MAGNETISM from the point of view of NEET
This is an important chapter but compared to quantity of syllabus less questions are being asked in NEET.
We get around 2 questions on an average on Magnetic effects of current and one question for Magnetism.
First point is about motion of charged particle in magnetic field. The force is given as
F = q (v × B), which confirms that force is perpendicular to magnetic field as well as velocity vector.
Means that force is perpendicular to plane containing the vectors v and B .
Direction of force can be found by the vector law as well as Fleming Rule. Pictorial depiction is given here
You can see we show the direction of DOT for vector coming out and as a CROSS X for vector into the plane.
Next is the path of charged particle when the velocity between v and B.
If the angle is 0,180, the force is zero and particle moves undeflected.
If the angle is 90 the force is radial and particle moves in circular path
If the angle is any other angle except 0,90,180 the path is helical.
The source of magnetic field is moving charge, i.e. current, while the source of electric field is charge. The moving charge produces electric and magnetic fields.
To find the direction and magnitude of magnetic field due to a current carrying conductor, we have with us the a law called Biot Savart Law.
We also have Ampere circuital law to find the magnetic field.
If you see the direction of magnetic field comes out to be the same.
Ampere circuit is to be used in pipes, or uniformly distributed current carrying conductors. Do refer to numerical in NCERT EXAMPLE 4.8
Now the force on conductor placed in Magnetic field F = I L x B in scalar form F = ILB sin θ,
Here importantly please do understand and remember that same direction currents result in attraction and opposite direction result in repulsion. Diagram gives you details
When we place coil in magnetic field (uniform) it experiences a torque, and that becomes the principle of DC motors, Galvanometer and ammeter and Voltmeter
IDEAL ammeter has ZERO resistance and is made by attatching small resistance in Parallel. The whole equipment is attached in SERIES because it has to measure current and current is constant in SERIES circuit.
IDEAL Voltmeter has INFINITE resistance and is made by attaching HIGH resistance in Series. The whole equipment is attached in PARALLEL because it has to measure VOLTAGE and it is constant in PARALLEL circuit.
Also important information for a charge particle moving in circular path is used in cyclotron. Here too electric field is there to accelerate and magnetic field to just bring back the particle in original DEE.
Solenoid and Toroid are two devices or system which can give strong magnetic field. The value of Magnetic field is m0nI, where n is the number of turns per unit length.
Remember toroid is the ONLY, system which produces magnetic field with any practical POLES.
And lastly you must know that smallest dipole ever possible is of a hydrogen atom and is termed as Bohr magneton.
MAGNETISM
Magnetism: Substances which have the property of attracting small pieces of Ni, Co Fe etc are called magnets and this property is called magnetism. These types of materials exist in nature and are called natural magnets. Pieces of Iron and other magnetic materials can be made to acquire these properties of natural magnets; such substances are called manmade magnets.
Properties of Bar Magnet: (i) attractive (ii) Directive (iii) repulsive (iv) Monopole does not exist (v) Obey inverse square law. Repulsion is surest test of magnetism. Whenever an iron piece is kept in the field of bar magnet, iron piece develops magnetic dipole moment due to induction and thus attraction starts.
Definitions:
- Magnetic field: Space around a magnet within which its can be experienced
- Uniform magnetic field: In which the strength of magnetic field is the same at all points of field.
- Magnetic poles: regions of apparently concentrated in the magnet where magnetic attraction is maximum.
- Magnetic axis: Line passing through the poles of a magnet.
- Magnetic Equator: Line passing through center of magnet and at right angles to magnetic axis
- Magnetic length: Distance between the poles of magnet [about 7/8th of geometrical length in bar magnet]
- Magnetic meridian: Vertical plane passing through magnetic axis of freely suspended magnet at a place.
- Magnetic moment: Torque acting on magnet when placed at right angles to a uniform magnetic field of unit strength.
- Geographic axis: line passing through geographical north & south poles. It is axis of rotation of earth.
- Magnetic axis: line passing through magnetic North and South Pole.
- Magnetic dipole: Consists of 2 unlike poles of equal strength separated by small distance. It is analogous to electric dipole
Elements of earth’s magnetic field:
Angle of declination: Angle between magnetic meridian and geographical meridian at that place, it is in the range of 200. It can be measured with magnetic compass.
Angle of dip or inclination: Angle between resultant earth’s magnetic and horizontal component of magnetic field. It varies from place to place; at the equator it is zero and at poles it is 00. It is measured with the help of dip circle.
Horizontal component: Is the horizontal component of resultant earth’s magnetic field and denoted by BH. It is maximum at equator and minimum at poles, maximum value is 900 and minimum 00
Isoclinic lines: joining the places of equal declination.
Isogonic lines: joining lines of equal dip.
Isodynamic lines: joining lines of equal horizontal component.
Neutral points: points where net magnet field due to magnet and magnetic field due earth is zero. When a bar magnet is placed with it’s north pointing towards geographic North. The neutral points are observed on equatorial line, and when magnet’s North is pointing towards geographic south neutral point is observed on axial line. With the help of these neutral points we can find the dipole moment of bar magnet, when magnetic field due to earth is known. In general if the magnet is turned by q (1800 in this case) the neutral point is rotated by q/2 (900 in this case).
Diamagnetic Substance: These are feebly repelled when placed in magnetic field. They have only paired electron (No Magnetic moment of their own)
Paramagnetic: These are feebly attracted when placed in magnetic field. They have unpaired electron & hence net dipole moment of their own.
Ferromagnetic strongly attracted when in magnetic field. They have nonzero magnetic dipole moment and individual interact in such a way that their dipole moments have common directions. These groups are called domains.
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
| 1. Effect in magnetic field
2. In a uniform field 3. In a nonuniform field 4. Permeability value 5. Effect of temperature 6. Susceptibility 7. Magnetic lines than outside 8. Example |
Shows slight repulsion
Aligns ^r to field Tends to move from stronger to weaker parts of field m £ 1 Diamagnetism not effected by temperature Small –ve value Slightly less than outside
Bi, Au, Cu, Hg, Sb |
Shows slight attraction
Aligns parallel to field Tends to move from weaker to stronger parts of field m ³ 1 Paramagnetism decreases with temperature Small +ve value Slightly more than outside Al, Pt, Cr, Mn, CuSO4 |
Shows strong attraction
Aligns parallel to field Tends to move from weaker to stronger parts of field m >>> 1 Ferromagnetism decreases with temperature Large +ve value Much more than outside
Fe, Co, Ni |
Curie Law: state that intensity of magnetization (M) is (i) proportional to magnetic induction (B) and inversely proportional to temperature (T) in Kelvin scale. The temperature at which magnetization ceases is called Curie temperature. Most of the metals have Curie temperature much less than their melting point, also when a substance is melted it does not retain its magnetization on re-solidifying
Hysteresis: is the phenomenon of lagging of magnetic induction (B) and intensity of magnetization (M) behind the magnetizing (H), when a specimen is taken through a cycle of magnetization.
PROPERTY OF MATERIAL FOR MAGNET
Permanent HIGH RETENTIVITY AND HIGH COERCIVITY HIGH PERMEABILITY
Electromagnet LOW RETENTIVITY and low COERCIVITY AND HIGH PERMEABILITY
Study the character when magnetic material (DIA, PARA or FERRO) is kept in magnetic field.