25.12 - Gotchas
Do conventional current arrows reflect the flow of negative or positive charge?Positive charge. Although in a typical current found in household
appliances and such, it is negatively charged electrons that flow, the convention is that the arrow indicates the direction of positive charge flow.
If electrons flow to the left, the conventional current arrow points to the right.
Current decreases when passing through a resistor and then increases again upon exiting. No. The current before, in and after the resistor is
the same.
A wire with a current has a net electrostatic charge. No. Although there is a net charge flowing by any given point, the overall wire is neutral.
A person walks by. The person contains electrons. Therefore, there is a current. No, the person is electrically neutral. Current is the flow of net
charge. In this case, there is no movement of net charge and therefore no current. If the person were electrically charged, there would be a
current as the person passed by.
25.13 - Summary
Electric current is the rate at which charge flows through a conductor. The symbol
for current is I and it is measured in amperes (A), where 1 A = 1 C/s.
Current usually consists of moving electrons, which have negative charge.
However, current is almost always represented in descriptions and diagrams as
conventional current, which is a flow of positive charges that would constitute the
same current, so the direction of conventional current is opposite to the actual
movement of electrons. Even though drawings often show the direction of current
with an arrow, current is a scalar.
Individual electrons in a wire travel much more slowly than the electric field that
propels them. Collisions with the atoms that make up the wire prevent the electrons’
continued acceleration, and cause them to follow a meandering zigzag path. The
drift speed is the average net speed of electrons along the wire. In household
wiring, it is about the speed of a snail.
Ohm’s law states the relationship between potential difference, current and
resistance. A resistor is an electrical component that can be used to regulate
current. A particular resistor is characterized by its resistance. Resistance is
measured in ohms (). 1 = 1 V/A.
Resistivity is a measure of how much the material resists current. For a simple wire made of a single material, the resistance is the resistivity of
the material times the wire’s length, divided by its cross-sectional area.
The temperature of a material can affect its resistivity.
Energy is dissipated, often as heat, when an electric current passes through a resistance. Electric power measures the amount of energy
consumed per unit time. It is equal to the current through a circuit element times the potential difference across it. Sometimes, as when a
battery is being charged, the energy consumed is stored rather than dissipated. For resistors that obey Ohm’s Law, the two equations shown in
the last line on the right may also be used to calculate power.
Definition of currentOhm’s lawǻV = IR
Electric power equations