If I wanted to use Fleming’s Left Hand Rule to find out the direction of the force exerted by the magnetic field around a straight current carrying wire, how would I go about doing it? The Right Hand Thumb Rule says the magnetic field lines are concentric circles around the wire, but how do I use this in Fleming’s Right and Left Hand Rules?
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So… if you have a wire, with current flowing through it, in an existing magnetic field, it will experience a force.
You can use either Fleming’s Left Hand Rule *or* the Right Hand Grip Rule to work out which direction things will go in.
We know that if you have a charged particle moving (so a current) in a magnetic field, that particle experiences a force. Fleming’s **Left Hand Rule**, where [we stick our fingers out at odd angles](https://en.wikipedia.org/wiki/File:LeftHandOutline.png), tells us how this works. The se**c**ond figure points in the direction of the **c**urrent (or the velocity of the charged particle), the **f**irst finger in the direction of the magnetic **f**ield, and the thumb points in the direction of the force the moving charged particles will experience (note that if our charged particle is negative the force will be in the opposite direction). See [the example in the top left of this page](http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/forwir2.html); we have the current, the magnetic field and the force; the wire will move up.
If the velocity and field aren’t at right-angles already, you can resolve one of them to find the component perpendicular to the other one (the same we would with forces in mechanics), and then use just that.
Alternatively, we know that if you have moving charge (or current) you get a magnetic field. The **Right Hand Grip Rule** (a simplified version of Ampère’s circuital law) tells you which way that magnetic field goes in relation to the moving charge. If you want to use this rule, think about how the magnetic field from the current, and the existing magnetic field will interact:
The general rule for interacting magnetic fields is that they will move so that the space where they are in the same direction is a big as possible, and the space where they are opposed is as small as possible.
So going back to [the top diagram here](http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/forwir2.html), the magnetic field from the magnets will flow North to South (so up and to the right, ish). The magnetic field from the wire will be curling around; on top of the wire the magnetic field will be heading down and left(ish), and below the wire it will be heading up and right(ish). So above the wire it will be going in the opposite direction to the magnetic field from the magnets, and below the wire it will be moving with them. The wire will then move to maximise the area below the wire (where the fields are ‘happy’), while minimising the are above (where they are ‘unhappy’); i.e. the wire will move up.
Whichever rule we use we get the same results. Which is good.
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Fleming’s Right Hand Rule is best avoided, as it confuses what is going on. You can always use Fleming’s Left Hand Rule and/or the Right Hand Grip Rule.
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