Lab 2: Week 3
Electromagnets and Transformers
PRE-LAB ASSIGNMENT: To be completed and turned in before lab. You should
come to lab having read through the description of the lab and thinking about how
the lab applies to what we are learning in class and fits into our everyday lives.
1) Give an example of where transformers are used in everyday life and what their
function is for the example you’ve chosen.
2) From what you have learned in class, assess whether or not you can harm yourself
working with the voltages you will be using in this lab.
3) Why will the battery drain quickly when you hook up the wires across it? How do
you avoid draining the battery in this lab? In this lab, you are going to study magnetism. To do this, you will construct your own
electromagnet and see what makes it stronger and weaker, and reverses its
direction. Then you will turn your electromagnet into a transformer.
Background Info—Electromagnets
An electromagnet is a coil of wire that’s magnetic only when an electric current flows
through it. To make an electromagnet even stronger, its wire is usually wound around
iron or another magnetic material. The field from the wire magnetizes the core (see text
pg 384 ff). The strength of this electromagnet increases when the current flowing around
the core increases. This can be increased by either sending more current through the wire
or increasing the number of turns around the core (that is wrapping the wire around the
core more times). The direction of the current flow determines the direction of the
magnetic field.
Background Info—Transformers (see text sect. 11.2)
A transformer allows you to transfer power from one closed circuit to another closed loop
circuit without having wires connect the two circuits. Specifically, it is an electrical
device that changes the alternating current in a circuit operating at one voltage and
produces an alternating current in another circuit at (often) a different voltage. The
operation of a transformer relies on two effects. The first effect, which we explore with
the electromagnet, is that an electric current will create a magnetic field. The second
effect is that a changing magnetic field will create an electric field. In a transformer, the
AC electric current in one circuit creates a varying magnetic field, and that varying
magnetic field creates an AC electric current in another circuit. The only connection
between these two circuits is a magnetic field. A transformer consists of primary and
secondary coils. Alternating current is sent into the primary coil and the “transformed”
current is taken out of the secondary. As discussed in the text, the ratio of voltages and
currents between the two coils depends on the ratio of the number of turns in the primary
and secondary coils. Transformers normally use the 60Hz AC line as their source of time
varying current. In this experiment you will create a time varying current simply by
rapidly connecting and disconnecting a battery. This gives you a crude AC current that
can still be used with a transformer.
Things you will Need:
3 wires 3 feet long
2 wires 12 feet long
at least 4 clip leads to hook up the circuit
Electrical tape to hold and insulate your wires
1 iron rod
1 wooden rod
1 aluminum rod
1 compass
1 permanent magnet
1 LED (light emitting diode)
small iron objects to pick up with your magnet…
(iron washers or sometimes paperclips will work)
1 6 Volt battery—
IMPORTANT Hooking the wires up across the battery will draw lots of
current. This will run down the battery within just a few minutes of continuous
use. The wire will also get quite hot. So to avoid running down your battery before
you can finish your experiment, you should carry out each measurement within just
a few seconds and then disconnect the wire across the battery as quickly as possible.
As always, if you think it is important to have something else, ask for it and if you make
a good argument for why it is needed, we will try to get it for you.
You and your group will work together to complete the lab and write up the group lab
report. For each lab, each person in your group will assume a job. As we go through the
semester, your group should rotate jobs, so you start each experiment with a new
job. Everyone should be helping with the hands-on stuff. For this first lab, divide up
jobs alphabetically, so closest to start of alphabet is manager.
o
The manager: This person is responsible for making sure that the group follows the
lab procedure and completes everything that is asked for in the lab.
o
The collector/recorder: This person is responsible for keeping the lab notebook for
the day, recording the observations observed by the group and the group’s
answers to the questions asked in the lab.
o
The skeptic: This person is there to question the results of the lab. Is everything
making sense? Are we taking the data correctly? Are the results and
conclusions reasonable? Did we skip a step?
Begin each lab report by titling the lab, listing your lab partners who are present, and
listing the jobs that each lab partner has assumed for the lab.
Procedure:
Part 1: Electromagnet
1) First, build an electromagnet. To do this, take one of your 3 ft lengths of
wire, and wrap it around the iron rod and secure the ends with electrical tape. Count the
number of turns you are able to wind onto the rod, and record that number in your lab
book, along with a sketch of your magnet.
Looking good:
Messy, but will still work:
Is your electromagnet acting like a magnet yet?
2) Next, hook up the wires to the battery and test the strength of your magnet using small
iron washers or paper clips. Can you pick up a small iron object with your magnet? How
many? If you reverse the direction of the current (switch which end of the wire is
connected to the positive terminal of the battery), does the magnet’s strength
change? Remember to always immediately disconnect your magnet from the
battery when you are not actively using it.
3) Study the direction of the magnetic field using a compass. First, look at your
compass. The compass consists of a permanently magnetized needle that can pivot
freely. In the absence of all other magnetic fields, the compass responds to the relatively
weak field of the Earth. Verify that your compass points in the same direction, no matter
how you rotate its case. If you’re not near any magnetic materials, this is marking the
direction of magnetic North. Now slowly bring a permanent magnet towards the
compass, and check that you can deflect the needle. Turn your permanent magnet around
end-to-end and see if the compass needle is deflected in a different direction. Why is this
happening?
Now hook your electromagnet to the battery and bring the compass toward the end of the
iron rod until the compass needle points toward or away from the end. Now, without
moving the compass or the magnet, reverse the direction of the current in the coil. What
happens to the compass? Why?
4) Does it matter if the core is iron or wood? Make a second electromagnet with your
second 3 ft piece of wire and the wooden core. How does it compare? Does it show any
signs at all of being a magnet if you look very carefully?
5) Does it matter if the core is aluminum? Make a third electromagnet with your last 3 ft
piece of wire using the aluminum core. How does it compare to the iron and wooden
cores? Which core makes the strongest electromagnet? What makes this material a good
core?
6) Does the electromagnet depend on the number of turns or the direction that the wire is
wound on the iron core? Think of experiments that can answer these questions. You
could use a variety of different approaches.
Part II: Transformer
7) Now you will build a transformer. For the primary coil it is best to combine two of
the 3 foot wires into one long wire and wrap them on the iron coil. Now take one of your
longer 12 foot wires and wind it over the top of the primary coil as shown in the figure
below. Count the number of turns as you wind. This will serve as your secondary
coil. Secure both ends with electrical tape. Do you have more turns in the primary or the
secondary coil? Is this a step-up or step-down transformer? What does that mean?
8) Test your transformer. Hook up the secondary coil to the LED, by connecting one end
of the secondary coil to one side of the LED, and the other end to the other side. Think
about the completed circuit you have just made (shown in blue below). The only things
in this circuit are a wire (the secondary coil) and a light. OK, the wire is part of the
transformer, but there’s no battery or power supply directly hooked up to the light. How
can you make the LED light up?
9) Hook up the primary coil (your original electromagnet) to the battery. Does the LED
light? The battery provides a DC current. However, you can turn your battery into an
AC source by rapidly connecting and disconnecting the wire from the battery over and
over. When you connect the battery, current starts to flow, when you disconnect the
battery, current stops flowing. Thus you are creating a rapidly changing current in your
primary coil which creates a rapidly changing magnetic field.
Try it. Does the LED indicate that current is flowing through the secondary coil of the
transformer? Describe under which conditions you can light the LED and in your report
be sure and explain this behavior.
10) Find out what happens if you reduce the number of turns of the secondary coil. (You
may have to change it by quite a bit to see a substantial effect.) Describe and explain this
behavior in your lab report.
11) In your lab report comment on how well your transformer would work if you wound
it on a wooden or aluminum core, but don’t do the experiment unless you have lots of
extra time.
You and your group can make a copy of what you wrote in your lab book to take home
with you. If you want to change your explanations of your results, you have one week to
turn them in and improve your grade.
Group Lab Report: Remember, your lab report should give an explanation of all of your
observations and measurements. Also, you need to think and try one additional
experiment for either the transformer or the electromagnet that will test your explanation.