Download Faraday`s Law and the Genecon

Faraday’s Law – Genecon and Transformer
Equipment:
genecon
ac power supply
connecting wires
voltmeter
stopwatch
full transformer set
Multi-meter
Overview:
In this lab we will further explore the phenomena associated with magnets and
coils of wire. A generator is a device which uses relative motion between wires and
magnetic fields. The voltage output of a DC generator such as the Genecon can be written
as: V() = 0.707NBA. where  is the rotational rate of the coil in rad/s, N = #turns of
wire on the coil, B = strength of the magnetic field in tesla, and A = area around which
the turns of wire are wound.
A. Preliminaries
1. A coil of wire has the following parameters, A = 98cm2, N = 2122. Write clear work
showing that if this coil is rotated at 688 rpm (revolutions per minute) in a 0.33 tesla field
that the DC output would be 349 volts.
2. If the rotational rate were increased by 10% what percentage change in output voltage
would be observed on this generator? _____ Why?
B. Experiment
3. Use your Genecon to produce 2.0 volts as steadily as you can for a time of 15 seconds.
DO NOT ATTEMPT TO PRODUCE MORE THAN 4.0 VOLTS AT ANY TIME. While
the Genecon is producing 2.0 volts count the number of turns of the handle during the 15
seconds that you time.
Number of turns of handle = _______________ in 15 seconds time (2.0 V output)
frequency in revs/s = (Number of turns of handle)/15s = ____________________ rev/s
 = angular speed in rad/s = 2f = _____________________ rad/s
4. The gears of the Genecon amplify the rotational speed of the generator wires by a
factor of 48 times over the rotational speed of the crank. Thus actual = 48. If we assume
that the Genecon has coil-area 1.0 cm2 in a field of 0.25 tesla, calculate the number of
turns of wire on the coil inside the Genecon using V = 0.707NBAactual. Show work.
Transformers
Equipment:
ac power supply
connecting wires
full transformer set
Multi-meter
Introduction:
Transformers transform one ac signal into another ac signal through a shared
electromagnetic wave interaction. The changing magnetic flux of the driven primary
circuit induces a current in the secondary passive circuit. If 100% of the changing
magnetic flux is shared by both circuits, then Faraday’s Law allows us to write:
Vp
Np

( BA) Vs

t
Ns
where Vp and Np are the voltage and number of turns on the primary circuit, and Vs and
Ns are the corresponding voltage and number of turns in the secondary circuit.
In reality, only a portion of the magnetic flux will be shared by the secondary circuit. We
will call this portion the efficiency of the transformer, and use the equation:
(1)
Vs V p

 efficiency . We will explore this relationship experimentally in this lab.
Ns N p
Procedure:
1. With the power supply off, connect AC output of the power supply to the coil with the
smallest number of turns. Connect the voltmeter in 20V ac~ mode to the secondary coil
with the smallest number of turns.
2. Turn the voltage adjustment all the way counter clockwise. Set the power supply to the
lowest range ac voltage. Turn on the power supply.
3. Adjust the voltage to about 5.0V. Record the voltage induced in the secondary coil.
4. Turn down the voltage and exchange your secondary coil with the coil with the next
higher number of turns. Readjust the voltage to 5.0V and record the voltage induced in
the secondary coil. Repeat for subsequently larger number of turn secondary coils.
Complete the remaining calculations in the table.
Np
Ns
Ns / N p
Vp
Vs
Vs / V p
Vs / V p
Ns / N p
Questions:
1. Which column in the table represents the efficiency of the transformer? ______ Show
this using equation (1).
2. A transformer has an efficiency of 80%. How many turns would have to be on the
secondary coil to achieve a voltage of 2400V if the primary coil has 500 turns and the
input voltage is 120V? Show formulas and work.
3. A student wants to create a 6.0V AA “battery” from a 1.5V AA battery. Can the
student do this using an 80% efficient transformer? If so, give an example transformer
coil ratio. If not, why not?