What is the difference between current and the ampere?

I have always thought that current has the same definition as the ampere.

When this problem comes up:

So I instinctively choose answer A. But according to the my book, B is the answer?!?!

But why? Isn't current the number of electrons that flow through a given point per second? So does this mean that current and amps have distinct definition? If so, why does current have the units --> amps? How does this work?

• It is like asking difference between mass and KG, current is a quantity and Amper is the unit ex:50mA of current. Commented Sep 28, 2020 at 3:26
• Your book is out of date. See Wikipedia. Commented Sep 28, 2020 at 3:34
• FYI: The force between two parallel conductors is proportional to the number of electrons passing a given point per second in each wire. en.wikipedia.org/wiki/Amp%C3%A8re%27s_force_law Commented Sep 28, 2020 at 3:37

It sounds like your book is outdated. The answer used to be B, but as of last year A is now correct.

The definition of the ampere in the 8th edition of the SI was:

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per metre of length.

So as you can see the 8th edition did in fact define the ampere in terms of the force between two parallel current carrying wires.

However, that changed with the new 9th edition:

The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 ×10−19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆νCs

So now the ampere is defined so that it is exactly $$1/1.602 \ 176 \ 634 \times 10^{-19}$$ electrons per second passing by a point.

Current is the rate of charge carriers passing a location, but the ampere is a unit of current and as such can in principle be defined using any experiment that produces a reliable amount of current. Now, with quantum-mechanically based resistance and voltage standards, it is possible to do several different experiments with high enough precision that it makes sense to simply define a fixed elementary charge.

https://www.bipm.org/en/publications/si-brochure/

Regarding the distinction between current and the ampere. The ampere is the SI unit of current. The ampere is said to have dimensions of current. This is similar to the idea that the meter is the SI unit of length. The meter is not length, it is a unit of length, and there are other non-SI units of length, like the mile and the inch. Similarly the ampere is not current, it is the SI unit of current and there are other unit systems that define current differently.

Current is a physical quantity describing the rate of flow of charge. The ampere is the SI unit of current.

Isn't current the number of electrons that flow through a given point per second?

Not quite, as the number of electrons is dimensionless, so you would need to multiply it be the electron charge in order to get current.

The question from your book is a little ambiguous, as the definition of the ampere actually changed recently, so your confusion is understandable. In the old definition, the ampere was defined by

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to $$2\cdot10^{−7}$$ newton per metre of length.

So, before 20 May, 2019, the answer B was correct. However, as of 20 May, 2019, the ampere is now defined by

The ampere, symbol $$\mathrm{A}$$, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge $$e$$ to be $$1.602176634\cdot10^{−19}$$ when expressed in the unit $$\mathrm{C}$$, which is equal to $$\mathrm{A\cdot s}$$, where the second is defined in terms of $$\Delta\nu_{\text{Cs}}$$.

So now, the answer A is correct.