- #1
Karol
- 1,380
- 22
Homework Statement
A point charge $$q_1=+20\cdot 10^{-9}[Coulomb]$$ is 5[cm] distance from charge $$q_2=-12\cdot 10^{-9}[Coulomb]$$.
An electron is released from 1[cm] distance from q2. what is it's velocity 1[cm] from q1.
Homework Equations
The potential=Voltage from a point charge: $$V=K\frac{q}{r}$$
The constant $$K=9\cdot 10^9$$
The work done to move from one point in the field to another: $$W=V\cdot q$$
The electron charge: $$e=1.6\cdot 10^{-19}[Coulomb]$$.
The electron mass: $$m_e=9.11\cdot 10^{-}[Kg]$$
The Attempt at a Solution
I solved:
I took the signs of the charges and calculated the potential:
##V_1=9\cdot 10^9\left(\frac{-12\cdot 10^{-9}}{0.01}+\frac{20\cdot 10^{-9}}{0.04}\right)=9\left(\frac{20}{0.04}-\frac{12}{0.01}\right)##
##V_2=9\left(\frac{20}{0.01}-\frac{12}{0.04}\right)##
Then i subtracted the potentials and translated the work into kinetic energy and found the velocity. it was wrong. but if i change the signs of the charges it comes out like in the book:
V=8.7E7[m/sec].
The solution that works:
##V_1=9\left(\frac{-20}{0.04}+\frac{12}{0.01}\right)##
##V_2=9\left(\frac{-20}{0.01}+\frac{12}{0.04}\right)##
Why does it work?