Charge & Current
REVIEW Electrons and protons have the same magnitude (amount) of charge, although the electron is negative and the proton positive. All electrons are identical to each other, and all protons are likewise identical to each other.
The charge on a single electron is very small --- too small to be convenient in electronics. For example, there are typically about 10,000,000,000,000,000 electrons per second flowing in the wires of a small, low-power circuit like a wristwatch. So, we use a larger unit to measure charge, called the coulomb.
1 coulomb = 6.24x1018 electrons
1 electron = 1.60x10-19 coulombs
Activities & Practice
to do as you read
Usually, in electronics work we aren't dealing with stationary (static) charges, but rather moving charge: electrical current. We'll use the variable I to stand for current. Current is defined as the rate of charge movement. Almost always, the moving charges are electrons (NOT protons or other charged particles).
The unit we use to measure current is the ampere, usually pronounced in shortened form as "amp".
1 amp = 1 coulomb per second
or, said even more briefly...
1 A = 1 coul/sec
So, saying that 1 amp is flowing through a wire is equivalent to saying 6.24x1018 electrons are flowing down the wire every second.
EXAMPLE An iPod Nano, playing audio only, uses about 25 mA of current. (a) What is that current, in amps? (b) How many electrons is that, per second?
(a) 1 milliamp is one-thousandth of an amp, or in other words, 1 A = 1000 mA.
(b)
1. Most electrical or electronic devices have a label on them somewhere, stating what voltage and current they require (or maximum current). The label on my laptop computer says the current it uses is 4.5A. (a) How many mA (milliamps) is that? (b) How many electrons per second is that?
2. The wireless WiFi network router in my house uses 500 mA of current. (a) How many amps is that? (b) How many electrons per second is that?
Current can be produced in a variety of ways, but the most common are batteries and generators.
Batteries use chemical reactions, and produce current that flows in a steady direction. This is called direct current, or DC.
Generators naturally produce current that constantly switches direction, back and forth. This is called alternating current, or AC. Because the electricity in your wall outlets comes from a generator at a power plant, it is AC. In the U.S., standard outlet current alternates back-and-forth 60 times per second, or 60 Hz. The voltage is 120 volts.
Because so many electrical devices need DC, many gadgets require the use of a power supply (often called a wall adapter or brick) that plugs into the wall outlet and converts the AC into DC. They usually also change the voltage.
3. Find the electrical specifications labels on 5 appliances or gizmos around your house. For each device, tell me what it is, what current it uses, what voltage it needs, and if it's AC or DC. Also give me the power ("wattage") it uses, if that is printed. Make a neat, logically-organized table.
NOTE: AC will be indicated on the label by "AC", "VAC" (volts, alternating current) or "50 Hz" or "60 Hz". There might also be a symbol that looks like a graph of a sine curve.
Do you remember above, when I said that in electrical circuits it's the electrons that are moving, not positive charges? Even though this is true, we usually pretend that there are positive charges that are moving in the opposite direction.
For example, in a DC circuit with a battery, electrons are flowing out of the negative terminal of the battery, traveling towards the positive terminal. We will pretend, however, that there are moving charges flowing through the wire from the positive toward the negative terminal of the battery. This is called conventional current.
4. On my camera's rechargable battery is printed the following: 7.4V 570 mAh.
(a) The "mAh" stands for milliAmp • hours, that is, milliAmp times hours. Based on the definition of the amp, what is the Amp • hour a unit of? What is it equal to? What, therefore, is a mAh equal to?
(b) What is 570 mAh equal to?
(c) How much total energy can this battery store?