# When is an Electric Field eqn set as - or + ?

• PerpetuallyConfused
In summary: To find the field due to a positive charge, set E = kQ/r^2 in this coordinate system, and to find the field due to a negative charge, set E = -kQ/r^2. In summary, the electric field at the origin is (8.00 + 6.00) Newtons per coulomb pointing in the direction of the positive x-axis and the y-axis.
PerpetuallyConfused

## Homework Statement

Two point charges are placed on the x axis.(Figure 1)The first charge, q1 = 8.00 nC , is placed a distance 16.0 m from the origin along the positive x axis; the second charge, q2 = 6.00 nC , is placed a distance 9.00 m from the origin along the negative x axis.

Find the electric field at the origin, point O.

Give the x and y components of the electric field as an ordered pair. Express your answer in Newtons per coulomb to three significant figures. Keep in mind that an x component that points to the right is positive and a y component that points upward is positive.

## Homework Equations

I know I have to use the equation E = kQ/r^2 and I have to calculate E for each charge.

## The Attempt at a Solution

So far I have E2 = kQ2/r2^2 and E1 = - kQ1/r1^2
I am confused on how I know when to set E = kQ/r^2 as positive or negative?

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PerpetuallyConfused said:

## Homework Statement

Two point charges are placed on the x axis.(Figure 1)The first charge, q1 = 8.00 nC , is placed a distance 16.0 m from the origin along the positive x axis; the second charge, q2 = 6.00 nC , is placed a distance 9.00 m from the origin along the negative x axis.

Find the electric field at the origin, point O.

Give the x and y components of the electric field as an ordered pair. Express your answer in Newtons per coulomb to three significant figures. Keep in mind that an x component that points to the right is positive and a y component that points upward is positive.
View attachment 220767

## Homework Equations

I know I have to use the equation E = kQ/r^2 and I have to calculate E for each charge.

## The Attempt at a Solution

So far I have E2 = kQ2/r2^2 and E1 = - kQ1/r1^2
I am confused on how I know when to set E = kQ/r^2 as positive or negative?
Conventionally, the field due to a positive charge is directed away from the charge and that due to a negative charge is directed towards the charge.

Assume any direction to be positive and find the resultant. Its sign will tell you whether it is along the positive direction or the negative direction.

Draw two arrows at the origin indicating the electric field due to each charge. Arrows to the right are positive, arrows to the left are negative. Add the two arrows as vectors.

cnh1995 said:
Assume any direction to be positive ...
The coordinate system posted by OP is already chosen so that positive x and y are "to the right" and "up".

## 1. What is an electric field equation?

An electric field equation is a mathematical representation of the strength and direction of the electric field at a given point in space. It is typically written as E = F/q, where E is the electric field, F is the force on a test charge, and q is the magnitude of the test charge.

## 2. When is an electric field equation set as negative?

An electric field equation is set as negative when the direction of the electric field is opposite to the direction of the force on a positive test charge. This means that the electric field is pointing towards the negative charge or away from the positive charge.

## 3. When is an electric field equation set as positive?

An electric field equation is set as positive when the direction of the electric field is the same as the direction of the force on a positive test charge. This means that the electric field is pointing towards the positive charge or away from the negative charge.

## 4. How do you determine the direction of the electric field using the electric field equation?

The direction of the electric field can be determined using the electric field equation by considering the sign of the charge that is creating the electric field. If the charge is positive, the electric field will point away from it. If the charge is negative, the electric field will point towards it.

## 5. Can the electric field equation be used to calculate the strength of an electric field due to multiple charges?

Yes, the electric field equation can be used to calculate the strength of an electric field due to multiple charges. In this case, you would need to calculate the electric field at a given point due to each individual charge and then add them together using vector addition.

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