Edward Thomas, Summer 1999
Abstract
We first make qualitative field plots of the B field
lines around a permanent magnet and a solenoid. The objective is to demonstrate
that the B field from a solenoid is basically the same as the B field from
a permanent magnet. The solenoid case is studied both with an "air core"
and also with an iron bar along its center. The iron bar is shown to increase
the field strength on the solenoid axis and to concentrate the field lines.
General background
We are all familiar with "permanent magnets" (and they have been known for about 2000 years). The theory of magnetism is not developed through study of permanent magnets but rather through study of magnetic fields produced by currents in wires.
Our first step is to show that there is no basic difference between the magnetic field produced by a permanent magnet and the field produced by current in a wire. Theory suggests that the magnetic field produced by a solenoid ( a long coil of wire) should be pretty much the same as that produced by a bar shaped "permanent magnet". Our first task is to plot the magnetic field lines around such a solenoid and around a bar magnet and confirm this. The concept of magnetic field lines is discussed in the text as Section 29-2.
When a magnetic material, such as iron, is placed in a magnetic field then the internal organization of the solid changes and it produces an additional field itself. As a result the field of a solenoid with iron inside it is altered from that for a solenoid with only air inside it. The field lines become more concentrated and the total field increases. In almost all industrial uses of solenoids to make magnetic fields (e.g. electric motors, transformers) iron is placed in the solenoid core to increase field and to shape the field line distribution. We will study what happens to the field when a piece of iron is placed in the solenoid.
Plotting of B fields.
A magnetic compass (used for determining direction to the earth’s poles) is a small bar magnet on a pivot axis. It can be shown that the compass "needle" will align itself with the B field at the point where the compass is located. Thus the line of the compass needle represents the direction of the B field at the point in space where you place the compass. It may not be a very precise indication since we are never quite sure which is the "point" in space where the field direction is being determined (is it one end, the other end, the middle??). But it gives a rough idea.
A crude way of plotting magnetic fields to is place the compass in the field, record the position of its ends, draw a line between these two positions, and take this as the direction of the field. This is how we plot fields in this present experiment. We use very small compasses called plotting compasses.
Apparatus
It might be helpful if you could bring a protractor to measure angles. If you do not have one then download and print our TN #25; cut out the protractor from the paper and bring it.
Finally it might be helpful also if you had pens or pencils of two different
colors (bring your own)