Ohm's Law - Resistance

[Sharie]

Ohm's Law - Resistance?

 

Below are the results of an experiment my group did at school while I was away sick. The experiment was to determine the resistance of an unknown resistor. The Resistor's actual resistance was 2.2 ohms, but the results (as you can see) are all over the place. The circuit included a voltmeter, ammeter and rheostat (which was adjusted with each trial) I have been reading and reading about Ohm's law, circuits and percentage errors, but I have only managed to confuse myself more. Basically, I need to explain the results and if the percentage errors are acceptable, but I am having trouble understanding all of this.

 

1. Can someone please explain why these results likely occurred (errors in the experiment)? Do the voltmeter, ammeter abd reheostat impact on resistance? and;

2. Are there any relationships between the results that I should highlight (like the amps dropping as the resistance increases in Trails 3 and 4 where the voltage is constant)?

 

I am so confused. Please help.

 

Trial, Current, Voltage, Resistance, Percentage Error

1 0.74, 1.5, 2.027, -7.863%

2 0.82, 1, 1.2195, -44.5681%

3 0.2, 0.5, 2.5, 13.53%

4 0.14, 0.5, 3.5714, 62.336%

[Xittenn]

1) Do the voltmeter, ammeter and rheostat have passive components within them?

 

2) Where did you get the resistors resistance from, what is its accuracy?

 

3) Where did the voltage come from, does it have any internal passive components? Active components?

[Sharie]

1) Do the voltmeter, ammeter and rheostat have passive components within them?

I am unsure of this as the teacher supplied the group with the equipment.

 

 

2) Where did you get the resistors resistance from, what is its accuracy?

 

The teacher sent an email out following the experiment that revealed the group's resistor's resistance.

 

3) Where did the voltage come from, does it have any internal passive components? Active components?

The voltage came from a power pack set at 4 volts.

 

 

Sorry, I am only a year 10 student and I was away the day the experiment was conducted, so I only have the group's notes to go on.

 

 

Hope you can help point me in the right direction.

[Xittenn]

Ahhh this is difficult because it is hard to help without giving you the answers, as this is homework.

 

1) your measuring devices do have passive components, do you feel this could be meaningful? how do you think a manufacturer might address this?

 

2) resistors have a range of values based on the coloured bands that they are marked with, knowing the bands can help you track error/uncertainty

 

3) power supplies also contribute to the problem because they do in fact contain active and passive components

 

You are in grade 10 so I wouldn't stress about the details, but try to take these points into consideration and write it out. Then reflect on it and make a decision if it sounds appropriate or not. Post your thoughts and someone will probably give you more feedback.

Edited February 26, 2012 by Xittenn

[Schrödinger's hat]

Suggestions, these are very useful for understanding what you did:

Draw (and post, it may help us give hints without giving answers directly) graphs of your results (current vs voltage).

Draw (and post) a circuit diagram.

Describe your apparatus in detail, how did you record each variable? Where did you source your errors? Maybe mark on the circuit diagram where/how the voltages/currents/resistances were measured. When did you measure your voltage/current?

Edited February 26, 2012 by Schrödinger's hat

[Joatmon]

There is something very wrong when you consider trials 3 and 4. The results suggest that under the same conditions concerning applied voltage you got two very different values of current. It's as if you took a reading, left the equipment connected, went back later and the current had changed significantly. Definitely not in accordance with Ohms law.

Edited February 26, 2012 by Joatmon

[swansont]

Ohm's Law - Resistance?

 

Below are the results of an experiment my group did at school while I was away sick. The experiment was to determine the resistance of an unknown resistor. The Resistor's actual resistance was 2.2 ohms, but the results (as you can see) are all over the place. The circuit included a voltmeter, ammeter and rheostat (which was adjusted with each trial) I have been reading and reading about Ohm's law, circuits and percentage errors, but I have only managed to confuse myself more. Basically, I need to explain the results and if the percentage errors are acceptable, but I am having trouble understanding all of this.

 

1. Can someone please explain why these results likely occurred (errors in the experiment)? Do the voltmeter, ammeter abd reheostat impact on resistance? and;

2. Are there any relationships between the results that I should highlight (like the amps dropping as the resistance increases in Trails 3 and 4 where the voltage is constant)?

 

I am so confused. Please help.

 

Trial, Current, Voltage, Resistance, Percentage Error

1 0.74, 1.5, 2.027, -7.863%

2 0.82, 1, 1.2195, -44.5681%

3 0.2, 0.5, 2.5, 13.53%

4 0.14, 0.5, 3.5714, 62.336%

 

It's curious that the voltage is in increments of 0.5V — do you know if this was a limit of the precision of the meter? That would be a large source of error, because (as your data show) the current varies a bit without the indicated voltage changing.

 

One very important note: "error" is not the difference between what you measure and the right answer; this is a concept that (in my experience as a TA) even college students have a hard time with. In actual science research you don't know what the "right" answer is. Errors give you the range of results consistent with your experiment.

 

If the voltmeter is indeed the limitation here, a large component of your true error is that the measured voltage is V ± 0.5 V. One way to analyze this is to graph V vs I, put the error bars in place and get your best fit; the slope gives you R. The limits of the line that still fit the data would be your error.