PHYS 2426 – Engineering Physics II
Faraday’s and Lenz’s Law
Leader: _________________________
Skeptic: _________________________
Recorder: __________________________
Encourager: ________________________
Materials
Pasco Galvanometer
Transformer Core
Cow Magnet
Bar Magnet
Masking Tape
200, 2 x 400, and 800 turn coil
Compass
Banana plug cable as needed
Alligator clip cable as needed
Introduction
In this lab we will explore Faraday’s Law. We will explore the basic phenomena of
induction and examine the negative sign in Faraday’s Law known as Lenz’s Law.
Part I – Faraday’s and Lenz’s Law
Procedure
Direction of current in the galvanometer
It is important in this activity to know the direction of the current in the galvanometer.
When positive current enters the red leads on the galvanometer and exits the common
lead, the needle will deflect to the right.
1. Set-up
Note: For a later section of this activity it is important that you connect the apparatus as
specified. So take your time and set up the apparatus correctly.
Notice how the turns are oriented on the 200 turn coil and which end of the coil is
connected to which plug. For reference purposes, call the end of the coil nearest you the
front. Position the coil so that it is in front of the galvanometer with its opening
perpendicular to the face of the galvanometer. Also, orient the coil so that the turns going
from front to back turn in a clockwise direction. Connect the front of the coil to the 5 mA
terminal on the galvanometer and the back of the coil to the common terminal on the
galvanometer.
Use the compass to identify the N and S pole of the cow magnet. Remember the N
pole of a magnet will repel the N pole of the compass. But don’t bring the strong cow
magnet too close to the compass or it will attract the compass needle no matter which
side is brought near. Label the N pole of the magnet for reference.
2. Observations
Quickly insert the N pole of the cow magnet into the front of the coil and observe the
needle of the galvanometer. Quickly pull the magnet back out and observe the needle of
the galvanometer.
Q1) What happened to the needle of the galvanometer when the magnet was inserted?
Q2) What happened to the needle of the galvanometer when the magnet was pulled back
out?
Reverse the direction of the magnet and try again.
Q3) Did you see any difference when you reversed the orientation of the magnet.
Repeat your observations for both orientations on the other side of the coil and
summarize your results in the table below. You should have a total of 8 observations.
Leave the last two columns blank for now.
Side of
Solenoid
(Front or
Back)
Pole
(N or S)
Direction of
Magnet
(into or out of
the solenoid)
Direction of
Needle on
Galvanometer
(R or L)
Sign of
current
(+/-)
Induced
Pole
(N/S)
Q4) List the cases where the galvanometer needle deflected to the left and to the right in
the space below.
Q5) Sketch diagrams for the cases where the galvanometer needle deflected to the left.
Be sure to indicate the orientation of the magnetic field and the direction of travel of the
magnet. Use your diagrams to explain how the situations that produced a deflection to
the left are similar.
Q6) Sketch diagrams for the cases where the galvanometer needle deflected to the right.
Be sure to indicate the orientation of the magnetic field and the direction of travel of the
magnet. Use your diagrams to explain how the situations that produced a deflection to
the right are similar.
Q7) If the magnet just sits inside the coil, do you see any deflection of the galvanometer?
Q8) What is happening to the magnetic field in the coil when you do see a deflection of
the galvanometer needle?
Question 8 is the essence of Faraday's law and why it was so difficult for Faraday to find
it. In his experiments Faraday had used steady magnetic fields to try and produce a
current, but it is a changing magnetic field that produces a current.
Lenz's Law
You should have noticed that there is a pattern when the deflection of the needle of
the galvanometer is to the left or the right. The result is known as Lenz's law.
Identify the terminals labeled + and – on the galvanometer. When a positive current
flows into the terminal labeled +, it will produce a deflection of the galvanometer needle
to the right.
Identify for each of your observations whether the current is + or – and fill in the
column of the table above for the sign of the current.
Q9) When you insert the magnet in the coil you produce a current. What does a current
in a coil produce?
Q10) Use the direction of current flow in the coil and your right hand rule for a solenoid
to determine the direction of the magnetic field induced by the current for each of the
eight observations you made above. For each of the 8 cases, label the pole induced on
the side of the coil where the magnet is being moved in the last column of the table
above.
Q11) In all cases when you inserted a N pole, what pole was induced on the side where
you inserted a N pole?
Q12) Would the induced pole try to attract or repel the inserted N pole?
Q13) In all cases when you removed a N pole, what pole was induced on the side where
you removed a N pole?
Q14) Would the induced pole try to attract or repel the removed N pole?
Q15) In all cases when you inserted a S pole, what pole was induced on the side where
you inserted a S pole?
Q16) Would the induced pole try to attract or repel the inserted S pole?
Q17) In all cases when you removed a S pole, what pole was induced on the side where
you removed a S pole?
Q18) Would the induced pole try to attract or repel the removed S pole?
Q19) Note that whenever you inserted a pole a like/opposite (circle one) pole was
induced, which would attract/repel (circle one) the inserted pole.
Q19) Note that whenever you removed a pole a like/opposite (circle one) pole was
induced, which would attract/repel (circle one) the removed pole.
Q20) Lenz's law is the observation of how the direction of the induced field compares to
the direction of the inducing field. The directions are _____________.
Magnetic Flux
So far we have only investigated a single magnet and coil. Let us investigate other
factors that might affect the induced current.
Q21) Try a weaker magnet such as the provided bar magnet. How does the strength of
the deflection compare when the bar magnet is inserted compared to when the cow
magnet is inserted?
Q22) Connect a coil with 400 turns. Try again with the cow magnet. How does the
effect compare?
Q23) Connect a coil with 800 turns. Try again with the cow magnet. How does the
effect compare?
Use the alligator clip wire to make a coil with 10 turns and connect it to the
galvanometer. Insert the cow magnet and note the size of the effect.
Wrap a coil again with 10 turns only make the radius of the coil half as big.
Insert the magnet and note the size of the effect.
Q24) How did the effect compare when the coil had half the radius? (Note: It can be
difficult to see a difference.)
You should have observed that the induced current was smaller if the magnet was
weaker, the number of turns was less and the area was smaller. This combination of
variables produces what is called the magnetic flux. The magnetic flux through a single
coil is defined as B = BnA, where n is a unit vector perpendicular to the area.
Remember that a dot product is defined as BnA = BA cos. If the magnetic field goes
through N coils, then the total flux is simply tot = NB . When you insert the magnet
into the coil, you change the flux because you change the magnetic field.
Q25) Are there any other ways to change the flux?
Q27) Try connecting a single piece of flexible wire across the terminals of the
galvanometer. Wrap three or four turns loosely around one end of the magnet. Rapidly
pull the ends of the wire so that you collapse the coil. Do you observe an effect? (Note
you will need to observe very carefully.)
Q28) In Q27 we saw that we could induce a current by changing the area. However
what are we actually producing when we change the flux? Find the SI units of
dB /dt. What kind of quantity has these units? Hint: what do you need to produce a
current?
You should have found in Q28) that the units of the rate of change of magnetic flux are
V, so we can sum up Faraday's law as the following: dB /dt = -E, where we have
denoted – as is customary – the induced potential by E for EMF. The – sign in Faraday's
law is Lenz's law. It means that the induced voltage is such as to try and oppose the
change in magnetic flex.
Q29) Faraday's law can be summed up as A _______________ magnetic flux induces
a/an ________________________, which __________________ the change.