Magnetism Review Answers

CP PhysicsMs. Morrison

1. Define the following vocabulary terms:

Magnetic field lines: lines of force representing the magnetic field around a magnet

Magnetic pole: the end of the magnet which produces magnetic force

Magnetic field: the space around a magnet through which magnetic force acts

1. Define the following vocabulary terms: cont’d

ALNICO magnet: a magnet made up of the elements of aluminum, nickel, and cobalt; these three metals when combined are ferromagnetic

Magnetic domain: a cluster of atoms which have aligned magnetic fields

Electromagnet: a current-carrying coil of wire

1. Define the following vocabulary terms: cont’d

Magnetic declination: the difference between the location of the true North pole and the magnetic North pole

Aurora borealis: also known as the Northern Lights, colored lights seen in higher latitudes due to charged particles interacting with Earth’s magnetic field in the upper atmosphere

1. Define the following vocabulary terms: cont’d

Transformer: an iron core with two coils of wire (primary and secondary), it changes voltage; step-up = increases voltage, step-down = decreases voltage

Electromagnetic induction: generating electricity by moving a coil of wire in a magnet’s magnetic field

2. How are magnetic poles similar to electric charges?

Like charges and poles repel Opposite charges and poles attract Both charges and poles exert forces

which can act over a distance – they do not have to have direct contact for the force to be experienced

3. What is the important difference between electric charges and magnetic poles?

Electric charges can be separated from each other – can remove negative charges from an object

Magnetic poles cannot be separated from each other – trying to break a magnet in half to separate the North from the South does not result in separate poles – just two smaller magnets

4. How is the magnetic field oriented around a magnet?

The magnetic field lines leave the north pole, curve around, and enter the south pole.

5. How can you “see” the magnetic field around the magnet?

You can see the magnetic field lines by sprinkling iron filings over the magnet. The filings will line up to show the field lines around the magnet.

6. Where is the magnetic field the strongest?

The magnetic field is strongest at the poles.

7. How are magnetic fields created?

Magnetic fields are created by moving charges, like electrons spinning or electrons flowing through a wire.

8. What is the smallest magnet?

An electron

9. Why are most materials not magnetic?

Most materials have atoms which have paired-up electrons so the magnetic fields of all the electrons cancel each other out so there is no remaining magnetic fields.

10. Why is iron magnetic? Each iron atom has 4 unpaired

electrons. Their magnetic fields add to each other to produce a strong magnetic field in each atom.

11. What is different about the magnetic domains in a permanent magnet versus a nonmagnetic material?

The domains in the permanent magnet line up one direction, while the domains in the nonmagnetic material are randomly arranged. When the domains line up, the magnetic fields add together creating a stronger permanent magnetic field.

12. Can a nonmagnetic material be made magnetic?

Yes, if the material contains some iron or ALNICO. It must be put into a strong magnetic field and it will be become temporarily magnetized. This is because the magnetic field will cause the random domains to line up. Once the magnetic field is removed, the domains will return to their random, unaligned state.

13. How can permanent magnets be destroyed or weakened?

Heating them, dropping them, or repeatedly hammering them. This causes the domains in the magnet to become unaligned and randomly arranged.

14. What did Oersted discover?

Oersted discovered that electricity moving through a wire caused a magnetic field to be created. He was conducting electric circuit experiments and had a compass near his circuit. When a electric current went through the circuit, the needle of the compass was deflected.

15. What happens to the magnetic field around a current-carrying wire if the current through it reverses direction?

The magnetic field will also reverse its direction.

16. How does an electric motor work?

A coil of wire is placed inside a permanent magnet.

When electricity goes through the wire, the coil of wire becomes an electromagnet and interacts with the magnet’s magnetic field.

The wire then spins. Every 180o the current reverses direction so that the wire keeps spinning and spinning. (Electromagnet’s magnetic field also keeps reversing.)

Motors convert electrical energy into mechanical energy.

17. What is thought to cause Earth’s magnetic field?

The motion of charged particles within the molten part of Earth’s core is thought to generate the Earth’s magnetic field.

18. Why is the presence of Earth’s magnetic field important?

It reduces the intensity of cosmic rays coming from the sun.

It traps the Van Allen Radiation belt which can sometimes interact with the Earth’s magnetic field to produce the Northern lights.

19. What factors affect the strength of an electromagnet?

The number of loops – more loops in the coil will increase the strength

The amount of current – the greater the current through the coil will increase the strength

The presence of an iron core – if an iron core is present it makes the electromagnet stronger

20. Who discovered electromagnetic induction?

Michael Faraday

21. What are two ways to generate current in a wire?

Move a wire in and out of a magnetic field

Move a magnet in and out of a coil of wire

22. How does an electric generator work?

A coil of wire is located inside a permanent magnet – just like a motor.

When you turn the coil of wire, electricity is generated because the wire is moving within a magnetic field.

So, electricity is produced when the coil of wire is spun within the magnet.

Generators convert mechanical energy into electrical energy. (opposite to motors)

23. What kind of current does an electric generator produce?

Alternating current (AC)

24. What three factors does the voltage produced by an electric generator depend on?

The number of loops in the wire – more loops will produce more voltage

The strength of the magnet – the stronger the magnet the more voltage produced

The speed of rotation of the wire – the faster it spins, the more voltage produced

25. What three factors does the strength of an electromagnet depend on?

See answer to question #19

26. How do you use your right hand to determine the magnetic field around a current-carrying straight wire?

OMIT

27. Draw a bar magnet and show its field lines.

The field lines should leave the north pole with arrows pointing out of the north pole, curve around, and then enter the south pole. So, the arrows should point at the south pole.

28. Draw the magnetic field lines between two north poles.

The lines will leave and move up and curve way from the north poles. They do not touch each other. The arrows point outwards.

For two south poles, you see a similar pattern with the lines curving up and away from the poles, but the arrows point towards the south poles. The arrows point inwards.

29. Draw the magnetic field lines between a north pole and a south pole.

The lines will move from the north pole to the south pole. So the arrows point to the south pole and away from the north pole.

30. A wire, 0.60 m long, is at right angles to a uniform magnetic field of magnetic induction equal to 0.400 T. The current through the wire is 4.00 A. What is the force that acts on the wire?

L = 0.6 m F = BILB = 0.400 T = (0.4)(4)

(0.6)I = 4.00 A F = 0.96 NF = ?

31. A wire, 2.0 m long, carries a current of 12 A. The wire is at right angles to a uniform magnetic field which has a force of 0.50 N. What is the strength of the magnetic field?

L = 2.0 m F = BILI = 12 A 0.50 = B(12)(2)F = 0.50 N 24 24B = ? B = 0.021 T

32. A wire is at right angles to a magnetic field, the force of which is 0.96 N. A current of 5.6 A flows through the wire. The induction of the magnetic field is 0.37 T. What is the length of the wire?

F = 0.96 N F = BILI = 5.6 A 0.96 = (0.37)(5.6)LB = 0.37 T 2.072 2.072L = ? L = 0.46 m

33. An electron with a charge of 1.6 x 10-19 C moves at right angles to a uniform magnetic field with an induction of 0.75 T. It has a velocity of 4.8 x 107 m/s, what is the force acting on the electron?

Q = 1.6 x 10-19 CB = 0.75 Tv = 4.8 x 107 m/sF = ?

F = BQv = (0.75)(1.6 x 10-19)(4.8 x

107)F = 5.76 x 10-12 N