# Thread: Left hand thumb rule and Right Hand thumb Rule

1. ## Left hand thumb rule and Right Hand thumb Rule

Can somebody please explain to me the right hand thumb rule and left hand thumb rule :S

2. ## Re: Left hand thumb rule and Right Hand thumb Rule

It is personal on which rules you use. I prefer to only use the right hand palm rule.

The thumb of your right hand always represents the current. (One current, One thumb)
The fingers of hand represent the magnetic field lines (Many lines, many fingers)
The direction of the palm is the direction of the force (Your palm is used to clip your brother or sister over the ear which is a force)

So for any problem you will point your thumb in the direction of the current, fingers in the direction of the magnetic field and palm in the direction of the force. When you orientate two then the third only has one direction possible.

If you are looking at induced currents then the only thing to remember is the force will go in the opposite direction to the movement as it is trying to slow it down.

If you are looking at a positive charge then you point your thumb in the direction of movement of the positive charge as they move in the direction of current.

If you are looking at a negative charge then point your thumb in the opposite direction to the movement as negative is the opposite to positive. (Some people prefer to use the left hand for negative but everything else remains the same)

3. ## Re: Left hand thumb rule and Right Hand thumb Rule

Actually 'helper' I think they are different rules but I’m pretty sure you don’t need the left hand rule cause only the right hand rule is in the syllabus.

Also there’s the right hand grip rule where in a current carrying conductor your thumb points in the direction of the current and your fingers curl around in the direction of the induced magnetic field. (Or vise versa)

4. ## Re: Left hand thumb rule and Right Hand thumb Rule

Originally Posted by denoz
Actually 'helper' I think they are different rules but I’m pretty sure you don’t need the left hand rule cause only the right hand rule is in the syllabus.
The right hand grip rule is specified for determining the direction of a magnetic field around a current carrying wire.

To determine the direction of a force or induced current in motors and generators there is no rule specified.

The rule I mentioned above is commonly known as the "right hand palm rule". It involves opening up the hand from the grip rule without changing the meanings of the fingers or thumbs.

There is another right hand palm rule that is used in surfing physics but involves changing the meanings of the fingers and thumb.

There is also Flemings left hand rule which is used in the same way.
There are then variations that may be used for negative charges and generators.

Other than the grip rule they are all used for the same thing in motors and generators.

As I said, I prefer using the right hand palm rule, so there isn't a need to change meanings. This is a personal preference and will depend on your texts, teachers, tutors and personal preference.

The rule I use is shown at
http://sol.sci.uop.edu/~jfalward/mag...cesfields.html

5. ## Re: Left hand thumb rule and Right Hand thumb Rule

http://sol.sci.uop.edu/~jfalward/mag...cesfields.html

now this IS a good motors & generators website

... reminds me of the time we saw sparks jump with a potential difference on a Van de Graaf generator, except on a better scale

Cutting a magnet in half
will not isolate a single
north or south. One
magnet becomes two,
then four, and so on.

This process will never
end; even when the last
electron spin dipole is
reached, it cannot be cut
to reveal a single north
or single south pole.

http://sol.sci.uop.edu/~jfalward/mag...esofmagnet.jpg
In which direction, if any, will the metal rod be deflected?
to the right relative to the battery ... with the right hand rule ?

6. ## Re: Left hand thumb rule and Right Hand thumb Rule

What you do is, in your physics exam, after you have actually answered the question that required the correct rule, very visibly hold up your hand whilst demonstrating THE WRONG RULE, as if you were figuring it out. People will then look at you and see you using the wrong rule and also use the wrong rule, getting the quesion wrong.

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