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# Electric field strength watch

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1. Hi I have some questions where I have to find the electric field strength. I've tried the questions two different ways (with 'or' separating the methods) and I don't know which method it correct.
2. (Original post by Je2)
Hi I have some questions where I have to find the electric field strength. I've tried the questions two different ways (with 'or' separating the methods) and I don't know which method it correct.
The second "method" is the correct one, because you are adding the field due to the two charges separately at that point.

The important point you have missed is that the field has direction.
In the case of, say, two positive charges, the force at the point midway between them due to one charge is in the opposite direction to the force due to the other. They are vectors, and in this case you have to subtract them to find the resultant.
When one is positive and the other is negative, the forces midway are in the same direction and must be added.
Remember that the field strength is defined as the force on a unit positive charge. So by inspection determine the direction at the mid point of the force due to the individual charges. That tells you whether they add or subtract.
3. Thank you! This is really useful information that I think my teacher should have mentioned
4. (Original post by Je2)
Thank you! This is really useful information that I think my teacher should have mentioned
Just be careful with the value of r in the formulas.

If the charges are 1m apart, then r = 1m in the formula to find the force of attraction or repulsion between them.
To find the field strength at a point midway between, you put r = 0.5m in each formula.
5. (Original post by Stonebridge)
The second "method" is the correct one, because you are adding the field due to the two charges separately at that point.

The important point you have missed is that the field has direction.
In the case of, say, two positive charges, the force at the point midway between them due to one charge is in the opposite direction to the force due to the other. They are vectors, and in this case you have to subtract them to find the resultant.
When one is positive and the other is negative, the forces midway are in the same direction and must be added.
Remember that the field strength is defined as the force on a unit positive charge. So by inspection determine the direction at the mid point of the force due to the individual charges. That tells you whether they add or subtract.
So for the first question I would add to get 324 N/C and for the second question I would subtract to get 45.0 N/C
6. (Original post by Je2)
So for the first question I would add to get 324 N/C and for the second question I would subtract to get 45.0 N/C
Yes. And r would be 0.5m in the 1st question and 1m in the second.
I haven't checked if you've done the calculation correctly, but the expressions are correct.

Edit. Checked answers and they are correct.
7. Thanks for the help!!

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Updated: November 16, 2013
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