Energy Bands

Although not often covered overtly in your typical physics classes, physics recognizes three types of mass:

1. Active gravitational mass
2. Passive gravitational mass
3. Inertial mass

They correspond to the m's in the formulae:

F = M*m/r^2

F = m*a

Thanks to what's called Einstein's equivalence principle we say that 1, 2, and 3 are numerically equal.

Now, what you're concerned with for effective mass is the inertial mass and the formula F = m*a. With a slight rearrangement we have m = F/a.

Electrons in a material will interact with the material, so that when you apply a force F to the electron (say via an electric field), the acceleration of the electron will be influenced not only by the applied force but its interactions with the atoms in the material.

Think of a ball bearing in syrup. Although the ball bearing still has the same gravitational mass, the effective mass is increased due to the friction of the bearing with the syrup (viscosity). To achieve the same acceleration as a ball bearing in free space you need to apply more force to the ball bearing in syrup, so the m in m = F/a, the effective mass, is larger.

Yes....bt still...how can u assume effective mass to be 0 and infinite in certain cases??

Hmm, I don't know if this is applicable, but an electron in an insulator can't be budged and one in a super conductor will move at the slightest provication.