Electric Current and Electric Potential

ELECTRIC CURRENT

As flowing water is called water current, flowing air is called air current similarly flow of charges is termed as electric current.

Electric current is expressed by the amount of charge flowing through a particular area in unit time. 

Thus, if a net charge Q, flows across any cross-section of a conductor in time t, then the current I, through the cross-section is

Since the electric current is defined by the scientist Andre Marie Ampere, hence the SI unit of electric current is named after him and the SI unit is Ampere ‘A’. Also, the SI unit of charge is Coulomb ‘C’ and that of time is seconds ‘s’.

• one ampere of electric current is current when 1 C of charge passes through a cross sectional area in 1 second.

• However, electrons were not known at the time when the phenomenon of electricity was first observed. So, electric current was considered to be the flow of positive charges and the direction of flow of positive charges was taken to be the direction of electric current.
• Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges.
• The electric current is measured by an instrument called Ammeter, which is always connected in series with the device for which the current is to be measured.

EXAMPLE: Find the electric current if 200 moles of electrons pass through a conductor for 5 minutes.
Solution: Given:

ELECTRIC POTENTIAL OF A POINT AND ELECTRIC POTENTIAL DIFFERENCE

The potential of a point is defined as the work done on bringing a unit positive charge from infinity to that point without accelerating.

• It is denoted by V. If A is any point in the space then its potential is written as V_A

We know that air flows between those points where there is a difference in air pressure. Similarly, a charge flows between two points when there is a difference in their potential. Hence, for flow of charges in a conducting metallic wire, the gravity, of course, has no role to play; the electrons move only if there is a difference of electric pressure – called the potential difference – along the conductor. This difference of potential may be produced by a battery, consisting of one or more electric cells. The chemical action within a cell generates the potential difference across the terminals of the cell, even when no current is drawn from it.

• When the cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor and produces an electric current. In order to maintain the current in a given electric circuit, the cell has to expend its chemical energy stored in it.

Thus, we define the electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to the other.

• The SI unit of electric potential difference is volt (V), named after Alessandro Volta (1745 –1827), an Italian physicist.
• One volt is the potential difference between two points in a current carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from one point to the other. Therefore,

• The potential difference is measured by means of an instrument called the voltmeter. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.