Find the magnitude of the electric field at point P

In summary, two identical spheres with the same charge at the vertices of an equilateral triangle will exert an outward electric field at point P. The horizontal force components are equal and opposite, resulting in a net electric field of 6910 N/C. The mistake in the calculation was taking the wrong distance between P and the midpoint between the two charges, resulting in an incorrect answer. The correct answer is B, with a net electric field of 5196 N/C.
  • #1
paulimerci
287
47
Homework Statement
Question posted below.
Relevant Equations
E = kq/r^2
There are two identical spheres with the same charge that are the vertices of an equilateral triangle. ##+3 \mu C## will exert an outward electric field, which is drawn in the FBD below (see the attached pic), Since the horizontal force components (1x and 2x) are equal and opposite at point P, we can cancel those two vectors, and the remaining y component vectors will give the resultant magnitude of the electric field at point P.
$$ E = \frac {kq}{r^2}$$
$$ E_{1y} = \frac {8.99 \times 10^9 \times 3 \times 10^-6 sin 60} {6.76}$$
$$ E_{1y} = 3455 N/C$$

$$E_{2y} = \frac {8.99 \times 10^9 \times 3 \times 10^-6 sin 60} {6.76}$$
$$ E_{2y} = 3455 N/C$$
$$ E _{net} = 6910 N/C$$
I'm not sure where I made a mistake because none of the answers match my answer. Can anyone point out where my mistake is?
 

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  • #2
What does r represent in your relevant equation? Think.
 
  • #3
haruspex said:
What does r represent in your relevant equation? Think.
r is the distance between the charge and the reference point (P).
 
  • #4
paulimerci said:
r is the distance between the charge and the reference point (P).
I see where I did a mistake I took the wrong distance (between P and the midpoint between two charges). r=3m, Is ##E_{1y} + E_{2y} = 2598+2598 = 5196 N/C##. So the answer choice is B?
 
  • #5
paulimerci said:
I see where I did a mistake I took the wrong distance (between P and the midpoint between two charges). r=3m, Is ##E_{1y} + E_{2y} = 2598+2598 = 5196 N/C##. So the answer choice is B?
Looks right.
 
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Likes paulimerci
  • #6
haruspex said:
Looks right.
Thank you!
 

FAQ: Find the magnitude of the electric field at point P

What formula do I use to find the magnitude of the electric field at point P?

The formula you use depends on the configuration of the charges. For a point charge, use Coulomb's law: \( E = \frac{k |q|}{r^2} \), where \( E \) is the electric field, \( k \) is Coulomb's constant, \( q \) is the charge, and \( r \) is the distance from the charge to point P.

How do I find the direction of the electric field at point P?

The direction of the electric field is given by the direction of the force that a positive test charge would experience at point P. For a positive source charge, the field points away from the charge, and for a negative source charge, it points toward the charge.

What if there are multiple charges? How do I find the resultant electric field at point P?

If there are multiple charges, calculate the electric field due to each charge at point P separately using the principle of superposition. Then, vectorially add these individual fields to get the resultant electric field at point P.

What units should the electric field magnitude be expressed in?

The magnitude of the electric field is expressed in Newtons per Coulomb (N/C) or equivalently in Volts per meter (V/m).

How does the distance from the charge affect the electric field at point P?

The electric field magnitude decreases with the square of the distance from the charge. Specifically, for a point charge, the electric field \( E \) is inversely proportional to the square of the distance \( r \) from the charge, as given by \( E = \frac{k |q|}{r^2} \).

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