Migratory birds use Earth’s magnetic field to guide them. Some people are concerned that human-caused magnetic fields could interfere with bird navigation. Suppose that a pair of parallel high-voltage lines, each carrying a current of 100A are 3.00 meters apart and lie in the same horizontal plane. Find the magnitude and direction of the magnetic field the lines produce at a point 15.0 m above the midpoint between the two lines in each of these two cases:

Solution :

The direction of current is into the page.

The magnetic field due to the wire is perpendicular to AP and the magnetic field due to the wire is perpendicular to BP. Thus, the net magnetic field at point P is towards the right.

The net magnetic field at point P is

The magnetic field is

The angle θ is

The net magnetic field at point P is

a). The direction of the current for both the wires is from north to south. The net magnetic field is towards the right when we are facing south.

The direction is towards the west.

b). Since, both the magnetic field are equal and opposite, so no magnetic field is produced.

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form