Calculate the charge on an oil droplet

AI Thread Summary
The discussion centers on calculating the charge of an oil droplet, with one participant noting a discrepancy between their result of 6.67 x 10^-19C and the book's answer of 3.2 x 10^-19C. Participants agree that the book likely contains an error and emphasize the importance of including the negative sign for charge and rounding to two significant figures. They suggest using a systematic approach by combining formulas before performing calculations. Additionally, they recommend including units in intermediate steps and working with symbols for clarity. Overall, the conversation highlights common pitfalls in physics calculations and encourages best practices.
Shaye
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Homework Statement
A tiny negatively charged oil drop is held stationary in the electric field between two horizontal parallel plates, as shown below (please see attachment). Its mass is 4.0x10^-15kg.

Question 15,b) Use the fact that the 2 forces balance to calculate the charge on the oil drop. (g = 10 N Kg^-1).
Relevant Equations
1. F - Ma
2. E = F/Q or E = V/D (Electric field strength)
The book gives an answer of Q = 3.2 x 10^-19C

I get an answer of 6.67 x 10^-19C. Workings below:

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Shaye said:
The book gives an answer of Q = 3.2 x 10^-19C

I get an answer of 6.67 x 10^-19C.
Hi @Shaye. I agree with your calculation. Looks like a mistake in the book.

Other points:
- don’t forget the minus sign for the charge;
- round the answer to 2 significant figures;
- in your intermediate step, when you find the value of the force, it’s good practice to include the unit (N);
- consider working in symbols and leaving the arithmetic to the end.
 
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Steve4Physics said:
- consider working in symbols and leaving the arithmetic to the end.
I agree. At this level, I would expect you to combine two simple formulas before plugging in the numbers:
$$mg = qE = \frac{qV}{d} \ \Rightarrow \ q = \frac{mgd}{V}$$Then you do that on a calculator.
 
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Steve4Physics said:
Hi @Shaye. I agree with your calculation. Looks like a mistake in the book.

Other points:
- don’t forget the minus sign for the charge;
- round the answer to 2 significant figures;
- in your intermediate step, when you find the value of the force, it’s good practice to include the unit (N);
- consider working in symbols and leaving the arithmetic to the end.
Thanks @Steve4Physics
 
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PeroK said:
I agree. At this level, I would expect you to combine two simple formulas before plugging in the numbers:
$$mg = qE = \frac{qV}{d} \ \Rightarrow \ q = \frac{mgd}{V}$$Then you do that on a calculator.
Yeah I should probably start to do this going forward @PeroK Good tip!
 
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