Question 1: Describe the concept of steady current as a flow of positive, negative or both. Define the unit of current.

## Definition of current

The time rate of flow of charge Q across any cross-section of the path of flow is defined as current. It is denoted by I.

(Steady current means continuous flow of free charge).

#### Explanation

Current is the result of flow of charges. The moving charge may be positive (i-e, holes or positive ions), negative (i-e, electrons or negative ions) or both positive and negative ions. The effect of the current is same in all cases. Mathematically,

In case current is due to the flow of both positive and negative ions, then

So we can conclude that;

1. In metallic conductors, current is produced due to the flow of conduction (or free) electrons. Free electrons can be set in motion by applying voltage across the metallic conductor. The negatively charged electrons will start moving toward the positive terminal. Thus a flow of electrons is set up.
2. In electrolytes, the current produced is due to the flow of positive as well as negative ions.
3. In semiconductors, the current is due to the flow of positive holes and electrons in opposite directions. Holes are the sites (places) of missing electrons and they act like positive charge.
4. In fluorescent discharge tube, current is due to the flow of positive and negative ions in opposite directions.

It is important to say that

1. Since electrons are constituents of matter, therefore, electric current is considered as matter in motion.
2. Actual direction of current is from negative terminal to positive terminal. However, the old convention that current flows from positive to negative terminal of the cell through the circuit is still accepted.
3. Substances like Copper and Silver etc have more free electrons and allow current flow easily. They are called conductors.

## Unit of Current

The SI unit of current is Ampere denoted by A (or Amp). It is defined as “One amp is the current when one Coulomb of charge flows across any cross-section of the conductor in one second.

The generally used sub-multiples of ampere are

1 milli ampere = 1 m A = 10-3 A

1 micro ampere = 1 μ A = 10-6 A