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XAVIER CATHOLIC COLLEGE
PHYSICS
MOTORS & GENERATORS
Worksheet 7: • 1.6a
Students : solve problems and analyse information about simple motors using: τ = nBIAcosθ
Torque on a Coil 1. A 5.0 cm square vertical loop with 50 turns of wire is in a magnetic field of intensity 0.02 T such that the plane of the coil is at right angles to the field. The loop carries a current of 4.0 A. a. b. c.
Calculate the magnitude of the force acting on each side of the coil. Calculate the maximum torque on the coil. Draw a diagram showing the coil in the field when the torque is maximum.
W7-1.6a-Torque on a Coil.doc
Page 1 of 4
2. A square coil, PQRS, with sides 0.05 m and 250 coils, carries 1.6 A and lies in a 0.4 T magnetic field directed from left to right. The plane of the coil is parallel to the field with PQ perpendicular to the field. a. b. c. d.
Calculate the magnitude of the force on side PQ. Calculate the magnitude of the force on side QR. The coil rotates so that PQ moves upwards. Deduce the direction of the current in it. Calculate the maximum torque on the coil.
3. An square ammeter coil has 80 turns of wire each of area 16.0 cm2. It is pivoted in a radial magnetic field of intensity 0.25 T. a. b.
Calculate the magnitude of the torque acting on the coil when 60 mA flows through it. lf the spring attached to the coil provides a restoring torque of 3.0 x 10-5 Nm for every degree the coil turns, how many degrees will the coil turn before it comes to rest?
W7-1.6a-Torque on a Coil.doc
Page 2 of 4
4. A coil is placed in a magnetic field as shown in the figure below.
(Shadwick, 2004: 9)
a. b.
Predict what happens to the coil when a current flows through it in the direction shown. Explain your answer. What change needs to be made to the coil to ensure it will continue to rotate while current is flowing through it?
W7-1.6a-Torque on a Coil.doc
Page 3 of 4
5. A square coil of side 6.0 cm lies horizontally in the plane of a uniform magnetic field of intensity 1.4 T as shown in question 4. The coil has 250 turns and carries 2.5 A. a. b. c. d.
Calculate the force on side DE. Calculate the maximum torque acting on the coil. Calculate the change in flux through the coil when it moves from its initial position through 90°. Predict what will happen to this coil after it rotates 90° if current keeps flowing through it.
Answers:
Note: 1a) 4 x 10-2 N ACKNOWLEDGEMENTS Shadwick, B. (2003) Surfing Physics Motors and Generators. Marrickville, Science Press W7-1.6a-Torque on a Coil.doc
Page 4 of 4
PHYSICS
MOTORS & GENERATORS
Worksheet 7: • 1.6a
Students : solve problems and analyse information about simple motors using: τ = nBIAcosθ
Torque on a Coil 1. A 5.0 cm square vertical loop with 50 turns of wire is in a magnetic field of intensity 0.02 T such that the plane of the coil is at right angles to the field. The loop carries a current of 4.0 A. a. b. c.
Calculate the magnitude of the force acting on each side of the coil. Calculate the maximum torque on the coil. Draw a diagram showing the coil in the field when the torque is maximum.
W7-1.6a-Torque on a Coil.doc
Page 1 of 4
2. A square coil, PQRS, with sides 0.05 m and 250 coils, carries 1.6 A and lies in a 0.4 T magnetic field directed from left to right. The plane of the coil is parallel to the field with PQ perpendicular to the field. a. b. c. d.
Calculate the magnitude of the force on side PQ. Calculate the magnitude of the force on side QR. The coil rotates so that PQ moves upwards. Deduce the direction of the current in it. Calculate the maximum torque on the coil.
3. An square ammeter coil has 80 turns of wire each of area 16.0 cm2. It is pivoted in a radial magnetic field of intensity 0.25 T. a. b.
Calculate the magnitude of the torque acting on the coil when 60 mA flows through it. lf the spring attached to the coil provides a restoring torque of 3.0 x 10-5 Nm for every degree the coil turns, how many degrees will the coil turn before it comes to rest?
W7-1.6a-Torque on a Coil.doc
Page 2 of 4
4. A coil is placed in a magnetic field as shown in the figure below.
(Shadwick, 2004: 9)
a. b.
Predict what happens to the coil when a current flows through it in the direction shown. Explain your answer. What change needs to be made to the coil to ensure it will continue to rotate while current is flowing through it?
W7-1.6a-Torque on a Coil.doc
Page 3 of 4
5. A square coil of side 6.0 cm lies horizontally in the plane of a uniform magnetic field of intensity 1.4 T as shown in question 4. The coil has 250 turns and carries 2.5 A. a. b. c. d.
Calculate the force on side DE. Calculate the maximum torque acting on the coil. Calculate the change in flux through the coil when it moves from its initial position through 90°. Predict what will happen to this coil after it rotates 90° if current keeps flowing through it.
Answers:
Note: 1a) 4 x 10-2 N ACKNOWLEDGEMENTS Shadwick, B. (2003) Surfing Physics Motors and Generators. Marrickville, Science Press W7-1.6a-Torque on a Coil.doc
Page 4 of 4
