Charges
Conservation of Charge
• Charges of an atom
• Neutral if protons = electrons
• Ions
– Positive if protons exceed
electrons
– Negative if electrons exceed
protons
How do materials gain and lose
electrons?
• Depends on valence electrons
– Does it gain or lose electrons
more easily?
» Positive vs. negatively
charged
• Examples:
– Fur and a rubber rod.
» Electrons are held
more firmly by rubber
» Given up easily by fur.
Check your knowledge:
• An object is electrically charged
when _________
• An objects charge is negative
when___________
• An objects charge is positive
when____________
Are electrons created or
destroyed?
• Principle of Conservation of Charge
Coloumb’s Law
• For charged particles or objects that are small
compared to the distance between them, the force
between the charges varies directly as the product
of the charges and inversely as the square of the
distance between them.
• Formula : F = k q1q2
• d2
•
• F = Net Force between the charges
• (attraction or repulsion). Units are in NEWTONS!
• k = Proportionality constant
• q1 = quantity of charge of one particle, coloumbs
• q2 = quantity of charge of another particle, columbs
• d = distance (d2 is distance*distance), meters
• What is a coloumb?
• unit of charge
• The charge of 6.24 x 1018 electrons
• The amount of charge that passes through
a 100 W bulb in 1 second.
Methods of charging
• Friction
• Induction
• Contact
Friction – Explain in your own
words charging by friction
questions
• a. If you rub a balloon with fur and it becomes
positive what charge does the fur have?
_______________
• b. During a physics lab, a plastic strip was rubbed
with cotton and became positively charged. The
correct explanation for why the plastic strip
becomes positively charged is that ...
– a. the plastic strip acquired extra protons from the
cotton.
– b. the plastic strip acquired extra protons during the
charging process.
– c. protons were created as the result of the charging
process.
– d. the plastic strip lost electrons to the cotton during the
charging process.
INDUCTION
Contact ( conduction)
• What happens when you slide your feet
over carpet and then touch your younger
brother?
Upon contact, electrons moved
from the negatively-charged object
onto the neutral object.
OR can flow from a neutral object
onto the positive object charging it.
Field diagrams
• We are able draw pictures to help us show
the electric field that surround charges.
The diagrams show us the direction and
size of the force
RULES
1. Away from positive charges and towards
negative charges:
Rules
2. The greater the charge, the
stronger the field.
Rules
3. Lines are drawn perpendicular to the
object.
4. Field lines NEVER cross:
Several electric field line patterns are shown
in the diagrams below. Which of these
patterns are incorrect? Explain why.
What’s wrong?
Consider the electric field lines shown in the
diagram below. From the diagram, it is apparent
that object A is ____ and object B is ____.
e. insufficient
a. +, + b. -, - c. +, - d. -, +
info
Exit ticket: Identify the charges on
the objects below