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Today we are going to look at:
Current: Measured in “Amperes” or “Amps” (A)
“The rate of flow of electrons through a circuit”
Voltage: Measured in “Volts” (V)
“Used to drive the electrons through the components”
Resistance: Measured in “Ohm’s” ( )
“A measure of how much voltage is required to let a
current flow”
Georg Simon Ohm (1787-1854)
Ohm formulated the relationship between
voltage, current & resistance as follows.
“The current in a circuit is directly proportional to the applied
voltage and inversely proportional to the resistance of the
circuit”
“Voltage (V) is equal to the current (I) multiplied by the
resistance (R)
If we use a triangle to help us, then we can arrange the formula to
make it easier to use.
V
I
R
This gives us the formula’s
V=IxR
I=
V
R
R=
V
I
Example 1: Voltage
Q: The resistance of a circuit is 12 and the current is 2A.
What is the voltage?
Step 1: Draw Triangle
Step 2: Select correct equation
V=IxR
V
I
R
Step 3: Plug in the numbers
Step 4: Calculate
V = ? I = 2A & R = 12
Step 5: Solution
V = 24V
Don’t Forget the units!
V = 2 x 12
Now you try one!
Q: The resistance of a circuit is 15 and the current is 3A.
What is the voltage?
Example 2: Current
Q: The voltage supply is 25V and the resistance is 5 . What is
the current
Step 1: Draw Triangle
I=
V
I
Step 2: Select correct equation
R
V
R
Step 3: Plug in the numbers
V = 25V
I=?
Step 4: Calculate
R = 5
Step 5: Solution
I = 5A
Don’t Forget the units!
I = 25  5
Now you try one!
Q: The voltage supply is 40V and the resistance is 10. What
is the current?
Example 3: Resistance
Q: The voltage supply is 300V and the current is 2A. What is
the resistance?
Step 1: Draw Triangle
R=
V
I
Step 2: Select correct equation
R
V
I
Step 3: Plug in the numbers
V = 300V
I = 2A
Step 4: Calculate
R=?
Step 5: Solution
R = 150 
Don’t Forget the units!
R = 300  2
Now you try one!
Q: The voltage supply is 500V and the current is 50A. What
is the resistance?