lecture 9

Ph272- General Physics II (Electricity and Magnetsism)

When you connect a battery to a circuit, the battery "pushes" the charges along the circuit, thus creating a current.
If the charges accounters no opposing forces, the charges would be accelerated by the electric field of battery. The charges would speed up thus increasing the current and increasing their kinetic energy. In essense the current "carries" the energy of the battery.
Most materials has electrical resistance because the charges collides with the vibrating atoms or impurities in the material.
These collisions counteract the acceleration due to the battery. A equilibrium situation is established such that the current is constant (or the speed of the charges are constant).
The energy carried by the charges are dissipated into the vibration of the atoms (in another word, heat).

Definition: Current = Charge passing through/time , Eq. 22-1.
From the microscopic theory, the current is related to the (average) velocity of the charge carriers, Eq. 22-2.

-> Do Problem 22-1, 22-2, P. 745.

V = IR or I = V/R

That is, the current is linearly proportional the voltage across the resistor.

Note: Not all materials obey Ohm's Law. For example, semiconductor device (say P-N junction) do not obey Ohm's Law.

-> Do Problem 22-9,22-10., P. 745

Note that the current is constant, the charges gain up kinetic energy from the E-field but dissipate the energy by colliding with the vibrating atoms (kinetic energy -> heat).
Power dissipated by a resistor P = IV
Power output from an ideal battery P = IV(output)=IV(terminal)
Power ouitput from a real battery : A real battery has internal resitance. Its terminal voltage depends on the ratio of the internal resistance and the total resistance (internal plus load resistance).

A drop in the terminal voltage indicates a bad battery (internal resistance got bigger).

-> Do Problem 22-19,22-21, P.745.

Resistance depends on the geometry of the resistor and the resistivity of the material
Resistivity is an instrinsic property of the material and it is independent of it size and shape
Analog: Difference between mass and mass density.
The resistivity of conductors, semiconductors, insulators, and superconductors have different temperature dependence.