# Need help on homework

1. An electron is released from rest in a uniform electric field and accelerates to the north at a rate of 120 m/s2. What are the magnitude and direction of the electric field?

2. f=ma

3. F=(9.11e-31)(120m/s^2)=1.1e-28

2.Two small nonconducting spheres have a total charge of 92.4 µC
a) when placed 1.00 m apart, the force each exerts on the other is 12.0 N and is repulsive. What is the charge on each? Smaller and Larger
b)What if the force were attractive?

If anyone could help me that would be great, i just started physics 2 a year after physics 1 so i am having some difficulties. I thought i did the first one right but the computer says i have the wrong answer, and i dont really know where to start on the second. Thanks

rock.freak667
Homework Helper
1. An electron is released from rest in a uniform electric field and accelerates to the north at a rate of 120 m/s2. What are the magnitude and direction of the electric field?

2. f=ma

3. F=(9.11e-31)(120m/s^2)=1.1e-28

So you have the force, now what is the electric field defined as?

2.Two small nonconducting spheres have a total charge of 92.4 µC
a) when placed 1.00 m apart, the force each exerts on the other is 12.0 N and is repulsive. What is the charge on each? Smaller and Larger
b)What if the force were attractive?

the electric force between two point charges, Q1 and Q2, separated by a distance r is given by:

$$F=\frac{Q_1 Q_2}{4 \pi \epsilon r^2}$$

You will need this equation and what is in bold

rl.bhat
Homework Helper
In the problem 1, force on the electron F = E*q = m*a where E is th electric field, q is the charge on electron. Now find E.
In the problem 2, q1 = q and q2 = 92.4 μC - q, if q1 and q1 are positive.
What should the charges for repulsive force?

For Number 1 would i just use F=QE where F=1.1e-28, Q=1.6e-19 then solve for E?

And for Number 2 wont i still have two unknowns, F and q? Sorry im just really lost on this one. thanks

rl.bhat
Homework Helper
And for Number 2 wont i still have two unknowns, F and q? Sorry im just really lost on this one. thanks

From the problem it implies that the attractive or the repulsive force has the same magnitude i.e. 12 N.

Thanks for the help i got the right answers. Is there an easier way then doing the quadradic equation though? It was a pain carrying all the numbers through

rl.bhat
Homework Helper
Thanks for the help i got the right answers. Is there an easier way then doing the quadradic equation though? It was a pain carrying all the numbers through
Since the sum of the charges is given, you cant avoid the quadratic equation.