Main content

## Physics library

### Unit 12: Lesson 1

Ohm's law and circuits with resistors- Introduction to circuits and Ohm's law
- Basic electrical quantities: current, voltage, power
- Resistors in series
- Resistors in parallel
- Example: Analyzing a more complex resistor circuit
- Analyzing a resistor circuit with two batteries
- Resistivity and conductivity
- Electric power
- Kirchhoff's current law
- Kirchhoff's voltage law
- Kirchhoff's laws
- Voltmeters and Ammeters
- Electrolytic conductivity

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Electrolytic conductivity

Liquids can also conduct electricity. Created by David SantoPietro.

## Want to join the conversation?

- Why would we use AC current to measure the resistivity of water? How does using AC prevent electrolysis?(19 votes)
- The electrodes alter their polarity 50 or 60 times a second. The heavy ions can not follow such high frequencies. So they can’t be discharged effectively. As a result the electrolysis is almost unnoticeable. But if water is the solvent, at each terminal small bubbles are noticed due to high mobility of H+ and OH-. They can migrate to the electrodes owing to their high mobility. Hence a mixture of H_2 and O_2 is formed at each electrode.

Credits: Parantap Nandi, Department—Electrical Engineering, West Bengal University of Technology (WBUT).(30 votes)

- Why was Edison so opposed to alternating current? I heard he killed an elephant with it and invented the electric chair just to try prove how dangerous it was.(3 votes)
- he opposed it because someone could use it to oppose his own inventions!(2 votes)

- at2:01, we suppose that the electrons go straight from one plate to the other, right?(4 votes)
- The
**ions**you mean. Positive and negative**ions**are the charge carriers here, not electrons. And no, here the system works differently, the positive ions move to the negative electrode and the negative ions move to the positive electrode. There they get**discharged**by gaining/losing electrons from/to the electrode. This is how electrolytes conduct electricity.(8 votes)

- Why would L/A have to remain the same, what if we brought the the plates closer. And why is it not the area of cross section the liquid?(4 votes)
- By maintaining constant L/A in your apparatus you can measure resistivity of different easily. Resistivity would be the only variable and everything else in the experiment is constant, so you can measure the resistance and figure out resistivity quickly!

Hope that helps.(5 votes)

- Can anyone tell me more about AC current. And the difference between AC and DC current.(3 votes)
- in DC the current goes 'directly' from positive to negative.

In AC, the direction 'Alternates' going back and forth. Like the electrons are vibrating in time with one another.

make sense?(5 votes)

- What is the difference between a Volt and a current? A current is obviously a flow of electrons but what is a volt and and why are there two meters like Ammeter and Voltmeter?(1 vote)
- Voltage is what makes the current. Voltage is analogous to water pressure for flowing water. Higher the pressure, the more the flow. Specifically voltage is the potential energy per unit charge, and the positive charges will want to flow from regions of high voltage to low voltage. Similar to how a ball will roll down a hill from a region of high gravitational potential energy to a region of low gravitational potential energy.(7 votes)

- AC is alternating current but what is DC(1 vote)
- DC stands for "direct current". It is the unidirectional flow of electric charge.(1 vote)

- So do you take the cross-sectional area of the plate or the cross-sectional of the conducting liquid?(2 votes)
- The cross section of the electrolytic solution along with the charge carrier density will determine how much current can flow.(1 vote)

- Why can we only look at the volume between the two plates and ignore the curved paths that would go outside it and come back? Does the additional liquid around that path not lower the resistance the same as parallel resistors in a circuit?(2 votes)
- Would the resistivity of the plates affect the total resistance seen in the system? If so, how is this accounted for?(2 votes)
- The resistance of the plates/wires would be modeled as a series resistance with the resistance of the solution.(1 vote)

## Video transcript

- [Voiceover] Most solids
offer some amount of resistance to the flow of current through them. This allows us to define
things like the resistivity, or the conductivity, but the
same is true for liquids. Consider this container full of a liquid. We can measure its resistivity. Now, if I took a battery,
and I put one lead here, and one lead here, if there
is a voltage and this liquid is able to conduct electricity,
then this current should be able to flow through
the intervening liquid over to the other side and then back up. Sometimes, this is done
with AC current, otherwise, you might get electrolysis and
then you get bubbles in here and that changes the liquid in some way. We want to measure the
resistance and the resistivity of the liquid, not of some altered liquid. So, sometimes you use AC, but
this is the general principal. Send in a voltage, a certain
amount of current will flow. How can we use that, to
determine the resistivity? Well, we know resistivity
is equal to the resistance that we measure times the
area divided by the length, and now you see there's
kinda of a problem length. I can imagine getting that. This length in here would
just be this distance. There's my length, because my "resistor," is this liquid in here. But, what's my area? So, this would be a bad experiment to do. If we want to measure the
resistivity, what we really want, is something where we
have a well defined area. Let me get rid of this. Imagine you had two plates. Take these two plates. You put them in the solution
you want to measure the resistivity of, so, we put them in here. Stick them into there. They have a well defined area. We've got those. We can measure those if we want. We set them apart some known
distance between them, L, and you hook them up to battery. So, take this one, hook
it up to a known voltage, hook the other side up to the other plate, and if this solution, if this
electrolytic solution in here can conduct electricity,
current will flow from this side to the other side, and you
can measure these quantities. You measure the length, that's easy. You measure the area. You got that. How do we measure the resistance? Well, we know the voltage. We can have a known voltage
of the battery up here, and you can stick ammeter in
here to measure the current. If I stick an ammeter,
ammeters measure the current. Now, I can just use Ohm's
law, and I know that the resistance is just going
to be the voltage divided by the current, and if plug
all these values into here, I can get an experimental value for the resistivity of this liquid,
sometimes it's called the electrolytic resistivity. Or, one over the electrolytic
resistivity would be the electrolytic conductivity. So, this would be the
electrolytic conductivity. So, this is an experimental way to do it. Honestly, you don't
even have to go through all that much trouble. You can just take a solution. First, put a solution in here
that has a known resistivity. That way, you can just do
this: R equals Rho L over A. If you know the resistivity,
and you can easily measure the resistance,
then you can just figure out what this constant is,
and this will stay the same. You just leave those
same plates in there with the same length and the same area. Put a new solution in there,
and that gives you this number, and this number staying the same. So, technically speaking, you
don't have to go in there, measure the area each time, and the length between these each time,
if you have some calibrated electrolytic solution where
it has a known resistivity. Or, you could use it the other way. If you had a solution
with a known resistivity, but there may be impurities
in there, or there may be dissolved salts,
or something, and you want to know what the concentration is. Well, that is going to
directly affect how much current will flow, and
it will directly affect the measured electrolytic conductivity. So, if you measure this,
and it comes out different from what you would expect
from a baseline solution. You can figure out what the concentration is of the conductive impurities
within this solution.