Current is measured in units calledampsoramperesorA. It is represented by the letterIin equations.

At 0V between two points, no electrons will move between those points. No current will flow if those points are connected by a wire. It's like putting a pipe between two ends of a flat lake: no water flows through the pipe.

A small voltage (like 1.5V from an AA battery) is like a fairly flat river. If you put a pipe (wire) between two points along the river, water will flow. But for a given size pipe, the amount of water per second won't be very big.

A high voltage (say, 120V) is like a waterfall along that river. If you put that same pipe between the top and the bottom of that waterfall, water will flow like crazy.

But voltage is not the same as capacity. Say you have a small stream flowing parallel to the river. If you put a small pipe from one point along each waterway to a set distance downstream, you will get the same amount of water flow in each of the two pipes. But if you replace the small water pipe with a big water pipe, the stream won't be able to provide enough water to fill the pipe, and you'll get more water in the river-pipe than the stream-pipe. The stream and the river result in the same drop in water pressure from one point to another (like a voltage), but can provide a different maximum amount of water (just like lower capacity batteries go dead earlier than high capacity batteries).

A battery is rated both with a voltage (like 1.5V) and a capacity (like 1600mAh). A dead battery has run out of charge - it has exhausted its store of electrons. It's like an empty water tower.

Voltage is measured in units calledvoltsorV. It is represented by the letterVin equations.

Resistance is measured in units calledOhmsor . It is represented by the letterRin equations.

Ok, what does this mean? Equations have meanings, you know. Let's say we get high voltage battery. That means

black | brown | red | orange | yellow | green | blue | violet | grey | white | |||||||

0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |

So for example blue-gray-yellow stripes correspond to 6(blue), 8(gray), and 4 zeros (yellow), which is 680000: . That's a lot of zeros to count, so people usually add prefixes to the units to remove zeros in sets of three. The most common prefixes are

The fourth stripe that's a little further from the others indicates the
tolerance, or how precisely the resistor's value actually matches what
the color code indicates. So if you have a 100 resistor with a
5% tolerance, that resistor could actually be anywhere between
95 to 105. What the resistor companies actually do is
they fabricate lots of resistors, then measure them and sort them by
tolerance. So in practice, if a resistor has a 5% band on it, its
value will be between 95 and 97.99999 or between 102.00001 and
105. All the resistors between 98 and 102 will have
been labelled as 2% resistors by the manufacturer and sold at a higher
price. The colors for tolerance are:

brown | red | gold | silver | none |

1% | 2% | 5% | 10% | 20% |

Batteries also differ in how
many electrons they store - their capacity - measured in
**m**illi**a**mp-**h**ours
(mAh). Voltage tells you how strongly the battery pressures the
electrons to flow. Capacity tells you how many of those electrons it
holds. The capacity of our batteries is about 1650mAh. This means that
if we were to use the battery to power one LED, drawing 20mA of current,
the battery would last over 80 hours.

We will be using two batteries. There are a couple of ways to connect batteries shown in Fig. 2.2.

Connecting batteries in
We will connect our batteries in **series**. The two 1.5V batteries
will give us 3V.

The LED component we're using actually contains two LEDs. The red LED faces one way and the green LED faces the other way. So depending on how you connect it in the circuit, one or the other will glow but not both.

Let's do an example calculation.

What do we plug in for **V**, the voltage? The two batteries are 1.5V each.
We've connected them in series. It's like stacking one water tower on
top of another: the result is twice as high. So our two 1.5V
batteries are like one 3.0V battery. So use .

What do we plug in for **R**, the resistance? You have three resistors in your kit,
each with a different resistance. The colored stripes on the resistor
indicate the value of the resistor. One of those resistors
is 680. So use . Note that the LED has some
resistance of its own. But the resistance of the LED is so low that
it gets dwarfed by the resistor, so we can ignore the LEDs resistance.

What do we plug in for **I**, the current? Well, that's what we calculate!
Ohm's Law will tell us how much current (I) to expect given the voltage
(V) provided by the battery, and the resistance (R) from the resistor
that we choose. It can help us pick the correct resistor. So let's do
some math:

That's 4.4mA, which we read out loud as 4.4 milli-amps.

What do you think will happen if we put in a smaller resistor? Look back at: . If we make R smaller, that's makes the whole fraction bigger. Look at an example:

So a smaller resistor (R) gives us a bigger current (I). Big current means more light. Try it! Put in a smaller value resistor!

ania alumni.rice.edu

http://ofb.net/~ania

2006-03-08