Optional Unit VII: Electromagnetism

B. Electromagnetism

Key Concepts

Oersted discovered that a magnetic field is created in the region of a current-carrying conductor.

The direction of the magnetic field lines is dependent on the direction of current flow through a conductor.

The magnetic field forms a series of concentric circles around a straight conductor.

The strength of the magnetic field diminishes inversely as the radial distance from the conductor increases. At any given distance from the conductor, the strength of the magnetic field increases directly with the amount of current in the conductor.

Ampere's Rule (right-hand rule) can be used to describe the direction of the magnetic field about a straight conductor. If the conductor is grasped with the right hand in such a way that the right thumb points in the direction of the current, the fingers wrap around the conductor in the direction of the magnetic field. (Some texts refer to the "right-hand rule". Avoid this convention to prevent unnecessary confusion.)

The strength of a magnetic field around a conductor can be intensified by coiling the conductor into a loop. A large number of loops form a coil, or solenoid (also referred to as a helix).

The magnetic field outside a solenoid resembles the field formed outside a bar magnet. Inside the solenoid the magnetic field consists of straight, uniformly spaced lines of flux.

Ampere's Rule for a solenoid (right-hand rule for a solenoid) states that if the solenoid is grasped in the right hand in such a way that the fingers curl in the direction of the current, the right thumb points in the direction of the north pole of the core. (Magnetic field lines point from south to north inside the core, in the same direction that the right thumb is pointing.)

The magnetic field strength increases enormously if a material with a high magnetic permeability is used for the core of the solenoid.

Magnetic permeability (µ) is the ratio of magnetic field strength with a particular core material to the magnetic field strength without that core material, in a vacuum.

Ferromagnetic materials have very high magnetic permeabilities.

The magnetic field strength (F) of a solenoid varies directly with the current (F à I) and also varies directly with the number of turns (N) per unit length (F à N). (I is the current.)

Learning Outcomes

Students will increase their abilities to :

1. Define the following terms: magnetic permeability, ferromagnetic .

2. State that the direction of the magnetic field around a current-carrying conductor depends on the direction of current flow through the conductor.

3. Illustrate that the magnetic field around a current-carrying conductor forms a series of concentric circles.

4. Recognize the effect that increased distance away from the conductor has on the strength of the magnetic field.

5. State Ampere's Rule (right-hand rule) for a straight conductor.

6. Use the right-hand rule to determine the direction of the magnetic field lines or the direction of the current for a straight conductor.

7. Explain that the strength of the magnetic field can be intensified by coiling the conductor into a loop.

8. Describe the shape of the magnetic field formed in the region outside of a solenoid.

9. State Ampere's Rule (right-hand rule) for a solenoid.

10. Apply the right-hand rule for a solenoid to determine the direction of the current through the coil or the magnetic polarity of the coil.

11. Recognize that the strength of the magnetic field of a solenoid can be increased enormously byusing a core with a high magnetic permeability.

12. Identify other factors which affect the strength of the magnetic field of a solenoid.

13. Transfer an understanding of electromagnetism to practical applications.

14. Solve problems relating to electromagnetism.

Teaching Suggestions, Activities and Demonstrations

1. Demonstrate that the strength of the magnetic field can be intensified by coiling the conductor into a loop.

2. Show students that the strength of the magnetic field of a solenoid can be increased enormouslyby using a core with a high magnetic permeability.

3. Experimentally investigate the magnetic field of a straight conductor or a solenoid.

4. Using a cathode ray tube or a Crookes tube, place a bar magnet near the tube. Observe whathappens. Reverse the direction of the magnetic field and repeat. Compare what happens in eachsituation. Does the deflection of the beam agree with the right hand rule for the Motor Principle? (Caution: x-rays are produced by this apparatus.)

   A more carefully controlled set-up can be used to find the charge-to-mass ratio of theelectron.

5. Design an experiment to investigate some of the factors which influence the strength of a magneticfield produced by a solenoid.

6. Suspend a magnet from a thread into glycerine, honey, or some other clear, viscous fluid. Gentlysprinkle iron filings on the liquid. The iron filings remain suspended, arranging themselves three-dimensionally in the magnetic field around the magnet. Draw a representation of the magnetic fieldpattern. Trying to remove the iron filings from the magnet may be difficult, but less so if you wrapthe magnet in plastic sandwich wrap beforehand. If an electromagnet can be set up for this demonstration, it not only helps to remove the iron filings, but it can also be used to illustrate the collapse of the magnetic field when the current to the electromagnet is discontinued.

   To show a two-dimensional magnetic field, place a magnet under a piece of paper or clearacetate. Sprinkle iron filings on the sheet. The resulting pattern can be drawn. Use two magnets under the sheet to show repulsion and attraction of magnetic fields.

7. A very fine colloidal suspension of iron filings in oil can be placed in a jar. Hold a magnet to the outside of the jar and make the metallic glob behave in unusual ways against the walls of the glass container.

8. Have students research theories of direction-finding and migration in animals based on the presence of magnetite in their cells. Seek ways in which this activity could be tied in with topics in Biology dealing with Animal Systems or Evolution.

9. A set of two crystalline ceramic cobalt-neodymium magnets are useful to have in the lab, because they are strong enough to reveal magnetic properties in substances such as a graphite pencil, or paper covered with graphite (as in a photocopied page).

10. Use a magnetic stud finder to show how metal objects behind a wall can be located.

11. The conventions used for defining current are inconsistent in student resources. In this Curriculum Guide, "conventional" current flow has been used. Current is considered to be the flow of positive charges, even though electrons are actually flowing in the opposite direction.

    If your student resources use electron current flow, then any references made to the right-hand rule will become the left-hand rule. Note as well that inconsistencies may exist between your physics resources and ones used for chemistry. Needless to say, students find these differences in the defined current (and the accompanying differences in sign conventions) to be very confusing.