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Ohm's law

Hello all,

Sorry for the repost but I could not get my schematic to show up correctly.

Hi everyone,

I have the following set up:

formatting link

The electric motor and a pressure sensor are running of the same 5V power supply. The transducer is connected to the power supply with

1.5m long 1 mm diameter cable with resistance of 0.02 ohms/meter.

The motor draws a current of approximately 2 amps when operating.

Now, what I have to do is calculate the drop in supply voltage across the pressure sensor when the motor is turned on.

Now, what I reasoned is that there will be a drop in both the wires:

So, the resistance in the wires are: 1.5 X 0.02 = 0.03 ohms

So the voltage drop when the motor is turned on would be:

2 X 0.03 = 0.06 V

However, it should be 0.06 X 2 = 1.2 V because there are two wires.

Is this reasoning correct? Or is there a special way to add them?

Cheers, Luca

Reply to
luca.pamparana
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By "cable", do you mean a single wire, or a pair: is the total resistance .03 ohms or .06 ohms?

That would be 0.12 V.

Yes, if the voltage is measured at the motor.

--
John
Reply to
John O'Flaherty

Great. Many thanks.

Reply to
luca.pamparana

One last question...

Why is it that the voltage drop is the same on each wire? One is connected to a 5V power supply and one is connected to the ground. How come the voltage drop is 0.06V for each of the wire?

Thanks, Luca

Reply to
luca.pamparana

Because current flows in a loop, you will have +A amps in one wire, and -A amps in the other, the sign just tells you the direction.

Reply to
a7yvm109gf5d1

It's the meaning of "voltage drop" : a change in voltage, which is independent of the absolute voltage. Voltage is a measurement between two points, one of which is taken as a zero reference. Now consider the voltages in your circuit, measured with the negative side of the power supply as the reference. Starting at the +5 V, going clockwise through the motor, you have +5 V, +4.94 V, +0.06 V, 0 V. Do you see how it works now?

--
John
Reply to
John O'Flaherty

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