Formula of Current: I = Q/t, SI Unit Ampere, Examples

The formula of current is $I = \frac{Q}{t}$ : the net charge Q that crosses any section of a conductor, divided by the time t it takes. One ampere is one coulomb flowing past every second.

Students meet this definition in the Class 10 electricity chapter and again in Class 12 and AP Physics. It is short, but most marks are lost on units: charge must be in coulombs, time in seconds, and milliamperes need converting before you do anything else. Below you will find the other two current formulas you will need, $I = \frac{V}{R}$ and $I = nAev_d$ , with examples.

Quick facts

Formula
$I = \frac{Q}{t}$
SI unit
ampere (A), a base unit
Dimensional formula
[M⁰ L⁰ T⁰ I¹]
Type
Scalar: it has a direction along the wire but does not add like a vector
From Ohm's law
$I = \frac{V}{R}$
Microscopic form
$I = nAev_d$
Measured with
An ammeter connected in series

Each symbol in the formula of current

Meaning of I = Q / t
Symbol	Quantity	SI unit
$I$	Current	ampere, A
$Q$	Net charge that passes	coulomb, C
$t$	Time for that charge to pass	second, s

Charge comes in packets of $e = 1.602\times10^{-19}$ C, so for n electrons $Q = ne$ . That gives the counting form $I = \frac{ne}{t}$ . Small currents are written in mA (10⁻³ A) or μA (10⁻⁶ A).

By convention, current points the way positive charge would move: from the positive terminal round the outside circuit to the negative. Electrons in a metal drift the other way.

Steps for current problems

Write what is given and convert: minutes to seconds, mC or μC to C, mA to A.
Choose the form: charge and time point to $Q/t$ , voltage and resistance point to $V/R$ .
Substitute with units and simplify.
If the question asks for the number of electrons, find Q first, then divide by e.

Use the calculator above for quick checks once you have converted everything to SI units.

Worked examples

Example 1: charge over a few minutes

A charge of 180 C passes through a heater element in 1.5 minutes. Find the current.

t = 1.5 × 60 = 90 s.

$I = \frac{180}{90} = 2.0\ \text{A}$

Example 2: counting electrons

A current of 0.50 A flows for 10 s. How many electrons pass a point?

Q = It = 0.50 × 10 = 5.0 C.

$n = \frac{Q}{e} = \frac{5.0}{1.602\times10^{-19}} \approx 3.1\times10^{19}$

Example 3: using Ohm's law

A 9.0 V battery is connected across a 450 Ω resistor. Find the current.

$I = \frac{V}{R} = \frac{9.0}{450} = 0.020\ \text{A} = 20\ \text{mA}$

Other current formulas you will need

Current in different situations
Situation	Formula
Definition	$I = \frac{Q}{t}$
Ohm's law	$I = \frac{V}{R}$
From power	$I = \frac{P}{V}$
In terms of electrons	$I = nAev_d$
Cell with internal resistance r	$I = \frac{\varepsilon}{R + r}$

The microscopic form is explained on the drift velocity page, and the battery form on the emf formula page. Ohm's law itself has its own Ohm's law page.

Units and dimension of current

The ampere belongs to the SI base set, alongside the metre, the kilogram and the second, so current is itself a base dimension written I. Its bracket form is simply [M⁰ L⁰ T⁰ I¹], explained in detail on the dimensional formula of current page. The coulomb is a derived unit: 1 C = 1 A s, which is why $Q = It$ gives coulombs directly.

Current is a scalar even though we draw arrows on circuit diagrams. At a junction you add currents as plain numbers, which is Kirchhoff's first rule.

Mistakes students make with current

Leaving time in minutes or hours. A phone battery rated in mA h must be converted to coulombs: 1 mA h = 3.6 C.
Thinking current is used up in a bulb. The same current flows in and out; energy is what is transferred.
Mixing the direction of electron flow with conventional current.
Connecting the ammeter in parallel in practical questions.

The class 10 electricity formula hub shows how current fits with voltage, resistance and power.

Frequently asked questions

What is the formula of current in physics?

I = Q / t, charge divided by time. With voltage and resistance known, you can also use I = V / R.

Which SI unit measures current?

The ampere (A). One ampere is a flow of one coulomb of charge per second.

Is electric current a vector quantity?

No. It has a direction along the conductor, but currents meeting at a junction add as ordinary numbers, not by vector rules, so it is a scalar.

How do you find the number of electrons from the current?

Multiply current by time to get the charge Q, then divide by e = 1.602 × 10⁻¹⁹ C.

What is the difference between current and current density?

Current is the total charge flow per second through a section. Current density is that current per unit area, J = I / A, and it is a vector.