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Alternating
Current:
An electric current that reverses its direction periodically.
Circuit:
A complete or partial path over which current may flow.
Resistance:
Property of a conductor by which it opposes the flow of an electric
current, resulting in the generation of heat in the conducting
material.
Voltage:
Electromotive force, or a difference in electrical potential,
expressed in volts.
Current:
The flow or rate of electric charge in a conductor or medium between
two points having a difference in potential, generally expressed
in amperes.
Current
Flow and Ohm's Law
Ohm's law is the most important, basic law of electricity. It
defines the relationship between the three fundamental electrical
quantities: current,
voltage,
and resistance.
When a voltage is applied to a circuit
containing only resistive elements (i.e. no coils), current flows
according to Ohm's Law, which is shown below.
I = V / R 
| Where: |
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I =
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Electrical Current (Amperes) |
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V =
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Voltage (Voltage) |
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R =
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Resistance (Ohms) |
Ohm's law states that the electrical current (I)
flowing in an circuit is proportional to the voltage (V)
and inversely proportional to the resistance
(R). Therefore, if the voltage is increased, the current
will increase provided the resistance of the circuit does not
change. Similarly, increasing the resistance of the circuit will
lower the current flow if the voltage is not changed. The formula
can be reorganized so that the relationship can easily be seen
for all of the three variables.
The Java applet below allows the user to vary each of these three
parameters in Ohm's Law and see the effect on the other two parameters.
Values may be input into the dialog boxes, or the resistance and
voltage may also be varied by moving the arrows in the applet.
Current and voltage are shown as they would be displayed on an
oscilloscope with the X-axis being time and the Y-axis being the
amplitude of the current or voltage. Ohm's Law is valid for both
direct current (DC) and alternating
current (AC). Note that in AC circuits consisting of
purely resistive elements, the current and voltage are always
in phase with each other.
Exercise: Use the interactive applet below to investigate
the relationship of the variables in Ohm's law. Vary the voltage
in the circuit by clicking and dragging the head of the arrow,
which is marked with the V. The resistance in the circuit can
be increased by dragging the arrow head under the variable resister,
which is marked R. Please note that the vertical scale of the
oscilloscope screen automatically adjusts to reflect the value
of the current.
See what happens to the voltage and current as the resistance
in the circuit is increased. What happens if there is not enough
resistance in a circuit? If the resistance is increased, what
must happen in order to maintain the same level of current flow?
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