Ohm’s Law Simplified
Probably one of the most complicated subjects for New Automotive students to understand is Ohm’s Law. There are only three factors to deal with in Ohm’s Law but the usage of them can be complicated. I used to teach Ohm’s Law to my students and about half of them got it right off the bat but most were usually confused after an hour or so. I hope to try and explain this in a simple and understandable way so the confusion will be gone.
Ohm’s Law Broken Down
Parts of George Simon Ohm’s Law
1. Voltage – Sign = E or V - Electromotive Force
a. Electrical Pressure or a Lack of electrons
2. Amperage – Sign = I or Intensity
a. Electron Flow
3. Resistance – Sign = R or Resistance
a. Opposition to Electron flow
4. The formula is simply:
V = I x R I = V / R R = V / I | V = Voltage (V) I = Current (A) R = Resistance (ohms) |
This can be applied to any simple circuit.
The Pie is representative of the Formula
Example # 1:
Volts = ? (Answer = 12 volts)
Amps = 2
Resistance = 6
Or 2 amps times 6 ohms = 12 volts
Example # 2:
Resistance = ohms ? (Answer = 6)
Volts = 12
Amps = 2
Or 12 Volts divided by 2 Amps equals 6 ohms
Example # 3:
Amps = ? (Answer = 2 amps)
Volts = 12
Resistance = 6 ohms
Or 12 Volts divided by 6 ohms = 2 Amps
Ok, Now you may ask “Why do I need to know this?”. Well the answer is that now today’s modern cars are relying on Amperage to make sure all components that are being monitored by the Computer are with in proper parmeters. So if resistance is present in one form or another, amperage will suffer as a result of bad components. It doesn’t matter what the voltage is because there are different components that operate using a variety of voltages. Amperage however is present at all times and regardless of the voltage it must remain constant or in some cases variable with in correct parameters.
Series Electrical Circuit Characteristics
In a series circuit amperage remains constant throughout the circuit.
Voltage is divided among the components in the circuit.
The sum of all voltages should equal source voltage.
How to compute the math involved in a series circuit. Let’s assume that the source voltage equals 12 volts. Resistor # 1 (R1) has a value of 6 ohms. Resistor # 2 (R2) has a value of 6 ohms. To figure the Total resistance in a series circuit because there is only one path for flow from point A (+) to point B(-), you need to add both resistance values together or a total of 12 ohms. So now we have two factors: 1. Voltage equals 12 volts, 2. Resistance total equals 12 ohms. The only factor left to compute is the Total amount of amperage running through the circuit. Utilizing Ohms Law to calculate amperage, we need to divide the 12 volts by the 12 ohms and we now have a total of 1 amp running through the circuit. The only thing left to calculate is how much voltage is being supplied to each resistor. To calculate this we need to multiply Each resistor by the amount of amperage in the circuit. (Remember that amperage remains constant throughout the entire circuit). Ok, so lets multply the resistance of resistor # 1 (R1) Times the amperage of 1 amp and R1 is getting 6 volts. ( 6 ohms times 1 amp = 6 volts). Multiply Resistor # 2 (R2) of 6 ohms times the amperage of 1 amp and R2 is also getting 6 volts. Remember that the sum of all voltages in the circuit must equal source voltage so we add R1 at 6 volts to R2 at 6 volts and we get a total of 12 volts which equals source voltage. It’s that simple???
So to review:
Source voltage was 12 volts
Resistor # 1 (R1) is 6 ohms
Resistor # 2 (R2) is 6 ohms
Our math looks like this:
Total resistance we add together or R1- 6 ohms +R2- 6 ohms = a total of 12 ohms
To calculate amperage -Ohms Law says Divide Voltage by resistance
Total Amperage is 12 volts divided by 12 ohms and we get Total of 1 amp
To calculate voltage Ohms Law says – Multiply Amperage times Resistance
Resistor # 1 (R1) voltage = 6 ohms times 1 amp and we get 6 volts at that resistor
Resistor # 2 (R2) Voltage = 6 ohms times 1 amp and we get 6 volts at that resistor
To calculate Source Voltage:
Add Resistor # 1 and Resistor # 2 voltages together and we get a source voltage of 12 volts .
Ta Da All done? Any questions just go back and read the above!
By using what some call the “Back Door Method” seeing as how we already Have 2 of the three factors (Volts and Resistance) , all we need to do now is calculate the Amperage for the circuit. We will do this by Dividing the Voltage by Resistor # 1 or 12 volts divided by 6 ohms, for an amperage output of 2 amps for the first leg (R1) of our schematic. Now we must divide our source voltage or 12 volts by Resistor # 2 which is 3 ohms for an amperage output of 4 amps for the second leg (R2) of our schematic. Once this is done we add our 2 amps from R1 and our 4 amps from R2 for a Total of 6 amps for the entire circuit.
Ohms Law Parallel Circuits
Parallel Circuit Characteristics.
1. Always have more than one path for flow.
2. Amperage varies throughout the circuit
3. Voltage remains constant throughout the circuit
4. The sum of all amperage should equal source Amperage.
5. If one path is interrupted, others will still function
6. Total resistance must be smaller than the smallest resistor in the circuit.
Review: Ohms Law Principles
1. Volts = Amperage times Resistance
2. Amperage = Volts divided by Resistance
3. Resistance = Volts divided by Amperage
Here we see the schematic for a Parallel circuit. Our source voltage is 12 volts. Let’s assume Resistor # 1 (R1) = 6 Ohms and Resistor # 2 (R2) = 3 Ohms.
We need to calculate the total resistance for the circuit so what we need to do next is to take the total amps for the circuit and Divide our source voltage of 12 volts by our Total amperage for the circuit of 6 amps for a total resistance of 2 ohms for the circuit. The total resistance for the circuit must be smaller than the smallest resistor in the circuit. As you can see our smallest resistor is 3 Ohms so our total resistance is smaller than the smallest resistor. This proves we did it correctly. Something to remember is as resistance goes down, Amperage goes up!
A lot of Instructors teach the 1 over R + the 1 over R Method. So if you want to see the calculations on that then e-mail me and I will post it!