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CH. 24 & 25 Review
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Short Answer: Answer in complete sentences
1. If all electrons create magnetic fields, why aren’t all materials magnets?
2. How are the forces between charges similar to the forces between magnetic poles?
3. Suppose you have two bar magnets. Only one of the magnets has a north and south poles labeled.
How would you determine which are the south and north poles on the unlabeled magnet?
4. An electrical wire carries current in a straight line from east to west. What is the direction of the
resulting magnetic field above the wire? What is the direction of the field below the wire?
5. If an electromagnet is used to pick up nails and other metal objects, what happens when the current
is turned off?
6. If a permanent magnet is dropped or struck by a hammer, it may lose its magnetism. Explain why.
7. How could you determine whether a bar of metal was magnetized or not? List two ways.
8. You have an electromagnet that is not quite strong enough. What are three changes you could make
to increase the strength?
True or False If false replace the italicized word in make the statement true.
9. The magnitude of the magnetic force on a current carrying wire depends on the strength of the
magnetic field, the current in the wire and the length of wire in the magnetic field.
10. When two parallel wires carry currents in opposite directions, their magnetic fields attract each
other.
11. A device used to measure very small electric currents is an ammeter.
12. In an electric motor, current is reversed every complete turn.
13. The speed of an electric motor can be controlled by varying the current flow.
Drawings:
14. Redraw the magnets below and sketch the position of the iron filings around the magnets and iron
bar in the diagram below.
15. Electrons are moving to the left in the wire shown in the diagram. In which direction does the north
pole of a compass point when it is:
(Hint: Sketch the B Field)
a. Above the wire?
b. Below the wire?
16. A force of attraction between two parallel wires is shown. The current in the upper wire is flowing
from right to left. Which way is the current flowing in the bottom wire? (Hint: Sketch the B Field)
17. When the north pole of a magnet is placed at the right end of the coil shown, it is repelled from the
electromagnet. What is the direction of current flow in the coil?
18. What is the direction of the force on the current carrying wire in the magnetic field shown below?
19. The direction of the force on a current carrying wire is shown in the diagram below. What is the
direction of the flow of current?
Calculations: Show all the steps necessary to solve the problem.
20. A wire 3.0 cm long lies perpendicular to a magnetic field with a magnetic induction of 0.4 T.
Calculate the force on he wire if the current is 5.0 A. What is the direction of the force?
21. A wire 1.5 m long is at right angles to a magnetic field.
a. What is the magnetic induction of the field if the force on the wire is 1.5 N and the current is 0.05 A?
b. What is the force on the wire if it is rotated 90.0?
22. An electron is moving east to west at 5x105 m/s through a magnetic field. A force of 4x10-6 N north is
acting on the electron. What is the magnitude and direction of the magnetic field?
23. What is the charge on a stream of particles moving at 2.0x106 m/s perpendicular to a magnetic field of
5.0x10-2 T? The force on the particles is 4.8x10-14 N. How many excess electrons?