Difference Between Ampere and Coulomb

Ampere and Coulomb are two measurement units that are used to measure the current. The current in a conductor is measured in Amperes, whereas Coulombs measure the amount of charge. One ampere is equal to the flow of one coulomb of a charge in a second. Unlike coulomb, which measures the amount of charge, ampere measures how fast the amount of charge is moving. This is the key difference between Ampere and Coulomb.

An electric current occurs inside a conductor when the charge carriers inside the conductor move through it under the effect of a voltage difference. A very common example of how the current occurs is the water flowing through a pipe. If the pipe is kept horizontally, there will be no flow inside it; if it is tilted at least slightly, it will create a potential difference between the two ends and water will start to flow through the pipe. The higher the slope, the higher the potential difference, hence, the higher the amount of water flows per second. Similarly, if the voltage difference between the two ends of a wire is higher, the amount of charge flows through will be higher, making high current.

CONTENTS
1. Overview and Key Difference
2. What is Ampere
3. What is Coulomb
4. Side by Side Comparison – Ampere vs Coulomb
5. Summary

What is Ampere?

The measuring unit of current, Ampere, is named after a French mathematician and physicist André-Marie Ampère who is considered as the father of electrodynamics. Amperes are also called as amps, in short.

Ampere’s force law states that two parallel electric wires carrying current impose a force on each other. International Systems of Unites (SI) defines one ampere based on this Ampere’s Force Law; “The ampere is that constant current which, 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”.

By Ohm’s Law, current is related to voltage as:

V = I x R

R is the resistance of the current carrying conductor. The power P consumed by a load relates to the current flows through it and the supplied voltage according to:

P = V x I

This can be used to understand the quantity of an ampere. Consider an electric iron having 1000 W rating, which is connected to the power line of 230 V. The amount of current it consumes to heat up can be calculated as:

P = VI
1000 W = 230 V ×I
I = 1000/230
I = 4.37 A

Compared to that, in electric arc welding, a current beam of nearly 1000 A is used to melt an iron rod. If a lightning bolt is considered, the current delivered by an average lightning flash is about 10,000 amps. But, a 100,000 amp lightning flash has also been measured.

Current is measured using Ammeter. Ammeter works in different techniques. In a moving-coil ammeter, a coil mounted along the coil’s diameter is supplied with the measured current. The coil is placed between two magnetic poles; N and S. As per Flemming’s Left-Hand Rule, a force is induced on a current carrying conductor which is placed in magnetic field. Therefore, the force on the mounted coil rotates the coil around its diameter. The amount of deflection here is proportional to the current through the coil; thus, the measurement can be taken. However, this approach requires breaking the conductor and placing the ammeter in the middle. Since this cannot be done in a running system, a magnetic method is used in clamp meters to measure both AC and DC currents without a physical contact with the conductor.

What is Coulomb?

The SI unit Coulomb, which is used to measure electric charges, is named after the physicist Charles-Augustin de Coulomb who introduced Coulomb’s law. Coulomb’s law states that when two charges q₁ and q₂ are placed r distance apart, a force acts on each charge according to:

F = (k_eq₁q₂)/r

Here, k_e is Coulomb’s constant. A Coulomb (C) is equal to the charge of roughly 6.241509×10¹⁸ number of electrons or protons. Hence, the charge of a single electron can be calculated as 1.602177×10⁻¹⁹ C.  Static electric charge is measured using an electrometer. As in the previous example of an electric iron, the amount of charge passes into the iron in one second can be calculated as:

I = Q/t
Q = 4.37 A ×1 s
Q = 4.37 C

During a lightning flash, around 15 coulombs of charge could pass a current of 30,000 A to ground from a cloud in a fraction of a second. However, a thunder cloud could hold hundreds of coulombs of charge during lightning.

Charge is also measured in ampere-hours (Ah = A x h) in batteries. A typical mobile phone battery of 1500 mAh (theoretically) holds 1.5 A x 3600s = 5400 C of charge, and to make a sense of the charge, it is expressed as the battery can provide 1500 mA current within an hour.

What is the difference between Ampere and Coulomb?

 Summery – Ampere vs Coulomb

Ampere is used to measure the flow of electric charges, unlike Coulomb, which is used to measure the static electric charge. Though Ampere is related to Coulomb by definition, Ampere is defined without using the charge, but using a force acting on a current-carrying conductor. This is the difference between Ampere and Coulomb.

Reference:
1. Lightning flashes and strokes. (n.d.). Retrieved May 29, 2017, from http://hyperphysics.phy-astr.gsu.edu/hbase/electric/lightning2.html
2. Ampere. (2017, May 28). Retrieved May 29, 2017, from https://en.wikipedia.org/wiki/Ampere
3. Coulomb. (2017, March 24). Retrieved May 29, 2017, from https://en.wikipedia.org/wiki/Coulomb#SI_prefixes

Image Courtesy:
1. “Ampere-def-en” By Danmichaelo (Public Domain) via Commons Wikimedia
2. “Galvanometer diagram” By TiCPU – (GFDL) via Commons Wikimedia