Notes for Magnetism | Physics

Magnetism

• In magnetism we study about the magnet and its properties

Magnet

• A magnet is a material which can attract iron objects.
• A natural magnet is an ore of iron (Fe₃O₄) called magnetite or lodestone.
• A magnet which is prepared artificially, is called an artificial magnet.

For Example

• A bar magnet, a horse-shoe magnet etc.
• A freely suspended magnet always aligns itself into North-South direction. Like magnetic poles repel and unlike magnetic poles attract each other.
• A current-carrying coil containing a soft iron core, is called an electromagnet.
• An electromagnet is utilised in electric bell, telegraph receiver, telephone diaphragm, transformer, dynamo etc.
• Permanent magnets are made of steel and temporary magnet or electromagnets are made of soft iron because steel cannot magnetised easily but when it is magnetised one time, cannot be demagnetised easily. The soft iron can be magnetised or demagnetised easily.

Properties of Magnet

• Attractive property A magnet can attract small pieces of magnetic substances like iron, steel, cobalt, nickel etc. The attraction is maximum at poles. Unlike poles attract and like poles repel.
• Directive property A magnet, when suspended freely, aligns itself approximately along geographical N-S line.
• Magnetic poles exist in pairs If a magnet is cut into two equal parts transverse to its length, then N and S-poles of the magnet do not get separated.

Magnetic Field

• The space in the surrounding of a magnet or a current carrying conductor in which its magnetic effect can be experienced, is called magnetic field.
• Magnetic lines of force is an imaginary line drawn in magnetic field at which a magnetic North pole will move, if it is free to do so.
• A tangent drawn at any point of a magnetic line of force represents the direction of magnetic field at that point.
• The magnetic flux linked with a surface is equal to the total number of magnetic lines of force passing through that surface normally. Its unit is weber.
• Magnetic flux, f = A. = BA cos q
• Magnetic Force Acting on a Charge
• Moving in Uniform Magnetic Field

The magnetic force on a moving charge in a magnetic field is given by F = Bqv sin q
where, B = magnetic field, q = charge, v = speed
q = angle between the direction of motion and magnetic field.
Magnetic Force Acting on a Current-Carrying Conductor Placed in Uniform Magnetic Field

• If a conductor carrying element / is placed in a magnetic field, the magnetic force on it is given by  F = Bill sin q

where, l = electric current flowing through the conductor  q = angle between the direction of current and magnetic field.
The direction of this force can be find out by Fleming's left hand rule which is given below.

• If we stretch the thumb, then the force finger and the middle finger of left hand in such a way that all three are perpendicular to each other and if fore finger represents the direction of magnetic field, middle finger represents the direction of current flowing through the conductor, then thumb will represent the direction of magnetic force.

Earth's Magnetism

• The earth has its own magnetic field and it resembles that of a magnetic dipole located at the centre of the earth. The pole near the geographic North of the earth is called the magnetic North pole. Similarly, the pole near the geographic South pole is called the magnetic South pole.
• The Earth's magnetic field diverts charged particle coming from space towards its poles and saves living beings from being severely harmed.
• Magnetic compass A magnetic needle which always direct in North-South (N-S) direction.
• Neutral point A point in a magnetic region where the net magnetic field is zero.

Magnetic Storm
Local disturbances in the earth's magnetic field which can damage telecommunication which are probably caused by lump of charged particles emanating from the sun is known as magnetic storm.
Coil Places in Uniform Magnetic Field
When a coil having number of turns N, each of area of cross-section A carrying current l is placed in a uniform magnetic field B, then a torque acts on it, which tries to rotate it.
Torque,  τ= NB/A sin q
Moving Coil Galvanometer

• A moving coil galvanometer is used to detect the presence of current and the direction of current in any circuit.
• When current is passed through a coil, suspended in a magnetic field, a torque acts on it. As coil rotates, a restoring torque acts on phospher bronze strip due to twist produce in it. In equilibrium, both torques become equal the pointer stops for a short moment and coil starts to rotate in opposite direction.

Ammeter and Voltmeter

• An ammeter is an instrument used to measure electric current. It is always connected in series. The resistance of an ideal ammeter is zero.
• A galvanometer can be converted into an ammeter by connecting a low resistance in parallel.
• A voltmeter is a device used to measure potential difference between two points in an electric circuit.
• The resistance of an ideal voltmeter is infinity. It is always connected in parallel.
• A galvanometer can be converted into a voltmeter by connecting a high resistance in series.
• A small resistance connected in parallel with the load resistance to reduce amount of electric current through resistor is called shunt.

Magnetic Substances

• There are three types of magnetic substances Paramagnetic, Diamagnetic and Ferromagnetic.

Paramagnetic Substances

• Those substances which are feebly magnetised in the direction of magnetic field when placed in strong magnetic field, are called paramagnetic substances. For example,- Aluminium, platinum, chromium, manganese, solutions of salts of iron, nickel, oxygen etc.

• These substances are attracted towards strong magnetic field in a non-uniform magnetic field.
• The magnetism of these substances decrease with increase in temperature.

Diamagnetic Substances

• Those substances which are feebly magnetised in the opposite direction of magnetic field when placed in strong magnetic field, are called diamagnetic substances. For example: Gold, Silver, zinc, copper, mercury, water, alcohol, air, hydrogen etc.

• These substances are attracted towards weak magnetic field in a non-uniform magnetic field.
• The magnetism produced in these substances does not change with increase or decrease in temperature.

Ferromagnetic Substances

• Those substances which are strongly magnetised in the direction of magnetic field when placed in it, are called ferromagnetic substances. For example –iron, nickel, cobalt, etc.
• The magnetism produced in these substances decreases with increase in temperature and at a particular temperature, called Curie temperature.
• At the Curie temperature, a paramagnetic substance becomes diamagnetic.
• The Curie law is Xm ∝1/T (where,   Xm= magnetic susceptibility of a paramagnetic substance and T = temperature)

• Curie temperature for iron is 770⁰C and for nickel is 358⁰