Left Hand Rules

by

Richard J. Terwilliger

Click on a

Created by

Richard J. Terwilliger

July 2001

around current bearing wires

Thumb points in the direction of electron flow.

Fingers curl around the wire in the direction of the

magnetic field.

The magnetic field in front of the wire points towards the top of the page.

The magnetic field behind the wire points towards the bottom of the page.

The magnetic field above the wire points into the page.

The magnetic field below the wire points out of the page.

Again, The thumb of the

left hand points in the direction of

electron flow

The fingers curl around the wire in

the direction of the

magnetic field

The magnetic field in front of the wire is to

the left

The magnetic field on the

left side of the wire is back

into the page

The magnetic field behind the wire is to

the right

The magnetic field on the right side of the wire is

pointed out of the page

Into the page is shown by

an X

Out of the page is

shown by a dot


an X

Out of the page is

shown by a dot


an X

Out of the page is

shown by a dot

The current flow is now to the left

Grasp the wire with your hand


The thumb points in the direction of

electron flow,the fingers curl

around the wire in the direction of the

magnetic field.





magnetic field.





magnetic field.

Each of the following diagrams shows a section of wire that has

been enlarged.

Associated with each wire is the direction of current flow

and the magnetic field around the wire.

Determine which of the following diagrams are correct.

Does the diagram at the right show the correct orientation of the magnetic field around the

current bearing wire?.

Is the diagram at the left correct?

The diagram is…

The diagram shows a compass placed above a

current bearing wire.

The compass needle points into the page

What is the direction of the current flow in the wire?

Click on your choice above.

Shown here is a loop of wire

connected to a potential source.

The electrons flow from the negative

terminal of the battery

through the wire and back to the positive

terminal.

through the wire and back to the positive

terminal.

We know that when current flows

through a wire a magnetic field is

formed.

to determine the direction of the magnetic field.

We use the

Fingers curl in the direction of the magnetic field

The arrows show the

direction of electron flow.

Grab the loop with your

Curl your fingers around the loop in the same direction

as the electron flow.

Your thumb now points

The magnetic field on the

outside of the loop is from the

north pole to the south pole

The magnetic field inside the

loop travels from the south

back to the north

If we place a compass inside

the loop it points in the

direction of the flux lines

Outside the loop a compass still points in the direction of the magnetic flux lines

I’m back!

Several LOOPS of wire are called a

We also use the

to determine the magnetic field around a coil.

To demonstrate the

we’ll start by building an electromagnet.

To build an electromagnet or solenoid we start with a

cylinder.

We could use one of the cardboard rolls found at the center of toilet paper rolls

If the inside of the cylinder is hollow it is said to have

an air core.

The front side of the coil is called the face

of the coil.

We will start creating an electrical solenoid by wrapping wire around

the core.

Each wrap is a loop of wire.

and all the loops form a

coil

Next attach a potential source

(battery)to the wire.

The current will flow from the negative

terminal

through the wire and back to the

positive terminal.

The current flowed up the back of the

coil

and down the front side or face of the

coil.

Current flowing through the coil creates a

magnetic field.

is used to determine the direction of the magnetic field.

The

The next few slides will show how to apply the

to this coil.

Grasp the coil with your left hand curling your fingers

around the coil in the direction

of electron flow.

Your thumb points to the

end of the coil

The magnetic flux lines come

out of the NORTH, go around and

into the SOUTH.

In what direction would a compass point if placed above the coil?

A compass will point in the same direction as the magnetic flux lines at that point.

Now we are going to replace this coil with another coil that

has the wire wrapped around in the opposite direction.

The battery will still be connected with the negative

terminal on the left.

Watch closely so you can see the difference.

Watch closely so you can see the difference.

The current still travels from the negative terminal

through the coil and back to to positive terminal.

Notice that the electrons travel up the face of the coil, over the top

and down the back

Notice that the electrons travel up the face of the coil, over the top

and down the back

Use the

to determine the NORTH end of the coil.

Grab the coil with your

Your fingers will follow the electron flow.

Curl your fingers over the top and down the

back.

You thumb points to the

end of the coil.

We now know the

end of the coil.

and the

around the coil.

Let’s try another example.

We’ll start with another coil.

The coil is attached to a potential source but the

polarity is unknown.

We do know that

is on the bottom of the coil

A

B

Using the

determine which is the negative terminal.

A B

A

B

Grab the coil with your

so you thumb points

A

B

Your fingers now curl in the direction of

A

B

Your fingers now curl in the direction of

A

B

B

The

must come from

A

B

B

The

must come from

A

B

Therefore is theB

Now the

There are 3 partsto the

An external magnetic field.

Remember that the magnetic field goes from

to

Either a charge moving across

the magnetic field.

Or current flow through a conductor that is in the

magnetic field.

A force acting on the moving charge or current bearing wire.

I will now show you how

to apply the

Point your fingers

Or the same direction as the

Your thumb points in the direction of negative

And the

acting on the current bearing wire or moving negative charge is

out of the palm.

First point your fingers of your left hand

Notice that your fingers point in the same direction

as the magnetic field shown by the symbol

Your thumb points in the direction of negative current flow

And the force acting on the moving charge or current bearing

wire is out of the palm.

So the force acting on the wire is

Let’s try another example

Shown here is a current bearing wire placed

between the north and south poles of a horseshoe

magnet.

The electron flow in the enlarged section of wire is

back into the page as shown by the arrows.

We can find the direction of the force on the wire

using the

Using your left hand point your fingers

Now, keeping your fingers pointed south, rotate your hand

so you thumb points in the same direction as the

current flow.

Now, keeping your fingers pointed south, rotate your hand

so you thumb points in the same direction as the

current flow.

The force on this section of wire is out of your palm or

Out of palm

Points at southNegative electron

flow

A current bearing wire is place between two bar magnets.

What is the direction of the force on the wire?

We know that the magnetic field between the bar magnets is from the north pole to the

south pole?

We also know that the current (electron flow) is out of the

negative terminal, through the circuit and back to the positive

terminal.

Therefore the current flow in the section of wire between the bar magnets

is toward the top of the page.

We can now use the

to find the direction of the force on the wire.

Point the fingers of your

in the direction of the magnetic field, south.

Fingers point south

Your thumb points in direction if the negative

current flow.

Fingers point south

The force on the wire is shown by a vector coming out of your

palm.

Fingers point south


palm.


palm.

Have fun using the

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Left Hand Rules