What is the Ampere?#

The ampere, often shortened to amp (and shown with the symbol A), is the standard unit for electric current in the world of science and engineering (the International System of Units, or SI).

Imagine electric current as a river flowing. The ampere measures how much water (electric charge) passes a certain point per second. A higher ampere value means more charge is flowing quickly.

It’s named after a brilliant French scientist, André-Marie Ampère, who did groundbreaking work on electricity and magnetism way back in the late 1700s and early 1800s. He’s considered one of the main founders of electromagnetism.

Electric Current and Charge: The Basic Connection#

Before we get deeper into the ampere’s definition, let’s quickly touch on what electric current is.

Electric Current: This is the flow of electric charge. In most electrical circuits we deal with, this charge is carried by tiny particles called electrons, which move through conductors like wires.

Electric Charge: This is a fundamental property of matter. Electrons have a negative charge, and protons have a positive charge. The amount of charge is measured in a unit called the coulomb (C).

The key link is: Current is the rate of flow of charge.

Think of it like this:

• Charge (Coulomb): The total amount of water in a bucket.
• Current (Ampere): How fast that water is flowing out of the bucket per second.

So, 1 ampere means 1 coulomb of charge is passing a point every second.

The Modern Definition of the Ampere (Since 2019)#

Scientists are always working to define units in the most precise and stable ways possible, often linking them to fundamental constants of nature. Since 2019, the ampere’s definition got an update.

Now, the ampere is defined by setting the value of the elementary charge (e) exactly.

Elementary Charge (e): This is the smallest possible amount of free electric charge you can have. It’s the magnitude of the charge on a single electron or a single proton. Its value is incredibly small: exactly 1.602 176 634 × 10⁻¹⁹ coulombs (C).

By fixing the elementary charge, we also fix the definition of the coulomb. Remember, the coulomb is the unit of charge. One coulomb is defined as the total charge of a specific, large number of elementary charges. How many? Just take 1 Coulomb and divide it by the charge of one elementary charge: 1 C / (1.602 176 634 × 10⁻¹⁹ C/e) ≈ 6.241 509 074 × 10¹⁸ elementary charges.

So, because 1 Ampere is 1 Coulomb per second (1 A = 1 C/s), the modern definition means:

An ampere is the electric current corresponding to a flow of 1/(1.602 176 634 × 10⁻¹⁹) elementary charges per second.

That’s about 6.241 509 074 × 10¹⁸ elementary charges flowing past a point every single second.

You can also think of it the other way around: 10¹⁹ elementary charges moving every 1.602 176 634 seconds would be a current of 1 ampere.

This definition ties the ampere directly to a fundamental constant (e) and the definition of the second (which is based on atomic clocks using caesium-133 atoms). This makes the ampere’s definition incredibly stable and reproducible anywhere in the world.

A Look Back: The Ampere’s History#

Understanding the history helps appreciate why the definition changed.

• Named for the Pioneer: As mentioned, the unit honors André-Marie Ampère. This was made official at an international meeting in 1881.
• Early Systems: Before the modern SI, other systems existed. The centimetre-gram-second (CGS) system had units for current. The ampere was originally set as exactly one-tenth of the CGS unit of current, which was called the “abampere”. The abampere was defined based on the magnetic force between wires carrying current. This size (1/10th) was chosen so that units derived from it in later systems (like the MKSA system) would be practical sizes for everyday use.
• The “International Ampere”: For a while, a practical way to realize (or physically create and measure) one ampere was defined based on chemistry! It was the amount of current needed to deposit a specific amount of silver (0.001 118 grams) per second from a silver nitrate solution. Later, more precise electrical measurements showed this “international ampere” was slightly different from the intended unit based on magnetic force – it was about 0.99985 A. This highlights the challenge of defining units based on specific experiments versus fundamental constants.

How We Used to Define the Ampere (Before 2019)#

For many years before the 2019 update, the ampere was defined in a completely different way, based on the magnetic force between two current-carrying wires.

Former SI Definition (Pre-2019): The ampere was the constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2 × 10⁻⁷ newtons per metre of length.

Let’s break that down:

• Why Magnetic Force? Ampère’s work showed that wires with current create magnetic fields around them, and these magnetic fields push or pull on other wires with current. Parallel wires with currents in the same direction attract each other; with currents in opposite directions, they repel.
• The Setup: The definition imagined an ideal scenario: two infinitely long, super-thin wires, exactly 1 meter apart, in a vacuum.
• The Force: If you put a certain current in both wires, they would push or pull on each other with a specific force per meter of their length. The definition fixed this force at 2 × 10⁻⁷ newtons for a current of 1 ampere.
• Impact on Vacuum Permeability (μ₀): This definition also had the effect of giving the vacuum magnetic permeability (μ₀) a defined value: exactly 4π × 10⁻⁷ Henry per metre (H/m). This constant describes how well a vacuum supports a magnetic field. Fixing μ₀ meant that the ampere was tied to the properties of electromagnetism in empty space.
• Impact on the Coulomb (Old System): In this older system, the coulomb was derived from the ampere and the second. One coulomb was simply the amount of charge that flows when a current of 1 ampere is maintained for 1 second (1 C = 1 A ⋅ s). This is the opposite relationship from the new definition!

Realizing the Old Definition in Practice#

Even though the definition involved idealized wires, scientists had ways to measure current very accurately based on this definition using complex instruments. One high-precision device was the Kibble balance, which measures electrical power against mechanical power.

However, for most practical high-precision work, the ampere was often realized (measured accurately) indirectly using Ohm’s Law.

Ohm’s Law: A fundamental relationship in electrical circuits stating that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, with the constant of proportionality being the resistance (R). Written as V = I * R.

Since highly stable and reproducible ways to define the volt (V) and the ohm (Ω) existed based on quantum mechanics (the Josephson effect for voltage standards and the Quantum Hall effect for resistance standards), engineers and scientists could use Ohm’s Law (I = V/R) to get a very accurate value for the ampere.

The new definition, which fixes the elementary charge, aligns better with these modern quantum electrical standards and avoids the concept of idealized wires.

The Ampere in Relation to Other Key Electrical Units#

In electrical engineering, the ampere doesn’t work alone. It’s part of a family of units. Here’s how it connects to some others you’ll encounter daily:

• Ampere (A) and Coulomb (C): As we know, 1 A = 1 C/s. Current is the rate of charge flow.
• Ampere (A), Volt (V), and Ohm (Ω): Ohm’s Law (V = I * R) is crucial. If you know the voltage across something and its resistance, you can find the current.

  Volt (V): The unit of electric potential difference or voltage. It’s like the “electrical pressure” that pushes charge through a circuit. 1 V = 1 Joule/Coulomb (J/C). Ohm (Ω): The unit of electrical resistance. It measures how much a material opposes the flow of electric current.

• Ampere (A), Volt (V), and Watt (W): Power (the rate at which energy is used or transferred) is calculated using voltage and current.

  Watt (W): The unit of power. 1 W = 1 Joule/second (J/s). In electrical terms, Power (P) = Voltage (V) * Current (I). So, 1 W = 1 V ⋅ A, meaning 1 A = 1 W/V.

• Ampere (A) and Second (s) defining the Coulomb (C): In the new SI, the elementary charge and the second are base definitions, and the coulomb is derived (C = A ⋅ s). But conceptually, it’s often easiest to think of A = C/s.

These relationships are fundamental to analyzing and designing electrical circuits.

Measuring Electric Current#

You can’t just see current flowing, so you need a tool to measure it.

Ammeter: This is the instrument specifically designed to measure electric current in amperes. To measure the current flowing through a component, you have to connect the ammeter in series with that component, so that the current flows through the meter.

Many multi-purpose test tools, called multimeters, can measure voltage, resistance, and current (among other things). When using a multimeter to measure current, you select the ‘amp’ or ‘mA’ setting and connect it in series. Be very careful when measuring current; improper connection can damage the meter or the circuit.

Practical Considerations in Electrical Engineering#

• Ampacity: This is a critical concept. Wires and components can only safely handle a certain amount of current before they overheat and get damaged or start a fire. The maximum safe current for a wire or device is called its ampacity. Choosing the right wire size and components for the expected current is a key part of electrical design.
• Current Levels Vary Hugely: The currents you deal with in engineering can range massively. Sensitive electronic circuits might operate with microamperes (μA, millionths of an amp) or milliamperes (mA, thousandths of an amp). Household appliances draw currents of a few amperes. Heavy-duty equipment, industrial motors, or power transmission lines can carry hundreds or thousands of amperes (kiloamperes, kA).
• Electric Shock: It’s important to remember that current flowing through the human body is what causes electric shock and injury. The severity depends on the amount of current and the path it takes, even small currents can be dangerous.

SI Prefixes for Ampere#

Like most SI units, the ampere can use prefixes to indicate larger or smaller values, which is super handy given the wide range of currents we see.

• kA: kiloampere (10³ A = 1000 A) - Used for very large currents like in power transmission or welding.
• A: ampere (10⁰ A = 1 A) - Common unit for many appliances and circuits.
• mA: milliampere (10⁻³ A = 0.001 A) - Used in smaller circuits, electronics.
• μA: microampere (10⁻⁶ A = 0.000 001 A) - Used in very sensitive electronics, sensors.
• nA: nanoampere (10⁻⁹ A)
• pA: picoampere (10⁻¹² A)

So, if a datasheet says a component draws 50 mA, it means it uses 0.05 amperes of current.

Wrapping Up#

The ampere is more than just a unit; it’s a fundamental concept representing the flow of electric charge. Understanding its definition, both the older, more tangible one based on force and the modern, more fundamental one based on elementary charge, is key. Knowing how it relates to volts, ohms, and watts, and how to measure it, are essential skills for anyone working in electrical engineering. Keep practicing with circuit problems, and you’ll get a solid feel for what an “amp” really means in the real world!