Homework Help: Don't know where to start here :-\

1. Mar 17, 2004

NIT14

How can I solve this problem?...

An electron is placed at point P in the electric field set up by a source charge, Q. Point P is located 50 cm from Q and has an electric field strength of 1.08x10^5 N/C directed away from Q. What is the magnitude of charge Q?

2. Mar 17, 2004

da_willem

Coulombs law:

$$E(r) = \frac{Q}{4 \pi \epsilon r^2}$$

gives the electric field strength E(r) at a distance r from a point charge Q. The electrical permittivity $$\epsilon$$ can be found in a tablebook. For vacuum or air it is approx. 8.85e-12 F/m.

Last edited: Mar 17, 2004
3. Mar 17, 2004

Chi Meson

Just a comment:
the fraction$$\frac{1}{4 \pi \epsilon}$$
is equal to the coulomb constant "k" (8.99 E9)

4. Mar 17, 2004

Chen

Only when:

$$\epsilon = \epsilon_0$$

If you are in a different medium there is a relative permeability $$\epsilon_r$$ in which case:

$$\epsilon = \epsilon_0\epsilon_r$$

And K is different.

5. Mar 17, 2004

Chi Meson

Agreed.

It was my assumption that the person asking the question was not yet at that level, and might have been taken aback by the use of epsilon when the textbook uses "k."