Electric Current: Definition, Formula, Unit and Examples
What Is Electric Current?
Electric current is the rate at which electric charge passes through a cross-section of a conductor. In simple words, it tells us how much electric charge passes a particular point in a given time.
Imagine water flowing through a pipe. The amount of water passing a point every second tells us the rate of water flow. Electric current is a similar idea, but it describes the flow of electric charge instead of water.
In a metal wire, electrons can move through the material. When a potential difference is applied across the wire, the electrons acquire a net drift. This directed movement of charge produces electric current.
Interactive Electric Current Diagram
Change the battery voltage and resistance. Observe how current and bulb brightness change. Red arrows show conventional current; blue arrows show electron drift in the opposite direction.
Interactive Electric Current Circuit
Adjust the battery voltage and resistance. Watch the circuit and learn how current, electron flow and brightness are related.
Explore the circuit
What is electric current?
Electric current is the rate at which electric charge passes through a cross-section of a conductor.
I = current in amperes (A), Q = charge in coulombs (C), and t = time in seconds (s).
For a changing current, average current is ΔQ/Δt; instantaneous current is dQ/dt.
Remember the direction
Conventional current: defined as the direction positive charges would move.
Electron drift in a metal: opposite to conventional current because electrons have negative charge.
SI unit: ampere (A). One ampere equals one coulomb per second: 1 A = 1 C/s.
Electric Current Formula
This formula is used when the current is constant over the time interval.
- I = electric current, measured in amperes (A)
- Q = electric charge transferred, measured in coulombs (C)
- t = time taken, measured in seconds (s)
If current changes with time, the average current over an interval is Iavg = ΔQ/Δt. The instantaneous current is I = dQ/dt, with the sign determined by the chosen current direction.
Other useful forms of the formula
Use Q = It to calculate charge when current and time are known. Use t = Q/I to calculate the time required when charge and constant current are known.
SI Unit of Electric Current
The SI unit of electric current is the ampere (A), named after physicist André-Marie Ampère.
| Physical quantity | Symbol | SI unit |
|---|---|---|
| Electric current | I | Ampere (A) |
| Electric charge | Q | Coulomb (C) |
| Time | t | Second (s) |
| Current density | J | Ampere per square metre (A/m²) |
Dimensional formula of electric current
Electric current is a fundamental SI quantity. Its dimensional formula is [I]. It cannot be expressed using only mass, length and time dimensions.
Note: The dimensional formula of current density is [I L⁻²], because current density is current divided by area.
Conventional Current and Electron Flow
Two directions are used when describing current in a metal wire. It is important not to confuse them.
| Conventional current | Electron flow in a metal |
|---|---|
| Defined as the direction positive charges would move. | Electrons carry negative charge and drift in the opposite direction. |
| In the external circuit, it is conventionally shown from the positive terminal towards the negative terminal. | Electron drift in the external metal wire is from the negative terminal towards the positive terminal. |
| Used in standard circuit diagrams and calculations. | Helps explain charge movement in metallic conductors. |
Conventional current is a defined direction; it does not mean electrons move in that same direction. In other materials, positive and negative charge carriers may both contribute to current.
Types of Electric Current
1. Direct Current (DC)
Direct current flows in one direction in a circuit. Batteries and cells provide DC. The current magnitude may vary in some DC circuits, but its direction does not reverse.
2. Alternating Current (AC)
Alternating current reverses direction periodically. Ordinary household mains electricity is AC. In many AC supplies, the current and voltage also vary in magnitude with time.
| Feature | DC | AC |
|---|---|---|
| Direction | Does not reverse | Reverses periodically |
| Example | Battery-powered circuit | Household mains supply |
| Common use | Portable electronics and battery circuits | Electricity distribution and household appliances |
Solved Numerical Problems on Electric Current
Example 1: Calculate current
Question: A charge of 20 C passes through a wire in 5 seconds. Find the current.
Given: Q = 20 C, t = 5 s
Formula: I = Q/t
Substitution: I = 20/5 = 4 A
Example 2: Calculate charge
Question: A current of 3 A flows for 8 seconds. Find the charge transferred.
Given: I = 3 A, t = 8 s
Formula: Q = It
Substitution: Q = 3 × 8 = 24 C
Example 3: Calculate time
Question: How long will it take for 60 C of charge to pass through a wire carrying a constant current of 5 A?
Given: Q = 60 C, I = 5 A
Formula: t = Q/I
Substitution: t = 60/5 = 12 s
Example 4: Current from the number of electrons
Question: Approximately 6.25 × 1018 electrons pass through a wire in one second. Find the magnitude of the current.
The magnitude of the charge on one electron is approximately 1.602 × 10−19 C.
Q = Ne = (6.25 × 1018) × (1.602 × 10−19) C ≈ 1.001 C.
I = Q/t ≈ 1.001/1 A.
Current Density
Current density describes how much electric current passes through a unit cross-sectional area. It is useful for understanding how concentrated the current is within a conductor.
- J = magnitude of current density
- I = current through the cross-section
- A = cross-sectional area in square metres (m²)
The SI unit of current density is A/m². Current density is a vector quantity; its direction is defined using conventional current. The formula J = I/A gives the magnitude when current density is uniform and perpendicular to the cross-section.
More generally, the current through a surface is calculated by integrating the current density over that surface.
Numerical example on current density
Question: A current of 2 A flows uniformly through a wire with cross-sectional area 4 × 10−6 m². Find the current density.
J = I/A = 2/(4 × 10−6)
What happens when a wire becomes thinner?
For the same steady current, current density increases when the cross-sectional area decreases, because J = I/A for uniform current density. In a metal with the same mobile charge-carrier density, the average electron drift speed also increases as the wire becomes thinner.
In a continuous wire carrying steady current, the current is the same through each cross-section when no charge is accumulating between them.
Common Mistakes Students Should Avoid
- Do not confuse electric charge (coulomb) with electric current (ampere).
- Use seconds with charge in coulombs and current in amperes.
- Remember that electron drift in a metal is opposite to conventional current.
- Current and current density are different quantities and have different units.
- For a changing current, distinguish average current from instantaneous current.
- A thinner wire does not automatically mean less current; current also depends on the circuit and its conditions.
Quick Practice Quiz
Choose an answer for each question, then select Check answers.
Frequently Asked Questions About Electric Current
What is electric current in simple words?
Electric current is the rate at which electric charge passes through a cross-section of a conductor.
What is the formula for electric current?
For constant current, I = Q/t, where Q is the charge transferred in coulombs and t is the time in seconds.
What is the SI unit of electric current?
The SI unit is the ampere (A). One ampere equals one coulomb per second.
Why is conventional current opposite to electron flow?
Conventional current is defined in the direction positive charges would move. Since electrons are negatively charged, their drift in a metal is opposite to conventional current.
What is the difference between AC and DC?
DC does not reverse its direction, while AC reverses direction periodically.
What is the SI unit of current density?
The SI unit of current density is ampere per square metre (A/m²).
Quick Revision
- Electric current: Rate of flow of electric charge.
- Formula for constant current: I = Q/t.
- SI unit: Ampere (A).
- Unit relation: 1 A = 1 C/s.
- DC: Current direction does not reverse.
- AC: Current direction reverses periodically.
- Current density: J = I/A for uniform perpendicular current density.
- Electron drift: Opposite to conventional current in a metal.
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