Straight Dope Message Board > Main Basic Electrical Question
 Register FAQ Calendar Mark Forums Read

#1
01-27-2004, 07:56 AM
 Munster Guest Join Date: Nov 2003
Basic Electrical Question

This is a problem which has been bugging me for some time now.

Suppose you got two ideal voltage sources, one 15V the other 10V say, and grounded the two negative terminals. Then you connected both positive terminals to the same node. What would the voltage at the node be? Wouldn't both voltage sources be trying to give the node a potential equal to there own?

Also wouldn't the same problem arise for two ideal current sources in the same branch of a circuit?

Seeing as I am talking about ideal sources feel free to infrom me on how non-idealities would make this problem solvable. In other words does the fact that there is no such thing as an ideal voltage source make this problem impossible to solve?
#2
01-27-2004, 08:01 AM
 GreyWanderer Guest Join Date: Oct 2003
It would short circuit. It's the same thing is connecting a wire from the 15V to directly to ground. Only the difference becomes 5V instead of 15V
#3
01-27-2004, 08:04 AM
 Munster Guest Join Date: Nov 2003
Dammit just realisesd that an earlier post of this question had posted even though the link timed out.

Cheers anyway.
#4
01-27-2004, 08:19 AM
 Patty O'Furniture Guest Join Date: May 1999
Assuming the two sources have the same current output capacity, the lower voltage source would lose in this tug of war as it tried to draw current from the other source. This is why you are told to replace batteries (cells) in pairs instead of singly.
__________________
Join Date: May 20, 1999
Location: Bangkok, Thailand
Posts: 8,513
#5
01-27-2004, 10:11 AM
 CurtC Guest Join Date: Dec 1999
A voltage source can be pretty well modeled by an ideal voltage source with a series resistance. The beefier sources have a lower resistance. This means that as it outputs more current, the voltage at the output drops, because voltage is dropped across that resistor.

So if you draw a circuit with the voltage sources modeled as a V with a series R, it's pretty easy to see what happens.

 Bookmarks

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is Off HTML code is Off
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Main     About This Message Board     Comments on Cecil's Columns/Staff Reports     Straight Dope Chicago     General Questions     Great Debates     Elections     Cafe Society     The Game Room     In My Humble Opinion (IMHO)     Mundane Pointless Stuff I Must Share (MPSIMS)     Marketplace     The BBQ Pit Side Conversations     The Barn House

All times are GMT -5. The time now is 04:01 AM.