# Can you help me with measuring electric charge?

1. Apr 23, 2014

### yhPscis

The electric charge is measured in Coulombs.

1 Coulomb is defined as "the charge of approximately 6.241×10^18 electrons" by wikipedia.

When I look up the charge of one electron, I get: 1.60217657 × 10^-19 coulombs

I'm confused by the fact that electronic charge is defined in terms of coulombs and not in terms of Newtons. It seems more logical to me that the charge of 1 electron should be defined in terms of Newtons, not coulombs, because charge is a physical property that expresses itself solely in the fact that it causes other charged matter to experience a force when in its vicinity (isn't it?) and Newton is the unit of force, so why isn't it defined in terms of newtons?

Mass is a physical property that expresses itself in the fact that it causes matter to resist a change in acceleration when it experiences a force, so it is very logically defined in terms of F/a, unlike with charge.

My exams are nearing, so this is very important to me, thank you!

Last edited: Apr 23, 2014
2. Apr 23, 2014

### Pharrahnox

It cannot be measured in force units, because the force on a charge is proportional to the inverse square of the distance between it and another charge. Therefore, the Newton value of it would be from one particular distance. Also, the force is dependant on the Coloumbs of at least 2 charged particles.

So if you were told that a charge had a force of 10N, what is the distance between that and another charge, what is the Coloumb value of charge1 and of charge2? There are three variables that could have an unlimited amount of different proportions of that 10N.

So, if the charge were to be measured in Newtons, you wouldn't be able to deduce much useful information other than the force, which is fairly easily calculated anyway with F = kQ1Q2/d^2

This is only addressing the meaningfulness (is that a word?) aspect of the units, not the actual definition of the unit. The Coloumb is the amount of electric charge, not force. If it was defined in terms of force, what would you use to calculate the Amperes of the flow of charged particles?

3. Apr 23, 2014

### sophiecentaur

Every unit of measurement that we use hangs on other units and all measurements use several quantities. It's all circular, in the end. We try to base our measurement system, when we can, on invariant things - like the speed of light, atomic energy levels and stable isotopes. The best we can hope for is repeatability - which is why the basic kg (a lump of Platinum in Paris) is not a very satisfactory standard of mass.

4. Apr 23, 2014

### UltrafastPED

I've used electrometers and charge amplifiers to measure electric charge; here is an application note on devices used to measure charge:

http://www.trekinc.com/pdf/3002-field-voltmeter.PDF

Come to think of it, I have also measured charge using the Millikan oil drop apparatus ... the old style beast with telescope, atomizer for the oil droplets, and a big capacitor. What a pain! 1969 was not a very good year ...

Looked a lot like this one: http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Ryan_McAllister/Slide3.htm

5. Apr 23, 2014

### Staff: Mentor

The SI unit of charge (the coulomb) is in fact defined (indirectly) by way of force.

One coulomb is the charge transferred by a current of one ampere in one second.

Two straight parallel wires, one meter apart, carrying equal currents and exerting a magnetic force of 2 x 10-7 newton on each other, by definition have a current of 1 ampere each.

In Gaussian units, the unit of charge is the statcoulomb: two charges of one statcoulomb each, separated by 1 cm, exert a force of one dyne on each other.

SI does things the way it does because it's intended as a practical system of units, and its definition of the ampere is easier to implement precisely in the lab than the Gaussian definition of the statcoulomb.

6. Apr 23, 2014

### DrGreg

If you'd read the Wikipedia article more carefully you would have noticed it didn't say "defined as", it said "is equal to". In the very next sentence it said "Its SI definition is the charge transported by a constant current of one ampere in one second", with a whole section on "Definition" later.

7. Apr 23, 2014

### dauto

Also, its NOT true that charges manifest themselves "solely in the fact that it causes other charged matter to experience a force when in its vicinity" Charges interact with electric and magnetic fields. These fields may be produced by charges in the opposite side of the universe, and propagate as electromagnetic waves. The definition of charge is through the Ampere - the unit of electric current, and a force between two currents is used in its definition. That force is not the same as the Coulomb force that you're thinking about. But granted, we could chose to define charge in terms of the Coulomb force by setting the Coulomb constant to 1. The Coulomb equation would be
$$F=\frac{q^2}{d^2}$$ giving charges the units meter times Newton1/2. That's still not the same thing as a force.

8. Apr 23, 2014

### Simon Bridge

I think post #1 involves end results rather than definitions ... dauto is correct that, correctly, charge is defined in terms of a current.

http://en.wikipedia.org/wiki/Charge_(physics [Broken])

So the unit of charge has to be defined in terms of current.
For electric charge, 1 coulomb per second is needed to produce a force of 1 Newton between currents 1m apart.

It just so happens that electric charge is quantized - the quantization leads to the results in post #1.

Last edited by a moderator: May 6, 2017