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## Homework Statement

The electrostatic potential energy of a system of point charges q1 = 1 µC, q2 = 2 µC, and q3 = 3 µC at the corners of an equilateral triangle whose side s = 30 cm is

a. 1.10 J

b. 0.990 J

c. 0.631 J

**d. 0.330 J**

e. 0.123 J

## Homework Equations

See below.

## The Attempt at a Solution

PE = k*q*q_0/(r)

PE_total = [(k*(1*10^-6 C)*(3*10^-6 C)/(.30 m)] + [(k*(1*10^-6 C)*(2*10^-6 C)/(.30 m)] + [(k*(3*10^-6 C)*(2*10^-6 C)/(.30 m)]

= k/.30 m*[(3*10^-12 C^2) + (2*10^-12) + (6*10^-12)] = 0.3296 J ??

## Homework Statement

Two flat parallel plates are a distance d = 0.40 cm apart. The potential difference between the plates is 360 V. The electric field at a point midway between the plates is approximately

**a. 90 kN/C**

b. 3.6 kN/C

c. 0.9 kN/C

d. zero

e. 3.6*10^5 N/C

## Homework Equations

See below.

## The Attempt at a Solution

E = delta V/d = 360 V/0.004 m = 90,000 V/m = 90,000 N/C ?

Thanks.