Eli5: In Flemings Left Hand Rule, what does Motion mean? The current direction and Field i understand but I’m so confused on where Motion is involved.

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Eli5: In Flemings Left Hand Rule, what does Motion mean? The current direction and Field i understand but I’m so confused on where Motion is involved.

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Think of it as “force”, not “motion”.

Given the particular current & field direction, the conductors is going to experience a force in the direction given by the rule. Since Fleming’s rules are for things that spin (motors/generators), you can safely assume that the rotor is going to move in the direction of the force unless something is holding the shaft. Hence “motion”.

*tl;dr: “motion” means “force” – the thumb tells you which way the wire will be forced/pushed.*

Fleming’s Left-hand Rule for Motors is a handy way of remembering how motors work when you can’t use vectors (and specifically the vector product). It is a simplification of the magnetic part of the [Lorentz Force Law](https://en.wikipedia.org/wiki/Lorentz_force).

The Lorentz Force Law tells you that if you have a particle with some charge *q* moving in some magnetic field **B** (a vector field) with some velocity **v** (also a vector), it experiences a force **F** (a vector) given by:

> **F** = q(**v** x **B**)

where the “x” is the vector cross product, which is where the weird right-angle stuff comes from.

So in the case of Fleming’s Left-hand rule for motors we switch out the *q* and **v** for **I** (noting that a current is the flow of charge) and we use “motion” as a substitute for “Force.”

In the Left-hand Rule you have the se**c**ond finger for **c**urrent, the **f**irst finger for **f**ield, and the thu**m**b for **m**otion.

But in the more generalised version the second finger would represent the velocity of the moving charge, the first finger still represents the field, and the thumb represents the magnetic force on it (if they are all at right-angles). Accepting that if your particle has a negative charge there will be a minus sign out the front, so the direction of the force will be flipped.

The nice thing about the Lorentz law is that it also works for generators/dynamos (instead of the right-hand rule).

You can also do almost all of this with just the right-hand grip rule (or Ampère’s circuital law to give it its fancy name).

A lot of vector mathematics was developed by 19th century physicists to help them get their theories of electromagnetism to work. You *can* study it to some extent without vectors, but it makes things a lot harder.