Register to reply

Why proton test charges and not electrons?

Share this thread:
Apr18-12, 06:41 PM
P: 1
When picturing electric fields, they are always based on a positive test charge. As you move on to potential difference and electrical currents, it is based more on electrons/negative charge. So, why aren't electric fields and fields lines based on a negative test charge instead of a positive test charge?

Thank you for your help!
Phys.Org News Partner Physics news on
Researchers demonstrate ultra low-field nuclear magnetic resonance using Earth's magnetic field
Bubbling down: Discovery suggests surprising uses for common bubbles
New non-metallic metamaterial enables team to 'compress' and contain light
Apr19-12, 03:12 AM
Sci Advisor
P: 2,438
This is just historical. For some reason one has defined what's now known to be carried by protons as the positive charge. There is a lot of confusion in the literature, talking about "technical direction of current" vs. "real direction of current" and similar gibberish.

It's way easier to just use the vector-field concept to understand these issues. The best is to use even relativistic four-dimensional notation right away. E.g., the electric four-current of a fluid with number density [itex]n_0[/itex] of charge carriers is given by

[tex]j^{\mu}=q n_0 u^{\mu}=q n_0 \gamma \begin{pmatrix}c \\ \vec{v} \end{pmatrix}, \quad \gamma=\frac{1}{\sqrt{1-\vec{v}^2/c^2}}.[/tex]

Here, [itex]q[/itex] is the charge of one particle ([itex]+e >0[/itex] for protons, [itex]-e<0[/itex] for electrons), [itex]c[/itex] the speed of light, [itex]n_0[/itex] the density of the fluid as measured in the local rest frame of the fluid cell, and [itex]\vec{v}[/itex] the flow-velocity field.

The sign of the total current through a cross section then is uniquely defined by the spatial components of this current-density vector and the orientation of the cross-sectional area:

[tex]I=\int_{A} \mathrm{d}^2 \vec{A} \cdot \vec{j}.[/tex]
Apr19-12, 05:18 AM
HW Helper
P: 5,243
They originally thought that electricity in wires was a flow of positive charges. I guess everything else derives from that erroneous conclusion.

Apr19-12, 04:15 PM
P: 617
Why proton test charges and not electrons?

Positive charges can also flow and create currents, for instance in solutions, in your body, in the ionosphere. Even in semiconductors, positive charges (holes) can flow and create currents. Just because electrons are the ones moving in metals and humans like to make useful electronic devices out of metals does not make the electrons special when in comes to electrical currents.

Register to reply

Related Discussions
What causes a proton to have a positive charge? Why do electrons have a negative char Quantum Physics 2
Electron and proton charges High Energy, Nuclear, Particle Physics 117
What happens when electrons crash into their proton Quantum Physics 24
What charges a proton? Quantum Physics 4
Electron & proton charges precisely equal General Physics 6