Paras

Episode 412: The force on a conductor in a magnetic field

Having reminded your students that magnetic fields can be found near permanent magnets and in the presence of an electric current, the next step is to show how the ‘field’ can be quantified. Again, students should know that a conductor carrying a current in a magnetic field will experience a force and will probably remember that Fleming's Left Hand Rule can be used to find the direction of that force.

Summary

Demonstrations: Leading to F = BIL. (15 minutes)

Discussion: Factors affecting the force. (15 minutes)

Discussion: Formal definitions. (20 minutes)

Student questions: BIL force calculations. (20 minutes)

Demonstrations:

Leading to F = BIL

Several quick experimental reminders are possible.

Tap 412-1: Forces on currents

TAP 412-2: An electromagnetic force

These lead on to a further experiment in which the relationship F=BIL can be established.

TAP 412-3: Force on a current-carrying wire

Discussion:

Factors affecting the force

The experiments above lead to the conclusion that the force F on the conductor is proportional to the length of wire in the field, L, the current I and the ‘strength’ of the field, represented by the flux density B. (There is also an 'angle factor' to consider, but we will leave this aside for now.)

Combining these we get F = BIL

(It can help students to refer to this force as the ‘BIL force’.)

Students will probably know that electric and gravitational fields are defined as the force on unit charge or mass. So by comparison, B = F/IL, and this gives a way of defining the 'magnetic field strength'. Physicists refer to this as the B-field or magnetic flux density which has units of

N A-1 m-1 or tesla (T).

A field of 1T is a very strong field. The field between the poles of the Magnadur magnets that are used in the above experiment is about 3 ( 10-2 T while the Earth's magnetic field is about 10-5 T.

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