# Find the electric field at a point outside of an insulating sphere?

#### hockey

1. The problem statement, all variables and given/known data
What is the electric field at a point .12m outside the surface of an isolated plastic sphere .23m in diameter that produces an electric field of 1350 N/C just outside the surface of the sphere?

2. Relevant equations
E = kQ/(r^2)

3. The attempt at a solution
First I found Q for the sphere with the above formula by substitution and found that it was 1.98375*10^-9C. From there, I thought that I could treat the sphere as a point charge and use the same formula to find the electric field, substituting as follows:
E= (9*10^9 Nm^2/C^2) (1.98375*10^-9C)/(.12m)^2 and got a result of 1239.8 N/C. This is not correct however, and I am not sure if it is because I am viewing the question wrong or not.

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#### gneill

Mentor
1. The problem statement, all variables and given/known data
What is the electric field at a point .12m outside the surface of an isolated plastic sphere .23m in diameter that produces an electric field of 1350 N/C just outside the surface of the sphere?

2. Relevant equations
E = kQ/(r^2)

3. The attempt at a solution
First I found Q for the sphere with the above formula by substitution and found that it was 1.98375*10^-9C. From there, I thought that I could treat the sphere as a point charge and use the same formula to find the electric field, substituting as follows:
E= (9*10^9 Nm^2/C^2) (1.98375*10^-9C)/(.12m)^2 and got a result of 1239.8 N/C. This is not correct however, and I am not sure if it is because I am viewing the question wrong or not.
Yes, you can treat the charged sphere as a point charge. But take a closer look at what you used as the distance from that charge for your second calculation. That distance was specified to be 0.12m outside of the original sphere... so where does that put it with respect to the "new" point charge?

"Find the electric field at a point outside of an insulating sphere?"

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