Charge & Current
A QUICK REVIEW OF OLD STUFF 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
AND NOW, SOME NEW STUFF 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.
Some examples of currents:
- A typical toaster uses about 7 amps.
- A given circuit in your house, all controlled by a single circuit breaker, can carry a maximum of 15 or 20 amps, depending on the circuit breaker. (Circuits breakers are safety devices designed to automatically shut off the current if too much flows. Too high a current can cause the wiring to catch fire.)
- The DVD player connected to my TV at home uses 140 milliamps (0.14 A).
- Car batteries (for an old-fashioned, internal-combustion non-hybrid, non-electric car) typically max out at between 500 and 600 amps. The battery of a car is used to power the small-yet-mighty electric starter motor that turns the engine in order to start it.
- An iPod Nano uses about 25 mA of current.
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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?
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1. If 37 coulombs of charge flows down a wire every 7.5 seconds, what is the current? SOLUTION
2. A typical lightning strike is about 40 coulombs of charge, typically consisting of four separate "strokes". (That's why lightning usually looks flickery.) Each stroke lasts about 30 microseconds. What is the current? SOLUTION
Here's way-cool video of lightning strikes filmed in super slow-motion.
3. 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? SOLUTION
4. 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? (c) How much charge flows through the router every minute? SOLUTION
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. (Why that is the case will be discussed when we talk about how generators work.) 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 Hertz (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.
5. 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? For example, in a battery-powered circuit, electrons flow away from the negative terminal of the battery, towards the positive terminal. However, during most of the 1800s no one knew that. During that century people invented the battery (Alessandro Volta in 1800), the generator (Michael Faraday, in the 1830's), the electric motor (by Anyos Jedlik in 1827), the lightbulb (by James Lindsay in 1835, and made practical in the 1880's by Thomas Edison), and cities were being wired for electricity; all this, before the electron itself was discovered, which was in 1899. In 1879, E.A. Hall had shown that the moving charges in circuits were negative, because of the effect of a magnetic field on the current, now called the Hall Effect. However, during all this time physicists and engineers had made the assumption that what was moving in the wires was positive charge. That tradition, or convention, was firmly established by the time it was discovered to be otherwise, and since it doesn't matter (after all, you can't see the moving charges) to this day we still speak of the charges as bing positive, moving away from the positive end of the battery towards the negative end. This is called conventional current.
6. 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?