Ohms Law Word Problems

Discussion in 'Homework Help' started by Windex, Feb 3, 2005.

Feb 2, 2005
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8.

A motor drawing 20 A of current is connected to a 220 V line through leads which have a resistance of 0.2 Ω for each lead.

What is the voltage available at the motor?

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I've gathered that I have 20 Amperes of current
I have 220 volts of electromotive force.

I don't know how much resistance I have there are plural words (each, leads). The thing is there is no number for how many leads there are.

Nor do I know if this is a parallel or series system or even comination.

Any ideas?

2. Firestorm Senior Member

Jan 24, 2005
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well, given ur problem id say u have 2 leads...assuming u kno ur formulas u should be able to figure this out...if not ill be glad to help u...thx l8er

-fire

3. pebe AAC Fanatic!

Oct 11, 2004
628
3
If this isn't a trick question then you have a motor taking 20A connected by a normal two-wire feed to a 220V supply.

Total restance of the two leads = 2 x 0.2ohms = 0.4ohms. Voltage dropped by them = 20 x 0.4 = 8 Volts. So voltage across motor = 220 - 8 = 212V

Feb 2, 2005
11
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How do you know that it's a two-wire feed?

This is interesting mainly because I was thinking the leads were seperate resistors. However, if they were seperate resistors then the voltage would decrease if it were only a series system. But it would have been nice to know how many resistors I have. Of course you do needs wire two wires to come from terminals to the terminals of the motor. I wasn't thinking about the individual wires themselves that carry the resistance. However, now that I think of it...

Nope, no specific resistance stated in my book for lead.

"What is the voltage available at the motor?" This shows that it's a series system because asking about available voltage means some has been dropped somewhere. And i'm guessing the voltage needs to drop somwhere. However the motor also becomes a load.. I'm so confuzzled.

But resistors do have a lead coating on each of the wires along with the inside of carbon. Perhaps the resistors are the leads?

G0 4h34d & h31|D fire

5. pebe AAC Fanatic!

Oct 11, 2004
628
3
You said a supply of 220V. That would either be DC, or a single phase supply. Both these connect via two leads. If the supply was 3phase (3 leads) then it would probably be 440V. You said the motor draws 20A of supply current. That must be carried by the two leads.

You said the leads had a resistance of 0.2ohms each. That is the lead resistance - not a separate resistor. I did not mention specific resistance which an entirely different thing.

:huh: ?? I think you are looking too deeply at a simple question. If 20A passes through 0.4ohms of leads there must be a voltage drop of 8V across the leads. That leaves 212V across the motor.

Feb 2, 2005
11
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This makes more sense. The european system is DC and 220. Direct current comes off the power source itself. European system is thus, DC. 2 lead wires create the resistance over a undetermined length which creates .2 Ω with a 2 A current going through the circuit. Ok. I've got it now. Thx

Plus it was stupid of me to think of a motor being turned by electrical energy, it's turned by magnetic or no?

SO HOW'S IT LOOK?????????

(8) 212 V

20 A = I
220 V = E
0.2 Ω = R

220 V is DC system.
Direct current requires 2 leads from each terminal.

2 leads x 0.2 Ω = 0.04 Ω
(Voltage dropped by the resistance) ER1 + ER2 = 20A * 0.4 Ω
ER1+ER2 = 20A * 0.4 Ω = 8 V
ER3 = Motor Load with voltage drop against it.
ER3 = ET  ER1+ ER2
ER3 = 220 V  4 V  4 V = 212 V
ER3 = 212 V

7. pebe AAC Fanatic!

Oct 11, 2004
628
3
Not quite. The european system is AC - not DC but 2 wire etc still apply.

Well, electrical energy makes the magnetic field that turns the motor.
It looks all right except for the bit "....Direct current requires 2 leads from each terminal". It requires TWO leads, TOTAL, from the supply. ie. ONE from EACH terminal.

8. Firestorm Senior Member

Jan 24, 2005
353
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well done pebe...and yes windex i think you were looking at the problem 2 indepth...not a bad question though ...l8er

-fire