Quick Revision for Class X Physics SA1



  1. Positive and negative charges: The charge acquired by a glass rod when rubbed with silk is called positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.
  2. Coulomb: It is the S.I. unit of charge. One coulomb is defined as that amount of charge which repels an equal and similar charge with a force of 9 x 109 N when placed in vacuum at a distance of 1 meter from it. Charge on an electron = -1.6 x 10-19 coulomb.
  3. Static and current electricities: Static electricity deals with the electric charges at rest while the current electricity deals with the electric charges in motion.
  4. Conductor: A substance which allows passage of electric charges through it easily is called a ‘conductor’. A conductor offers very low resistance to the flow of current. For example copper, silver, aluminium etc.
  5. Insulator: A substance that has infinitely high resistance does not allow electric current to flow through it. It is called an ‘insulator’. For example rubber, glass, plastic, ebonite etc.
  6. Electric current: The flow of electric charges across a cross-section of a conductor constitutes an electric current. It is defined as the rate of flow of the electric charge through any section of a conductor.                                                                    

       Electric current = Charge/Time     or        I = Q/t

Electric current is a scalar quantity.

  1. Ampere: It is the S.I. unit of current. If one coulomb of charge flows through any section of a conductor in one second, then current through it is said to be one ampere.                                                                                                                         1 ampere = 1 coulomb/1 second    or      1 A = 1C/1s = 1Cs-1                                                                                                                                                                                                              1 milliampere =    1 mA = 10-3 A                                                                                                                                                                                 1 microampere = 1µA = 10-6 A
  2. Electric circuit: The closed path along which electric current flows is called an ‘electric circuit’.
  3. Conventional current: Conventionally, the direction of motion of positive charges is taken as the direction of current. The direction of conventional current is opposite to that of the negatively charged electrons.
  4. Electric field: It is the region around a charged body within which its influence can be experienced. Continue reading “Quick Revision for Class X Physics SA1”

Working of a capacitor

Capacitor and Battery Analogy

Capacitors are analogous to Batteries because they both are used to store and charge electrical energy, though they both work differently.

Batteries have two terminals one positive and one negative. And they involve chemical reactions to produce electrons at one terminal and absorb at the other. While capacitors are not able to produce charge or electrons, but only stores them. Capacitors are much simpler in comparison to batteries.

The terminals in the capacitors are connected to two metallic plates separated by an insulator (materials which do not conduct electricity) or dielectrics. A capacitor can be easily built at home using metallic foils and air or piece of paper as dielectric. The capacitor thus formed will have a very low tendency to store charge.

Also, the major difference between battery and capacitors can be that Capacitors can easily dump their charges in a fraction of seconds while batteries may take minutes to discharge.


The dielectrics used in the capacitors can be any insulator which polarizes on the application of electricity. However Mica, porcelain, Teflon, Cellulose, Ceramic, Glass and even Air are used in practical capacitors which are best suited for the functions of the capacitor.  The size and the type of dielectric used determine the application for which a capacitor may be used.

Capacitors which have air as a dielectric are used in tuning circuits in the radio and other devices, while capacitors with ceramic dielectrics are used for high frequency purposes such as in Antennas.  For high voltage capacitors glass dielectrics are used. Capacitors are manufactured to serve many purposes like a small plastic capacitor used in the Calculators to Ceramic Capacitors used in Supercomputers.

What happens on connecting a capacitor to a circuit? :

The figure shows the circuit symbol of a capacitor.

When we connect a capacitor to a battery, the plate connected to the positive terminal of a battery loses electrons to the battery and the plate attached to the negative terminal absorbs the electrons that the battery produces. Once the capacitor is charged fully the voltage across the capacitor becomes equal to that of the battery.  Small capacitors hold small amount of charge but large capacitors have the ability to store charge in large amounts.

We can also observe a working capacitor in nature, in the form of lightning, the cloud and the ground being the two plates of capacitor and lightning is the releasing of charge between the two plates. It is obvious that when the capacitor is that large, huge amount of charge can be stores in it.

Let’s now take up a capacitor connected in series with a bulb and a battery as shown in the figure.  If the capacitor used here can store large amount of charge or we can say if we are using large capacitor we can notice that

  • The bulb will light up as current flows through it to charge up the capacitor , But we can observe the bulb going dim as the capacitor gets charged to its capacity and when the capacity is reached the bulb goes off .
  • Now if the battery is replaced by a wire, the current starts flowing from its one plate to the other and the bulb will light up initially and then go dim as the capacitor starts discharging. The bulb goes off again when the capacitor is totally discharged.

The Capacity of a capacitor to store charge or capacitance of a capacitor is measured in terms of Farad. 1-Farad is the capacitance of a capacitor which can store 1 coulomb of charge at 1 volt.  A one farad capacitor can be very large it can be as big as a 1 liter soda bottle, this being the reason why capacitance is usually measured in microfarads and general usage of small capacitors.

Applications of Capacitors:

  • Capacitors are used for high frequency and speed applications such as in lasers to get highly bright and instantaneous flashes.
  • In tuning circuits along with inductors to match a frequency.
  • As noise filters and to block DC components of the current.
  • In flash lights of a camera.
  • For signal processing.

Safety and Hazards
Capacitors can retain charge for a longer period after being disconnected from a circuit, this charge can be fatal  and is very dangerous this is the reason why Televisions have a “Do Not Open” warning over them as they use large capacitors for tuning purposes and these capacitors store large amount of charge which can even kill .

Electric Charges and Field :Physics Revision Test for Class XII


electric chargesElectric Charges and Field Test Paper

Max. Marks: 30                 Time: 75 minutes

(Please take care to complete the test in the prescribed time limit)

One mark Questions

  1. Define electric flux.
  2. How is the force between two charges affected when dielectric constant of the medium in which they are placed increases?
  3. Name the physical quantity whose SI unit is NC-1
  4. Why two electric lines of force never intersect each other?
  5. Define dielectric constant of a medium.
  6. What is an ideal dipole?
  7. Write the SI unit of electric dipole moment.
  8. Calculate the number of electrons making a charge of -0.1 mC.

    2 Marks questions

  9. Vehicles carrying inflammable materials have metallic ropes (or chains)touching the ground while in motion. Explain why is it necessary?
  10. Derive an expression for the electric field intensity at a distance ‘r’ from a point charge ‘q’
  11. Two point charges 4 μC and 2 μC are separated by a distance 1 m in air. At what point on the line joining the two charges is the electric field intensity zero?
  12. Derive an expression for the work done in rotating a dipole through an angle θ in a uniform electric field.

    3 Marks Questions

  13. Show that, in a uniform electric field, a dipole experiences only a torque, but no net force. Derive an expression for the torque experienced by a dipole in a uniform electric field.
  14. Write three points of difference between mass and charge.
  15. An electric dipole of length 2 cm is placed with its axis making an angle 60 degrees with respect to a uniform electric field of 105 N/C. If it experiences a torque of 8 √3 Nm, calculate the
    (i) magnitude of charge on the dipole, and
    (ii) potential energy of the dipole.

    5 marks Questions

  16. Using Gauss’ theorem, derive an expression for the electric field intensity at a point due to a uniformly charged spherical conducting shell when the point is
    (a) outside the sphere
    (b) on the sphere
    (c) inside the sphere