##
How to verify
Biot-Savart Law ¡H

The magnetic field set up at point P by the current element *i d***s**

The vector* *d**B **is** **perpendicular both to d**s **(which
is the direction of the current)
and to the unit vector **r **directed from the element** **to
the point P.

The magnetic of d**B **is inversely proportional to r^{2 },
where r is the distance from the element to the point P.
However, the current must form a loop. You will not have only a small
current element.

All the current elements along the current loop can contribute to the
magnetic field at point P.

How do we measure magnetic field d**B** just due to a small current
element?

Here is an ideal!

There is no magnetic field in the direction of the current element.

The above two green lines meet at blue dot P.

Current elements from green lines will not produce magnetic field at
point P.

The magnetic field at point P is due to current elements from two yellow
line sections.

We can keep one of the current element much further away from point
P.

Now we have a way to measure the magnetic field just due to one current
element.

Click near the **right yellow** line section and drag the mouse
up/down to change its length.
Click near the **left yellow** line section and drag the mouse up/down
or left/right

to change its length and location.

The point P moves when you make the above changes.
The magnetic field at point P is shown at the top left corner.

Click at the DC current source to change its polarity.

You can enter value for the current source into the text Field.

The moving black points represent the electrons moving with average
drift velocity.

If you enter a larger value for the current , the electrons will move
faster.

Keep increasing the value for the current, until you see the electrons

Why is that ? Is there something wrong with the program?
Look at tires of a car when it is accelerating!

Those tires seem to be rotated faster and then change direction ...
!

Your suggestions are highly appreciated! Please click *hwang@phy03.phy.ntnu.edu.tw*

Author¡GFu-Kwun Hwang, Dept. of physics, National Taiwan Normal University

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