1. Aim: How do Charges behave in an Electric Field?
They repel due to a force… what affects the size of this force? The charges given to the pith balls and the distance between them.
Do Now: What is the charge on 7.2x1011 protons?

2. How can I describe charge behavior?
Without being too technical, what is being generated by my machine?
An electric field!
How do we know?
What is that stuff inside?
Simple Answer: Electricity generates an electric field where electrons can then be accelerated outwards once they are given enough energy by the electricity induced across a cathode at the center that acts as a conductor. Electrons flow into the cathode, but are then stripped when enough energy is given to excite them. The electrons then leave an ion trail that other lost electrons follow. This shows the electrons behavior in an electric field. The resulting ion trail is due to the ionization of gas inside the ball.

3. Electric fields
A region of space where electric force acts on a charge.
It is a vector quantity.
If we put a charge in that field, we should be able to predict how the field will affect it.
Gravity is a non-electric field.
F = qE…. Looks a lot like F = mg, right?

4. E-Fields
Calculate the magnitude of the electric field strength at a point in a field where an electron experiences a force with a magnitude of 1.0x10-15 N. The mass of an electron is 9.1 x kg. What is this electrons acceleration?

5. Electric field around a point charge.
The electric field is in the same direction
that a positive charge would move. e- P+
Let students attempt to draw field lines, but help scaffold. Draw on board with one proton and ask for direction and place all around charge.
Use a Tiny Imaginary Proton (TIP) to see which way
it would move near the object in question.

6. Electric Field Mapping
Field lines NEVER CROSS + -

7. Examples-Draw the Electric Fields P+ e- P+ P+
Always point Positive to Negative!
If I place a point A close to an electric field going out, what can we say about the source of the field?
What feels a greater force at A? Proton or electron? Same! Neutron feel anything?

8. When asked for direction…
The direction of an E-Field is based ONLY on your known facts of physics!
Do not use your calculation to determine the direction! E-field is always treated as a magnitude in calculation, so NEVER negative.
An electron always travels in the direction opposite an electric field.
An electric field is opposite an electrons motion.

9. Practice
What is the force and direction acting on an electron placed in an external electric field where the electric field is 100 N/C due East?
Direction: Since the field is East and we are dealing with an electron, the direction of it’s force must be WEST!
F = qE
F = (1.6x10-19C)(100 N/C) = 1.6x10-17 N

10. An electric field acts on a positive 6. 9μC charge with a force of 0
An electric field acts on a positive 6.9μC charge with a force of 0.55N left. What is the magnitude and direction of the electric field?
The direction must also be left because the electric field is in the same direction a positive charge would move.
F = qE E = 7.97 x 104 N/C

11. How can we describe the E-field between oppositely charged parallel plates?
If I placed 3 charges with the same mass at different points between the plates, which one would feel the greatest force?
NONE! All the same.
What is I shot an electron in between? Where would it move? If I shot an electron and a proton, which would hit first?
Milikan Oil Drop Experiment… Measured charge of electron by setting charge on plate equal to mass of oil drop. Figured out electrons were in increments of 1.6x10-19C charge.
Uniform field, constant force except near edges
Determine the path of a positive test charge coming in from the left side.

12. Envelope Activity
With a partner attempt the following questions and write your answers in your notebook.

13. Envelope Activity
Calculate the force on an 5C charge in an electric field of strength 3x103 N/m.
Draw the electric field between two negative charges. *Remember T.I.P!!!
A positive Alpha particle is shot between two parallel plates. Which plate does it hit? Why?

14. Summary How do we describe charge?
How do we define the direction of an electric field?
How do objects gain a charge?
Lose or gain ELECTRONS!