2011 April 8
by roma

This expands on the concepts I described in the April 7 Confusion post.

-Voltage is the energy per unit charge between 2 points. Energy is measured in joules, charge is measured in coulombs. Incidentally, this is very similar to the definition of an electric field: the force per unit charge. However, the between 2 points part of the voltage definition is very important.
-In the term electromagnetism, the term electro actually refers to voltage.

Using the latter definition, I can go back to problem P4, which I described earlier. Once again, two loads are wired in series with a battery. The question is:

After the current goes through the battery, it then flows through Element #1. Element #1 has 9 volts across it. How much energy is transferred to Element #1 as the charge flows through it?

With this definition, 9 volts across it simply means that there are 9 joules / coulomb between the two ends of element 1. We know that 2 coulombs of charge move between the battery ends. Therefore, 18 joules of energy are transferred to the battery.

I can also go back to the problem in chapter 3 of Electricity Demistyfied.

4. Suppose that the timer above, which has no resistance, is replaced by another timer that has resistance equal to that of the lamp. If nothing else in the circuit changes, how will the voltage across the lamp (not the voltmeter reading) be different when the switch is closed, as compared with the voltage across the lamp in the original circuit when the switch is closed?
a. It will be the same.
b. It will be half as great.
c. It will be 1/4 as great.
d. More information is needed to answer this question

The resistance in that segment of the circuit has doubled. Half of the energy previously available to the lamp is now eaten up by the timer. So only half the energy is available for the lamp, so the voltage across the lamp is only half as great. The key to understanding this is to keep the definition of voltage in mind.

Information courtesy of William J. Beaty, http://amasci.com/ele-edu.html.

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