### Ohm's Law

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Messages: 1 - 50 of 96
• #### Message 1.

Posted by Lawrence Jones (U4805414) on Wednesday, 11th August 2010

I was quite shocked whilst watching this week's edition of University Challenge to observe that neither the team from Newnham College (Cambridge), nor Southampton university were familiar with Ohm’s law. The question: two 1 ohm resistors are connected in parallel, what is the resultant resistance?

Why is it that engineers and scientists are expected to posses a reasonable knowledge of the arts, yet the opposite doesn't appear to apply to arts graduates? R4 is a major offender in this area, especially Woman's Hour. I often shake my head in despair whilst reading the Woman's Hour newsletter.

• #### Message 2

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Posted by Eddie (U14033965) on Wednesday, 11th August 2010

1 ohm if my memory serves me right.

• #### Message 3

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Posted by SteveH (U14288326) on Wednesday, 11th August 2010

1/2 ohm.

• #### Message 4

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Posted by oldpythonboy (U10772388) on Wednesday, 11th August 2010

Why did Mr Ohm marry Mrs Ohm?

• #### Message 5

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Posted by oldpythonboy (U10772388) on Wednesday, 11th August 2010

1 ohm if my memory serves me right.

Yup

In parallel 1/R=1/R1+1/R2

In series R=R1+R2

• #### Message 6

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Posted by oldpythonboy (U10772388) on Wednesday, 11th August 2010

Sorry Steve

I've just realised you are right. I posted before I used my equation.

1/R=1/1+1/1=2
R=1/2

• #### Message 7

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Posted by SteveH (U14288326) on Wednesday, 11th August 2010

Come on oldpythonboy, you've got the formula right just put the values in.

• #### Message 8

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Posted by Eddie (U14033965) on Wednesday, 11th August 2010

Oops 1/2 is right.

• #### Message 9

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Posted by grandpacliffy (U14287151) on Wednesday, 11th August 2010

I think it is 0.5 ohms

• #### Message 10

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Posted by abacus9900 (U14531191) on Wednesday, 11th August 2010

Think of two 1 ohm resistors in parallel as presenting two paths for the electric current to follow. With just 1 resistor the current only has one path to follow. With two resistors there are two paths to follow hence more room, so in the case of two 1 ohm resistors in parallel (that is to say two resistors side by side), mathematically, the overall resistance is exactly half of just a single 1 ohm resistor, i.e. 1/2 ohm.

If the resistors were in series, i.e. connected to one another one after the other, then the current sees only one path but a path made up of two 1 ohm resistors, i.e. 2 ohms.

• #### Message 11

, in reply to message 7.

Posted by oldpythonboy (U10772388) on Wednesday, 11th August 2010

Because he couldn't resistor.

Going back to the OP I don't suppose talking about Ohm's Law in Starbucks or down the pub happens too often so you can be pretty sure you won't be caught out.

Though, you might be in a conversation about a novel or a visit to an art gallery in the said places.

• #### Message 12

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Posted by abacus9900 (U14531191) on Wednesday, 11th August 2010

oldpythonboy, to be fair, no matter how intelligent people are, you can't expect them to know everything. I think we should be impressed by how much they do know.

• #### Message 13

, in reply to message 10.

Posted by Hy Dranger (U14569978) on Wednesday, 11th August 2010

I knew the 1/2 ohm bit but I'm stumped on the supplementary question of why Mr Ohm married Mrs Ohm.

Anyway it all goes to show that you can lead a student to the fountain of knowledge but you can't make them think. There was a similar case some years ago when some MIT graduates were handed a battery, torch bulb and a piece of wire but were unable to make the bulb light!

• #### Message 14

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Posted by abacus9900 (U14531191) on Wednesday, 11th August 2010

Why is it that engineers and scientists are expected to posses a reasonable knowledge of the arts, yet the opposite doesn't appear to apply to arts graduates?

Aren't they fundamentally different spheres?

• #### Message 15

, in reply to message 13.

Posted by oldpythonboy (U10772388) on Wednesday, 11th August 2010

Hi Hy Dranger

I knew the 1/2 ohm bit but I'm stumped on the supplementary question of why Mr Ohm married Mrs Ohm.

See start of mesage #11

• #### Message 16

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Posted by SteveH (U14288326) on Wednesday, 11th August 2010

Sorry oldpythonboy, our posts must have crossed earlier.

• #### Message 17

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Posted by Blue_Biro (U4004117) on Wednesday, 11th August 2010

>> Because he couldn't resistor. <<

But when it came to the drinks at the wedding, they found the spirits a bit of a disappointment. You see, they found the kirsch off.

• #### Message 18

, in reply to message 13.

Posted by abacus9900 (U14531191) on Wednesday, 11th August 2010

Anyway it all goes to show that you can lead a student to the fountain of knowledge but you can't make them think. There was a similar case some years ago when some MIT graduates were handed a battery, torch bulb and a piece of wire but were unable to make the bulb light!

Being good at theory and practice does not always go hand in hand.

• #### Message 19

, in reply to message 17.

Posted by abacus9900 (U14531191) on Wednesday, 11th August 2010

But when it came to the drinks at the wedding, they found the spirits a bit of a disappointment. You see, they found the kirsch off.

Oh dear.

• #### Message 20

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Posted by hoddles off into the sunset (U14129169) on Wednesday, 11th August 2010

My wife was told many years ago by her Physics teacher "you will never get anywhere in life without knowing Ohm's Law."

She has proved him wrong.

• #### Message 21

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Posted by abacus9900 (U14531191) on Wednesday, 11th August 2010

My wife was told many years ago by her Physics teacher "you will never get anywhere in life without knowing Ohm's Law."

If your wife was hoping to be an electrical engineer then her physics teacher was right. Was she?

• #### Message 22

, in reply to message 17.

Posted by oldpythonboy (U10772388) on Wednesday, 11th August 2010

You see, they found the kirsch off

Now, we don't want to see quips about 'AC/DC' or 'removing shorts' do we?

• #### Message 23

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Posted by hoddles off into the sunset (U14129169) on Wednesday, 11th August 2010

My wife was told many years ago by her Physics teacher "you will never get anywhere in life without knowing Ohm's Law."

Quoted from this message

If your wife was hoping to be an electrical engineer then her physics teacher was right. Was she?

Clearly not - though his thought processes see to be similar to those of the OP.

• #### Message 24

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Posted by oldpythonboy (U10772388) on Wednesday, 11th August 2010

Clearly not - though his thought processes see to be similar to those of the OP.

Who was he, Sherlock Ohms?

• #### Message 25

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Posted by Blue_Biro (U4004117) on Wednesday, 11th August 2010

>> Now, we don't want to see quips about 'AC/DC' or 'removing shorts' do we? <<

Certainly not. If Lee, my cross-dressing sibling, got wind of it... Well, having a tranny sister can be trying, and there's no point in trying to shock lee.

• #### Message 26

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Posted by abacus9900 (U14531191) on Wednesday, 11th August 2010

Oh my, this is getting worse.

• #### Message 27

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Posted by Blue_Biro (U4004117) on Wednesday, 11th August 2010

Don't worry, abacus, I can't think of any more.

Well I can but they're a bit contrived.

Er...

• #### Message 28

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Posted by abacus9900 (U14531191) on Wednesday, 11th August 2010

A live wire, eh?

• #### Message 29

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Posted by Principled (U8899543) on Wednesday, 11th August 2010

Abacus
Being good at theory and practice does not always go hand in hand.
Or in engineering speak:
They can calculate the square root of a biscuit tin, but haven't got the common sense to work out how to take the lid off!
P

• #### Message 30

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Posted by Blue_Biro (U4004117) on Wednesday, 11th August 2010

>> Or in engineering speak:
They can calculate the square root of a biscuit tin, but haven't got the common sense to work out how to take the lid off! <<

Is that real engineering speak?

It's just, as an analogy or metaphor, it doesn't really work.

• #### Message 31

, in reply to message 30.

Posted by RCWhiting (U11440738) on Thursday, 12th August 2010

I am a scientist, I can answer many of the UC questions each week.
I find the maths/physics generally the most difficult and it seems that the teams do too.
Most questions are (i) do you know the answer (ii)can you recall it quickly?
Maths questions are, firstly Paxman shouting "know the answer before you buzz", secondly you are expecting an opponent to beat you.
But most importantly they involve more stages.
(a) have you registered the often complex information?
(b) do you know the method to use?
(c) can you recall it?
(d) Can you plug in the info and produce an answer?
In this case I did get 0.5 ahead of the teams but often fail with these type of questions.

• #### Message 32

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Posted by Mr_Red (U4022339) on Friday, 13th August 2010

I worked with a colleague who watched UC avidly. He agreed we could answer as good as most teams (in the comfort of our armchairs).

But I had to stiffle a laugh when he told me he kept his own score.

• #### Message 33

, in reply to message 27.

Posted by oldpythonboy (U10772388) on Friday, 13th August 2010

Blue_Biro

Well I can but they're a bit contrived.

You mean like this?

Tainting the Family Tree

The Smith's were proud of their family tradition. Their ancestors had come to America on the Mayflower. They had included Senators and Wall Street wizards.

They decided to compile a family history, a legacy for their children and grandchildren. They hired a fine author. Only one problem arose - how to handle that great-uncle George, who was executed in the electric chair.

The author said he could handle the story tactfully.

The book appeared. It said "Great-uncle George occupied a chair of applied electronics at an important government institution, was attached to his position by the strongest of ties, and his death came as a great shock."

Wudubelieveit!

• #### Message 34

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Posted by Blue_Biro (U4004117) on Friday, 13th August 2010

So if you hang a crime writer, do you describe them as a master of suspense?

And have I misunderstood the term "cutting edge"...?

• #### Message 35

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Posted by StuartG (U14275984) on Friday, 13th August 2010

Fri, 13 Aug 2010 22:06 GMT, in reply to oldpythonboy in message 33

Good one!

• #### Message 36

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Posted by oldpythonboy (U10772388) on Friday, 13th August 2010

So if you hang a crime writer, do you describe them as a master of suspense?

A knotty problem. Just let me check the noose channels.

And have I misunderstood the term "cutting edge"...?

Only if you are having trouble with your sirloin steak!

It's not too late to change your knife around.

• #### Message 37

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Posted by Pumblechook (U6852342) on Saturday, 14th August 2010

Think I will call my new house 'Ohm'.

• #### Message 38

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Posted by Pumblechook (U6852342) on Monday, 16th August 2010

I suspect those crowing about Ohm's Law don't know much electrical theory beyond it.

What do we actually mean by 230 Volt mains when it is a constantly changing voltage..a sine wave?

• #### Message 39

, in reply to message 38.

Posted by StuartG (U14275984) on Monday, 16th August 2010

Mon, 16 Aug 2010 22:55 GMT, in reply to Pumblechook in message 38

RMS values?
The mean value is 0?
square root of 2 is (from memory) 1.4142 times 230 ummm lots more?
but that's not Ohms Law?
Cheers,
StuartG

• #### Message 40

, in reply to message 39.

Posted by piscator (U3320021) on Tuesday, 17th August 2010

Mnemonic for the root of two

I wish I knew the root of two

• #### Message 41

, in reply to message 38.

Posted by Blue_Biro (U4004117) on Tuesday, 17th August 2010

>> I suspect those crowing about Ohm's Law don't know much electrical theory beyond it. <<

Where on earth did THAT assumption come from, Pumblechook???

>> What do we actually mean by 230 Volt mains when it is a constantly changing voltage..a sine wave? <<

Count me as one of the ones who knows about Root Mean Square. It's been years, many years, but I can still dredge up memories of three-phase (star and delta), step-up and step-down transformers (we don't want high current coz P=(I^2)R would mean a lot of power lost in the lines). I can draw a common emitter transistor amplifier circuit, and maybe even an astable multivibrator at a stretch. I can draw a functional sketch of a transistor (PNP, NPN, or, at a stretch, an FET). I can calculate the reluctance of a capacitor or inductor of a given value for a given frequency, and (probably) calculate Q-factor. I can work out the value of resistors in parallel and/or in series.

I can do this without referring to a text book or looking it up online. I know I would recall a great deal more if I spent a couple of hours with a text book - which I don't think is too bad for a qualified teacher of English as a Foreign Language.

And I'm guessing the others on this thread probably know more than me about electrical theory.

• #### Message 42

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Posted by StuartG (U14275984) on Tuesday, 17th August 2010

Tue, 17 Aug 2010 18:34 GMT, in reply to Blue_Biro in message 41

"astable multivibrator"
Sounds fun...
Cheers,
StuartG

• #### Message 43

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Posted by Blue_Biro (U4004117) on Tuesday, 17th August 2010

Not as fun as it sounds, alas...

• #### Message 44

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Posted by abacus9900 (U14531191) on Tuesday, 17th August 2010

What do we actually mean by 230 Volt mains when it is a constantly changing voltage..a sine wave?

Like Blue_Biro, I have forgotten a fair bit of electrical theory but I believe 230 volts is equivalent to what a direct voltage would be, i.e. one that was not constantly changing. As Blue_Biro mentioned it is known as the 'root mean square' and I think it is the square root of the mean of the squares of all the values of the varying sinusoidal voltages (but I could be wrong). It's obviously useful to know what an equivalent direct voltage would be for an AC one.

• #### Message 45

, in reply to message 42.

Posted by abacus9900 (U14531191) on Tuesday, 17th August 2010

"astable multivibrator"

The basic circuit is an amplifier with two stages using transistors. The output is fed back to the input via alternating current (AC). The first transistor stage is conducting current while the other is not (off). The stages alternate states, one conducting, the other non-conducting at a specific frequency determined by other components. I think such a device is used for timing purposes.

• #### Message 46

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Posted by Blue_Biro (U4004117) on Tuesday, 17th August 2010

It can be used for timing purposes, or to make a pair of lights flash alternately, or to generate square waves. IIRC the frequency is determined by the resistor and capacitor combination. I once used one to drive a speaker as part of a burglar alarm system. Happy days.

• #### Message 47

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Posted by abacus9900 (U14531191) on Tuesday, 17th August 2010

Thank you BB.

• #### Message 48

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Posted by StuartG (U14275984) on Tuesday, 17th August 2010

Tue, 17 Aug 2010 19:50 GMT, in reply to abacus9900 in message 47

You all will remember this, first introduced by Signetics, and now made by most.
cheap, versatile, monostable or multivibrator. Bit heavy on current, but more recent updates use less (CMOS). 8 pin dil. You've guessed it 555 timer.
Cheers,
StuartG

• #### Message 49

, in reply to message 48.

Posted by Lawrence Jones (U4805414) on Tuesday, 17th August 2010

Message 48

You all will remember this, first introduced by Signetics, and now made by most.

Ahhh, good old Signetics - loved designing with the NE568 150 Mhz PLL Great also so see someone mention a discrete astable

Great responses everyone and thank you.

• #### Message 50

, in reply to message 49.

Posted by Pumblechook (U6852342) on Tuesday, 17th August 2010

You are all too clever. Must think of a harder question.

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