• Support PF! Buy your school textbooks, materials and every day products Here!

Simple circuit question

  • Engineering
  • Thread starter ƒ(x)
  • Start date
  • #1
324
0

Homework Statement


I'm going to try drawing the circuit. Numbers are added to it for reference. I apologize for the formatting. I'm not sure how to do white space. Edit: for VB the + is on top.

VA and VB are switches.

I need to figure out the voltage across VA and VBB when they are:
1. open, open
2. open, closed
3. closed, open
4. closed, closed

1-------(+ VA -)-------(+ 8 V -)-------2--------------3
|_______________________________|___________|
12Ω____________________________15Ω_______(+ VB -)
|_______________________________|___________|
6-----------------------------------------5--------------4




Homework Equations


V = IR
Kirchhoff's Laws


The Attempt at a Solution



1. When they are both open there isn't a circuit. So, VA = 0 V and VB = 0 V.
2. Again, no circuit.
3. There is a circuit. It has a total resistance of 27 Ω (I don't know if this matters). VA = -8 V and VB = 0 V.
4. I'm not sure about this one. There are two loops, but loop 1256 has a total resistance of 27 Ω and loop 1346 has a total resistance of 12 Ω. So 1256 gets 27/39 = 9/13 of the current and voltage and 1346 gets 12/39 = 4/13 of the current and voltage? So VA = -8*9/13 V and VB = -8*4/13 V?
 
Last edited:

Answers and Replies

  • #2
NascentOxygen
Staff Emeritus
Science Advisor
9,243
1,068
1. When they are both open there isn't a circuit. So, VA = 0 V and VB = 0 V.
Circuit or no circuit, does not give the answer. If a voltmeter (or any device) were to be connected between the 2 terminals of the switch in question, would that form a path for current to flow through the meter? If so, what voltage would it register? That's the question you must address.
 
  • #3
NascentOxygen
Staff Emeritus
Science Advisor
9,243
1,068
Code:
If you wish to construct ASCII diagrams, enclose those lines
between [b][[/b][b]code][/b] and [b][/code[/b][b]][/b] statements to display in a
fixed-width font.
 
  • #4
CWatters
Science Advisor
Homework Helper
Gold Member
10,528
2,294
1. When they are both open there isn't a circuit. So, VA = 0 V and VB = 0 V.
You need a circuit for current to flow. You don't need a circuit for a voltage to be present. For example a 9V battery still produces 9V even when it's disconnected from the circuit.
 

Related Threads for: Simple circuit question

Replies
3
Views
468
Replies
3
Views
1K
Replies
1
Views
1K
Replies
2
Views
1K
Replies
2
Views
1K
Replies
13
Views
3K
Replies
1
Views
894
Top