Edward Thomas, Summer 1999
Questions these experiments will enable you to answer: Is the magnetic field of a solenoid qualitatively similar to the magnetic field of a permanent magnet? What effect does placing a magnetizable material (iron) in the core of a solenoid have on its magnetic field? Does the Biot-Savart Law correctly predict the magnetic field near the end of a solenoid?
General Comment
We are going to be plotting B fields around various objects. It is important to make sure that no magnets or bits of iron are close to the location on the bench where you are trying to make these measurements. Move everything away and also keep your plotting apparatus at least 20 cm from the power supply (which generates B fields). The plotting of B fields with plotting compasses is rather crude and not very accurate; our main objective is to get a general feel for the B field structure.
In principle magnetic fields may wipe out information on magnetic storage devices such as computer discs and credit cards. Keep all such items away from the magnets and from the solenoid ( we suggest at least 20 cm). Our own tests have shown that the fields produced by the permanent magnets used here are not sufficiently strong to corrupt a computer disc.
Experiment #1: Field Plot of Permanent Magnet
Lay one of the permanent magnets on a piece of paper. We will plot the B field lines around the magnet with a plotting compass and will determine B field direction at a couple of points.
We suggest you arrange things as follows. Align the magnet axis with one of the grid lines and position it so that it is partly off the paper (we would suggest that about 2 inches of the magnet length be on the paper). It might help things to draw an outline of the magnet on the paper. Take the small plotting compass place it as close as possible to the end of the magnet and on the axis of the magnet. Make a mark on the paper at the ends of the magnet needle (or at least as close as you can get). Move the compass out by one diameter so that the end of the needle closest to the magnet is now at one of the spots you have just marked; mark the position of the other end of the compass. Keep repeating this across your paper. Join each pair of spots with a straight line and you will have a B field line. Repeat this to make at least four such lines starting at different points close to the magnet end and going off to the right at different angles from the magnet axis between say 0 degrees (which you just carried out) and 90 degree. These lines give a qualitative idea of the field at least on the right hand side of the magnet axis.
In principle we would also like to know the field on the left hand side also. Is there any point in doing this? Will it just be a mirror image of the plot made to the right?
Finally go to a position 5 cm along the magnet axis (away from its end) and 5 cm to the right; estimate as best you can the direction of the field at this point and express it as an angle from the axis (a paper protractor might be helpful here, or you can use the arctangent with measuree "rise" and "run"). Also find the angle at a point zero cm along the axis and 5 cm to the right.
Experiment #2: Field Plot for an Air Cored Solenoid.
You are now to plot the B field produced by a solenoid. The technique and layout is exactly the same as for the permanent magnet.
On the bench is a pair of solenoids, one inside the other; also inside the smaller diameter solenoid is a solid rod of iron. Select the inner solenoid (smallest diameter--thick wires) and remove the metal rod. Place the solenoid on another sheet of graph paper in a similar manner to the permanent magnet (it helps to let the square block at the end of the solenoid hang over the edge of the bench). Connect it to the power supply (For operational instructions see Technical Note #23) and set up a current of 5 Amps through the solenoid. Now plot the B field for this situation using the same procedures as in Experiment #1. Again measure the direction of the B field at the (same) two points referred to in Experiment #1 (but now of course measured from the end of the solenoid).
Experiment #3: Field Plot for an Iron Cored Solenoid.
You are now to replace the iron bar in the solenoid and plot the B field for this case. The technique and layout should be exactly the same as for the air cored plot of Experiment #2.
Perhaps this is most instructive if done on the same sheet of paper as Experiment #2 so that the differences (if any) can be clearly demonstrated. We suggest that you use the same sheet and make the plot with a different color or different symbols. Keep the solenoid current the same at 5 A. Again determine field direction at the two geometrically specified points.
Consider how this plot differs, qualitatively, from the plot when the core of the solenoid is air.
Experiment #4: Addition of Fields.
We shall now study the field distribution for the permanent magnet and solenoid acting together with the axes of the two objects being on the same line and the fields opposing.
Place the permanent magnet and the air cored (not iron cored) solenoid on a new piece of graph paper with their axes on the same line. Arrange things so that the facing "poles" are the same (both North or both south) and they are separated by about 8 inches (20 cm). Again with a 5 A current in the solenoid plot the B fields in the space between the magnet and the solenoid using the same techniques as before. Locate, on the line joining the two poles, the point at which the field is zero (this can be located only very roughly to an accuracy of about the size of the plotting compass) and record its distance from the solenoid.
From this plot which object, solenoid or magnet, produces the greatest
B field?