#$&*
Phy 122
Your 'question form' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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Signs on charge
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I cannot find the exact problem again - I accidentally close a window and lost the previous submit question that had the exact wording copied - but it was similar to this one from Problem set 2 #4
Problem
A battery with negligible internal resistance maintains a potential difference of 8 volts in a uniform conducting wire 1.5 meters long. What force is experienced by a conduction electron in this wire (electron charge has a magnitude of 1.6 * 10^-19 Coulomb)?
Solution
From the charge (Coulombs) and voltage (Joules / Coulomb) we can find the amount of work done on a given charge over the length of the wire. Then from the work and distance we can easily find the force.
The work done is 8 J/C (1.6 x 10^-19 C) = 12.8 x 10^-19 J = 12.8 x 10^-19 N m.
If 12.8 x 10^-19 N m of work are done by a uniform force over a distance of 1.5 meters, the force is 12.8 x 10^-19 N m / ( 1.5 m) = 8.533 * 10^-19 N.
Generalized Solution
The work done on charge q through potential difference `dV is `dW = q `dV.
If this work is done over distance `dx, then we see that the average force exerted on q is Fave = `dW / `dx = q `dV / `dx.
The quantity `dV / `dx is the average potential gradient, also called the average electric field, over the path followed by the charge.
Note that the force on the charge is always in the direction of the wire, as is the distance `dx.
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In the question I lost, the charge on the electron was not expressed in magnitude, but with a charge of -1.6 * 10-`19 Coloumb. The answer to the question was positive - the negative charge seemingly had no affect on the calculation. Is that because of the statement in the last sentence of the general answer Note that the force on the charge is always in the direction of the wire, as is the distance `dx.
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@&
The work done on the charge is the product of the charge and the field. The force on the charge and its displacement will always be in the same direction. If the charge is negative, the force on it will be in the direction opposite the field, as will its displacement. If the charge is positive both the force and the displacement will be in the direction of the field.
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#$&*
Phy 122
Your 'question form' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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Defer Labs
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I now have 3 remaining labs: Two due today 4/14 and one due next week 4/21.
I have gotten behind on the labs do to some work projects eating up my weekends.
I know I can have all three labs completed by 4/28 and request that I b able to defer the due dates until then.
Thanks,
@&
That won't be a problem.
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