Chapter 19:
Electric Charges and Currents
19-1: Electric Charge
19-2: Static Electricity
Electric Charge
• All matter is made up of
atoms, and atoms are
made up of tiny particles
called protons, neutrons,
and electrons.
• Both protons and electrons
have a basic property
called charge - protons are
positive and electrons are
negative.
Charge and Force
• A force of attraction pulls objects together - this
exists between 2 oppositely charged particles
(negative-positive)
• A force of repulsion pushes objects apart - this
exists between 2 like charges
(neg-neg, pos-pos)
Rule for Charges:
Like charges repel each other, unlike charges
attract each other
Electric Field
• Definition: the arrangement of field lines that
determine the force per unit charge a small stationary
charge would experience at each point in space
• The pattern of an electric field depends on the
number, size, sign, and location of charges.
• The electric field is visualized by showing lines of force
• Force is stronger where lines are closer together occurs when charges are close
• The direction of the force depends on whether the
placed charge is positive or negative
Electric Field - Like Charges
Electric Field - Unlike Charges
Electric Field - Charges
19-2: Static Electricity
• Electrons may be loosely held to their atoms so they can easily be separated from their
atoms.
• An entire object can acquire a charge
• A neutral object acquires an electric charge
when it either gains or loses electrons.
Methods of Gaining or Losing Charges:
• Friction: removes the
charges from an object
and places them on
another object.
Methods of Gaining or Losing Charges:
• Conduction: electrons flow through one
object to another by direct contact.
Methods of Gaining or Losing Charges:
• Induction: charges are rearranged and
line up in different positions.
Conductors and Insulators
• Conductors: material which permits electrons to flow
freely or transfers heat more easily than other
substances. Ex: metals – silver, copper, aluminum
• Insulators: material made up of atoms with tightly
bound electrons that are unable to flow freely – do not
conduct electric charges well.
Ex: rubber, glass, wood, plastic, dry air
• Polarized: partial alignment of negative and positive
charges in molecules in response to an electric field
Electric Discharge
• Transfer of electrons from one object to another
results in a static charge on each object.
• Static electricity is the build up of unbalanced electric
charges on an object
• Electrons that move from one object to another
eventually leave the object
• Electric discharge: loss of static electricity as electric
charges move off an object
Lightning
• Charges separate within the
thunderclouds
• There is a build up positive
and negative charges in
different parts of the cloud
• Objects on Earth become
electrically charged by
induction
• Charges move away from the
cloud, positive charges are left
closest to ground – transfer of
electrons is lightning
• Lightning can be cloud to
cloud
• Thunder is the expansion of air
from the heat produced by
lightning
Lightning Rods and Grounding
• Benjamin Franklin – lightning rods
• Grounding: the Earth is extremely large and is a good
conductor of electric charge. Earth can easily accept
or give up electric charges.
• Lightning rod is put on top of buildings. A wire is run
from the rod to the ground where lightning can pass
through safely. The charge then goes into Earth.
Lightning Rods and Grounding
Electroscope
• Can detect electric charges
• Consists of 2 leaves, a metal knob, and a metal rod
• A charge is put onto the knob which then flows down the metal
rod into the leaves separating them.
• If an electroscope is touched by a negative charge, the
negative charge flows down the rod into the leaves, giving both
leaves a negative charge and causing them to separate (like
charges repel). Same thing will happen if a positive charge is
put to the electroscope.