Electrostatics is the first chapter of Class XII Physics and carries around 7 marks in CBSE exams. In fact without complete knowledge of electrostatics total electrodynamics will be very difficult.
Let us understand the important topics in this chapter:
This chapter starts with the concept of charge and to be very specific stationary charge.
We have three basic properties of charges
- Qunatisation : Charge on any body exists as integral multiple of smallest charge, charge of electron or mathematically Q = ne
- Conservation : In an isolated system charge remains conserved. Mathematically it is stated as ΔQ = 0
Coulomb’s law states about the force between the two charges. The effect of medium on electrostatic force.
Maximum force is between charges when they are kept in vacuum. In any medium the force decreases by the factor of dielectric constant.
Definition of 1 Coulomb: It is the amount of charge, which when kept at a distance of one meter from another identical charge exert a force of 9 × 109 N.
Electrical field intensity at a point is defined as the force experienced by a unit positive charge, placed at that point.
Dipole is defined as two equal and opposite charges kept at finite distance.
Dipole moment is defined as the product of either charge and the distance between them. It has the unit of Coulomb Meter.
Derivation of electric field due to dipole on axial line and equatorial line is again very important.
When dipole is placed in uniform electric field, it will not experience any force. It will experience only torque unless the angle is either zero or 180°.
Derivation of torque and potential energy is also important. Remember that when angle is zero, then the configuration is having stable equilibrium. But when angle is 180° it is having unstable equilibrium.
Electric flux is defined as the dot product of electric field intensity and area vector. It has unit of Nm2/C
But Gauss law is applicable only for closed surface. It states that flux enclosed in a closed surface is equal to charge divided by permittivity.
It helps in calculating the electric field and flux for symmetrical surfaces. In this context the derivation of electric field due to linear charge configuration and uniformly charged sheet is also important.
Electrostatic potential of a point is defined as the amount of work done in bringing a unit positive charge from infinity to that point against electrostatic force without acceleration.
Its unit is volt.
Potential energy of two charge is defined as the amount of work done in bringing charges from infinite distance to given distance.
Electric force, Electric field intensity, Electrostatic potential and Potential Energy, all of these four basic quantities follow the principle of superposition. Force and electric filed are vectors and Potential and potential energies are scalar.
Here we should remember that one more unit of electric field is there based on potential, because electric field is also equal to negative of potential gradient. This unit is termed as Volt/meter.
Now we start from the concept of capacitor. Capacitor is a device to store energy in the form of electric field. Though the net charge of capacitor is zero, but both terminals of capacitor store equal and opposite amount of charges.
The capacitance of a capacitor is defined as the ratio of charge to potential. So capacitance can have the unit of Coulomb per Volt. SI unit of capacitance is Farad.
Capacitance of capacitor is numerically equal to charge required to raise the potential by 1 Volt. And the definition of 1 Farad is defined here now. If 1 Coulomb raises the potential of capacitor by 1 Volt then the capacitance is said to be on 1 Farad.
Capacitor can be classified on the basis of shape of plates of capacitor i.e. parallel plate capacitor, spherical capacitor, cylindrical capacitor etc.
Capacitor can also be classified based on material filled in, called dielectric. Air capacitor, mica capacitor etc.
Most common is parallel plate capacitor. The value of capacitance of parallel plate capacitor depends on area of plates ‘A’ and distance between the plates‘d’.
To increase the capacitance, we use more than one capacitor and make combinations. In parallel combinations net capacitance gets added and in series it is added in inverse.
For parallel connections we must see that the both terminals of capacitor are joined, and for series connections ONLY one end is connected.
The energy of capacitor is given as ½ CV2 = ½ Q2/C = ½ QV
One should not get confused by statement, that first formula states energy increases as C increases but second formula states energy decreases as C is increased and third formula states that energy is independent of C. Let us recollect that first formula states energy increases as C is increased BUT only when Voltage V is kept constant. Similarly Second formula states that energy is inversely proportional to capacitance C PROVIDED charge Q is constant.
To increase the capacitance, dielectric material is inserted between the plates. So in this case the capacitance gets increased by the dielectric constant ‘k’
Actually in this case the electric field inside the dielectric slab becomes E/k while E is the electric field outside the dielectric.
Now one more type of questions are asked in which the capacitors are added, either with battery connected or removed. If the battery is connected then the net potential remains the same and charge might vary. But if the battery is disconnected, then the charge remains constant.