# Simple circuit question

Discussion in 'OT Technology' started by terminator1010, Nov 7, 2005.

1. ### terminator1010Eld

Joined:
Dec 23, 2001
Messages:
4,891
0
Location:
Hordanes Land
I've got a 100Kohm resistor connected to the posive terminal of a 9V battery. This is an open circuit. The voltage across just the battery is 8.6V, the voltage from the resistor to the negative terminal of the battery is ~7.5V. I'm wondering why there is a ~1.1V voltage drop across the resistor if there is no current flowing through the resistor (cause its an open circuit)

2. ### High VoltageGuest

r u trying to get me to do your homework for you

3. ### TillsLets Go Flyers

Joined:
Dec 8, 2001
Messages:
12,684
0
Location:
Los Angeles

When you are when you are taking the measurement, what are you doing to the circuit?

4. ### terminator1010Eld

Joined:
Dec 23, 2001
Messages:
4,891
0
Location:
Hordanes Land
LOL, I wish my homework were this simple.

I'm not doing anything to the circuit, it remains an open circuit. (i'm completing the cicuit with the voltmeter when taking the voltage at the open end of the resistor and the negative terminal of the battery. Getting 7.5V) This seems to defy the most basic laws of circuit anaylsis. Open circuit = no current flowing = no voltage drop across resistor. In theory I should be getting 8.6V when i'm actually getting 7.5V.

5. ### TillsLets Go Flyers

Joined:
Dec 8, 2001
Messages:
12,684
0
Location:
Los Angeles
Current flows positive to negative. If you are measuring from the "Open" end of the resistor (the closed end being attached to the positive side of the source) to the negative side, you are measuring the voltage after the resistor hence the drop in voltage.

6. ### ScrapinSiNew Member

Joined:
Jul 4, 2005
Messages:
155
0
DING! That is Correct!

7. ### twistidBanged By Super ModelsModerator

Joined:
Jul 15, 2001
Messages:
41,165
327
Location:
The Kansass/Oklahomo Border
DC current always flows from negative to positive, where AC current flows back and forth between positive and negative.

8. ### terminator1010Eld

Joined:
Dec 23, 2001
Messages:
4,891
0
Location:
Hordanes Land
I don't quite understand this. There is no current flowing in this circuit though. So what causes the voltage drop? Also when I put the voltmeter across just the resistor, I get zero volts. meaning no voltage drop. Why is this?

9. ### TillsLets Go Flyers

Joined:
Dec 8, 2001
Messages:
12,684
0
Location:
Los Angeles
I have a DC power source with a positive +5V and a +12V, which way is the current flowing?

10. ### TillsLets Go Flyers

Joined:
Dec 8, 2001
Messages:
12,684
0
Location:
Los Angeles
When you are placing the meter across the neg terminal and the open end of the resistor you are completeing the circuit hence current is flowing. The voltage drop that you see is from the 100kOhm resistor.

If you attach the resistor to the neg term of the battery then take a measurement from the positive side to the open end of the resistor you should see full voltage.

11. ### terminator1010Eld

Joined:
Dec 23, 2001
Messages:
4,891
0
Location:
Hordanes Land
Nope, tried putting the resistor on the neg side, same voltage, sign reversed of course. Current does actually flow from negative to positive. Current is the flow of electrons. The negative terminal or ground provides the electrons and they flow to the abscence of electrons (positive voltage). There is a current convention though that states you can take current flow from positive to negative as most people use.

12. ### TillsLets Go Flyers

Joined:
Dec 8, 2001
Messages:
12,684
0
Location:
Los Angeles
Very Good...now the question is why the voltage drop is still there? (I expected it to be)

We are both right and wrong depending on which viewpoint that you take but I am right Pos to Neg . Lets save this debate for a later time

13. ### twistidBanged By Super ModelsModerator

Joined:
Jul 15, 2001
Messages:
41,165
327
Location:
The Kansass/Oklahomo Border
http://easyelectronics.tripod.com/dcac.htm
Conventionally, dc current is regarded as being from 'positive' to 'negative' of a battery or any other dc source (such as a dynamo). It is a 'stream' flow, just like the water flow in the hydraulic circuit, but the 'stream' is actually composed of sub-atomic particles or electrons. Unfortunately, after convention had established the 'positive to negative' flow definition it was found that this electron stream flow was actually from negative to positive. This does not matter for most practical purposes, but for an understanding of how transistors and other solid-state devices work it is necessary to appreciate this 'reverse' working

http://www.school-for-champions.com/science/dc.htm
Direct current or DC electricity is the continuous movement of electrons from an area of negative (-) charges to an area of positive (+) charges through a conducting material such as a metal wire. Whereas static electricity sparks consist of the sudden movement of electrons from a negative to positive surface, DC electricity is the continuous movement of the electrons through a wire.

http://www.xtant.com/html/techSupport/electronics.cfm
The movement of free electrons from one atom to another is called current flow. Current always flows from negative to positive. A good analogy can be made with a water hose. The hose may be considered a conductor, and the water moving electrons. The current, measured in amperes, would be the volume of water passing by a point along the hose in a given amount of time. The higher the volume, the more current.

i could go on...

14. ### terminator1010Eld

Joined:
Dec 23, 2001
Messages:
4,891
0
Location:
Hordanes Land
I suspect it has something to do with the internal resistance of the source, or the non-idealties of the voltage source.

15. ### ScrapinSiNew Member

Joined:
Jul 4, 2005
Messages:
155
0
how about drawing up some schematics to show us exactly where you are measuring,and what your talking about?

16. ### terminator1010Eld

Joined:
Dec 23, 2001
Messages:
4,891
0
Location:
Hordanes Land
Too much work. Its not that hard to understand, resistor attached to battery.

17. ### TillsLets Go Flyers

Joined:
Dec 8, 2001
Messages:
12,684
0
Location:
Los Angeles
You are ignoring the electon holes which travel from positive to negative and produce current. As I said before we were both right and wrong depending on the viewpoint. If you have any questions on the matter feel free to ask them and I will explain it.

Last edited: Nov 12, 2005
18. ### TillsLets Go Flyers

Joined:
Dec 8, 2001
Messages:
12,684
0
Location:
Los Angeles
You need to make sure the 9v is an energizer

In a properly working battery, I don't believe you should see a voltage drop that large due to the source.

Have you calculated what the voltage drop over the resistor should be in your circuit?

19. ### twistidBanged By Super ModelsModerator

Joined:
Jul 15, 2001
Messages:
41,165
327
Location:
The Kansass/Oklahomo Border
here's a simple example: when you wire an amp, you put the fuse on the + side close to the battery. this one protects from a short in the wiring... if the wire from the fuse to the amp shorts out, it will pop the fuse to stop from completing the circuit because dc current flows - to +... if it flowed the other way you'd have to fuse the ground instead.

Joined:
Dec 8, 2001
Messages:
12,684