How did scientists figure out that the charge of the electron was indeed negative? I know how the cathode ray tube experiment works, but how did Thompson know that the plate that the cathode ray beam was attracted to was positive, meaning the cathode ray was negative? What is the history behind positive and negative charges leading up to the cathode ray experiment. Ben Franklin postulated that a body with excess electricity was positive as in a surplus of electricity, and that surplus flows from positive to negative. But how and when was it discovered that electricity flows from negative to positive?
[Physics] Why is charge of the electron negative
chargeconventionselectronshistory
Related Solutions
Fleming's right-hand rule applies when a conductor is moving in a magnetic field and the current is induced. However, in this case, we have a charged particle moving through a non-varying magnetic field, so it's Lorentz magnetic force law that applies best here. The simple form, in which there is only a magnetic field component (and no externally applied electrostatic field) is this: $${\bf F} = q({\bf v} \times {\bf B})$$ in which ${\bf F}$ is the force vector, $q$ is the charge of the moving particle, ${\bf v}$ is the velocity of the particle and ${\bf B}$ is the magnetic field vector. If we apply the right hand rule to this (the direction of any cross product of vectors can be visualized by using a right hand rule), we see that ${\bf v}$ is to the right when viewing your diagram and ${\bf B}$ is coming out of the page (in the direction of N to S of the magnet) and so the resulting ${\bf F}$ is pointing down.
However, because the charge $q$ is negative, the sign of the resulting vector ${\bf F}$ is reversed, so the force is upward. The diagram is incorrect and should show reversed N and S poles of the magnet (or equivalently, reversed electrode polarity).
Note, too, that Fleming's left-hand rule (current and magnetic field are given, and force is the result) will also work for direction here because, as you correctly point out, Fleming's left-hand rule is given in terms of "conventional current" (from + to -) rather than electron flow, and electron flow is clearly left-to-right in the diagram. For the force to be pointing down, again the poles must be reversed.
Here is a similar diagram figure 2.4 from publisher Prentice-Hall which correctly shows the opposite polarity for the electrodes.
Get together a collection of charges. As many different ways to generate a charge as you can think of. Go ahead and invite your friends so they can think of some more. (As a practical matter you make static charges just before you use them, but still...)
Now, test them pair wise to see if they attract or repel one-another. Keep careful records.
Find the largest set that are all mutually attractive and the largest set that are all mutually repulsive.
You'll find that the attractive set has exactly two members (though you can make many different examples of this set) and the repulsive set consists of half (either half!) the charges you've created.
Ponder that for a while. It also gives you the answer to how like charges respond to one another (though you can get that directly by preparing two similar charges).
Best Answer
Benjamin Franklin proposed electric fluid theory and considered electric current to be flow of a charged fluid. He meant to use positive to denote a surplus of the fluid, negative as a deficit of it. No one knows how he came up with the choice, but it became the convention and as a result lead also to the labeling of charge. I know of no fact that could lead him to that choice. It might just be random. Years later Thompson discovered electron, and according to already established convention, it had a negative charge.