Discharging Capacitor experiment Watch

Paypurr
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We did our experiment a few days ago, and when working out the time constant from the graph I got 140s when i calculated it with the values of the capacitor and resistor I got 100s

When I did the charging experiment I got the time constant as 104, which is very close to the calculated value.

Anyone have any ideas on why it’s so different, we were told to calculate it from a curved graph so maybe I went wrong there. But I’m worried that I’ve made a silly mistake and this counts towards my practical endorsement so I need to pass it
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JammieDodger27
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You've probably got a range of 20+% on this experiment because the capacitors themselves likely have a percentage uncertainty of around 20%, before you include any other errors.

The biggest issue is that you're trying to calculate this from a curved graph. This is not the way we're taught to do it as part of physics A-Level.

Can you make it a straight line graph using logs?
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JammieDodger27
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(Original post by BobbJo)
I do agree that a straight line graph using logs makes this easier + more accurate

But I believe it is unwise to go against the instructions given
"we were told to calculate it from a curved graph"
It would tell you if the error is in the graph or the experiment
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Eimmanuel
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(Original post by Paypurr)
We did our experiment a few days ago, and when working out the time constant from the graph I got 140s when i calculated it with the values of the capacitor and resistor I got 100s. ...
What is the experiment that you are doing? Charging or discharging? What is the graph that you are plotting to obtain the time constant?

(Original post by Paypurr)
…..Anyone have any ideas on why it’s so different, we were told to calculate it from a curved graph so maybe I went wrong there. But I’m worried that I’ve made a silly mistake and this counts towards my practical endorsement so I need to pass it
Can you provide the specific instructions of the graph that you are supposed to plot and how do you calculate the time constant.
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DownbeatAquos
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(Original post by Paypurr)
We did our experiment a few days ago, and when working out the time constant from the graph I got 140s when i calculated it with the values of the capacitor and resistor I got 100s

When I did the charging experiment I got the time constant as 104, which is very close to the calculated value.

Anyone have any ideas on why it’s so different, we were told to calculate it from a curved graph so maybe I went wrong there. But I’m worried that I’ve made a silly mistake and this counts towards my practical endorsement so I need to pass it
First of all, your answer is incredible considering you were calculating from a curved graph, considering the way you calculate the time constant is by using the gradient (So I have no idea how you calculated a singular value for gradient), what you can do to fix it, is to replace your values for the y-axis with ln values of them, which will convert your graph to a straight line, then calculate the gradient, which is the time constant. Also calculate uncertainty using the range of your repeats divided by 2 and draw range bars and 2 lines of worst fit, to calculate your uncertainty for the time constant, and see if your true value lies between your 2 calculated values
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Paypurr
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(Original post by JammieDodger27)
You've probably got a range of 20+% on this experiment because the capacitors themselves likely have a percentage uncertainty of around 20%, before you include any other errors.

The biggest issue is that you're trying to calculate this from a curved graph. This is not the way we're taught to do it as part of physics A-Level.

Can you make it a straight line graph using logs?
For the discharging experiment we were told to do logs, I preferred it that way, we did the experiment with a timer. But the charging one we did with a data logger, and I don’t have access to the files so I only have a printed sheet of a curve graph so I have no values in a table or anything to go from. The guy next to me just rearranged the formula and chose a point and used that. I just don’t like the percentage different I got, I think I recall my tutor saying that as long as it’s under 50% it’s okay. But the method of obtaining the value concerns me and I don’t want to be marked down for not doing it properly that’s why I thought I would ask, it’s AQQ for you
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Paypurr
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(Original post by Eimmanuel)
Can you provide the specific instructions of the graph that you are supposed to plot and how do you calculate the time constant.
It’s for charging, we used a data logger. Voltage on Y axis and time on X axis. I have no table of values, because of the data logger and I don’t have access to the files. I used a point that lied on the curve and put them values into the equation THEN rearranged to find CR. I don’t think this is the correct way, but we haven’t got much guidance on this experiment. I think I may try and get access to the original files and plot logs. But I would appreciate if you could comment on how I obtained the value in the first place, is it even possible to do that?
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Paypurr
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(Original post by DownbeatAquos)
First of all, your answer is incredible considering you were calculating from a curved graph, considering the way you calculate the time constant is by using the gradient (So I have no idea how you calculated a singular value for gradient), what you can do to fix it, is to replace your values for the y-axis with ln values of them, which will convert your graph to a straight line, then calculate the gradient, which is the time constant. Also calculate uncertainty using the range of your repeats divided by 2 and draw range bars and 2 lines of worst fit, to calculate your uncertainty for the time constant, and see if your true value lies between your 2 calculated values
Thank you for your advice, I did plot logs for the discharging experiment, but used a data logger for charging so didn’t record a table to results as our tutor said because we used a data logger we didn’t need to. I am going to try and access the graph files and plot logs. Thanks again!
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Eimmanuel
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(Original post by Paypurr)
It’s for charging, we used a data logger. Voltage on Y axis and time on X axis. I have no table of values, because of the data logger and I don’t have access to the files. I used a point that lied on the curve and put them values into the equation THEN rearranged to find CR.
Is the curve smooth to you? If yes, how do you choose the point?
Do you still have access to the printed graph as you mentioned in post # 6?


(Original post by Paypurr)
…. I don’t think this is the correct way, but we haven’t got much guidance on this experiment. I think I may try and get access to the original files and plot logs. But I would appreciate if you could comment on how I obtained the value in the first place, is it even possible to do that?
I may try to access the original files to plot the ln graph rather than log graph.

As you are using data logger, your percentage uncertainty for the time constant is pretty high. When you use data logger, I believe you set the time interval to register the experimental results. Say that you set the time interval to 5 seconds, make sure the point that you choose to compute time constant are multiple of 5 seconds NOT something like 52 seconds, 111 seconds, etc.

In addition, you should use more than 1 point to compute the average value of the time constant to reduce the uncertainty.

Based on what you have mentioned in post #5, I suspect that you have used the incorrect value of V0. A common mistake for students. It would be good that you show your working before I go into it.
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Paypurr
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(Original post by Eimmanuel)
I may try to access the original files to plot the ln graph rather than log graph.

As you are using data logger, your percentage uncertainty for the time constant is pretty high. When you use data logger, I believe you set the time interval to register the experimental results. Say that you set the time interval to 5 seconds, make sure the point that you choose to compute time constant are multiple of 5 seconds NOT something like 52 seconds, 111 seconds, etc.

In addition, you should use more than 1 point to compute the average value of the time constant to reduce the uncertainty.

Based on what you have mentioned in post #5, I suspect that you have used the incorrect value of V0. A common mistake for students. It would be good that you show your working before I go into it.
Hi again!

I plotted the ln graph and worked out the CR from the gradient and it came to 151 (3sf)

The high reading may just be due to uncertainty, because using the stop watch I got a much closer value for time constant (when discharging)
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Eimmanuel
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(Original post by Paypurr)
Hi again!

I plotted the ln graph and worked out the CR from the gradient and it came to 151 (3sf)

The high reading may just be due to uncertainty, because using the stop watch I got a much closer value for time constant (when discharging)

Can you show the graph of the ln graph and voltage vs time?
I am quite surprised that the uncertainty or error is much larger for the data logger.
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Eimmanuel
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(Original post by Paypurr)
Hi again!

I plotted the ln graph and worked out the CR from the gradient and it came to 151 (3sf)
Did you plot the graph using a spreadsheet program such as Excel or Google Sheets?


(Original post by Paypurr)
...The high reading may just be due to uncertainty, because using the stop watch I got a much closer value for time constant (when discharging)
If the uncertainty of the time constant is found to be larger using a data logger, it is suggesting that the setup has some faults.
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