curve B is inverted relative to curve A, exchange the banana pins in terminals P and Q and repeat item 1.2 of this procedure.

4.

CLK on GRAPH SETUP (8) then print the graph of one of the peaks [instructions in PASCO Interface and Computer Primer]

QUESTIONS

Q1

The shape of curve A is not quite a `square wave', why not? HINT: this is an LR circuit with a time constant $\tau = L/R$

Q2

The curve that describes the voltage across the primary coil, (curve B) is similar to curve A. Why? HINT: think of two approximately equal resistors in series.

Q3

The curve that describes the voltage across the primary coil, (curve B) is similar to curve A, but it shows `spikes'. Why? HINT: the voltage across this coil is due to two factors, R and L.