1. Electrostatics Forces and Fields
The study of stationary charges and their interactions

2. Electric Charge Two types of charge – “positive” and “negative”.
Charge is measured in “coulombs” - C
“e” – “elementary charge” – the amount of charge on a proton or electron
e = x C (+ for proton;- for electron)
“q” or “Q” is the variable used for charge

3. Law of Charges - Opposite charges attract, like charges repel
Law of Conservation of Charge – charge cannot be created or destroyed only transferred. So the net amount of charge produced in any process is zero.
Conductors allow electrons to move freely, Insulators do not!

4. Methods of Charging
Charging by friction scrapes electrons off one object and places them on the other
Charging by induction requires no contact This works only with conductors.
Charging by contact requires a physical connection (insulators and conductors)

5. Polarization
Polarization: a “separation of charge” is induced in an insulator by the influence of a charged object

6. Electroscopes – a device to detect charge

8. “Sharing” charge - charged objects will transfer charge to come to a better equilibrium
If two charged conductors are connected (such as by a conducting wire) they will share charge to achieve a more balanced (shared) charge.
-8mC
6mC
1mC
1mC

9. Electric Forces – Coulomb’s Law
+q1 F + r
Coulomb’s constant
k = 9 x 109 Nm2/C2 F +
+q2

10. Force interactions – “superposition”
Linear – charges on an axis
Find Fnet on q3
q q q3
30 cm
20 cm - + -
-8.0mC mC mC
F3,1 = 1.2N Fnet = -F32 + F31 = -2.7N N = -1.5N
F3,2 = 2.7N The Fnet is 1.5N and points to the left
(Note: found magnitude only)

11. Right angles Find Fnet on q3 at origin F31 = 140N F32 = 330N
Use Pythagorean theorem and tan-1
to find net force and direction y
F32
F31 +
52cm -
q3 = 65mC
q3 = -86mC
30cm +
q2 = 50mC

12. Superposition – the net force of a system of chargesb

13. Electric Fields ( N/C)
Electric fields - vectors
Electric fields - drawn always in a direction that is away from “+” and toward “-”.
Definition: force per unit charge

14. A charge, Q, produces a field in space
A charge, Q, produces a field in space. This exerts a force (N) on a small “test”
charge “q” placed in the field.
Note: Q can
be either + or –
but the small
q is always + q +Q r
The E-field can also be calculated using the strength of the charge “Q”
and the distance to a particular point in the field.

15. E-Field lines Field lines drawn away from positive and toward negative
Field lines do not cross
Number of lines is proportional to charge strength

16. Interaction of field lines between charges of equal magnitude and opposite sign

20. Electric Potential (Volts)
Energy per unit charge at a given point due to an Electric Field.
1 volt = 1 joule per coulomb (1 V=1 J/C)
Potential Difference is required to make current flow.

21. Work in an Electric Field
“W” is the work required to move a charge through a potential difference in joules
Remember – work is equal to the change in PE
“q” is the magnitude of the charge in Coulombs
“V” is the potential difference in volts

22. Capacitors A device used to store charge
Capacitance is measured in “farads”
“C” is capacitance in farads
“Q” is stored charge in Coulombs
“V” is potential difference in volts
most common type is parallel plate capacitor.

23. A charged capacitor stores energy. The electric energy stored
when a neutral capacitor is charged with an amount of
charge – Q – from a potential (battery) of V is:
energy (PE) = ½ QV
Combining this expression with the definition of capacitance
gives:
PE = ½ QV = ½ CV2 = ½ Q2/C