Right-Hand Rules:  A Guide to finding the Direction of the Magnetic Force


Solutions

1.  A proton is travelling with a speed of 5.0 x 106 m / s, when it encounters a magnetic field that is perpendicular to the velocity of the proton with a magnitude of 0.40 T.  Make a sketch of this situation indicating the directions of the velocity of the proton, the magnetic field and the magnetic force.

ANSWER:

First, let us assume that the proton is moving from the left edge to the right edge of this page.  Also, we can define the magnetic field direction perpedicular to the velocity of the proton and as moving out of this page.  Using RHR #1:

the magnetic force pushes the proton toward the bottom edge of this page.  A sketch indicating the directions of the proton's velocity, the magnetic field and the magnetic force looks like this:

2.  Here, a long, straight wire carries a current of 3.0 A.  A particle with charge q = +6.5 x 10-6 C moves parallel to the wire at a distance of r = 0.050 m and a speed of v = 280 m / s.  Determine the magnitude and direction of the magnetic field experienced by the charge.

ANSWER:

As the current I moves through the wire, the movement of electrons induces a magetic field around the wire.  The direction of this magnetic field can be determined using RHR #2:

As the current moves away from you the reader, a magnetic field circles the wire - moving in a clockwise direction.  The charge q is pushed toward the bottom of this page.
The magnitude of the induced magnetic field can be determined using the mathematical equation, B = (moI) / (2pr).  So for this situation:
B = (moI) / (2pr)
B = (mo x 3.0 A) / (2p x 0.05 m)
B = 1.2 x 10-5 T.

Remember, this field interacts with the moving charge to produce a force that would change the trajectory of charge's velocity.

References:

Cutnell, J.and Johnson, K. (1998), Physics, Vol. 2, Wiley: NY, p. 631, 33, 46, and 49.
[Back to Seat Experiments Index][Back to RH Rules]