Introductory Physics Volume Two
Introductory Physics Volume Two
Introductory Physics Volume Two
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
58 Circuits 3.3<br />
Definition: Current<br />
The electric current, I, is defined to be the amount of charge that<br />
flows per time.<br />
I = dq<br />
dt<br />
The unit of current is coulombs per second. This<br />
combination of units is called the Ampere or Amp,<br />
abbreviated as just A: (1A = 1C<br />
1s ).<br />
Definition: Current Density<br />
Let A be a small area that is normal<br />
to the flow of current in a particular<br />
region. Let I be the current flowing<br />
through the area A. The current density,<br />
⃗J, is a vector in the direction of the flow<br />
of current. The magnitude of the current<br />
density is equal to the current per area.<br />
J = I A<br />
⊲ Problem 3.1<br />
A light bulb draws a current of 1.0 mA from a battery.<br />
(a) How many electrons pass through the bulb in 160 seconds?<br />
(b) The filament has a radius of 0.20 mm. What is the current density<br />
through the filament?<br />
§ 3.3 Ohm’s Law<br />
When an electric field is applied to a conductor the free charges in<br />
the conductor begin to move. If the conductor is isolated the charges<br />
will distribute themselves so that the field inside the conductor is zero<br />
and then the charges will cease to move. One the other hand, if the<br />
conductor is not isolated but part of a circuit, as the filament was in<br />
the flashlight circuit, then a continual flow of charge can be sustained,<br />
and the electric field in the conductor will not be zero.
Change language