Magnetism 1. In this presentation you will: explore how magnets can be used to investigate magnetism. Next >

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1 Magnetism 1 In this presentation you will: explore how magnets can be used to investigate magnetism

2 Introduction Magnetism was recognized as a naturally occurring phenomenon by Greek philosophers as long ago as 600BC. For centuries, magnetic compasses have been used for navigation. Today, magnetism is made use of in every form of modern technology. Computer hard drive Using a few magnets and some other equipment, a number of simple experiments can be carried out to investigate magnetism.

3 Magnetic Poles The ends of a magnet are called poles. The Earth acts as a giant magnet. If a bar magnet is free to move, one end will always point towards the Earth's magnetic North Pole. This end of the magnet is termed its north pole. The other end is its south pole. You can try this out by suspending a bar magnet on a piece of string to see which way it points.

4 Question 1 A magnet is suspended so that it is free to rotate. Which end of the magnet turns to face the Earth's magnetic North Pole? A) Its north pole B) Its south pole C) It will depend on where the magnet is placed D) You cannot say

5 Question 1 A magnet is suspended so that it is free to rotate. Which end of the magnet turns to face the Earth's magnetic North Pole? A) Its north pole B) Its south pole C) It will depend on where the magnet is placed D) You cannot say

6 Compass A compass needle is a magnet and will align with the Earth's magnetic field. One end turns toward the magnetic North Pole, the other end toward the magnetic South Pole. A simple compass can be made by magnetizing a needle and then carefully floating it on water.

7 Attraction and Repulsion You can use two bar magnets to investigate the force between the poles of a magnet. Try comparing the force between like poles and unlike poles. Observe how the force changes with the distance between the poles.

8 Attraction and Repulsion For two magnets, the following rules apply: Unlike poles brought close together attract each other. Like poles brought close together repel each other.

9 Question 2 Two magnets are held with their ends facing each other. They are trying to repel each other. How are the magnets held? A) Only so a north pole faces a south pole B) Only so a north pole faces a north pole C) So a north pole faces a north pole, or a south pole faces a south pole D) You cannot say

10 Question 2 Two magnets are held with their ends facing each other. They are trying to repel each other. How are the magnets held? A) Only so a north pole faces a south pole B) Only so a north pole faces a north pole C) So a north pole faces a north pole, or a south pole faces a south pole D) You cannot say

11 Magnetic Materials You may have noticed that some materials will interact with a magnet and some will not. You can use a bar magnet to investigate a range of different materials to see which are affected by a magnet. Steel can Aluminum can Good materials to investigate include iron, steel, copper, and aluminum. You could also try a range of coins.

12 Ferromagnetic Materials The magnetic field of a magnet can bring about (induce) magnetism in certain materials. This induced magnetism causes the material to become attracted to the magnet. Iron Steel Cobalt These materials are known as magnetic or ferromagnetic materials.

13 Ferromagnetic Materials Common ferromagnetic materials are: elements such as iron, cobalt, and nickel alloys of these elements, such as steel or alnico (an alloy of aluminum, nickel, and cobalt) Iron Steel Cobalt

14 Ferromagnetic Materials Ferromagnetic materials fall into two categories: hard magnetic materials soft magnetic materials You can use a horseshoe magnet to test a range of materials. Iron nails (soft) Steel paper clips (hard)

15 Ferromagnetic Materials Hard magnetic materials, such as steel, cannot be easily magnetized. However, they retain their induced magnetism when the magnetizing field is removed. Hard magnetic materials are used to make permanent magnets. Iron nails (soft) Steel paper clips (hard)

16 Ferromagnetic Materials Soft magnetic materials, such as iron, can be magnetized easily. However, they lose this induced magnetism when the magnetizing field is removed. Soft magnetic materials are used to make temporary magnets. Iron nails (soft) These materials are often used in the cores of electromagnets.

17 Magnetic Field The magnetic field is the region surrounding a magnet where the magnetic force can be detected. This can be plotted using a compass and craft paper. Place the magnet on a sheet of white craft paper. The plotting compass is used to show the magnetic field around a magnet.

18 Magnetic Field Move the plotting compass around the magnet, marking a dot at the point of the compass needle as you go. The magnetic field can then be drawn out by connecting the dots and using arrows to indicate the field direction.

19 Mapping a Magnetic Field A magnetic field can also be mapped using iron filings sprinkled onto a piece of paper, for example, white craft paper with a bar magnet underneath. The iron filings form into a pattern around the magnet. Gently tapping the craft paper will help Bar magnet White craft paper White craft paper Iron filings the iron filings align.

20 Mapping a Magnetic Field A magnetic field is made up of lines of force. The lines are closer together at the poles of the magnet where the field is strongest. More iron filings are attracted to the poles of the magnet No iron filings are attracted to the central region of the magnet Other iron filings form lines that loop between the poles

21 Question 3 The magnetic field around a bar magnet is plotted. In which direction should the arrows on the magnetic field lines point? A) Towards the north pole of the magnet B) Away from the north pole of the magnet C) Away from the south pole of the magnet D) You cannot say

22 Question 3 The magnetic field around a bar magnet is plotted. In which direction should the arrows on the magnetic field lines point? A) Towards the north pole of the magnet B) Away from the north pole of the magnet C) Away from the south pole of the magnet D) You cannot say

23 Magnetic Effect of an Electric Current Another way to generate a magnetic field is to use electricity. Conductor Magnetic field If a current is passed through a wire (conductor), then a magnetic field forms around the wire. Current flow away from you The magnetic field direction is dependent upon current direction. The strength of the magnetic field is proportional to the size of the current. Field direction Current flow towards you

24 Electromagnets Imagine a copper wire is shaped into a loop and connected to a voltage source. A current will flow to form a magnetic field. If more loops are made, the magnetic field will become stronger. This is like putting magnets next to each other. The strength of the magnetic field is directly proportional to the number of turns.

25 Electromagnets In a coil, the individual circular fields combine to cover the entire length of the coil. The field pattern produced by this is similar to that of a permanent magnet. The field lines inside the coil are parallel, showing the field strength is constant within the coil. Using a soft-iron core makes the field stronger. Together, the coil and the core form an electromagnet.

26 Question 4 What happens when a current flows through a wire? A) Nothing happens B) An equal current flows in the opposite direction C) A magnetic field is created around the wire

27 Question 4 What happens when a current flows through a wire? A) Nothing happens B) An equal current flows in the opposite direction C) A magnetic field is created around the wire

28 Summary In this presentation you have seen: how bar magnets can be used to investigate magnetic poles and magnetic materials how horseshoe magnets can be used to investigate hard and soft magnetic materials how a compass and iron filings can be used to map the magnetic field around a magnet how copper wire can be used to make an electromagnet End >