Motor Effect and the 3rd Right Hand Rule

3rd right hand rule:

  1. point thumb in direction of conventional (positive) current flow (I+)
  2. point fingers in direction of permanent or external magnetic field (B)
  3. point palm in direction of the magnetic force (FB)

Motor Effect:

The right hand rule can be used to demonstrate the motor effect, the principle behind all electric motors. In amotor, a permanent magnetic field exerts a torque on a current-carrying coil as predicted by right hand rule #3, causing it to rotate about an axis.

The magnetic force on a conductor can be calculated using the formula:

FB = magnetic force in Newtons (N)

B = magnetic field intensity in Tesla (T)

I = current flow in amps (A)

L = length of conductor in the field in metres (m)

Note: Tesla is the unit for magnetic field intensity, named for Nikola Tesla.

Moving Charges in Magnetic Fields:

A current does not necessarily have to travel through a conductor to experience the effects of a magnetic field. A single charged particle that is moving constitutes a current flow and will feel the effects of a magnetic field that it passes through. Maximum force occurs when the particle travels at right angles to the field. All of our questions will assume this is the case.

The magnetic force on a moving charged particle can be calculated using the following formula:

FB = magnetic force in Newtons (N)

q = charge of particle in Coulombs (C)

v = speed of particle in m/s

B = magnetic field intensity in Tesla (T)

A Note about Conventional vs. Electron Current Flow:

The right hand rules are based on conventional, or positive, current flow. However, electrons actually flow in the opposite direction. To use the right hand rules, you must use positive flow, so if a problem presents you with a negative current or particle, you can do one of two things:

1.  point your thumb of your right hand in the opposite direction of the negative flow indicated (the positive direction)

2.  use your LEFT hand, where your thumb points in the direction of negative flow (nothing else changes)