The magnetic field set up at point P by the current element i ds turn out to be The above equation is called the Biot-Savart law. The vector dB is perpendicular both to ds (which is the direction of the current) and to the unit vector r directed from the element to the point P.

The magnetic of dB is inversely proportional to r² , 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 dB 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
• changing directions -> moving faster -> changing direction ...!

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 ...!