MAGNETISM A Brief History of magnetism In historic times it was recorded that some rocks from a region known as Magnesia (in Turkey) were attracted to each other. These rocks were called ‘lodestone’ or magnet.
Properties of Magnets 1. 2.
Magnets attract some metals known as magnetic or ferromagnetic materials. These are iron, steel, nickel and cobalt. If a magnet is suspended form a thread in air, it will always turn so that one end of the magnet points towards the North Pole of the Earth. This is the North-seeking or the North Pole of the magnet. The opposite end of the magnet pointing towards the South Pole of the Earth is the South-seeking or the South Pole of the magnet.
3.
Magnets have two poles: North Pole and South Pole.
4.
The strength of the magnet is greatest at its two poles.
5.
Like poles repel and unlike poles attract. This is known as the LAW OF MAGNETISM.
Magnetic Induction When a magnet is brought near an iron bar, the iron bar is attracted towards the magnet. This can be explained by the theory of magnetic induction. The theory states that when a North Pole of a magnet is brought near one end of an iron bar, the end closest to the magnet becomes South Pole and the far end of the iron bar becomes North Pole. These poles on the iron bar are known as induced poles because they were not there before the magnet has created them. This phenomenon is known as MAGNETIC INDUCTION. The magnet’s North Pole attracts the South Pole of the iron bar because according to law of magnetism unlike poles attract. Induced N-pole
Iron bar before
Induced S-Pole
Iron bar after
magnet attraction
Methods of Magnetizing 1.
One-Stroke Method ( single touch method) One end of a magnet is rubbed against an iron bar in the manner shown. The process is repeated quite a few times. The iron bar becomes a magnet with poles as shown. NOTE: The magnet must be taken to an appropriate height before returning it to the iron bar so that the alignment of the domains produced in the iron bar are not disordered when the magnet is taken back to start position.
Iron bar
Before (non-magnet)
magnet
After (now it is a magnet)
Double-Stroke Method ( double touch method)
2.
Two magnets are rubbed as shown. Opposite poles of a magnet are are formed on the two ends of the iron bar.
3.
Electrical method. This is the best method to magnetize. It forms strong magnets.
Pass a direct current (d.c.) through a solenoid with an iron bar placed inside it.
Iron bar solenoid
Methods of Demagnetizing 1. 2. 3.
Hammering the magnet will demagnetize the magnet. Heating the magnet will demagnetize the magnet. Passing an alternating current (a.c.) will demagnetize the magnet. This is the best way to demagnetize. The bar magnet is placed inside a solenoid just like in the diagram above and instead of d.c. an alternating current is passed through the solenoid. An a.c. current flows in both directions that would continuously alter the direction of the domains inside the magnet and demagnetize it. The magnet is drawn out of the solenoid while the a.c. current is flowing.
magnet
a.c. supply Magnetic Field: It is a region around a magnet in which another magnet or magnetic material experiences a force. Examples of magnetic fields:
Screening/Shielding: When an iron ring is placed in a magnetic field, it absorbs all the lines of force. This is property is used to protect appliances from strong magnetic fields.
Iron ring
Plotting field lines with a plotting compass Place a bar magnet on a white sheet of paper. Put a small plotting compass near the north end of the magnet. Mark the south end and the north end of the compass needle with a dot. Lift the compass and place its center on the next dot and mark the north end of the needle. Continue to do so until you reach the south end of the magnet. lift the compass and the magnet. Join the dots to draw the the field line. Repeat the above procedure to plot more fields lines on both side of the magnet. Purpose of using a small compass: This allows the dots to be plotted close to each other and it is easy to draw the field lines. Also it is more sensitive to weak magnetic fields.
Field strength and direction Some important facts emerge when plotting lines of force:
1. - Lines of force NEVER cross. 2. - Lines of force are CONTINUOUS. 3. - Lines of force always form individual CLOSED LOOPS around the magnet. 4. - Lines of force have a definite DIRECTION from North to South. 5. - Lines of force that are close together indicate a STRONG magnetic field. 6. - Lines of force that are farther apart indicate a WEAK magnetic field.
Difference between properties of iron and steel.
Iron (Soft magnetic material)
Steel (Hard magnetic material)
It magnetizes easily
It is difficult to magnetize
It demagnetizes easily
It is difficult to demagnetize
It forms temporary magnets
It forms permanent magnets
Uses of temporary magnets: electromagnet, electric bell, galvanometer, relay switch, reed switch, circuit breaker.
Uses of permanent magnets: motors, to take out pieces of metal out of the eye or stomach.
Uses of electromagnet in audio/visual tapes: The tape is made of a magnetic material. The sound is converted into electrical and then an electromagnet is used to form a certain magnetic pattern on the tape. When the tape is run over a head of a cassette player, the magnetic pattern is read by an electromagnet and converted back to electrical and sound signals. The same process takes place in videotapes.
The Tape Recorder The simplest tape recorders are very simple indeed, and everything from a Walkman to a high-end audiophile deck embodies that fundamental simplicity. The basic idea involves an electromagnet that applies a magnetic flux to the oxide on the tape. The oxide permanently "remembers" the flux it sees. A tape recorder's record head is a very small, circular electromagnet with a small gap in it, like this:
This electromagnet is tiny -- perhaps the size of a flattened pea. The electromagnet consists of an iron core wrapped with wire, as shown in the figure. During recording, the audio signal is sent through the coil of wire to create a magnetic field in the core. At the gap, magnetic flux forms a fringe pattern to bridge the gap (shown in red), and this flux is what magnetizes the oxide on the tape. During playback, the motion of the tape pulls a varying magnetic field across the gap. This creates a varying magnetic field in the core and therefore a signal in the coil. This signal is amplified to drive the speakers.
Field due to current in a straight wire If we pass a current down a wire we find that we can map the magnetic field lines using a compass. The main characteristics of the magnetic field are: • The field lines are circular and are centered on the wire • The strength of the magnetic field decreases as we move away from the wire • The direction of the magnetic field reverses if we reverse the direction of the current • The strength of the magnetic field is proportional to the magnitude of the current.
Right-hand Rule The magnetic field direction can be remembered by using a right-hand rule. This rule is sometimes known as Maxwell’s screw rule.
With your right hand, point your thumb along the direction of the current and then curl your fingers. The direction in which your fingers curl represents the direction of the magnetic field. This is known as a right-hand screw rule because, if you take a right-hand screw, you must turn it in the direction of the magnetic field to make it bore into wood along the direction of the current.
Field due to a current in a single loop coil
Field due to a solenoid
The field due to a current flowing around a circular coil is the sum of the fields due to each segment of the coil. The result is a magnetic field which threads the coil in the direction shown. Once again the field direction is given by the right-hand rule.
Applications of Electromagnets Relays
Computers and electronic circuits are low voltage, low power systems. It is often necessary for computers or other electronic circuits to control high currents or voltages. To connect them directly is impossible. Instead they control mechanical mechanisms that operate switches in the other circuit. These switches are known as relays. In a relay a switch is opened or closed by an electromagnet controlled by the computer. In the example below, the closing of the switch on the low current circuit turns on the electromagnet. The magnetic field attracts the arm of the switch on the high current circuit. The bulb lights up. This switch opens and cuts the current flowing in that circuit.
Bells
In an electric bell the bell clapper is part of the circuit. Normally the clapper rests against an electrical contact. When the button is pressed current flows through the circuit and the solenoid is energized. The magnetic field produced pulls the clapper towards the pole of the solenoid and the clapper strikes the bell. This movement also immediately breaks the circuit and current ceases to flow through the solenoid. The clapper springs away from the bell and touches the electrical contact again, remaking the circuit; this cycle is repeated as long as the bell push is pressed.