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Unit 8 : MAGNETISM Year 7, 2012 LSS, MSPSBS

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observe that a freely-‐suspended magnet comes to rest in a North-‐South direction state that a magnet has two poles infer that like poles repel and unlike poles attract distinguish between magnetic and non-‐magnetic materials state that iron, steel, nickel and cobalt are magnetic materials infer that there is a magnetic field around a magnet observe and draw magnetic field patterns using iron filings: i) around a magnet ii) between two like poles arranged end to end iii) between two unlike poles arranged end to end

list some of the uses of magnets in everyday application.

Magnets and their properties A magnet exerts a force known as magnetic force. This force attracts iron objects. Magnets have different shapes and sizes. The three common types of magnets are:

• Bar magnet • Horseshoe magnet • Magnadur magnet

Bar magnet Horseshoe magnet Magnadur magnet A bar magnet exerts a magnetic force which can attract paper clips, pins, and iron filings. These objects will mostly cling to the two ends of the magnet, showing that magnetic force is strongest at the two ends. These two ends are the two poles of the magnet. If a magnet is suspended freely it will rest in a north-‐south direction. The poles of the magnet are called the ‘north-‐seeking’ pole (N-‐pole) and the ‘south-‐seeking’ pole (S-‐pole) of the magnet. INTERACTIONS OF MAGNETIC POLES If two like poles (N-‐pole and N-‐pole or S-‐pole and S-‐pole) are brought close together, the magnets push each other away. The magnets repel each other. If two bar magnets of opposite poles are brought near each other, it will attract each other. This repulsion and attraction is due to a magnetic force. A magnetic force is the force exerted by a magnet.

A magnet is a solid which attracts iron objects and points north-south when suspended freely.

Lodestone or loadstone is a naturally magnetized piece of the mineral magnetite. They are naturally occurring

magnets.

Learning Outcomes:


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Magnetic Materials

Magnetic materials are used to make magnets. Most magnets are made of iron or steel but do not behave exactly the same way because iron is easy to magnetise and loses its magnetism easily. Steel, on the other hand, is harder to magnetise and does not lose its magnetism easily. ‘Alnico’ magnet is a very strong magnet, which is made from a mixture of aluminium (Al), nickel (Ni), and cobalt (Co). Differences between magnetic material and non-‐magnetic material:

Magnetic material Non-‐magnetic material Substances which are attracted to a

magnet. Substances which are not attracted to a

magnet. Examples:

• Cobalt (Co) • Steel (an alloy of iron) • Iron (Fe) • Nickel (Ni)

Examples: • Aluminium • Copper • Tin

• Rubber • Wood • Plastic • Glass

DISTINGUISHING BETWEEN MAGNET AND MAGNETIC MATERIALS 1. The north and south poles of a magnet are brought near the same end of the tested bar.

2. If attraction takes place in one end and repulsion in the other end, then the tested bar is a magnet.

3. If both ends of the magnet attract the end of the bar, then the tested bar is a magnetic material.

DIFFERENCES BETWEEN A MAGNETIC MATERIAL AND A MAGNET

MAGNETIC MATERIAL MAGNET Attracted to a magnet Attracts magnetic materials

Both ends of the magnetic material are attracted to a magnet

Only one end of a magnet attracts to another magnet and the other end repels

Repulsion test is the only test to identify a magnet from a magnetic material.

Properties of magnets: 1. It points to the North-South direction when suspended freely. 2. Like poles repel each other. 3. Unlike poles attract each other.


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Magnetic Fields

Magnetic fields cannot be seen but it can be detected using iron filings. The tiny pieces of iron line up in the magnetic field.

Magnetic field lines shown by iron filings on a bar magnet.

The shape of the magnetic field around the magnet is shown by lines. This is known as the magnetic field’s lines of force.

Arrows on the lines point away from North and towards South.

The lines of magnetic force do not cross each other.

The closer together the lines of forces are, the stronger the magnetic field is.

Small compasses called the plotting compass can be used to show the magnetic field around a bar magnet instead of using iron filings. The needle in each compass turns and lines up along the field lines. MAGNETIC FIELD LINES BETWEEN TWO ATTRACTING MAGNETS

The region in the center of the two bar magnets (labeled X) has no magnetic field as the two opposing fields cancel each other out. This is called the neutral point.

A magnetic field is a region around a magnet, which cannot be seen, where magnetic forces acts on: (i) other magnets and (ii) objects made of magnetic materials.


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MAGNETIC FIELD LINES BETWEEN TWO REPELLING MAGNETS

X is the neutral point. When two bar magnets with like poles are positioned alongside each other (North and North; South and South), as shown on the figure on the left, there are two neutral point regions.

THE EARTH’S MAGNETIC FIELD

The Earth behaves as if it contains a giant bar magnet. The Earth has a magnetic North pole and a magnetic South Pole. Compasses work because bar magnets can line up in the Earth’s magnetic field and point north. If we place a compass on Earth, it will point to the North because the needle of the compass is also a magnet and the N-‐pole of the magnet is attracted to the Earth’s magnetic South Pole. If we suspend a bar magnet freely, the N-‐pole of the magnet will also point north because it is actually the Earth’s Magnetic South Pole, and the N-‐pole of the magnet is attracted to the Earth’s Magnetic South Pole.

Uses of Magnets

1. Magnetic buttons and refrigerator doors These are used to hold pieces of paper onto surfaces such as the outside of refrigerators. The surface is made of magnetic materials, usually iron.

2. Magnetic door stoppers A door stopper consists of a strong magnet and a piece of iron which are used to hold a door in place.

3. Magnetic ink on cheque and bank cards Magnetic ink is made by mixing a magnetic substance with a liquid. The mixture is used as ink to mark numbers on cheques.

4. In hospitals During accidents, people might get pieces of iron or steel in their eyes. In hospitals, doctors use strong magnets to remove these pieces of iron or steel.

5. In car engines A car engine has many moving parts made of steel. Friction between these moving parts produces fine pieces of steel. In some cars, a magnetic plug in the engine removes these pieces of steel to maintain the performance of the engine.

6. Loudspeakers Magnets in the loudspeakers are used to make the cone vibrate to make sound waves in the air.

7. Small electric motors Magnets in the electric motors help to convert electrical energy into mechanical energy. In larger electric motors, electromagnets are used.

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