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Ohms law
Voltage, Current and Resistance
 Ohms law connects the voltage across a device
and the current flowing through it to its
resistance.
 This presentation will teach you how to apply
ohms law to some real situations. But before we
start looking at formulas, let's remind ourselves
about what voltage, current and resistance are.
 Let's consider a circuit as a tank of
water with an out flow pipe.
 The amount of water in the tank,
pushing down on the water below it, is
the potential difference or voltage. The
width of the pipe leaving the tank is the
resistance (the smaller the pipe the
bigger the resistance). The amount of
water that flows out of the tank each
second is the current.
 With this comparison, the rate at which
the water flows out of the tank is given
by the potential difference of the water
in the tank and the width the outlet
pipe.
 Ohms law states the equivalent observation for an
electronic circuit.
 Ohms law tells us that the flow of current (I) in a
circuit is given by the voltage (V) across the circuit
divided by the resistance (R) of the circuit.
 Or in short
 This equation can be rearranged to give three
formulas:
 Don't worry if your not good at maths, just
remember the ohms law triangle. With this cover
the letter you need and the position of the
remaining two letters gives you the equation.
 So in the example below we have covered I to
leave V/R
 Let's look at selecting a current limit resistor for
an LED.
 Like diodes LEDs drop some voltage across them,
this is typically 1.8 volts for a standard LED. If we
are running off a 5 volt supply then the voltage
across the resistor Vr = 5 - 1.8 = 3.2 V
 LEDs normally need about 10 mA (0.01 Amps) to
operate, so we can work out the resistance using R
=V/I
 So R = 3.2 / 0.01 = 320 ohms.
 Suppose we have a battery operated circuit that
uses a push button switch as shown in the circuit
diagram. When the switch is pressed current will
be flowing through the resistor.
 If a lot of current flows through the resistor the
batteries won't last long. If we are using a 9 volt
battery and the pull up resistor is 1K we can
calculate the current usage while the switch is
pressed.
 I = V / R = 9 / 1,000
So I = 0.009 Amps = 9 mA