**Coulomb’s law :**

If two point charges q_{1 }and q_{2} are separated by a distance r in a medium’ then the force of attraction or repulsion F between them is given by,

F = {1\over 4 \pi \epsilon _o \epsilon _r} \times {q_1q_2\over r^2}[\katex]

- Here, ε
_{0}= 8.854 x 10^{-12}C^{2}N^{-1}m^{-2} - ε
_{r}= 1 (for air or vacuum)

**Electric potential difference :**

(a) If W work is done to bring Q coulomb of positive charge from infinity to a particular point in an electric field, then the potential at that point,

V = W/Q or, W = V.Q

(b) If W joule of work is done to move a coulomb of charge from a point A to another point B, then the potential difference between A and B,

V_{A}- V_{B} = W/Q

**Electro-Motive Force (EMF) :**

If W work is done to move Q charge around a complete circuit, then EMF of the cell,

EMF = W/Q

**Electric current :**

If a charge 'Q' flows through a cross-section of a conductor in 't' seconds, then the magnitude of current,

Current (I) = Charge (Q) / time (t)

**Ohm's Law:**

Potential Difference (PD) = Current (I) × Resistance (R)

**EMF and internal resistance of a cell:**

V = E – I.r

**Notation Meaning**

- E = EMF
- I = Current
- r = Internal resistance
- V = Terminal voltage

**Resistance of a conductor :**

Resistance (R) = ρ {l\over A}[\katex]

**Notation Meaning**

- ρ = resistivity or specific resistance
- l = length of conductor
- A = area of cross-section