Electricity Basics
Electricity is, in simplest terms, a flow of electrons due to a change in charge
of atoms. Some characteristics of electricity are how much pressure it exerts
(Voltage), how fast it is moving (Current), and whether there is anything
slowing it down (Resistance).
Voltage is the difference in charge between two points.
Current is the rate at which charge is flowing.
Resistance is a material's tendency to resist the flow of charge (current)
When describing voltage, current, and resistance,
a common analogy is a water tank. In this
analogy, charge is represented by the water
amount, voltage is represented by the water
pressure, and current is represented by the water
flow. So for this analogy, remember:
Water = Charge
Pressure = Voltage
Flow = Current
Consider a water tank at a certain height above
the ground. At the bottom of this tank there is a
hose.
The pressure at the end of the hose can represent voltage. The water in
the tank represents charge. The more water in the tank, the higher the
charge, the more pressure is measured at the end of the hose.
We can think of this tank as a battery, a place where we store a certain
amount of energy and then release it. If we drain our tank a certain
amount, the pressure created at the end of the hose goes down. We can
think of this as decreasing voltage, like when a flashlight gets dimmer as
the batteries run down. There is also a decrease in the amount of water
that will flow through the hose. Less pressure means less water is flowing,
which brings us to current.
We can think of the amount of water flowing
through the hose from the tank as current. The
higher the pressure, the higher the flow, and
vice-versa. With water, we would measure the
volume of the water flowing through the hose
over a certain period of time. With electricity, we
measure the amount of charge flowing through
the circuit over a period of time. Current is
measured in Amperes (usually just referred to as
"Amps"). Amps are represented in equations by
the letter "I".
Let's say now that we have two tanks, each with
a hose coming from the bottom. Each tank has
the exact same amount of water, but the hose on
one tank is narrower than the hose on the other.
We measure the same amount of
pressure at the end of either hose, but
when the water begins to flow, the flow
rate of the water in the tank with the
narrower hose will be less than the flow
rate of the water in the tank with the
wider hose. In electrical terms, the
current through the narrower hose is
less than the current through the wider
hose. If we want the flow to be the same
through both hoses, we have to increase
the amount of water (charge) in the tank
with the narrower hose.
This increases the pressure (voltage) at the end of
the narrower hose, pushing more water through the
tank. This is just like an increase in voltage that
causes an increase in current.
Now we're starting to see the relationship between
voltage and current. But there is a third factor to be
considered here: the width of the hose. In this
analogy, the width of the hose is the resistance. This
means we need to add another term to our model:
Water = Charge (measured in Coulombs)
Pressure = Voltage (measured in Volts)
Flow = Current (measured in Amperes, or "Amps"
for short)
Hose Width = Resistance (measured in Ohms)
We can't fit as much volume through a
narrow pipe than a wider one at the same
pressure. This is resistance. The narrow
pipe "resists" the flow of water through it
even though the water is at the same
pressure as the tank with the wider pipe.
Ohm’s Law
In 1827, Georg Ohm, a German physicist, discovered that voltage, current,
and resistance are all related. Ohm’s Law is best represented by the equation
V=IxR
where V is voltage (measured in Volts), I is current (measured in Amperes or
Amps), and R is resistance (measured in Ohms). If two of the values are
known, we can calculate the third missing value.
V
V
This is a digital multimeter. It
measures several things:
• Voltage (V)
• Current (amperes)
• Resistance (Ω)
• Continuity (check for circuit
completeness)
Your multimeter has several parts:
• Dial
• Probes
• Ports
• LCD display
To use your multimeter, plug the
probes into the ports at the
bottom.
• The RED probe should go in the
RIGHT PORT.
• The BLACK probe should go into
the CENTER port.
Measuring Voltage
Let’s test some batteries to see
how much charge they have left.
1. Turn the dial to the left so that
the pointer is pointed at 20V
2. Touch the RED probe to the
POSITIVE (+) side of the battery.
3. Touch the BLACK probe to the
NEGATIVE (-) side of the battery.
4. Read the numbers on the LCD
display. You should get a number
that is close to 9V.
5. Repeat the process with the other
batteries in your box.
Build this circuit using the materials in your box. Test the voltage at the
points indicated. Is it the same everywhere? Explain your results.
A
C
B
Measuring Resistance
1. Turn the dial to the 2 kΩ
setting.
2. Take the resistor out of the
circuit you previously built.
3. Straighten out the legs of
the resistor.
4. Place a probe on either end
of the resistor and then
read the numbers on the
LCD display.
Note that when you turned the dial to 2 kΩ, your multimeter read 0.465
That means this resistor has a value of about 465 Ω.
When you use a multimeter to test resistance, the meter will read one of
three things, 0.00, 1, or the actual resistor value.
If the multimeter reads 1, it's overloaded. You will need to try a higher
range such as 20 kΩ mode or 200 kΩ. There is no harm if this happens, it
simply means the range knob needs to be adjusted.
If the multimeter reads 0.00 or nearly zero, then you need to lower the
range to 2 kΩ or 200 Ω.
Measuring Current
To measure current we have to physically interrupt the flow of current and
put the meter in the circuit. To do this, put the copper wires into the circuit
as shown in the diagram below.
Place one probe on each of the bare
copper wires. Our multimeter is acting
as a piece of wire -- we've now
completed the circuit, and the LED will
light up. All multimeters take readings
over time and then give the average, so
expect the reading to change.
The color of the probes does not matter.
If the current reading is negative, either
ignore the negative sign, or switch the
probes.
Measuring Continuity
Continuity is essentially checking to
see if a circuit is closed. If it is not
closed, it may be broken, so
continuity can help us determine if
the circuit is bad or if a component
is bad.
Turn the multimeter dial to the
diode symbol.
To test continuity, first we need to make sure that the meter will sound a
tone when a circuit is complete. To do this, simply touch the two probes
together. You should hear a faint tone being emitted from the multimeter.
Take one of the LEDs from your box. Place the RED probe on the LONG leg
(anode) of the LED, and the BLACK probe on the SHORT leg (cathode) of
the LED. If the LED is not burned out, it should light up.