The motion of these electrons is like that of a ball in play in a pinball machine. The ball
moves fairly randomly as it rebounds from bumpers and other obstacles. However, the
surface of a pinball table slants downward, so despite the erratic motion caused by the
bumpers, a component of the gravitational force pulls the ball down toward you over
time. (In short, the maker of the pinball game uses gravity to ensure that the ball “drifts”
downward and “drains”.)
There is no simple relationship between drift speed and the amount of current in various
conductors. A great number of charge carriers moving slowly can result in a larger
electric current than a smaller number of charge carriers flowing quickly.
There is a final point to be made about the erratic motion of electrons in the current.
Energy transfers from the electrons to the atoms during the collisions. The collisions
increase the random (thermal) motion of the atoms. This explains why the temperature
of the filaments in the car headlights increases when a current flows through them:
Energy is being transferred from the electrons in the current to the atoms that make up
the filaments. The filaments become hot and emit light. The other parts of the wiring
increase in temperature as well, but the system is designed to maximize the
temperature increase in the filaments and minimize it elsewhere.
If the switch is 1.0 meter away
from the headlights, and charge
carriers move at the speed
shown above, how long will it
take for the headlights to light
once the switch is thrown?
Nearly instantaneous
·Field acts along entire wire very quickly
25.3 - Ohm’s law and resistance
Resistance: The ratio of the
potential difference across a
conductor to the current
through it.
Resistor: An electrical
component often used to
control the amount of current
flow.
Resistance is defined as the potential difference across two points on a conductor
divided by the current flowing through the conductor. At the right, we use a common
electrical component called a resistor to illustrate this concept. There is a potential
difference across the resistor and current flowing through it. Divide the potential
difference by the current and you have calculated the resistance of this resistor.
The resistance of a resistor such as those shown above or to the right is constant. The
resistor is made of ohmic materials, which are empirically known to obey Ohm’s law.
Increase the potential difference and the current increases proportionally. The
resistance does not change. The linear relationship between potential difference and
current is shown as the first equation in Equation 1.
Since resistance equals potential difference divided by current, its unit, the ohm, is volts
per ampere. Resistance is represented by , the Greek letter omega.
Resistors are not the only components that resist the flow of current. The filament in a
light bulb or the coils of an electric hot plate both function as resistors. The term
“resistor” broadly refers to any element that is a source of resistance.
Again, water is a good analogy. The potential difference driving current in a wire
resembles the pressure exerted on water in a pipe. Increase the pressure, and the
water flow increases. Different pipes have different amounts of resistance. For instance,
one with a rough interior wall would have greater resistance to water flow than one with
a smooth wall.
Materials that do not obey Ohm’s law are called non-ohmic. Many components used in
modern circuitry are made of non-ohmic materials. For instance, a diode has little
resistance to current flow in one direction, and great resistance to current flow in the
other.
Georg Ohm published his major work, including what we now know as Ohm’s law, in
- His theories were greeted with skepticism and his career was slow to progress.
Why it took so long for his work to be appreciated is hard to say. Perhaps it is because
the law is empirical as opposed to a fundamental law of nature. It is fair to note that many major leaps forward in physics were met with
Resistors wired into a computer circuit board. Each resistor
has a color code that indicates its resistance in ohms.Resistor
Component used to regulate current
Resistance
Potential difference divided by current