Quick Quiz 6.5
A spherical balloon contains a positively charged object at its center. As the balloon is inflated to a greater volume while the charged object remains at the center, does the electric potential at the surface of the balloon (a)increase,
(b)decrease, or (c)remain the same?
Does the electric flux through the surface of the balloon
(d)increase,
(e)decrease, or
(f)remain the same?
Quick Quiz 6.6
In Figure, take q1 to be a negative source charge and q2 to be the test charge. If q2 is initially positive and is changed
to a charge of the same magnitude but negative, the potential at the position of q2 due to q1
(a)increases
(b)decreases
(c) remains the same.
Course of lectures 
«Contemporary Physics: Part1»
Lecture №12
Current and Resistance.
Direct Current Circuits.
Electric Current
(7.14)
(7.15)
The current is the rate at which charge flows through this surface.
It is conventional to assign to the current the same direction as the flow of positive charge.
The direction of the current is opposite the direction of flow of electrons.
Electric Current
Microscopic Model of Current
(7.16)
Resistance
Consider a conductor of cross-sectional area A carrying a current I. The current density J in the conductor is defined as the current per unit area.
(7.17)
(7.18)
Resistance
A current density J and an electric field E are established in a conductor whenever a
potential difference is maintained across the conductor.
Ohm’s law |
|
(7.19) |
|
|
|
the conductivity of the conductor
For many materials (including most metals), the ratio of the current density to the electric field is a constant σ that is independent of the electric field producing the current.
Resistance
(7.20) |
The resistance of the conductor |
|
(7.22)
(7.21)
resistivity
Resistance
Resistance