Magnetic Force

If a charged particle is projected in a magnetic field, it experiences a magnetic force. By projecting the particle in different direction from the same point P with different speeds, we can observe the following facts about the magnetic field force.

a.       Force experienced by the moving charge is directly proportional to the magnitude of the charge
 i.e.       F α q

b.       Force experienced by the moving charge is directly proportional to the component of velocity perpendicular to the direction of magnetic field
i.e.       F α v sinθ

c.       The magnitude of the force F is directly proportional to the magnitude of the magnetic field applied
 i.e.       F α B

On combining all factors we get,

F = kq v sinθ B
Here k=1 is proportionality constant.
F= q v sinθ B.
Here we can see that v and B follows the vector product hence force is perpendicular to v and B.

 Direction of the force can be predicted by Right Handed Screw Rule or Right Hand Rule.

 
By measuring the magnetic force F acting on a charge q moving at a speed v, we can obtain B. If v=1, q=1 and sinθ =1 or θ=90º then F = 1x 1x B x 1 =B

Thus the magnetic field induction at a point in the field is equal to the force experienced by a unit charge moving with a unit velocity perpendicular to the direction of magnetic field at that point.

Special Cases

a.       If θ =0º or 180º the F = q v Bsin(0) = 0. Its means, a charged particle moving parallel to the direction of magnetic field, does not experience any force.

b.      If θ =90º or 180º the F = q v Bsin(90) = qvB(maximum force). Its means, a charged particle moving along a perpendicular to the direction of magnetic field, it experiences maximum force.

c.       If v =0, then F=q v sinθ B = 0. It means, a charged particle is at rest in a magnetic field, it experiences maximum force. It experiences no force.

UNIT OF MAGNETIC FIELD B
SI unit of magnetic field is tesla (T) or Weber /meter2
1T = 1NA-1m-1
1 Gauss = 10-4 T
Dimensions of B = [MA-1T-2]

We have a magnetic field of the order of 10-5 T near to earth surface

Magnetic field is also called as Magnetic Induction or Magnetic Flux Density.

Sensitiveness of Potentiometer

Sensitiveness of Potentiometer means is the smallest potential difference can be measured with the help of Potentiometer.

Sensitivity of Potentiometer can be increased by decreasing its potential gradient. The same can be achieved by

1.       By increasing the length of potentiometer wire.

2.       If the potentiometer wire is a fixed length, the potential gradient can be deceased by reducing the current in the potentiometer wire circuit with help of rheostat.

Magnetic Dipole

Magnetic dipole consists to two unlike poles of equal strength and separated by a small distance.
The distance between the two poles of the bar is called magnetic length of the magnet. It is a vector directed from S-pole to N-Pole and represented by 2l.

                                                                                         
 Magnetic dipole is the product of the strength of either dipole (m) or the magnetic length (2l) of the magnet. It is represented by M.

 It is vector quantity directed from South to North Poles.
SI unit of M is ampere-meter2 or joule/tesla.

Current Loop as a Magnetic Dipole

A current loop act as a magnetic dipole as it has both south and north pole.

Current loop act as Magnetic dipole 

When we see a current carrying loop from top, current direction is anticlockwise and has north polarity. If, lower face of loop is seen from bottom then current direction is clockwise and have south polarity. Overall it behaves as a magnetic dipole depends upon

Physical Significance Of Dipoles

Physical significance of dipole is that it is used to predict whether a molecule is polar or non polar.

In most molecules, the centre’s of positive charges and of negative charges lie at the same place. Therefore, their dipole moment is zero and that molecule is considered as non polar.

CO2 and CH4 are of this type of molecules. However, they develop a dipole moment when an electric field is applied.

But in some molecules, the centres of negative charges and of positive charges do not coincide. Therefore they have a permanent electric dipole moment, even in the absence of an electric field. Such molecules are called polar molecules.

A water molecule, H2O, is an example of this type

Electric Dipole

An electric dipole is a pair of equal and opposite point charges q and –q, separated by a distance 2a. The line connecting the two charges defines a direction in space.

i.e. p = q × 2a

By convention, the direction from –q to q is said to be the direction of the dipole. The mid-point of locations of –q and q is called the centre of the dipole.

The total charge of the electric dipole is obviously zero as it contains two equal and opposite charge which cancels each other . This does not mean that the field of the electric dipole is zero.

 The electric field due to a dipole  falls off, at large distance, faster than like 1/r2 (the dependence on rof the field due to a single charge q).

The electric field produced by a dipole is known as dipole field.

Properties of Electric Field Lines

An electric field line is, in general is the path followed by a +ve charge when kept in an electric field.

An arrow on the curve is obviously necessary to specify the direction of electric field from the two possible directions indicated by a tangent to the curve. A field line is a space curve, i.e., a curve in three dimensions.

Properties of Electric field lines: –

1. The electric lines of force are imaginary lines.

2. A unit positive charge placed in the electric field tends to follow a path along the field line if it is free to do so.

3. The electric lines of force came out from a positive charge and came in a negative charge.

4. The tangent to an electric field line at any point gives the direction of the electric field at that point.

5. Two electric lines of force can never cross each other.Because If they do, then at the point of intersection, there will be two tangents. It means there are two values of the electric field at that point, which is not possible.

    Further, electric field being a vector quantity, there can be only one resultant field at the given point,  represented by one tangent at the given point for the given line of force.

6. Electric lines of force are closer (crowded) where the electric field is stronger and the lines spread out where the electric field is weaker.

7. Electric lines of force are perpendicular to the surface of a positively or negatively charged body.

8. Electric lines of force contract lengthwise to represent attraction between two unlike charges.

9. Electric lines of force exert lateral (sideways) pressure to represent repulsion between two like charges.

10.The number of lines per unit cross – sectional area perpendicular to the field lines (i.e. density of lines of force) is directly proportional to the magnitude of the intensity of electric field in that region.

11. Electric lines of force do not pass through a conductor. Hence, the interior of the conductor is free from the influence of the electric field.

i.e. E in a conductor is 0

12. Electric lines of force can pass through an insulator.

Magnetic properties of solids

Every substance has some magnetic properties associated with it. The origin of these properties lies in the electrons. Each electron in an atom behaves like a tiny magnet.
 Its magnetic moment originates from two types of motions
1. its orbital  motion around the nucleus
2. its spin around its own axis .
 Electron being a charged particle and undergoing these motions can be considered as a small loop of current which possesses a magnetic moment.  Thus, each electron has a permanent spin and an orbital magnetic moment associated with it.  On the basis of their magnetic properties, substances can be classified into five categories:
 (i) paramagnetic
(ii) diamagnetic
(iii) ferromagnetic
(iv) antiferromagnetic
 (v) ferrimagnetic.

 (i) Paramagnetism:
1) Paramagnetic substances are weakly attracted by a magnetic field.
2) They are magnetised in a magnetic field in the same direction.
3) They lose their magnetism in the absence of magnetic field.
4) Paramagnetism is due to presence of one or more unpaired electrons which are attracted by the magnetic field.
 O2, Cu2+, Fe3+, Cr3+  are some examples of such substances.

(ii) Diamagnetism:
1) Diamagnetic substances are weakly repelled by a magnetic field. H2O, NaCl and C6H6 are some examples of such substances.
2) They are weakly magnetised in a magnetic field in opposite direction.
3) Diamagnetism is shown by those substances in which all the electrons are paired and there are no unpaired electrons.
4) Pairing of electrons cancels their magnetic moments and they lose their magnetic character.

(iii) Ferromagnetism:
1) A few substances like iron, cobalt, nickel, gadolinium and CrO2 are attracted very strongly by a magnetic field. Such substances are called ferromagnetic substances.
2) Besides strong attractions, these substances can be permanently magnetised.
3) In solid state, the metal ions of ferromagnetic substances are grouped together into small regions called domains. Thus, each domain acts as a tiny magnet.
4) In an unmagnetised piece of a ferromagnetic substance the domains are randomly oriented and their magnetic moments get cancelled.
5) When the substance is placed in a magnetic field all the domains get oriented in the direction of the magnetic field. and a strong magnetic effect is produced.
6) This ordering of domains persist even when the magnetic field is removed and the ferromagnetic substance becomes a permanent magnet.

(iv) Antiferromagnetism:
1) Anti-ferromagnetism have domain structure similar to ferromagnetic substance, but their domains are oppositely oriented and cancel out each other’s magnetic moment
e.g. MnO

(v) Ferrimagnetism:
1) Ferrimagnetism is observed when the magnetic moments of the domains in the substance are aligned in parallel and anti-parallel directions in unequal numbers .
2) They are weakly attracted by magnetic field as compared to ferromagnetic substances.
Fe3O4 (magnetite) and ferrites like MgFe2O4 and ZnFe2O4 are examples of such substances.
3) These substances also lose ferrimagnetism on heating and become paramagnetic.

Difference between potentiometer and voltmeter

Potentiometer	Voltmeter
Potentiometer measure emf of cell very accurately.	Voltmeter measure emf of cell approximately.
Potentiometer does not draw any current from known emf source while measuring current.	Voltmeter draw current from known emf source while measuring current.
While measuring emf, resistance of potentiometer become infinite.	While measuring emf, resistance of voltmeter becomes very high but measurable.
In the potentiometer sensitivity is high.	In the voltmeter sensitivity is low.
It is based on null deflection method.	It is based on deflection method

Exceptional behaviour of graphite

Graphite’s exceptional properties: –

1. Graphite is soft while covalent solid  are hard.
2. It is a conductor of electricity while covalent solids are non conductors. 

Reason for its exceptional behaviour : –
Its exceptional properties are due to its typical structure.

Structure of graphite 

 Carbon atoms are arranged in different layers and each atom is covalently bonded to three of its neighbouring atoms in the same layer. The fourth valence electron of each atom is present between different layers and is free to move about. These free electrons make graphite a good conductor of electricity. Different layers can slide one over the other. This makes graphite a soft solid and a good solid lubricant