Key Concept: Magnetism

Sample Questions: (As usual, I left out questions that were too similar to ones that were already included)

1. (JUNE 2000: 416): A magnetic compass is placed at one of the ends of a bar magnet. Which of the following diagrams correctly shows the direction in which the compass needle will point?

 

2. (JUNE 2000: 416): Two electromagnets are placed end to end. In which of the following diagrams do the electromagnets attract each other?

 

3. (JUNE 1999: 416): Which of the following devices works by using magnetic fields?

A. light bulb

B. baseboard heater

C. loudspeaker

D. alkaline battery

4. (JUNE 1998: 416): Two magnets are brought close to each other. Which of the following best illustrates the magnetic fields surrounding these magnets?

 

5. (JUNE 1998: 416): An electric current flows through a straight wire and produces a magnetic field. Which of the following diagrams correctly represents this magnetic field?

 

Notes:

Magnetic Poles

Pole

Pole

Result

North (N)

North(N)

repulsion

South (S)

South (S)

repulsion

North(N)

South (S)

attraction

Magnetic Fields

These consist of lines of force that run from the north pole to the south pole. ( see question 4C .) They never cross, and the more lines you have the stronger the magnet.

When there are two fields side by side, the lines run from North to south again, but no loop is made through "empty space."

Types of Materials

  1. Ferromagnetic material: This is a material that sticks to a permanent magnet. Examples include iron, cobalt and nickel (easy to remember because they are all consecutive in the periodic table: atomic numbers 26, 27, and 28) There are other ferromagnetic materials such as Nd (used in quality speakers), but the guys responsible for writing provincial exams don't seem to know about them. Every atom has moving charges so on its own it has magnetic dipoles ( North ends and South ends), but few elements, which consist of "zillions" of atoms, are ferromagnetic because the fields cancel out either within the atom or between atoms. But ferromagnetic elements have "domains", groups of atoms whose magnetic dipoles are aligned. Nearby magnets create even more organization by aligning the domains.
  2. Non-magnetic materials include sulfur, plastic, gold, copper and aluminum. These will not stick to a magnet. They cannot form domains because of the way their electrons are arranged.
  3. Temporary magnet: this consists of a ferromagnetic material such as iron, which while sticking to a magnet, can pick up other ferromagnetic materials. If the material is pure, it will lose its magnetism as soon as it detaches itself from the permanent magnet. This is because there are no impurities within the temporary magnet to lock the "domains" into place
  4. Permanent Magnet: These include compasses or horseshoe magnets made of Al, Ni(nickel) and Co(cobalt). Notice the Al is not ferromagnetic. It's there to lock the magnetic domains into place. What do we mean? When a mixture of those three elements is placed inside an electromagnet, the domains get organized nicely so that the N and S poles of nickel and cobalt's individual domains all strengthen each other. The Al locks the domains into place, so that when the current is cut off, the material remains magnetic. The same happens with steel because steel contains the impurity carbon which serves the same role as aluminum.

The Relationship between Electricity and Magnetism

Electricity can be generated by moving magnets (like Hydro's generators), and with the correct setup, you can get motion from having electricity move in a loop between two magnets. That's because magnetism results from the motion of electrons. The poles always exist at an angle of 90o to the direction of the electrons.

(1)If electricity moves through a straight wire, the left-hand rule reveals the direction of a magnetic field. (revealed by sprinkling iron dust on a cardboard placed around wire. The direction is obtained from a compass needle: where the needle points is where the field is going.)

(2) A solenoid is a loop of wire, and the individual magnetic fields strengthen each other. The North pole of the overall field is revealed by the left hand rule again, but this time the thumb does not point in the direction of the (+) but in points North. The other fingers wrap themselves in the same way that the electrons do, and remember they flow from (-) to (+).

In the above diagram we have more than just a solenoid. A bar has been placed inside the coil, so we really have an electromagnet. To make this magnet stronger, we can

(a) increase the current

(b) place a ferromagnetic material inside the solenoid

(c) increase the number of loops