chapter1 physics class 12 ncert electric charge

electric charges
coulombs law

1. Chapter-1 Electric Charges & Fields PHYSICS CLASS- 12 TH CLASS BY :- Er . Navneet Jangid

2. Introduction Electricity is a branch of physics which deals with presence & flow of charge in any material body. This branch is further divided into two main categories:- Static electricity or Electrostatics Current electricity

3. Electrostatics Thus Electrostatics is a branch of physics which study the forces, fields & potentials arising from static charges. This branch is also known as Static electricity or Frictional electricity. Static because here we study charges at rest , & Frictional because the main cause of production of charges is friction i.e. when two insulating bodies are rubbed against each other they led to transfer of electrons from one body to other body. Electro means electric Static means rest

4. Some common examples of static electricity are:- Lightning in sky Bits of paper get attracted towards comb when it is rubbed with any silk piece Sensation of electric shock while opening the door & many more are there.

5. What is electric charge? It is an inherent property possessed by material bodies due to which they are able to exert & respond to any type of electrical force on any other object. There are two types of charges:- a) Positive charge (+) b) Negative charge (-) Both charges have same magnitude of 1.6×10 -19 C but they differ in sign & this property which differentiates both charges is known as polarity of charges. Electric charge is a scalar quantity & is measured in C (Coulomb) which is named after a very renowned physicist Charles Augustin de Coulomb. Properties of electric charge:- Like (same) charges always repel each other & unlike (different) charges attract.

6. Methods of transfer of charge are:- By Rubbing:- It is a method of transfer of charge between two insulating bodies when they are rubbed against each other. Hence charge transferred in this is very small in amount. Also it is called as charge transferred due to friction. Different materials possess different number of electrons. If two same nature bodies are rubbed against each other then no transfer of charge takes place whereas if two different nature bodies are rubbed then there is transfer of charge & the one who is gaining electrons acquire negative charge & the other one who is losing electrons acquire positive charge. (a) (b)

7. 2) By Conduction :- It is a process of transfer of charge between two bodies when they are in actual contact with each other. The body having more number of electrons will transfer its charge to another body having less charge when they are brought in actual contact with each other i.e. charging by Touching . Thus in this way when among the two bodies when one body is earth then that sharing of charge with earth is called as Grounding.

8. 3) By Induction:- This phenomenon of charging an uncharged body by bringing a charged body near it without making any direct contact between two bodies.

9. Hence materials are classified into two categories Conductors :- Those material which can conduct charge from one end to other. Examples are metals, human bodies, earth etc. Insulators:- those material which can’t conduct electricity i.e. they are poor conductors of electricity. Examples are glass, rubber, wool, plastic etc. However, in case of insulator when an electric field is applied, induced charges appear on their surface for some time i.e. why they are also called as dielectrics . Hence dielectrics are defined as insulating materials which transmit electric effects without conducting. In case of conductor when some charge is transferred then it distributes readily over its entire surface whereas in case of insulator if some charge is put then it stays at the same place.

10. Coulomb’s Law

11. Vector form of Coulomb’s Law

12. Superposition principle It states that force on any charge due to a number of charges is vector sum of all forces on that charge due to other charges taken one at a time. The individual forces are unaffected due to presence of other charges.

13. It is a vector quantity whose direction is always from positive charge to negative charge. Its SI unit is N/C & cgs unit is Dyne/stat coulomb & its dimensional formula is [MLT -3 A -1 ]. A(+Q) B(+q) Hence using superposition principle we can also calculate the value of total electric field acting on a point when a number of charges are present.

14. Electric Field Lines It is an imaginary path either straight or curved of an electric field such that tangent to it will give the direction of electric field at that point. They are always continuous curves but they don’t form closed loops. In case of single positive charge they are always directed radially outwards whereas for single negative charge they are always directed radially inwards. Field lines of different charges are shown

15. Electric Dipole

16. Electric Flux It is a measure of electric field lines crossing this area. It is a scalar quantity . Its unit is Nm/C 2 or V-m & its dimensional formula is [ML 3 T -3 A -1 ]. There are two types of flux one is inward flux which is taken to be positive & other is inward flux which is taken to be negative.

17. Continuous charge distribution Charges on surface of system are located so close together that they are supposed to be continuous distribution of charges. In a system the continuous distribution is along Some line Over a surface Throughout a volume

18. Gauss’s Law It states that total electric flux over any closed surface enclosing a volume in vacuum is 1/ ε times the total charge enclosed by the surface.

19. Applications of Gauss’s Law Field due to an infinitely long straight uniformly charged wire :- Consider an infinite line which has linear charge density λ . Using Gauss’s law let us calculate the value of electric field over a distance ‘r’ from the line charge.

20. Electric field intensity due to a thin infinite sheet of charge

21. Electric field intensity due to a uniformly charged spherical shell:- Consider a thin spherical shell of radius R & centre O. Let a charge Q be distributed uniformly over the surface of shell. Now we have to calculate the value of field intensity at three different regions of the shell.

22. Numericals for practice Two bodies A & B carry charges -3µC & -0.44µC . How many electrons should be transferred from A to B so that they acquire equal charge? Force of attraction between two point charges placed at a distance ‘d’ is F. What distance apart should they be kept in same medium, so that force between them is 2F? Three point charges are placed at the following points on x-axis: 3µC at x=0, -4µC at x= 50cm & -5µC at x= 120cm. Calculate the force on -4µC charge. An infinite line charge produces a field of 19 x 10 -4 N/C at a distance of 5cm. Calculate linear charge density. A plastic rod of length 2.2m & radius 3.6mm carries a negative charge of 3.8 x 10 -7 C spread uniformly over its surface. What is the electric field near mid point of rod, at a point on its surface? A particle of mass 10 -4 Kg & charge 5µC thrown at a speed of 20m/s against a uniform electric field of strength 2 x 10 5 N/C. How much distance will it travel before coming to rest momentarily? Two charges +20µC & -20µC are held 1cm apart. Calculate electric field at a point on the equatorial line at a a distance of 50cm from the centre of dipole. A dipole consisting of an electron & proton separated by a distance of 4 x 10 -10 m is situated in an electric field of intensity 3 x 10 5 N/C at an angle of 30° with the field. Calculate the dipole moment & the torque acting on it.