Electric Current

Just as a flow of water is produced when water particles move continuously in a particular direction, electric current is produced when charged particles, such as electrons or ions, begin to move in an ordered manner in a definite direction.

A metallic conductor contains many free electrons. These electrons are always moving randomly, but this random motion does not produce a net electric current in any particular direction. When a potential difference is created between the two ends of the conductor, the free electrons acquire a directed drift. As a result, electric current flows through the conductor.

👉 Definition of Electric Current

The amount of charge flowing through a given cross-section of a conductor per unit time is called electric current.

If an amount of charge Q flows in time t, then—

where,

• I = electric current
• Q = charge flowing through the conductor
• t = time

Therefore,

Electric current is a scalar quantity. A direction of flow is associated with current, but currents are not added according to the rules of vector addition.

👉 Direction of Electric Current

The direction in which positive charge would move is taken as the direction of conventional current.

In metallic conductors, the main mobile charge carriers are electrons. Since electrons are negatively charged, their direction of motion is opposite to the direction of conventional current.

Remember:

• Conventional current: higher potential → lower potential
• Electron flow: lower potential → higher potential

👉 SI Unit of Electric Current

The SI unit of electric current is ampere (A).

If 1 coulomb of charge flows through a cross-section of a conductor in 1 second, the current is 1 ampere.

Therefore,

✏️ Example 1

If 20 coulombs of charge flow in 10 seconds, find the current.

Answer: 2 A

✏️ Example 2

A current of 5 A flows for 5 minutes. How much charge passes through the conductor?

Answer: 1500 C

👉 Direct Current and Alternating Current

📚 Direct Current (DC)

A current that flows continuously in the same direction is called direct current.

📚 Alternating Current (AC)

A current whose direction reverses periodically at regular intervals is called alternating current.

In the following parts of this chapter, simple DC circuits are mainly used to explain electric current, resistance, and Ohm's law.

👉 Ohm's Law

When the temperature, material, and other physical conditions of a conductor remain constant, the potential difference across its ends is directly proportional to the current flowing through it.

or,

If the constant of proportionality is represented by R, then—

This relation is known as Ohm's law. Here, R is the electrical resistance of the conductor.

From Ohm's law—

and,

📚 What Does It Mean?

For the same potential difference, a larger resistance allows less current to flow, while a smaller resistance allows more current to flow.

👉 Unit of Resistance

The SI unit of resistance is ohm (Ω).

If a potential difference of 1 volt across a conductor produces a current of 1 ampere, the resistance of that conductor is 1 ohm.

Larger and smaller units include:

The nature of resistance and the factors on which it depends are discussed in detail on the Electrical Resistance and Resistivity page.

👉 Ohmic and Non-ohmic Conductors

📚 Ohmic Conductor

A conductor that obeys Ohm's law at a fixed temperature and under fixed physical conditions is called an ohmic conductor. Many metallic conductors show this behaviour over a suitable range.

📚 Non-ohmic Conductor

A material for which V and I are not directly proportional is called non-ohmic. Semiconductor devices are common examples.

👉 V-I Graph

For an ohmic conductor, when temperature and other physical conditions remain unchanged, . Therefore, the graph of potential difference against current is a straight line passing through the origin.

For a non-ohmic material, the graph is not a straight line through the origin.

Important for exams: When you are given a V-I graph, first check whether it is a straight line passing through the origin.

Solved Examples

✏️ Example 3

The potential difference across a conductor is 50 V and its resistance is 10 Ω. Find the current.

Answer: 5 A

✏️ Example 4

A conductor has a resistance of 10 Ω and carries a current of 5 A. Find the potential difference.

Answer: 50 V

✏️ Example 5

A current of 2 A flows through a conductor when a potential difference of 10 V is applied across it. Find its resistance.

Answer: 5 Ω

✏️ Example 6

An electric bulb of resistance 125 Ω is connected to a 250 V supply. Find the current.

Answer: 2 A

🧠 Quick Revision

⭐ Charge is represented by Q, electric current by I, time by t, potential difference by V, and resistance by R.

⭐ SI unit of charge: coulomb (C); SI unit of current: ampere (A); SI unit of potential difference: volt (V); SI unit of resistance: ohm (Ω).

⭐ Electric current:

⭐ Charge:

⭐ Ohm's law:

⭐ Resistance:

⭐ Current:

⭐ 1 A = 1 C/s

⭐ 1 Ω = 1 V/A

⭐ In a metallic conductor, electron flow and conventional current are in opposite directions.

✍ Try It Yourself

🔹 If 12 C of charge flows in 4 seconds, find the current.

🔹 A current of 3 A flows for 2 minutes. Find the total charge transferred.

🔹 A current of 6 A flows under a potential difference of 24 V. Find the resistance.

🔹 A current of 2.5 A flows through a resistance of 8 Ω. Find the potential difference.

🔹 Two resistors have resistances in the ratio 2:3 and are connected separately to the same potential difference. What is the ratio of the currents through them?