Overview…• Testing of a magnet• Induced magnetism• Theory of magnetism• Methods of magnetisation & demagnetisation• Plotting / Drawing of magnetic field lines
Testing of Magnet One end of specimen The other end of Specimen brought near to N specimen brought pole near to N pole Bar Magnet Attraction occurs Repulsion occurs (or repulsion occurs) (or attraction occurs) Soft Iron Rod Attraction occurs Attraction occurs(magnetic material) Wooden Rod Nothing happens Nothing happens
Testing of magnetThus• If attraction occurs for both ends of the material, then it must be a soft iron (or any magnetic material)• If repulsion occurs for one side of its end, then it must be a magnet.• Only repulsion between a specimen and a magnet allows us to conclude that the specimen is a magnet
Induced Magnetism• When a piece of unmagnetised magnetic material (iron or steel) touches or is brought near to a permanent magnet, it is attracted and becomes a magnet itself. The material is said to have magnetism induced into it.• Materials that has magnetism induced into it are called induced magnet.• The process by which the unmagnetised magnetic materials become magnets is called magnetisation.
Induced MagnetismHow do we know thetwo ends of the ironsnails are of Northpolarity?
Induced MagnetismPlace the North poleof a bar magnet nearthe two ends of theiron nails.The iron nailsexperienced a greaterrepulsion.
Induced Magnetism Temporary. They lose their magnetism once the permanent magnet is drawn away from it.
Background knowledgeTheory of Magnetism• In an unmagnetized state, the domains all point in different directions.• Their magnetic effects cancel each other.
Background knowledgeTheory of Magnetism• All the domains of a fully magnetized material point in the same direction.
Method of magnetization• The process of magnetization converts a piece of steel into a permanent magnet.• Note: Methods of magnetisation produces permanent magnet, unlike induced magnets. Thus, steel must be used for all magnetisation and not iron
Method of magnetization• Magnetization by stroking Single stroke method Double touch method• Magnetization using an electric current.
SS S SS NN NNNN Steel Bar S The pole produced at the end of the magnetized steel is of the opposite polarity of the stroking pole.
Steel Bar What are the poles induced in the steel bar above? Click on either of the diagrams below. N S S N
The steel bar to beN S magnetized is placed inside a solenoid. When a direct current (d.c.) is passed through the solenoid and then turned off, Use the Right-Hand the steel bar becomes Grip Rule to determine magnetized when the polarity of the magnetized steel. removed from the solenoid.
Ways of increasing the Magnetism• 1) Increase the current (ie Use a stronger battery)• 2) Increase the number of coils / turns in the solenoid
Methods of Demagnetization• The methods of demagnetisation removes all magnetism from a permanent magnet. – Heating – Hammering (east-west direction) – Using an A.C current.• Note:We only demagnetised permanent magnets.So all specimen described here are eitherpermanent bar magnets or magnetised steeland not iron
Heat magnet to red-hot. Then cool.As in heating, the molecules are set to vibration,causing the magnetic alignment to be lost.
Hammer magnet vigorously.By heating, the molecules vibrate vigorouslythus causing the magnetic alignment to be lost.This is a very quick way to remove magnetism
solenoid • Place a magnet or magnetized steel inside a 12 V a.c. solenoid connected to an alternating current (a.c)• Turn on the current and slowly remove themagnet 2 to 3 metres away from the solenoid.
Magnetic Field Magnetic field lines were introduced by Michael Faraday (1791 – 1867) who named them lines of force.
Magnetic FieldIt is a region around a magnet whereother magnetic objects experience amagnetic force.
Experiment: To plot the field lines of a magnetic field Use of compass.
Experiment: To plot the field lines of a magnetic field Procedure: 1. Place the bar magnet at the centre of the piece of paper so that its N-pole faces North and its S-pole faces South.
Experiment: To plot the field lines of a magnetic field 2. Starting near one pole of the magnet, the positions of the ends, N and S, of the compass needle are marked by pencil dots X and Y. The compass is then moved until one end is exactly over Y and the new position of the other end is marked with a third dot.
Experiment: To plot the field lines of a magnetic field 3. Repeat the process of marking the dots. Join the series of dots and this will give the plot of the field lines of the magnetic field.
Experiment: To plot the field lines of a magnetic field Precautions: - Check that the plotting compass is in good working order. - Ensure that there is no strong magnetic field (other than the Earths magnetic field) around the plotting compass.
Magnetic Field Lines used to represent the direction of magnetic field pattern.
Magnetic Field Lines1) Outside the magnet, the lines start from the N pole and end at the S pole.2) The lines can never cross each other.3) The lines are closer to each other in a stronger magnetic field.
How would the magnetic field patterndiffer if there are two magnets withlike poles facing each other?
X is known as the neutral point. No magnetic field lines at X.
Neutral point:It is the region between two magnetswhere there are no magnetic field lines.This is because the field due to onemagnet cancels out that due to the other. X is known as the neutral point. No magnetic field lines at X.
X X X is known as the neutral point. No magnetic field lines at X.
Magnetic Properties of Iron and Steel Iron SteelIt gets easily magnetized It is hard to get magnetizedIt gets easily It is hard to getdemagnetized demagnetizedUsed for temporary Used to make permanentmagnets such as induced magnetsand electromagnetsKnown as soft-magnetic Known as hard magneticmaterial material