# Homework Help: Electric field as a function - potential

1. Jun 9, 2008

### scholio

electric field as a function -- potential

1. The problem statement, all variables and given/known data

assume that the electric field in space is given by E = E_o*e^(-r/R) where r is the radial distance away from the origin and E_o and R are constants. E points away fro the origin. Calculate the electric potential at any point r if zero potential is taken at r = +infinity.

i should get electric potential V(r) = E_o*R*e^(-r/R)
2. Relevant equations

point charge electric potential V = V(r) = kq/r where k is constant = 9*10^9, q is charge, r is distance

electric potential difference deltaV_AB = V_B =V_A = - [<integral>E*dr] from r_A to r_B

3. The attempt at a solution

what does it mean that E points away from the origin, how does knowing that affect the problem?

what is the integral of the E function, how integrate the e^(-r/R) portion specifically?

if i let r = infinity, then in the E function then e^(-infinity) = 0, so the function goes to zero

any tips on how to get started appreciated...

2. Jun 9, 2008

### alphysicist

scholio,

The integral is:

$$\Delta V_{ab} = -\int\limits_{a}^{b} \vec E\cdot d\vec r$$

and so since there is a dot product inside the integral, you need to know the (relative) directions of $d\vec r$ and $\vec E$ in order to write down the integral for this particular problem.

3. Jun 9, 2008

### scholio

the problem states that E points away from the origin. and r is the radial distance away from the origin. so does that make E negative and dr positive?

so now do i sub in the function in for E in the integral, how do i integrate e^(-r/R)?

since i want to calculate for electric potential V, is the charge at the origin, thus a point charge?

4. Jun 9, 2008

### HallsofIvy

No, it means that r and E both point in the same direction- so their dot product is just the product of their lengths.

Use the substitution u= r/R.

??There is no mention of a "charge at the origin", just an electric force field- no mention of what causes the force field.