# Trying to see if I understand Ohms law

• open
In summary, if you want to run a power mosfet at 17 amps from a 12 volt battery, you would need to connect it with a 2.2 ohm resistor in series. Anything else in series (such as another power mosfet) would create an unsafe current draw.
open
Hello
I'm trying to get my head around ohms law, and have a question.
If I have a motor that draws 12volt 100amp, and I would like to drop the amps down to 30, would I put a 360ohm resister in line.
12 * 30 = 360
V * I = R

Side note can you get a resister that is 480 watt?

Thank you

P.S I've got some nicchrome wire that I could cut to that ohm, but it would glow red hot. Would that happen to all types of reisiters that could do these.

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open said:
Hello
I'm trying to get my head around ohms law, and have a question.
If I have a motor that draws 12volt 100amp, and I would like to drop the amps down to 30, would I put a 360ohm resister in line.
12 * 30 = 360
V * I = R

Side note can you get a resister that is 480 watt?

Thank you

No.

The motor has a resistance of 12 / 100 or 0.12 ohms. Assume this is constant.
If you wanted to put 30 amps through this you would have a voltage across the motor of
30 amps * 0.12 ohms or 3.6 volts.

This would leave 12 - 3.6 or 8.4 volts across a series resistor which would have 30 amps flowing in it so it would be 8.4 volts / 30 amps or 0.28 ohms.
The power of the resistor would be 8.4 * 30 or 252 watts.

Note that this would not be a good thing to do to a motor. It may not even rotate with only 3.6 volts across it.

Incidentally, V * I = R is not correct. V * I = power, or V / I = R

No.

The motor has a resistance of 12 / 100 or 0.12 ohms. Assume this is constant.
If you wanted to put 30 amps through this you would have a voltage across the motor of
30 amps * 0.12 ohms or 3.6 volts.

This would leave 12 - 3.6 or 8.4 volts across a series resistor which would have 30 amps flowing in it so it would be 8.4 volts / 30 amps or 0.28 ohms.
The power of the resistor would be 8.4 * 30 or 252 watts.

Note that this would not be a good thing to do to a motor. It may not even rotate with only 3.6 volts across it.

Incidentally, V * I = R is not correct. V * I = power, or V / I = R

Thanks for ther quick repley.
If a powermosfet = 50 volt 17 amp, and is in serial with a 2.2ohm reisiter, will it make 17amp at 12 volt, or will a resisiter at .7ohm in serial make 17amp at 12 volt(V/I=R).

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open said:
Thanks for ther quick repley.
If a powermosfet = 50 volt 17 amp, and is in serial with a 2.2ohm reisiter, will it make 17amp at 12 volt, or will a resisiter at .7ohm in serial make 17amp at 12 volt(V/I=R).

If you put a 2.2 ohm resistor across a 12 volt supply, it will draw a current of 5.45 amps (12 volts / 2.2 ohms = 5.4545 Amps.)

Putting an extra 0.7 ohms in series would drop the current to 12 /(2.2 + 0.7) or 4.14 amps.

Putting just the 0.7 ohm resistor across the battery would draw a current of 17.14 amps, as you suggest.

A MOSFET does not generate voltage. This one can stand 50 volts when it is not conducting and it can conduct 17 amps when it is acting as a short circuit. The 50 volts and the 17 amps cannot happen at the same time or the MOSFET would be dissipating 850 watts. (50 volts * 17 amps = 850 watts). This is a lot of power for a small MOSFET and may blow it up.

OPEN,
Please do not try any of these ideas in practice until you are sure of what you are doing. It seems likely that your task involves vehicle electrics, or a vehicle-type 12V battery.

Be aware that such batteries can pass enormous currents in the case of a short-circuit.
This is a very serious fire hazard, especially when petrol (gasoline) is nearby.

## 1. What is Ohm's Law?

Ohm's Law is a fundamental principle in physics that describes the relationship between voltage, current, and resistance in an electrical circuit. It states that the current through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance between them.

## 2. How do I use Ohm's Law to calculate voltage, current, or resistance?

To calculate voltage, current, or resistance using Ohm's Law, you can use the formula V = IR, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. You can rearrange the formula to solve for any of the three variables, depending on what information you have.

## 3. Can Ohm's Law be applied to all electrical circuits?

Yes, Ohm's Law can be applied to all electrical circuits, as long as they follow the basic principles of a linear relationship between voltage, current, and resistance. However, it may not be accurate in certain cases, such as when dealing with non-ohmic materials or complex circuits.

## 4. Does temperature affect Ohm's Law?

Yes, temperature can affect Ohm's Law and the resistance of a material. Most conductors have a positive temperature coefficient, meaning their resistance increases as temperature increases. However, some materials, such as semiconductors, have a negative temperature coefficient and their resistance decreases as temperature increases.

## 5. How is Ohm's Law used in practical applications?

Ohm's Law is used in various practical applications, such as designing and troubleshooting electrical circuits, calculating power and energy consumption, and selecting appropriate components for a circuit. It is also an essential concept for understanding more complex electrical principles and laws.

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