Electric Current and Luminous Intensity

Introduction

Of the seven SI base units, five — the metre, kilogram, second, kelvin and mole — appear in almost every chemistry calculation. The remaining two, the ampere for electric current and the candela for luminous intensity, turn up less often, but they complete the system. The ampere matters whenever chemistry and electricity meet, as in batteries, electroplating and electrolysis. The candela describes brightness as seen by the human eye. This page introduces both briefly so that you know what they measure and why they belong in the list.

Core explanation

The seven base units. The SI is built on seven base quantities: length (metre, m), mass (kilogram, kg), time (second, s), temperature (kelvin, K), amount of substance (mole, mol), electric current (ampere, A) and luminous intensity (candela, cd). Every other SI unit can be made by combining these.

Electric current. An electric current is a flow of electric charge. In a metal wire the moving charges are electrons; in a solution or molten salt they are ions. Current tells you how much charge passes a point each second . The SI unit is the ampere (symbol A), often shortened to "amp". Current is measured with an ammeter placed in series in a circuit.

Charge and current. Electric charge is measured in coulombs (C). A current of one ampere means that one coulomb of charge passes a point every second. So:

charge (C) = current (A) × time (s)

A current of 2 A flowing for 10 s transfers 20 C of charge.

How the ampere is defined. Since 2019 the ampere has been defined by fixing the value of the elementary charge , the charge on a single proton, at exactly 1.602176634 × 10⁻¹⁹ C. One ampere therefore corresponds to about 6.24 × 10¹⁸ elementary charges passing a point each second. You do not need to memorise the exact number, but you should know that the definition is tied to a constant of nature rather than to an object.

Typical currents.

Situation Approximate current --- --- Digital watch a few microamperes Mobile phone while charging about 1–3 A Kettle about 10 A School electrolysis experiment about 0.1–1 A

Luminous intensity. The candela (cd) measures how bright a light source appears in a particular direction, weighted to match the sensitivity of the human eye. The eye is most sensitive to green-yellow light, so a green lamp can appear brighter than a red lamp giving out the same power. An ordinary wax candle gives out roughly one candela, which is where the name comes from. The candela is defined by fixing the luminous efficacy of green light of frequency 540 × 10¹² Hz.

Where each appears in chemistry. Current is central to electrochemistry: the amount of product formed in electrolysis depends on the charge passed. Light measurement in chemistry usually uses instruments that measure absorbance or radiant power rather than the eye-weighted candela, so the candela is rarely used in the laboratory.

Formulae

charge Q (C) = current I (A) × time t (s). The symbol for current is I; the symbol for charge is Q.

Step-by-step reasoning

To find the charge passed in an electrolysis experiment:

1. Read the current from the ammeter in amperes. 2. Convert the time into seconds (minutes × 60). 3. Multiply current by time. 4. Give the answer in coulombs, C.

Visual explanation

Imagine a wire cut through by an imaginary window. Count the charged particles crossing the window each second: that count, multiplied by the charge on each particle, is the current. Double the number crossing per second and the current doubles.

Real-world analogy

Current is like the flow of water in a river measured in litres per second at a bridge. The water itself is the "charge"; the rate at which it passes under the bridge is the "current". A slow stream and a flood differ in rate, not in what water is.

Real-world example

In electroplating, a thin layer of metal such as nickel or silver is deposited onto an object. Operators control the current in amperes and the time in seconds, because together they set the total charge, and the charge decides how thick the metal coating becomes.

Why?

Why is current a base unit rather than being built from others? Electric charge cannot be expressed using only length, mass, time and temperature. A separate base quantity is needed for electrical measurements, and current was chosen because it is easy to measure accurately with instruments.

Common misconception

"Current gets used up as it goes around a circuit." In a simple series circuit the current is the same at every point. Energy is transferred to components, but the charge keeps flowing and is not consumed.

Worked example

Question: A current of 0.50 A is passed through copper(II) sulfate solution for 30 minutes. How much charge flows?

Reasoning: Time = 30 × 60 = 1800 s. Charge = current × time = 0.50 × 1800 = 900 C.

Answer: 900 C (9.0 × 10² C).

Quick check

1. What are the SI base unit and symbol for electric current? Answer: The ampere, symbol A.

Exam focus

Remember that time must be in seconds when using Q = I × t. Examiners often give time in minutes to test this. Be able to list all seven base units with their symbols, and do not confuse the candela (cd) with the unit of charge, the coulomb (C).

Advanced insight

The Faraday constant, about 96 500 C mol⁻¹, is the charge carried by one mole of electrons. It links the ampere to the mole and allows chemists to calculate exactly how many moles of a metal are deposited in electrolysis from the current and time alone.

Summary

The ampere (A) is the SI base unit of electric current, the rate of flow of charge. One ampere transfers one coulomb per second, so Q = I × t with time in seconds. The candela (cd) is the base unit of luminous intensity, brightness as perceived by the eye. Current is important in electrochemistry; the candela is rarely used in chemistry.

Practice questions

1. List the seven SI base units with their symbols. Answer: Metre (m), kilogram (kg), second (s), kelvin (K), mole (mol), ampere (A) and candela (cd). 2. Calculate the charge transferred when 2.0 A flows for 5.0 minutes. Answer: Time = 300 s, so charge = 2.0 × 300 = 600 C. 3. What particles carry the current in a solution during electrolysis? Answer: Ions, moving towards the electrodes. 4. Why can a green light appear brighter than a red light of the same power? Answer: Luminous intensity is weighted by the eye's sensitivity, and the eye is more sensitive to green light than to red.