__CHAPTER 3 CURRENT ELECTRICITY__

- Current through a given area of a conductor is the net charge passing per unit time through the area.

- To maintain a steady current, we must have a closed circuit in which an external agency moves electric charge from lower to higher potential energy. The work done per unit charge by the source in taking the charge from lower to higher potential energy is called the electromotive force of the source.

- Ohm’s Law: The electric current I flowing through a substance is proportional to the voltage V across its ends i.e. V ∝ I or V= RI where R is called the resistance of the substance. The unit of resistance is ohm. 1 Ω = 1 V A
^{-1}.

- The resistance R of a conductor depends on its length
*l*and constant cross-sectional area A through the relation.

R =(ρ l)/A

where ρ is called the resistivity of a material and depends on the temperature and pressure.

- Current density (j) is the amount of charge flowing per second per unit area normal o he flow.

- Ohm’s law is not a fundamental law of nature. It fails if:
- V depends on I non-linearly.
- The relation between V and I depends on the sign of V for the same absolute value of V.
- The relation between V and I is non-unique.

- Kirchhoff’s Rules:
- Junction rule: At any junction of circuit elements, the sum of currents entering the junction must be equal to the sum of currents leaving it.
- Loop rule: The algebraic sum of changes in potential around any closed loop must be zero.

- The potentiometer is the device used to compare potential differences.