# Ohm's law

Discussion in 'Electronic Design' started by [email protected], Jan 16, 2008.

1. ### Guest

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:

http://farm3.static.flickr.com/2098/2197444321_ffd34dd2d4.jpg?v=0

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

2. ### John O'FlahertyGuest

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.

3. ### Guest

Great. Many thanks.

4. ### Guest

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

5. ### Guest

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.

6. ### John O'FlahertyGuest

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?  