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Cheat sheet · JEE Preparation Physics

Current Electricity cheat sheet

The whole chapter on one page: key formulas, facts and the points to remember, for quick revision before a test.

JEE · Physics

Cheat sheet

Class 12 · Chapter 3Current Electricity

Kirchhoff: junction rule (charge), loop rule (energy).

1. Basics

I=neAvdI = neAv_dV=IRV = IR
R=ρlAR = \rho\dfrac{l}{A}ρ=mne2τ\rho = \dfrac{m}{ne^2\tau}

Temperature: ρT=ρ0[1+α(T−T0)]\rho_T = \rho_0[1 + \alpha(T - T_0)].

2. Cells

V=ε−IrV = \varepsilon - Ir
I=εR+rI = \dfrac{\varepsilon}{R + r}
  • Series: ε=ε1+ε2\varepsilon = \varepsilon_1 + \varepsilon_2, r=r1+r2r = r_1 + r_2.
  • Parallel cells: equivalent emf is a weighted mean.

3. Networks

  • Series: R = R₁ + R₂; parallel: 1/R = 1/R₁ + 1/R₂.
  • Kirchhoff's junction rule: ΣI = 0; loop rule: ΣΔV = 0.
  • Wheatstone bridge balanced: P/Q = R/S.
P=VI=I2R=V2RP = VI = I^2R = \dfrac{V^2}{R}

4. Meter Bridge

RS=l100−l\dfrac{R}{S} = \dfrac{l}{100 - l}

5. Key Points

  • Drift velocity ~ 10⁻⁴ m/s.
  • Ohm's law fails for diodes.
  • Mobility μ = v_d / E.
  • Resistivity of metals rises with temperature.

Remember: I = neAv_d | V = ε − Ir | Kirchhoff | Wheatstone

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