Download Electomagnetism: Galvanometer

Electomagnetism: Galvanometer
Lab Plan
In this lab we will demonstrate the relationship between electricity and magnetism and the
resulting force produced. We will also discover how to create and detect magnetic fields. A
Galvanometer is basically a current detector that utilizes the relationship between electricity
and magnetism to determine the direction and magnitude of current. Originally they were
used to find faults in telecommunication cables. Galvanometers are analog meters that have
recently been replaced by analog to digital convertors for some uses. However galvanometers
are still utilized in positioning and control systems.
Parts Choice
Here are the materials we will need to construct our galvanometer:
About 8 feet of insulated wire (18-24 gauge)
Metal Sewing Needle
3 feet of thread
Permanent magnet
9-volt battery
A small strip of paper
Scissors
Lab Write up
Galvanometer
First we need to magnetize the needle by rubbing one side of the permanent magnet
against it about 30 times in the same direction. Next cut the paper into a small arrow
and stick the needle lengthwise into the paper arrow as shown below:
Figure 1: needle and arrow shaped paper
Now we need to put some thread through the top edge of the arrow so it can hang
freely. With the wire we make a loop about three inches in diameter. We continue to
wrap the wire in the same pattern as the original loop until we have a coil of about five
loops. We need to keep some wire free on both ends. Using the thread we tie the wires
together. Next we tie the thread attached to the paper to the top of the loop. The
paper and needle should hang freely inside the middle of the loop as shown below:
Figure 2: Copper wire connected to battery and arrowed paper
Now we need to connect the wire to the battery. Attach one end of the wire to the
positive terminal of the battery, and the other wire end to the negative terminal. Pay
careful attention to the paper arrow. The phenomenon we see here is known as the
Lorentz Force:
where
B is the magnetic field (in teslas)
q is the electric charge of the particle (in coulombs)
v is the instantaneous velocity of the particle (in meters per second)
× is the vector cross product
Figure 3:Real life picture
We can determine the direction of the magnetic field by using the right hand rule:
Figure 4: Right Hand Rule
Question 1: Why did the arrow turn out from the loop? Current through a wire
produces a magnetic field around the wire. When current flows through a loop, it
produces a magnetic field inside of the loop. Since the needle is magnetized, it tries to
align itself with the magnetic field produced by the current in the loop.
Question 2: How does a compass work? A compass works in the same fashion. The
needle inside of a compass is magnetized. That needle tries to align itself with the
magnetic field produced by the earth.
Question 3: What direction is the current flowing in the loop? The current will be
flowing counter clock wise if the arrow is pointing outward and clock wise if the arrow is
pointing inward.
Question 4: What would happen if you turned the loop changing the direction of the
current? The arrow will move to the new direction of current. This is because the
direction of the magnetic field is dependent on the direction of the current.
Question 5: What rule allows us to determine the direction of current? Why?
The right hand allows us to determine the direction of the magnetic field. The Lorentz
Force equation provides this by taking a cross-product.
Conclusion
Hopefully you are now familiar with the relationship between electricity and magnetism and
how the two forces combine to produce the electromagnetic force. In fact electromagnets,
also known as solenoids, can become quite sophisticated and are used in a variety of
applications. It’s hard to imagine life without electromagnetism at work. For further
exploration of electromagnetism try using wires with various resistances. You can even
construct your own electromagnetic compass.