- #1
Terminal Voltage Formula Doesn't Apply?
In summary: The formula does work, but you need to understand its limitations and the assumptions it relies on. In summary, The question is asking why the terminal voltage formula does not work in this scenario. By using the formula, Vt = E - Ir, the answer should be 1.2 V, but the actual answer is 1.8 V. This is because the formula assumes that the current is flowing out of the positive terminal of the battery, but in this scenario, the current is flowing in the opposite direction. This changes the polarity of the potential drop across the internal resistor, resulting in a larger total potential drop across the battery terminals. Therefore, in order to find the correct terminal voltage, both the potential drop across the internal resistor
- #2
mirandab17
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any ideas?
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- #3
mirandab17
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Okay, sure I will!
Any thoughts on the initial question though?
Any thoughts on the initial question though?
- #4
SammyS
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That's the answer you would get if the current were flowing in the direction opposite what is given in this problem.mirandab17 said:
- #5
gneill
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mirandab17 said:
Label the potential drops on the circuit according to the current direction. Add them up. What do you get?
- #6
mirandab17
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Isn't there only one potential drop? Across the internal resistor?
- #7
SammyS
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There is the 1.5 Volt cell that's also between the terminals. Right?mirandab17 said:
- #8
gneill
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mirandab17 said:
Well, when you "walk" through the circuit in the direction of the current flow, do you have a potential rise or a potential drop as you pass though the battery?
- #9
mirandab17
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That counts as a potential drop too? Tell me, what exactly is a potential drop?
Anyway, okay, if I do that.
Potential drop across the internal resistor is V=IR = (.6)(.5) = .3
Adding that to the EMF (another potential drop) is 1.8 V.
Oh, well there's the answer then. :)
Anyway, okay, if I do that.
Potential drop across the internal resistor is V=IR = (.6)(.5) = .3
Adding that to the EMF (another potential drop) is 1.8 V.
Oh, well there's the answer then. :)
- #10
mirandab17
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So you have two potential drops within the battery?
- #11
gneill
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mirandab17 said:
It depends upon the current direction through the battery and the direction that you "walk" through the circuit. While the polarity of a cell does not change, whether you consider it to be a potential drop or a potential rise depends upon the direction you're going.
The polarity of the potential change across a resistor on the other hand clearly depends upon the direction of the current flowing though it. So both the current direction and the direction you choose to follow around the circuit matter.
EDIT: Note that your "Terminal Voltage Formula" assumes that the battery is producing a current that flows out of its positive terminal. In that case the internal resistance causes a voltage drop in the direction of the current flow, and so decreases the voltage that you see at the battery terminals. When the current is forced to flow in the other direction (battery charging), the potential drop across the internal resistance due to the current has the same polarity as that of the cell. So then the battery terminals have a larger total potential across them.
- #12
mirandab17
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Hmm... okay... so what would happen if the internal resistor was on the left hand side? Would there still be two potential drops?
I'm just confused because I haven't had to deal with adding up the potential drop across the internal resistor and EMF value itself before in order to find terminal voltage. I thought the terminal voltage equation would just work really simply.
I'm just confused because I haven't had to deal with adding up the potential drop across the internal resistor and EMF value itself before in order to find terminal voltage. I thought the terminal voltage equation would just work really simply.
- #13
gneill
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The location of the internal resistance doesn't matter. In fact, in a real battery it's distributed in various places in the chemistry of the cell. The simplified model that we work with has an ideal cell in series with a 'lumped' resistance which we call the internal resistance of the battery. Since the cell and resistance are in series, the same current flows through both so the sum of the potential changes is the same regardless of the order in which you add them.mirandab17 said:
It's good to know where a formula comes from and what assumptions it relies on so that you can judge its applicability in different situations. In many cases its easier (and safer!) to apply simple, basic circuit analysis rather than memorize a number of particular-case formulas.
- #14
mirandab17
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Is this an exception though? Because there have been so many other scenarios where I haven't had to do this. Why now did we have to add up the potential drops? As in, why didn't the formula work?
- #15
Dadface
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Imagine being given two batteries one of 9V and the other of 6V.There are two ways that these can be connected in series.One way gives a total voltage of 15V and the other way(with one of the batteries turned round) gives a total voltage of 3V.Sketch it out with the polarities marked.
Now look again at the advice given above and think about how the voltages add,with the current flowing one way and subtract with the current flowing the opposite way.
(if you get confused about current direction remember that there is a charging circuit)
Now look again at the advice given above and think about how the voltages add,with the current flowing one way and subtract with the current flowing the opposite way.
(if you get confused about current direction remember that there is a charging circuit)
- #16
gneill
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mirandab17 said:
See the EDIT addition that I made to post #11 in this thread.
FAQ: Terminal Voltage Formula Doesn't Apply?
1. What is the terminal voltage formula and why doesn't it always apply?
The terminal voltage formula is used to calculate the voltage difference between the positive and negative terminals of a battery or circuit. However, it may not always apply because there are other factors that can affect the voltage, such as internal resistance and external loads.
2. What is internal resistance and how does it affect terminal voltage?
Internal resistance is the resistance within the battery or circuit itself. It is caused by the materials and design of the battery or circuit and can cause a drop in voltage. This means that the actual terminal voltage may be lower than the calculated value from the formula.
3. Can external loads affect the terminal voltage?
Yes, external loads such as resistors, motors, or other components in the circuit can affect the terminal voltage. These loads can cause a drop in voltage due to the current passing through them, resulting in a lower terminal voltage than the calculated value from the formula.
4. How can we account for internal resistance and external loads when using the terminal voltage formula?
To account for these factors, we can use a modified version of the terminal voltage formula that takes into consideration the internal resistance and external loads. This modified formula is known as the "internal resistance model" and provides a more accurate calculation of the terminal voltage.
5. Are there any other factors that can affect the terminal voltage?
Yes, there are other factors that can affect terminal voltage, such as temperature, age of the battery, and variations in the materials used. These can cause fluctuations in the voltage and may not always align with the calculated value from the formula.