3. Electric Charge
Facts about charge:
• There are 2 types basically, positive (protons) and negative (electrons)
• LIKE charges REPEL and OPPOSITE charges ATTRACT
• Electrostatic forces – both attractive and repulsive forces – are the foundation of our existence!
• They are responsible for holding the building blocks of matter (atoms and molecules)
together in perfect balance!
4. Electric Charge – The specifics
•The symbol for CHARGE is “q”
•The unit is the COULOMB (C),
Some important constants: named after Charles Coulomb
•If we are talking about a SINGLE
charged particle such as 1 electron
or 1 proton we are referring to an
ELEMENTARY charge and often
use, e , to symbolize this.
Particle Charge Mass
Proton 1.6x10-19 C 1.67 x10-27 kg
Electron 1.6x10-19 C 9.11 x10-31 kg
Neutron 0 1.67 x10-27 kg
5. Charge is “CONSERVED”
Charge cannot be created or destroyed only transferred
from one object to another. Even though these 2
charges attract initially, they repel after touching. Notice
the NET charge stays the same.
These are examples of IONS – they have a net charge not
equal to zero.
• atoms gain more electrons OR
• Lose electrons
6. Conductors and Insulators
The movement of charge is limited by the substance the charge is
trying to pass through. There are generally 2 types of
substances.
Conductors: Allow charge to move readily though it.
Insulators: Restrict the movement of the charge
Conductor = Copper Wire
Insulator = Plastic sheath
7. Which one is which?!!!
One of these isolated charged spheres is copper and the other is
rubber. The diagram below depicts the distribution of excess
negative charge over the surface of two spheres. Label which is
which and support your answer with an explanation.
8. Charging and Discharging
There are basically 2 ways you can charge something.
2. Charge by friction
3. Induction
“BIONIC is the first-ever ionic formula
mascara. The primary ingredient in
BIONIC is a chain molecule with a
positive charge. The friction caused by
sweeping the mascara brush across
lashes causes a negative charge. Since
opposites attract, the positively charged
formula adheres to the negatively
charged lashes for a dramatic effect that
lasts all day.”
9. Triboelectric series
• Tells us about the electron affinity (aka
an object’s “love”) of electrons
• To the right: objects higher up tend to
give up electrons easily
• Have lower electron affinity
• So if you rub an object at the top again
an object on the bottom, you will get a
transfer of charge!
10. Induction and Grounding
The second way to charge something is via INDUCTION, which requires NO
PHYSICAL CONTACT.
11. Induction and Grounding
The second way to charge something is via INDUCTION, which requires NO
PHYSICAL CONTACT.
We bring a negatively charged rod near a neutral sphere. The protons in the sphere
localize near the rod, while the electrons are repelled to the other side of the sphere. A
wire can then be brought in contact with the negative side and allowed to touch the
GROUND. The electrons will always move towards a more massive objects to increase
separation from other electrons, leaving a NET positive sphere behind.
12. Electroscope
• The electroscope typically consists of a conducting plate or
knob, a conducting base and either a pair of conducting
leaves or a conducting needle.
• It can be used to determine charge!
13.
14. Test your KNOWLEDGE
• Suppose that a negatively charged balloon is used to charge an electroscope. Explain
in terms of electron movement what is happening in each step.
16. Electric Force
The electric force between 2 objects is symbolic of the
gravitational force between 2 objects. RECALL:
1
Fgα Mm Fg α 2
r
1 qq
FE α q1q2 FE α FEα 1 2 2
r2 r
k = constant of proportionality
Nm 2
k = Coulomb constant = 8.99 x109 2
C
q1q2
FE = k 2 → Coulomb' s Law
r
17. Example: Coulombs Law
• Two balloons are charged with an identical quantity and type of charge: -6.25 nC. They
are held apart at a separation distance of 61.7 cm.
• How many electrons are contained within one of the charged balloons?
• Determine the magnitude of the electrical force of repulsion between them.
3.9x1010 electrons
9.4x10-7 N repulsive force
18. Example
Calculate the separation distance between a 12C charge and a 6C
charge is the electric force between them is 136.5 N
q1q2 q1q2
Fe = k 2 → r = k
r Fe
(12)(6)
r = (8.99 ) 9
= 68,900.31 m
(136.5)
