1. uniform electric field region
An electron has a speed v. Calculate the magnitude and direction of a uniform electric field that will stop this electron in a distance D.
Initial -e v
uniform electric field region D
Final -e
v=0
uniform electric field region

2. uniform electric field region
An electron has a speed v. Calculate the magnitude and direction of a uniform electric field that will stop this electron in a distance D.
Direction of E -e v
uniform electric field region
Force on electron is opposite to the direction of the electric field.
This problem could be solved fairly easily using kinematics.

3. uniform electric field region
An electron has a speed v. Calculate the magnitude and direction of a uniform electric field that will stop this electron in a distance D. D
Initial
Final -e -e v E
v=0
uniform electric field region
If we do an energy calculation, we don’t have to worry about vectors and kinematics.

4. uniform electric field region
Initial
Final -e -e v E
v=0
uniform electric field region
Do you understand that these E’s have different meanings?
Use magnitudes if you have determined direction of E by other means.

5. uniform electric field region
Initial
Final -e -e v E
v=0
uniform electric field region
V=-ED is negative because the electron is moving away from a high-potential region towards a low-potential region. That’s why it is brought to rest. It doesn’t “want” to move from high potential to low potential.

6. You can also use kinematics to solve this problem. Why?
Possible variations on the previous problem (the “same” problem, but students would disagree)…
…calculate the closest the electron would come to another electron...or…
…calculate the minimum initial speed an electron needs to escape from a proton a distance D away.