K. Craig Fall 2009
Several thousand years ago, the Greeks discovered that certain metallic rocks from the district of
Magnesia in Asia Minor would attract iron, and attract or repel similar rocks – hence the name magnet! The mysterious force that magnets generate fascinate everyone, young and old. Today, magnets do a lot more than hold notes to refrigerators. A multitude of hidden magnets provide the driving force of much of today’s technology. The forces that drive the motors in our appliances, that generate music in our earphones and CD players, that form the images on our TV screens, that store information on computers, and that deliver electricity to our homes to operate all these technological wonders are all provided by magnets.
The topic of magnets spans many areas of interest. Examples are:
Astronomy – magnetic fields of planets, stars, sun.
Biology – magnetic bacteria, magnetic fields of the brain
Geology – magnetic rocks and continental drift
Medicine and Health – magnetic resonance imaging, power lines and cancer
Technology – computers, maglev trains, credit cards
Physics – the structure of matter, the effects of electric and magnetic fields, particle accelerators
Warfare – magnetic mines, radar
Entertainment – toys, games, magic
This document describes the basic facts about this mysterious force along with simple, hands-on demonstrations that each of you can do!
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K. Craig Fall 2009
Some facts about magnetism have been known for hundreds of years. They are:
1.
If free to rotate, permanent magnets point approximately north-south .
Demonstration:
Magnets always have two poles called north and south. If you hang a magnet from a string and allow the magnet to pivot freely, the north pole of the magnet will point towards the earth’s geographic north and the south pole will point towards the earth’s geographic south.
A compass is just a magnetic needle on a pivot.
2.
Like poles repel, unlike poles attract . Demonstration: If you have two magnets and hold the north poles towards each other or the south poles towards each other, you will feel a repulsive force. If you hold the north pole of one magnet near the south pole of the other magnet, you will feel an attractive force.
3.
Permanent magnets attract some things (like iron and steel) but not others (like wood or glass). Magnetic forces attract only magnetic materials . Demonstration: Hold a piece of iron near a permanent magnet and the iron will be attracted to the magnet. Hold a piece of glass or wood, a nonmagnetic material, near a permanent magnet and there is no attractive force.
4.
Magnetic forces act at a distance, and they can act through nonmagnetic barriers .
Demonstration: Tie a paper clip to a piece of string. On top of a glass turned upside down, place a permanent magnet with one end hanging over the side of the glass. Place the paper clip near the magnet. The magnet pulls the paper clip to it. With your finger on the string pressing it to the ground, pull the free end of the string under your finger. The paperclip will separate from the magnet but will float in space. The magnet force is acting at a distance!
Next place the magnet inside the glass against the side of the glass. Place the paper clip on the outside of the glass near the permanent magnet. It stays there! Magnetic forces can act through nonmagnetic barriers.
5.
Things attracted to a permanent magnet become temporary magnets themselves .
Demonstration: Take a paper clip and place it near a permanent magnet. The paper clip is attracted to the magnet and it “sticks” to the magnet. Now place a second paper clip near the first paper clip; it “sticks” to the first paper clip. The first paper clip has become a temporary magnet. Try a third paper clip on the second paper clip. There is a limit as to how many paper clips you can string together.
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K. Craig Fall 2009
Some facts have only been known since the 19 th
century. Up until the year 1820, everyone thought magnetism and electricity were completely separate. In that year, the Danish physicist
Hans Oersted discovered that a compass needle was deflected by an electric current. For twelve years after Oersted’s discovery, scientists looked for the complementary effect: how to make a magnetic field produce a current. At last, in 1832, Michael Farady made a suggestion: move the magnet! So these new facts about the force are:
6.
A coil of wire with an electric current flowing through it becomes a magnet . Demonstration:
Take a piece of wire and attach the ends of the wire to the terminals of a battery. Be careful as the wire will get hot quickly as electric current passes through the wire and heats it up.
Hold the wire near the needle of a compass. The compass needle is deflected! The electricity in the wire has generated a magnetic field.
7.
Putting iron inside a current-carrying coil greatly increases the strength of the electromagnet . Demonstration: Take an iron nail and wrap a wire around it. Hold the ends of the wire to the terminals of a battery. Again be careful as the wire will get hot quickly as electric current passes through the wire and heats it up. Bring the nail near another nail or magnetic object. The electromagnet you have built will pick up the nail and even several nails.
8.
A changing magnetic field induces an electric current in a conductor (like copper) .
Demonstration: Take a loop of wire and connect it to a sensitive current meter that displays current by the deflection of a needle. Hold a magnet stationary inside the loop of wire. The current meter needle does not move. Now move the magnet! As the magnet moves, the current meter needle moves. The moving magnet inside the loop of wire has caused a current to flow in the wire. Michael Faraday would be proud!
9.
A charged particle experiences no magnetic force when moving parallel to a magnetic field, but when it is moving perpendicular to the field it experiences a force perpendicular to both the field and the direction of motion . Demonstration: This is a tough one! A charged particle is tough to find! Since current is charge in motion, we can demonstrate this together with fact # 10.
10.
A current-carrying wire in a perpendicular magnetic field experiences a force perpendicular to both the wire and the field . Demonstration: Let’s build a motor! All we need is a D-size battery, two large paper clips, a rubber band, about 2 feet of magnet wire, a small refrigerator magnet, and some sand paper. The motor rotor turns because electricity is flowing in the wire and the wire is in a magnetic field.
Challenge Question: If you take a strong magnet and drop it down a copper (nonmagnetic but a good conductor of electricity) pipe, the magnet floats slowly down the pipe without touching the sides of the pipe. What is slowing the fall of the magnet?
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