SUMMARY
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 (i.e., from one terminal of the source to the other) is called the electromotive force, or emf, of the source. Note that the emf is not a force; it is the voltage difference between the two terminals of a source in open circuit.
Current is the rate of flow of electrons in a conductor. Current or Intensity of current is the quantity of charge that flows through a conductor in a circuit in one second.
The SI unit of electric current is Ampere (A).
Ammeter is the device to measure current.
Potential difference is the difference of potential between the two ends of a conductor. Current only flows, if there is potential difference between the ends of the conductor. Potential difference is the work done in moving a charge between the ends of a conductor.
The SI unit of potential difference is Volt (V).
Voltmeter is the device to measure voltage.
Voltmeter is connected in parallel, across the two points between which the voltage is to be measured. An ammeter is connected series in a circuit.
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: 1Ω = 1 V A –1 .
V ∝ I or V = R I