Current electricity (revision sheet)

1. Ohm’s Law : V=IR

2. Resistivity : ρ=(A/l)R

3. Temperature coefficient of resistance : α=( ρ- ρ0)/t = (R-R0)/t

4. Resistance in series : R=R1+R2+R3+…….

5. Resistance in parallel : R = 1/R1+1/R2+……

6. Terminal potential difference V=IR=E-Ir

7. Cells in series E=E1+E2+…..

8. Electrical energy W=EIt

9. Heat produced in a resistor H=I2Rt = VIt

10. Power dissipated in resistor P=VI + I2R

11. Maximum current in n rows of m cells each : Imax=mE/2R = nE/2r

12. Wheatstone bridge when balanced : P/Q = R/S

13. Kirchoff’s Law : (1) For junction I1+I2+I3+……=0 (2) For a loop I1R1+I2R2+I3R3+……= sum of potential drops

14. Shunt resistance required to convert galvanometer into ammeter to read upto I amperes: S=(IgG)/(I-Ig) where G is the galvanometer resistance and Ig is the current through galvanometer to produce full scale deflection

15. Current when n cells are connected in series I=nE/(R+nr)

16. Current when n cells are connected in parallel I=nE/(nR+r)

17. For balanced potentiometer E1/E2=l1/l2