NCERT Class 12 Physics Physics Part-I: Chapter 3 — 1 INTRODUCTION
This chapter introduces the concept of electric current as the flow of charges, contrasting natural phenomena like lightning with steady currents in devices like torches. It defines electric current mathematically, explaining both steady and instantaneous currents using the formula I = dq/dt. The SI unit of current, the ampere, is introduced, with examples of current magnitudes in lightning and nerves. The chapter then discusses electric currents in conductors, explaining how free electrons in metals carry current when an electric field is applied. It differentiates between solid conductors, where electrons are the charge carriers, and electrolytic solutions, where both positive and negative charges move. The behavior of electrons in conductors without an electric field, involving thermal motion and collisions with ions, is also described. This foundational understanding is crucial for comprehending electrical circuits and devices in physics.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 12 |
| Subject | Physics |
| Book | Physics Part-I |
| Chapter | Chapter 3 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 726 |
Learning outcomes
- Define electric current and its SI unit.
- Differentiate between steady and instantaneous currents.
- Understand the concept of charge carriers in conductors.
- Explain the role of electric fields in creating current.
- Describe the motion of electrons in conductors.
Vocabulary
| Word | Meaning |
|---|---|
| Electric current | The flow of electric charge. |
| Steady current | A current that remains constant over time. |
| Instantaneous current | The current at a specific moment in time. |
| Conductor | A material that allows electric charge to flow through it. |
| Ampere | The SI unit of electric current. |
| Charge carriers | Particles that carry electric charge, such as electrons or ions. |
| Thermal motion | Random movement of particles due to heat. |
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Practice questions
- What is the SI unit of electric current? Answer: The SI unit of electric current is the Ampere (A).
- Define instantaneous current. Answer: Instantaneous current is defined as the limit of the ratio of charge to time interval as the time interval approaches zero (I = lim(Δt→0) ΔQ/Δt).
- What are the charge carriers in solid conductors? Answer: The charge carriers in solid conductors are typically negatively charged electrons.
- Give an example of a natural phenomenon involving electric current. Answer: Lightning is an example of a natural phenomenon involving electric current.
Practice MCQs
Q1. What is the definition of steady current?
Explanation: Steady current refers to a current that does not change its value over time.
Q2. The SI unit of electric current is:
Explanation: The Ampere (A) is the standard international unit for measuring electric current.
Q3. In metals, electric current is primarily carried by:
Explanation: In solid conductors like metals, the free electrons are responsible for carrying the electric current.
Q4. Which of the following is an example of a device with a steady electric current?
Explanation: A torch uses a battery to provide a steady and continuous flow of electric current.
Q5. The formula for instantaneous current is:
Explanation: Instantaneous current is defined as the rate of change of charge with respect to time, represented by the derivative dQ/dt.
Frequently asked questions
What is electric current?
Electric current is the flow of electric charges. It is defined as the rate at which electric charge flows through a particular area.
What is the difference between steady and instantaneous current?
Steady current is constant over time, while instantaneous current is the current at a specific moment in time, calculated as the limit of charge flow over an infinitesimally small time interval.
What is the SI unit of electric current?
The SI unit of electric current is the Ampere (A).
What causes electric current in conductors like metals?
In metals, electric current is caused by the movement of free electrons when an electric field is applied.
Are there other types of conductors besides metals?
Yes, electrolytic solutions are another type of conductor where both positive and negative charges can move.
What happens to electrons in a conductor when no electric field is present?
When no electric field is present, electrons move randomly due to thermal motion and collide with fixed ions, but there is no net flow in any particular direction.
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NCERT Class 12 Physics — Physics Part-I — Chapter 3 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.