Transcript Presentation

Chapter 16:Electricity
Section 1 – What is Electricity?
• Definition: electricity – energy transferred as
protons or electrons flow.
– Electricity is caused by unbalanced electric
charge.
eeee- e- e
e- e
e- ee- e-
• Definition: electric charge - a property of
matter based on the number of electrons on
an object.
– Objects can gain or lose electrons to change their
charge.
Benjamin Franklin
identified the property of electric charge and
gave it the positive/negative name!
• Electric charge can be positive, negative, or neutral.
Negative: more – than +
Neutral: Same number of + and –
++ +
+ +-
+ --+
-+ -
Positive: more + than –
+
+ +
- - -
Some materials allow charges to flow, while other
objects stop the flow of electrical charges.
• Definition: electrical conductor – a material
that allows charges to move freely.
• Definition – electrical insulator – a material
that does not transfer charges easily.
 CONDUCTOR
NSULATOR 
Insulators / Conductors
• Some conductors:
– All Metals
– Salt Solutions (electrolytes)
– Silver is the best metal conductor!
• Some Insulators:
– Wood / Cloth
– Glass ; Plastic; Rubber; Air
Electric force is a property that is responsible
for most of the common forces that we
experience, like friction.
– Definition: electric force – the attractive or
repulsive force between objects due to their
charge.
• Electric force exists as an invisible field around
charged objects.
• It causes:
•Opposite charges attract.
+
A positive electric
field pushes
outward.
•Like Charges repel.
-
A negative electric
field pulls inward.
That explains why opposites attract!
+

+
The arrows are pointing in the same direction…
-


+
-
• Definition: static electricity – a build up non-moving
electric charges on an object.
– Static electricity is often produced by friction!
– Friction causes electrons to stick to some materials…
• As more and more electrons build up, the static charge
increases.
Static Electricity
• Static discharge occurs when the static electricity
flows away from its source.
• Definition: electric induction – the process by which
a charged object causes another object to become
charged.
– Example: combing your hair causes static charge to form
on the comb.
– The comb can then be used to charge pieces of paper.
Chapter 16: Electricity
Section 2 – Electric Current
When electrons begin moving, they form a
stream of electricity, called a current.
• Definition: electrical current – a measure of the
# of electrons moving through a conductor.
Current is measured in amps (A).
MORE ELECTRONS = MORE CURRENT
Electricity is often discussed in terms of voltage.
– Do you have any idea what voltage is?
• Definition: voltage – the push that causes
electric current to flow.
– Voltage measures the amount of work that must
be done to move a charge.
• Voltage has units of volts, V.
Yes, work has to be done to move electrons!
– The voltage on a battery, for instance, tells how
much work that battery can do.
Electric Current
• Do you think a 1.5 volt battery can power a car?
• Of course not!!
• The car needs a 12 volt battery.
• The 1.5 v battery doesn’t
produce enough force to
move all of the electrons
needed to run the electronics.
Speaking of batteries…
• Definition: battery – a device able to convert
chemical energy into electrical energy.
– Batteries DO NOT store electricity!
So, how do batteries work?
– Each battery contains an electrolyte (an acid), a
cathode, and an anode.
• Definition: cathode – the + end of a battery.
• Definition: anode – the - end of a battery.
• A neg. charge
collects on the
anode.
• The charge flows
to the cathode
when connected
by a wire.
• Each of the batteries below have the SAME
voltage, but are used in different electronics.
• What is different about them?
THEY PUSH DIFFERENT AMOUNTS OF
ELECTRONS…WITH THE SAME AMOUNT
OF FORCE.
• Its kinda like…
Moving a small
amt. of water at 10
mph.
Think AAA battery…
Moving lots of
water at at 10
mph.
Think D-Cell battery…
The water is moving at the same speed, but different
amounts of water are being moved!
The amount of water represents current (amps)…in case you didn’t
figure that out yet.
When a current cannot flow through a material
easily, it is encountering resistance.
• Definition: resistance – opposition to the flow
of electric current.
– In other words, resistance slows down or stops
the flow of electrons.
– Electric insulators cause almost 100%
resistance.
• Enough resistance to stop the flow of electricity.
Electric Current
• Resistance is a form of friction.
– Resistance can cause some electric energy to be
converted into thermal energy…
Electric stoves work by
using that fact!
Electricity flows through
The coil, which causes
resistance.
Why is knowing resistance important?
• Resistance is how incandescent light bulbs
work.
Electricity travels through the wire
coil and encounters resistance.
The coil becomes hot from the
resistance, and begins to glow.
Inside the bulb, argon gas keeps the
filament from catching on fire.
Most filaments are made from the
element Tungsten.
• LED bulbs and fluorescent bulbs do not
require as much electric current as incan.
bulbs.
• So…LEDs and Fluorescent bulbs are very
“energy efficient”!
Chapter 16:Electricity
Section 3 – Circuits
What is a circuit?
• When you flip the light switch, the light comes
on…obviously.
• What does the switch have to do with it??
– The switch on the wall is part of a circuit.
– When it is switched to the ON position, it allows
electric current to flow into the light bulb.
• Whenever current flows, it encounters
resistance.
– We can measure this resistance with:
• The Resistance Equation (Ohm’s Law):
–Resistance = Voltage/Current
–R = V/I
–Resistance is measured in Ohms (Ω).
Circuits
– Definition: circuit – a set of components that
allow one or more complete paths for electric
current to flow.
– Any time electricity is flowing, a circuit is present.
Circuits
• When electricity is able to flow freely through a
circuit, that circuit “closed”.
• Incomplete circuits will not allow electricity to flow
freely.
– They are called “open” circuits.
Types of Circuits
– Definition: series circuit – a circuit that forms a
single pathway.
– Electricity can only flow one way in a series
circuit.
Types of Circuits
• Definition: parallel circuit – a circuit that forms
multiple pathways for electricity.
– Parallel circuits can have a piece removed from
them and still allow electricity to flow in another
part of the circuit.
Resistance in Circuits
• It is useful to know the resistance of objects in
circuits. There are two rules used to calculate it:
• Series circuits have a total resistance of:
– RT = R1 + R2 +R3…..
– In other words, you add up the value of all the resistors.
• Parallel circuits have a total resistance of
– RT = 1/(R1 + R2 + R3…..)
– That’s all there is to it.
Circuit Components
• Circuits consist of many different things.
• The most common components of a circuit are:
–
–
–
–
–
Power Supplies
Wires
Resistors
Bulbs
Switches
• Definition: resistor – a device that blocks SOME (not
all) electric current in a circuit.
• Appliances like TV’s, lights, computers…these are all resistors.
Electric Current in wires….
• Electric resistance explains why electronics
get hot as they run.
• Even conductors cause some resistance.
• As a wire gets thicker, there is less resistance.
Lots of Resistance
Some Resistance
Little resistance
Electric Current in wires….
• As a wire gets shorter, there is less resistance.
Lots of Resistance
Some Resistance
Little resistance
Electric Current in wires….
• As a wire gets thicker it can carry more
electric current.
Basic Circuit Symbols (copy these to notes!)
Resistor
Wire
Battery or
Power Source
Bulb
Open Switch
Closed Switch