Magnetic fields and motors

Question 1

With drawings, how is the strength of a magnetic field represented?

Answer 1

The strength of a magnetic field is represented by how tightly packed the lines are.

Question 2

How can the direction of a magnetic field around a current-carrying wire be worked out.

Answer 2

• Thumb points in the direction of current.
• Curled fingers point in the direction of the field.

Question 3

Draw the magnetic field lines around a coil of wire.

Question 4

Draw the magnetic field lines around a selenoid.

Question 5

Draw a wire passing perpendicular to a magnetic field.

• What direction is the force on the wire
• What orientation of the wire would produce no force?

Answer 5

• If the current is parallel to the flux lines, no force acts.

Question 6

Explain flemings left hand rule.

Answer 6

The first finger points in the direction of the uniform magnetic field, the second finger points in the direction of the conventional current. Then your thumb points in the direction of the force (In which motion takes place).

Question 7

What is the equation F = BIL?

Answer 7

The size of the force, F, on a current-carrying wire at right angles to a magnetic field is proportional to the current, the length of the wire and the strength of the magnetic field B.

Question 8

What is magnetic field strength/flux density?

Answer 8

Magnetic field strength is a vector quantity with both direction and magnitude.

It is measured in teslas, T.

T = Wb/m²

Question 9

Explain how a split-ring commutator motor works.

Answer 9

• Using a split-ring commutator, the current in a loop can be reversed each time the loop becomes vertical.
• This allows the loop to rotate steadily.