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CBSE Class 12 Physics Current Electricity Formula

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Last updated date: 22nd Mar 2024
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CBSE Class 12 Physics Chapter 3 - Current Electricity Formula - Free PDF

Free PDF download of Physics Class 12 Chapter 3 - Current Electricity Formulas Prepared by Expert Teachers at Vedantu.com. To Register Online Physics Tuitions on Vedantu.com to clear your doubts from our expert teachers and solve the problems easily to score more marks in your CBSE Board exams.

Competitive Exams after 12th Science

Current Electricity - What is Electricity?

Current electricity or electricity is the branch of Physics that deals with the dynamic/moving state of charges. Some daily life examples of current electricity are as follows:

  • Charge flows through the filament of a bulb.

  • Charge flows through the charging cable to charge the laptop. 

  • Lightening is also an example of current electricity as the charge from the cloud falls on earth. It is an example of atmospheric electricity.


Important Formulas of Current Electricity - Chapter 3 Class 12

Current electricity formulas list is given below for quick revision.

Specification

Formula

Electric current

I = Q/t (in ampere)

where I = electric current, Q = Charge flow, t = time taken to flow charge

Current Density

j = I/A =nevd (in ampere/metre2)

where  j = current density, I = current, A = cross-section area of the conductor

Drift velocity of free electrons

vd  = aτ = $\frac{eE}{m}τ$ (m/s)

where a = acceleration, τ = relaxation time, e = charge of electron, E = electric field, m = mass of electron

Relaxation time

τ = $\frac{𝜆}{v_{rms}}$

Where 𝜆 = mean free path of an electron

Mobility of an electron

μe = $\frac{j}{neE}$ (in m2V-1s-1)

Electric resistance

R = tan ፀ = V/I (in ohm or volt/ampere)

Resistance of a wire of length l and cross-section A

R = $\frac{m}{ne^{2}τ}$ . $\frac{l}{A}$


Electrical resistivity or Specific resistance

ρ = R(A/l) = $\frac{m}{ne^{2}τ}$ (in ohm-metre)

Equivalent resistance

1. Resistance in series 

R = R1+R2 +R3 +R4 +..........R


2. Resistance in parallel

$\frac{1}{R}$ = $\frac{1}{R_{1}}$ + $\frac{1}{R_{2}}$ + $\frac{1}{R_{3}}$.....$\frac{1}{R_{N}}$

Electric power

P = VI = I2R = $\frac{V^{2}}{R}$ (in watt)

EMF (Electromotive force)

E=dW/dQ (in volt or joule/coulomb)

Relation between EMF, Voltage, and Resistance of a circuit

E = I (R+r)

where R = external resistance and r = internal resistance

Electric cell

E = $\frac{W}{Q}$ . V = $\frac{W_{ext}}{Q}$ = IR

Combination of cells

In series  I = $\frac{nE}{nr + R}$ 

In parallel I = $\frac{nE}{r + nR}$ 

In mixed grouping I = $\frac{mnE}{nr + mR}$ 

Combination of cells

For maximum current

R = $\frac{nr}{m}$   , Imax = $\frac{1}{2}$ . $\frac{mE}{r}$

Krirchhiff’s laws

ΣI = 0, ΣIR = ΣE

Wheatstone bridge

$\frac{P}{Q}$ = $\frac{R}{S}$

Metre bridge

$\frac{P}{Q}$ = $\frac{l}{100 - l}$

Potentiometer

E = Kl, $\frac{E_{1}}{E_{2}}$ = $\frac{l_{1}}{l_{2}}$, r= R($\frac{l_{1}}{l_{2}}$ - 1)


For bridges and electric cells, symbols have their usual meaning. 

These formulas are given to the students looking for last-minute revision. These formulas are also very helpful for the students who are preparing for competitive entrance exams like JEE or NEET. Students can also revise the complete syllabus of Physics Class 12 following the revision notes of Class 12 Physics.