Transcript Ch 19 Powerpoint - Electric Charges

Chapter 19
It’s Electric!
Going Back to
Basics
A charge is not something
you can see, weigh, or
define, but you can observe
how charge affects the
behavior of particles.
Charge & Force
• The difference between 2 charges has a lot do with the forces they
exert
• Recall – a force is a push or pull on an object
• Charges particles exert pushes & pulls
Opposites Attract
• A force of attraction exists between
oppositely charged particles
• This is the force responsible for
holding the negatively charged
electrons in the outer orbital shell
that surrounds the positively charged
nucleus
Like Charges Repel
• A force of repulsion exists between particles of the same charge
• This is the force responsible pushing objects apart
Electric Field
• Extends outward through space from every charged particle
Electric Field
• When a charged particle moves into the electric field of another
charged particle, it is either pushed or pulled
Developing a Charge
• A stable atom has an equal # of protons & electrons, so the
overall charge is zero because the positives cancel out the
negatives
• For an object to become charged, it needs to either gain or lose
electrons creating an ion
• positive ions = a loss of electrons
• negative ions = a gain of electrons
Law of Conservation of Charge
• A neutral object acquires an electric charge when it gains
or loses electrons
• The charge is neither created nor destroyed
• Charge is only being transferred from one object to
another
Methods of Charging
1. Friction – rubbing two objects
together, one gains electrons while
the other loses electrons
(This is how a balloon acquires a charge that
allows it to stick to a wall.)
Methods of Charging
2. Conduction – electrons flow from one object
to the next by direct contact
• Conductors, like most metals, allow electric charges to flow
freely
• Insulators, do not allow charges to flow freely (think about
plastic handles on power tools!)
Conduction
Methods of Charging
3. Induction – rearrangement of electric charges
•
•
Think back to the balloon sticking to the wall
The positive charges move closer to the surface of the wall &
the negatively charged balloon is attracted to the surface
Induction
Static Electricity
• the transfer of electrons from one object to another
without further movement
• static means stationary (not moving)
• it is the build up of electric charges on an object
• once built up though, the charges do not flow
Electric Discharge
•
•
•
•
static electricity will eventually leave the object
this loss of electric charge is called electric discharge
can occur quickly or slowly
usually, extra electrons will escape onto nearby water
molecules in the air
Static Shocks
• on dry days there are fewer water molecules for the
electrons to escape
• objects are more easily charged & tend to lose their charge
quickly
• this creates a shock, a spark of light, or a crackle of noise
• lightning is a dramatic example of the discharge of static
electricity
Lightning is an Example of
Static Electricity
• It cannot be used as a source of
electricity because the current
does not flow, it is just rapidly
discharged
Lightning
• Lightning occurs when
electrons jump through the air
creating intense light & heat.
• Thunder, the loud boom,
happens because the rapidly
heated air expands
Lightning
• It occurs between two areas of
different charge
• from cloud to cloud
• from one part of a cloud to
another part of the same cloud
• from cloud to ground
How Lightning Works
• In your typical storm cloud, the negative charge at the bottom of
the cloud enhances the positive charge at the ground.
• The cloud wants to "complete the circuit" and sends out a stepped
leader toward the ground. That further enhances the positive charge
near the ground, and objects may form a streamer of positive charge
that reaches up, trying to make the connection.
• In Benjamin Franklin’s famous experiment with a kite in
1752, he tried to prove that lightning was electrical & flew
a kite during a thunderstorm.
• He tied a metal key onto the string
and, as he suspected it would,
electricity from the storm clouds
flowed down the string, which was
wet, and he received an electrical
shock.
Franklin Discovers . . .
• that electricity moves quickly through certain materials, and that
a pointed surface attracts better than a flat surface
• He suggested placing pointed metal rods above the roofs of
buildings & running to the ground as protection from lightning
• The Earth is a good conductor & can easily accept or give up
electrons so it acts as a grounding device
Lightning Rods
The Electroscope
• instrument used to detect an electric charge
• the thin metal leaves hang straight down from the metal
rod when uncharged
• when charged, the leaves spread apart to indicate the
presence of like charges (remember like charges repel)
Is there a charge
present? How can
you tell?
Does this show
charging by friction,
conduction or
induction?