A current of 5 A flows through a straight piece of wire that passes through 4 m of uniform magnetic field of flux density 1.5 T, at right angles to the direction of the field.What is the magnitude of the force experienced by the wire?
SAMPLE ANSWER 20A
Using the formula for force on a current-carrying conductor in a magnetic field, making sure all values are in their standard unit: F IlB (5)(4)(1.5) 30 The wire experiences a force of 30 N.
SAMPLE PROBLEM 20B
A rectangular loop of wire, carrying a current of 0.5 A, is placed in a uniform magnetic field of flux density 6 T. The loop of wire, with dimensions 30 cm 10 cm, is free to rotate about a central axis as shown in Fig 20.12. (i) Calculate the magnitude of the force acting on each of the 30 cm sides of the loop. (ii) Calculate the torque of the couple about the central axis when it is in the position shown in Fig 20.12. (iii) Describe what happens to the torque of the couple as the loop rotates through 90. (iv) Will the loop of wire stop moving when it has rotated through 90 from the position in Fig 20.12? Explain. (v) What would happen if the direction of the current changed as the loop rotated through 90?
SAMPLE ANSWER 20B
(i) F IlB (0.5)(0.3)(6) 0.9. The wire experiences a force of 0.9 N.
(ii) T Fd (0.9)(0.1) 0.09 The torque of the couple is 0.09 Nm.
(iii) The torque of the couple decreases to zero as the loop rotates through 90 because the perpendicular distance between the lines of action of the forces decreases to zero.
(iv) The loop of wire will not stop moving when it has rotated through 90 because its momentum will carry it through that position.
(v) If the direction of the current changed as the loop rotated through 90, a couple would be produced again that would cause the loop to continue its rotation.
0.5 A B = 6 T Force F into of page 0.5 m Force F out of page 10 cm Fig 20.12 Axis
