# Current Electricity | Class 9 Physics Formula

Coulomb’s law :

If two point charges q1 and q2 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, ε0 = 8.854 x 10-12C2N-1m-2
• εr = 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,

VA- VB = 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