Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 10)

Ohm’s Law: You just observed that when you have less
resistance in the circuit, more current flows (making the
LED brighter). The relationship between voltage, current,
and resistance is known as Ohm’s Law (after George
Ohm who discovered it in 1828):
Current =
Resistance: Just what is Resistance? Take your hands
and rub them together very fast
. Your hands should feel
warm. The friction between your hands converts your
effort into heat. Resistance is the electrical friction
between an electric current and the material it is flowing
through; it is the loss of energy from electrons as they
move between atoms of the material. Resistors are
made using carbon and can be constructed with different
resistive values, such as the seven parts included in your
Electronic Playground. If a large amount of current is
passed through a resistor then it will become warm due
to the electrical friction. Light bulbs use a small piece of
a highly resistive material called tungsten. Enough
current is passed through this tungsten to heat it until it
glows white hot, producing light. Metal wires have some
electrical resistance, but it is very low (less than 1
Ω per
foot) and can be ignored in almost all circuits. Materials
such as metals which have low resistance are called
conductors. Materials such as paper, plastic, and air
have extremely high values of resistance and are called
insulators.
Resistor Color Code: You may have seen the colored
bands on the resistors and may be wondering what they
mean. They are the method for marking the value of
resistance on the part. The first ring represents the first
digit of the resistor’s value. The second ring represents
the second digit of the resistor’s value. The third ring tells
you the power of ten to multiply by
, (or the number of
zeros to add). The final and fourth ring represents the
construction tolerance. Most resistors have a gold band
for a 5% tolerance. This means the value of the resistor
is guaranteed to be within 5% of the value marked. The
colors below are used to represent the numbers 0
through 9.
Use the color code to check the values of the seven
resistors included in your Electronic Playground. (The
values are marked next to them on the box). They are all
5% tolerance.
The Variable Resistor: We talked about how a switch is
used to turn the electricity on and off just like a valve is
used to turn the water on and off. But there are many
times when you want some water but don’t need all that
the pipe can deliver, so you control the water by adjusting
an opening in the pipe with a faucet. Unfortunately, you
can’t adjust the thickness of an already thin wire. But you
could also control the water flow by forcing the water
through an adjustable length of rocks, as in the rock arm
shown below.
In electronics we use a variable resistor. This is a normal
resistor (50K
Ω in your Playground) with an additional arm
contact that can move along the resistive material and
tap off the desired resistance.
There is a scale printed next to the dial on the variable
resistor which shows the percentage of the total
resistance that is between springs 49 and 50. The
remaining resistance will be between springs 48 and 49.
The resistance between springs 48 and 50 will always be
50K
Ω, the total resistance.
Now let’s demonstrate how this works.
MORE ABOUT RESISTORS
-10-
Example of Color Code
Orange Red
Violet Gold
27 X 10 3 = 27,000 Ω,
with 5% Tolerance
COLOR VALUE
Black 0
Brown 1
Red
2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9
Rock Arm
Variable Resistor
Insulating Base Material
Wiper Contact
Thin Layer of
Resistive
Material
Stationary
Contact
Movable
Arm
Leads
Variable
Resistor
Symbol for Variable
Resistor
Voltage
Resistance