Welcome to Australia's only Vintage Radio and Television discussion forums. You are not logged in. Please log in below, apply for an account or retrieve your password.
Australian Vintage Radio Forums
  Home  ·  About Us  ·  Discussion Forums  ·  Glossary  ·  Outside Links  ·  Policies  ·  Services Directory  ·  Safety Warnings  ·  Tutorials

General Discussion

Forum home - Go back to General discussion

 Ohms Law. Can any one here explain it in detail.
« Back · 1 · 2 · Next »
 Return to top of page · Post #: 1 · Written at 12:01:17 PM on 5 July 2018.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1575

This is a law I have broken many times lol. But seriously can any one here explain it. Its one of the things I would like to learn if it it makes things easier.


 
 Return to top of page · Post #: 2 · Written at 1:31:01 PM on 5 July 2018.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

 
 Return to top of page · Post #: 3 · Written at 1:45:55 PM on 5 July 2018.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 883

Hi Carl, you really need a good grasp of quantum physics...............i'm only kidding.............

Ohms law is a simplification by observation and fits what you see into a 3 terminal equation...............

Na............it is an attempt to make intangible things into a simple form and compare with other things you can see and feel.

Like when they draw a parallel between electricity and water flowing.

Your basic things are then pressure (volts), flow (amps) and drag (ohms) if you can think of water being electricity.
Don't worry about electrons and atomic structure that's all a simple model to explain what is really witchcraft (quantum physics).

Then this guy called Ohm worked out that the higher the current the hotter or redder things got, or if you "narrowed" the pipe the resistance went up (kinked the hose) and the current went down, (no squirt) and that sort of relation between them and found that when you used units that his mates Mr Ampere and Mr Volt had also worked out the three could be expressed at each other and dammit, when they checked their gauges the simple equation held true, well pretty well anyway. Except some dimwit kept calling volts E or V so watch that one.

So after all that forget any crap and just keep repeating to yourself E equals I time R or I equals E over R or R equals E over I like a dumb parrot like I did about 10,000 times until it stuck in my head.
BUT to really make it stick you need to set up a real life circuit involving say a battery (Volts), A resistor (Ohms) and connect in a voltmeter and an ammeter and with known values of your volts and ohms and amps you can demonstrate to your self that..Lo!.....the equation actually holds water (that's a pun). I think I did that back in electricity 101 and was amazed when the equation was close. It still works for me 60 years later so I guess nothing has changed.
If you know what HT voltage you have to a valve and you know what the screen current is, using those 2 itmes v, a, you can go r equals E times I and get so many ohms, for the screen dropper resistor!. The answer may be in milli or mega but that just depends on whether you have milliamps or some fraction multiplier.

Don't worry it only took me 20 years or so to understand 3 terminal equations, and 4 terminal equations are outside of my IQ envelope.

Did that help or just make it worse?
Fred.


 
 Return to top of page · Post #: 4 · Written at 3:28:23 PM on 5 July 2018.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1911

Easy way to remember it:

E
-------
I R

So, from this, E = I times R
I = E / R
R = E / I


 
 Return to top of page · Post #: 5 · Written at 4:26:05 PM on 5 July 2018.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

As I said it's simple in practice, but you need to be careful of the units. Make sure you're calculating in amps, volts and ohms, or be aware of the powers of 10 involved.


 
 Return to top of page · Post #: 6 · Written at 8:30:29 PM on 5 July 2018.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6352

Except that some years ago the politically correct class in the trade changed E (electromotive force) to V (voltage) so many or even most modern sources will express Ohms Law as V=IR rather than E=IR.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 7 · Written at 9:13:57 PM on 5 July 2018.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 412

People are getting vey confused here.

It should be E=IR that is E= I times R.

And then transposed to what you want, where E = potential difference(voltage)(electromotive force).
I= Amperes, and R = Ohms.


 
 Return to top of page · Post #: 8 · Written at 9:24:47 PM on 5 July 2018.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6352

Indeed. Corrected.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 9 · Written at 12:35:06 AM on 6 July 2018.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1380


 
 Return to top of page · Post #: 10 · Written at 10:00:44 AM on 9 July 2018.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 766

QUOTE: Like when they draw a parallel between electricity and water flowing.

Your basic things are then pressure (volts), flow (amps) and drag (ohms) if you can think of water being electricity.


For extra fun, remember that the electrons actually flow in the opposite direction of electric current... Blame Ben Franklin for this one, he was messing around with kits in thunderstorms, and basically he guessed wrong when he described electric current flow. And this error is so deeply embedded in the electrical engineering technology world that it can never be corrected. Yes, current goes to the cathode inside a valve/ tube, but electrons are emitted off the cathode when it's hot enough and when some other nearby electrode is positive.

Here's something else people always get wrong first time: You have a heater string that adds up to 120V (in the USA). Runs off the American powerline of 120VAC. Now say I have a rectifier diode in series with the series string. Consider that the heater string is the same as a big power resistor. And the power consumed over time when the diode is present in series is half the power that is consumed when the diode is not in circuit. Okay... What voltage should I apply to the resistor without the diode to get half power?

Say the powerline voltage is 120VAC. The resistor is 100 ohms. power = (V^2)/R = 14400/100 = 144 watts. The rectifier diode will make this power be 72 watts (half the time the resistor is "off"). Okay, I'd like to know what voltage to use on the resistor, without the diode, to make it consume 72 watts. It would be 72 = (v^2)/100 -> 72*100 = V^2 = 7299 = V^2. Okay, take the square root of 7200 to get what V must be. Thus V = 84.85V. People would guess it would have been 60V, but there's a square root going on here... I've built radios with Compactron tubes where I got the heater voltages to add up to 85V, and used a rectifier diode off the powerline to run their heaters. The heaters glow as usual, and the cathodes also glow and emit electrons as usual as well.


 
 Return to top of page · Post #: 11 · Written at 5:15:22 PM on 9 July 2018.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6352

Back many, many years now, when I did the electrical trades course as part of my apprenticeship we were (thankfully) taught the distinction between electron flow and current flow. Fleming's Right Hand Rule proves that current flow is from + to -. The proof is also easily seen in an electroplating machine where gold, attached to the positive side of a circuit gets deposited on the tin, nickel, or whatever that is attached to the negative side.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 12 · Written at 10:20:21 PM on 9 July 2018.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

In electron tubes, the space charge around the cathode is filled with free electrons, not 'positrons', and they are attracted to the positive plate.


 
 Return to top of page · Post #: 13 · Written at 2:21:04 PM on 11 July 2018.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 766

In semiconductors, we sometimes talk about "holes", which are essentially gaps or missing electrons. And that these "holes" act like positive charges, flowing in the same direction that current does.


 
 Return to top of page · Post #: 14 · Written at 8:41:27 PM on 3 August 2018.
Rick Bartlett's Gravatar
 Location: Ballarat, VIC
 Member since 4 July 2018
 Member #: 2260
 Postcount: 15

I still get confused with Ohm's law, but I always have a chart on hand for 'in case' I can't
remember. I think the above descriptions are excellent and it's I find to remember different
letters and it's meaning:
e.g
Ohms which is 'R' for Resistance
Volts can be 'V' or 'E', that gets me all the time.
Watts can be 'P', as in Power
So many different ways of calculating the same equations that it's misleading if you are
not familiar with all of the different terminology. I find this when looking at dated schematics
compared to modern too, even the use of different symbols.
'Fred Lever' describes Ohms Law quite excellent above.
If you type in Ohms Law into 'Google' and look at images, you'll find some great charts there
that can help too.
https://i.pinimg.com/originals/c6/bf/f3/c6bff368edf9b7890309e2aff8f78e50.gif


 
 Return to top of page · Post #: 15 · Written at 10:05:46 PM on 3 August 2018.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4347

There is a Magazine now, called Diyode. (Jaycar sells it) WEB and print. It is running a series on the basics. It would be well worth following up on. The thing with EMF is that it is more to do with currentless voltage like the charge on a cap.


 
« Back · 1 · 2 · Next »
 You need to be a member to post comments on this forum.

Sign In

Username:
Password:
 Keep me logged in.
Do not tick box on a computer with public access.