Ohms Law Problem

I cannot undersatd when you multiply current by resistance that you get current.

Voltage.

Hit the books again! It will be worth it, you need to know it. Too basic to not to know.

I = V/R.

Not sure what you are asking--Voltage drops may need to be subtracted from the(voltage) total to produce the answer. It is simple unless you don't stick to the "law".

Good luck and don't freak out---It is an expanding universe for us and what we know is our passport. You will be fine.

Good morning,

This is a link to the theory of Ohm's Law

http://www.grc.nasa.gov/WWW/K-12/Sample_Projects/Ohms_Law/ohmslaw.html

A little more sophisticated one is here

http://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.intro.html

Inside that site is a link that will take you to a very nice Walter Fendt applet which allows you to practice calculations.

After completing that tutorial, go to this link to check your understanding.

http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/ohmlaw.html

Good luck on your interview. What school are you graduating from?

Ambition. The soldier's virtue. - Anthony and Cleopatra

Thnx all for your reply. I must clarify some mistakes in my post. I am in my first year of my degree course so 4 years in total. I am looking for parttime work and have interview in january, just helping out and maybe fix something - get experience I hope

I hope when I am in my 4 year of course that I understand ohms law. Thank yu all for kind responses. I also am learning english so please forgive my bad language.

Valerie

Welcome to CR4, Valerie. I hope that you'll consider registering with the site. It's a great resource.

are you learning english ?....so do i.....

where are you come from ?where do you live ?

First, forget about electricity, because you cannot see or handle it directly, and think about a very similar problem of running water through a pipe from a tank. The higher the tank is above the outlet of the pipe greater the flow. The narrower the pipe the slower the flow. So you could say that a narrow pipe resists the flow of the water. If the flow is laminar, we can represent this as:

Water_Flow = Driving_Height divided by Pipe_Resistance.
using more standard technical terms:
Current = Potential/Resistance

Now let us look at the electrical equivalent. Crudely, we are looking at flow of electrons; this is called current. Similarly to the pipe* and electrical_resistor restricts the flow.
The Voltage applied* is to electricity what height (or potential energy) is to mechanical movement. So, just as the pipe for laminar flow, we can write:
Electrical_Current = Electrical_Potential/Electrical_Resistance
Obviously, we can rearrange this as Potential = Current x Resistance
Electrical Current is measured in Amps, Electrical Potential is measured in Volts, and Resistance is measured in Ohms, so this is often loosely presented as
Voltage = Current x Resistance (and the units match - check them on Wikipedia)

A final comment: Ohm's law applies to what are known as "linear resistors". There are a lot of these about, but many important devices (including motors, transformers, lamps and transistors) do not really behave like that at all.

*Pedantically, that is incorrect - we apply a potential that is measured in Volts

The higher the tank is above the outlet of the pipe greater the flow.

A little confusing--

The size of the pipe should illustrate the volume/current and resistance, but height of the liquid or head pressure would reflect voltage, not current--there would be more flow of liquid(current), but only because of the increase in pressure(voltage) and not because of a change in pipe diameter(resistance).

For goodness sakes, you are expecting the posting to tell you the "why" before it tells you the "what". After that, it does seem to be quite specific about what you can treat as Voltage and what as current.

Valerie,

You probably should not follow this advice to learn electricity from the water analogy. It's OK for grade school students, but I understand that you are a University student? As soon as you take fluids (or possible freshman physics) you'll run into too many differences that will tend to confuse.

Valerie -

As I read your question, you have not reached the grade-school starting point for Ohms law. But you do appear to have dimensional analysis skills that enable you to reject the worst junk. However, that will not substitute for physical insight.

As regards initial insight, considering low-velocity water flow through pipes with volume-flow-rate substituting for current (Amps) and height of water (pressure head) substituting for potential (Voltage) would be one of the less-bad starting points. So long as you do not expect this analogy to be valid for everything...

I didn't mean to make a big deal out of it, but the plain truth is that if you learn electricity as a water analogy, some instructor is gonna have a heck of a hard time trying to explain that you really don't have a flow of little electrons down that wire in the way you have a flow of water molecules down the pipe. If you're gonna be a EE, learn electricity as electricity.

Would you care to provide a reference that teaches this as you would wish - and in a way that a first-timer can visualise? (N.B. I was originally taught using a sand-water analogy, but including various caveats. Although it never caused me a problem, I would certainly embrace a more direct method)

However, so far as I know, other than QM issues about particle identity, current flow behaviour in metals is so close to pressurised water flow through a sand filter that there is no need to unlearn anything.

OK. But, before I go into how I would do it, let me tell you the problems I see and you can point out if I'm wrong.

The biggest problem, in my opinion, is that height is not the analogue to Voltage; pressure is. Of course you can relate height to pressure with a little bit of hand-waving and under the breath muttering about density, but a beginner will often have trouble with that concept.

If you want to take the analogy beyond the visualisation point, I think you will find that 'head' (the proper term for the effect of height) is equivalent to 'built-in potential' and 'pressure' is equivalent to 'applied potential'. To my mind, the only issue here is that in both cases the distinctions and conversions between the two are beyond the scope of elementary descriptions.

I'll grant you that, and I have to say that adding the sand certainly helps - that's not commonly done in the US. The other point that I would make is that most people in my experience learn electricity before they learn fluids. Thus, if there is an analogy to be made, it would be better to explain fluids using the electricity analogy.

I do understand that visualization has great value. For example, if you wanted to explain why two resistors in parallel have less equivalent resistance, there are some good visualization techniques and water flow through a resisting medium is clearly useful. But, this is a specific analogy and one where the limitations can be explicitly stated.

We were taught mechanics (including elementary low-velocity fluids) first. For now, I believe this to be sensible, as you can arrange experiments so that you can see what is happening (glass tubing, coloured markers to display local flow, etc). But I look forward to seeing a direct electrical method with comparable ease of experimental visualisation - then I will be able to recommmend this instead.

We may be talking apples and oranges. If you were taught any amount of applied hydraulics, that is at odds with the typical American physics course. But, let's put the student in a freshman physics class, using Hugh Young's textbook (a good standard in the US). So, clearly in the first semester they will be introduced to fluids, but not to flow in pipes except for special cases, and not to static head and velocity head. But, and this is the real fly in the ointment, they will learn Poiseuille's Law.

Young, by the way, does a decent job of showing how to visualize resistance, but if I were teaching the course, I would do a supportive lab, having them make 50' and 100' and 200' coils of #20 awg and #22 awg wire and measure V=IR. Then, I'd do series and parallel resistances in the next lab. To my mind, meters are a great visualization, particularly for a EE.

But, if I wanted to do a more thorough, careful understanding of Ohm's Law, I'd do the historic thread, going back to Cavendish and Davy.

Agreed about the necessary experiments - we did all that in high-school labs as well - except that we used rheostats rather than cutting coils (tidier and less wasteful, but equally informative as you can calculate the length directly).

I also agree with your perspective on understanding the way the ideas developed. Indeed, I regard this as crucial for a number of reasons. First, it prepares the student for the new concepts; second, (s)he learns that (and how) human understanding of our world has developed in the past (and hopefully that there will be more to learn), and third, demonstrates the interchange between experiment, phenomenological ideas and mathematics. However, I believe the most important effect (at least for engineers) is that this cements the concept of 'mechanism' - something that about 90% of new engineering graduates whose details we see do not have, and that appears almost impossible to retrofit.

Regarding Poiseuille's law - we encountered this in order to have a reasonably simple method of measuring viscosity - and we had to document the integrations in our notebooks. Is this what you mean?

Again, our perspectives may be different as we see different education systems, but Poiseuille's Law is usually used as the starting point for hydraulic resistance in US courses, and that R⁴ and the boundary layer effects puzzles the heck out of good students when you compare it to Ohmic resistance.

I seem to recall that James Clerk Maxwell used hydrostatic and hydrodynamic analogies to derive what have come to be known as Maxwell's Laws; the four equations that completely describe all electromagnetic phenomena, from dc to light.

Fourth year? Are you at high school hoping to go on to university - or are you pulling our legs?

Oh ! very tricky question to pull all us !

But, you sure need to learn more .... that is the formula you have learnt in your school is applied more linenetly....pl. forgive my english.... But you should make an experiment for erifying the formula you are learning..... to astonishment, you shall find some errors may be in decimels or may be repeat readings have different results... How come this?

the simple formula we are taught Volts = I * R or I = V/R have drifting current values recorded with time as the resistance element starts heating up gradually.....

So, one thing is sure, the Linear Ohm's Law holds true for at specified temperature and specified Climatic conditions....

So, whenever you want to calculate the current, pl. ask for Ambient temperature conditions.

wow, what is a forth year elctrical engineering student? Can you show us your college name? where is it?

I cannt tell you what the ohm law is , but I can suggest you go to local law court to accuse your college or school authorities. and then get your money back.

your college or school must cheat your student's money.

cn,

She said it wrong. She's a first year student in a four year program.

Happy New year.