Magnets and Magnetic Fields Chapter 26

A Magnet attracts certain materials to itself. A magnet will attract Iron, Steel, Nickel, Cobalt and some alloys of these. A magnet has no noticeable effect on other materials. A Bar Magnet is strongest at each end. Dip a bar magnet into iron filings or a box of pins. It attracts the filings or the pins to itself. Most cling on at each end of the magnet. The regions of greatest strength at each end are called Magnetic Poles.

If a Bar Magnet is suspended freely it will line up approximately North-South. The pole of the magnet that always points North is called the North-Seeking Pole or the North Pole. The pole that points south is called the South-Seeking Pole or the South Pole.

Magnetic Poles occur in Pairs. You cannot have a single pole on its own. For every north pole there is always a south pole. The strength of the north pole is the same as the strength of the south pole. Like poles repel and unlike poles attract The North Pole of one magnet repels the North Pole of another. The South Pole of one magnet repels the South Pole of another. The North Pole of one magnet is attracted to the South Pole of another magnet.

A Magnet causes some materials brought near it or touching it to become magnetised. This magnetism is called Induced Magnetism. If the magnet is taken away some materials (called permanent magnets) hold on to their magnetism but others (called temporary magnets) lose most of it. Hard steel holds onto magnetism very well whereas soft iron does not. Ordinary nails are usually made from soft iron and do not retain their magnetism very well.

What is a Magnetic Field? A Magnetic Field is any region of space where magnetic forces can be felt. What is the Direction of a Magnetic Field ? The Direction of the Magnetic Field at a point is the direction of the force on a north pole if it was placed at that point.

What is a Magnetic Field Line? A line drawn in a magnetic field so that the tangent to it at any point shows the direction of the magnetic field at that point is called a Magnetic Field Line. What are Magnetic Poles? Magnetic poles are the regions at each end of a magnet where the magnetic forces are greatest. Magnetic poles are always found in pairs.

A Plotting Compass is a small magnet that can rotate about a vertical axis. If no other magnets are nearby it will line up North-South. If another magnetic field is present it will deflect the compass needle from its N-S position. If the other field is strong enough the compass needle will line up almost parallel to the field rather than North-South.

The Magnetic Field around a Bar Magnet The magnetic field lines start at the north pole and end at the south pole. The magnetic field lines never cross each other. Near the poles - where the magnetic field is strongest - the lines are close together. Further away, where the field is weaker, the lines are far apart.

Iron Filings showing the Magnetic Field around a Bar Magnet

The magnetic field around a U-shaped Magnet Earth’s Magnetic Field, pointing approximately North

What is the Magnetic Effect of an Electric Current? Every Current-Carrying Conductor has a Magnetic Field around it caused by the current. As long as the current is flowing the Magnetic Field exists. If the current stops flowing the Magnetic Field disappears.

Experiment to show the Magnetic Effect of an Electric Current Set up the equipment with the wire lined up North-South. The plotting compass also lines up N-S. Close the switch, sending current through the wire. The compass needle will deflect. Reverse the direction of the current and the needle deflects in the opposite direction. Open the switch, no current flows, the magnetic field disappears and the compass again lines up N-S. Conclusion: Every current carrying conductor has a magnetic field around it caused by the current.

The Magnetic Field around a Long Straight Wire

The Right-Hand Grip Rule If a Right Hand clasps a conductor with the Thumb pointing in the direction of the Current Then the Fingers give the direction of the Magnetic Field around the conductor. The thumb points in the direction of conventional current, i.e. from + to -

A Circular Loop of Wire carries a current in the direction shown. Using the Right-Hand Grip Rule at a number of points on the wire shows us the shape of the magnetic field around the loop. The side of the loop facing us behaves like a South Pole (the magnetic field lines are going in) The other side is like a North Pole (the magnetic field lines are coming out).

The Magnetic Field around a Current-Carrying Coil (or a loop)

The Magnetic Field around a Current-Carrying Solenoid

What is an Electromagnet? A Solenoid carrying a current and containing a soft iron core is known as an Electromagnet. Electromagnets are used in: Scrap yard cranes Electric motors Electric bells Moving coil loudspeakers Induction coils By turning the current on or off the magnet can be turned on or off. By varying the size of the current the strength of the magnet can be varied.

A powerful Electromagnet lifting scrap Iron

A Magnetic Compass shows the direction of the Earth’s Magnetic Field and it is used in navigation. The Earth’s Magnetic Field forms a protective layer (from charged particles) around the Earth.

The Magnetic Compass has been used for hundreds of years in marine navigation, since it enables you to know direction. The angle between True North and Magnetic North is also of importance. Charts and maps used in navigation have its value in the locality of the chart noted on them since navigators must allow for it in their calculations.