1 Physics 2: HSC Course, 2nd edition (Andriessen et al, 2003), Chapter 7

MODULE: MOTORS AND GENERATORS

Chapter 7: Generating electricity (Questions, pages 130-2)

1. Faraday was able to produce an electrical current by moving a magnet relative to a coilof wire. This occurred because he changed the magnetic flux through the coil.

2. Magnetic flux ΦB is a measure of the amount of magnetic field passing through a givenarea. It depends on the strength of the field (otherwise known as the magnetic fluxdensity) B, as well as the surface area A that it is passing through. It is calculated as theproduct of the magnetic field component perpendicular to the area.flux Φ = B x A

B = AΦ

3.(a)

A = 1.5 m2

B = 2.0 TΦ = 3.0 Wb

(b)A = 0.75 m2

B = 0.03 T Φ = 2.3 x 10-2 Wb(c)

A = 0.04 x 0.03 m2

B = 5 x 10-3 T Φ = 6.0 x 10-6 Wb(d) As area is parallel to B, Φ = 0

4. (a) To the right(b) To the left(c) No current, if both move at the same speed, as there is no relative movement betweenthe coil and the magnet.

5. The faster the magnet moves relative to the coil, the greater the magnitude of theinduced current.

6. The magnet could be moving towards the loop with the S pole facing the loop, or themagnet could be moving away from the loop with the N pole facing the loop.

7. (a) Current to the left (note the variation in winding)(b) No induced current(c) To the right (on opening, the current is opposite to maintain B)

2 Physics 2: HSC Course, 2nd edition (Andriessen et al, 2003), Chapter 7

8. Clockwise current. Each side has a current produced so as to result in a force outwards(F = BIl)

9. Clockwise. When the current flows in the outer loop, the induced magnetic field withinthe loop is into the page. As the resistance is increased, the current in the outer loopdecreases and the strength of the field is reduced. Therefore, the strength of the magneticfield inside the inner loop reduces. The induced current in the inner loop flows in such away as to oppose the change, that is, increase magnetic field in the same direction byadding to it. The induced current in the inner loop will be clockwise, the same as in outerloop.

10. (a) Clockwise (Induced I decreases B in as has B out)(b) Anticlockwise (If I is decreasing, the opposite current is induced)(c) Clockwise (Opposite current decreasing)(d) Anticlockwise

11.(a)

Φ = BA = 0.50 x 0.07 x 0.04 = 1.4 x 10-3 Wb

(b) Anticlockwise. (When removed, the flux is decreasing inside the loop. Inducedcurrent opposes the effect and has B out of the page, therefore anticlockwise.)

12.(a) Φ = BA

= 1.5 x 10-3 x 0.065 x 0.065 = 6.3 x 10-6 Wb

(b) Each loop has the same flux change so emf would be 25 times greater.(c) Anticlockwise. (Induced current has B out of the page to restore decreasing B.)

13.• Change the external magnetic field strength.• Remove the coil from the external magnetic field.• Distort the shape of the coil so that the area of the coil changes.• Rotate the coil about an axis so that the area perpendicular to the field changes.

14. (a) Back emf is an electromagnetic force that opposes the main current flow in acircuit.(b) When the coil of a motor rotates, a back emf is induced in the coil due to its motion inthe external magnetic field.(c) The direction produces an opposite torque to that of the current supplied and thereforeis in the opposite direction. If this was not the case, the current would increase and themotor coil would go faster and faster forever. The principle of conservation of energystates that energy cannot be created or destroyed, but it can be transformed from one formto another.

3 Physics 2: HSC Course, 2nd edition (Andriessen et al, 2003), Chapter 7

(d) If no load is attached to the motor (it does not push anything), then the speed ofrotation of the motor increases until the back emf equals the supply emf (the maximumspeed of rotation) and no current flows through the motor coil. There is no force on thecoil so it does not increase its speed of rotation. If there is a load, the motor is sloweddown and this results in a smaller back emf.(e) Overloading the motor slows it down, reducing the back emf. This allows almost allthe supply emf to be effective and produce a very high current flowing through the coilthat can overheat the wires until they melt, or the insulation burns off. For this reason,when a motor is switched on and starts to turn slowly, a shunt resistor is added to thecircuit to lower the current.

15.

emfsupply – emfback = IR

(a)240 – 0 = I x 5

I = 48 A

(b)240 – 237 = I x 5 I = 0.6 A

16.

R = 10 ΩEmfsupply = 240 VI = 2 A(a) 240 – 0 = I x 10

I = 24 A

(b) 240 – emfback = 2 x 10 emfback = 220 V

17.(a) An eddy current is a circular or whirling current induced in a metal conductor that isstationary in a changing magnetic field or that is moving through a magnetic field. Theyresemble the eddies or swirls left in the water after a boat has passed by.(b) Eddy currents can be produced:

• when there is a magnetic field acting on part of a metal object and there is relativemovement between the magnetic field and the object

• when a conductor is moving in an external magnetic field• when a metal object is subjected to a changing magnetic field.

(c) Heating occurs due to the collisions between moving charges and the atoms of themetal as well as through the direct agitation of atoms by a magnetic field changingdirection at a high frequency. The increased vibration of the atoms is heat.

4 Physics 2: HSC Course, 2nd edition (Andriessen et al, 2003), Chapter 7

18. (a)<new diagram figure 7A to come>Consider a positive charge (pretend it will move) being pulled down the page, i.e. has vdown the page and B out of page. The force on it will be to the left. Therefore current onsheet in B will be to the left and anticlockwise current is induced.

(b) Up the page

19. See the textbook pages 128-9.