Download MODULE: MOTORS AND GENERATORS 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 coil
of 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 given
area. It depends on the strength of the field (otherwise known as the magnetic flux
density) B, as well as the surface area A that it is passing through. It is calculated as the
product of the magnetic field component perpendicular to the area.
flux Φ = B x A
Φ
B=
A
3.
(a)
(b)
A = 1.5 m2
B = 2.0 T
Φ = 3.0 Wb
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 between
the coil and the magnet.
5. The faster the magnet moves relative to the coil, the greater the magnitude of the
induced current.
6. The magnet could be moving towards the loop with the S pole facing the loop, or the
magnet 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)
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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 within
the loop is into the page. As the resistance is increased, the current in the outer loop
decreases and the strength of the field is reduced. Therefore, the strength of the magnetic
field inside the inner loop reduces. The induced current in the inner loop flows in such a
way as to oppose the change, that is, increase magnetic field in the same direction by
adding to it. The induced current in the inner loop will be clockwise, the same as in outer
loop.
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. Induced
current 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 a
circuit.
(b) When the coil of a motor rotates, a back emf is induced in the coil due to its motion in
the external magnetic field.
(c) The direction produces an opposite torque to that of the current supplied and therefore
is in the opposite direction. If this was not the case, the current would increase and the
motor coil would go faster and faster forever. The principle of conservation of energy
states that energy cannot be created or destroyed, but it can be transformed from one form
to another.
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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 of
rotation of the motor increases until the back emf equals the supply emf (the maximum
speed of rotation) and no current flows through the motor coil. There is no force on the
coil so it does not increase its speed of rotation. If there is a load, the motor is slowed
down and this results in a smaller back emf.
(e) Overloading the motor slows it down, reducing the back emf. This allows almost all
the supply emf to be effective and produce a very high current flowing through the coil
that 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 the
circuit 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 Ω
Emfsupply = 240 V
I=2A
(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 is
stationary in a changing magnetic field or that is moving through a magnetic field. They
resemble 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 relative
movement 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 the
metal as well as through the direct agitation of atoms by a magnetic field changing
direction at a high frequency. The increased vibration of the atoms is heat.
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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 v
down the page and B out of page. The force on it will be to the left. Therefore current on
sheet in B will be to the left and anticlockwise current is induced.
(b) Up the page
19. See the textbook pages 128-9.
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Physics 2: HSC Course, 2nd edition (Andriessen et al, 2003), Chapter 7