Transcript Chapter 13 Electricity

Chapter 13 Electricity
How do you determine the electric
charge of an object?
If more electrons - than protons +
will have - negative charge
If more protons + than electrons will have positive + charge
What are the basic characteristics
of electric forces?
Hold the particles of matter together
The amount of force depends on the
amount of charge on each object
Creates a field around the object similar
to a magnetic field
Depends on the distance between objects:
The amount of force drops by the square of
the distance( if the distance doubles, the
charge drops to 1/4 or ½ 2)
What are the 3 methods for
charging an object:
Friction
Conduction
induction
How does friction work and give
an example?
When 2 objects rub against each
other, rubbing off electrons
This creates an imbalance
Examples:
sliding around on trampoline
shuffling on carpet
Clothes tumbling in a dryer
How does conduction
work ?
A charged object directly
touches an uncharged
object, and the electrons
flow across, causing an
imbalance
How does induction work?
The charged object comes close to,
BUT does not directly touch the
uncharged object.
The like charges are repelled and
move away, so each end of the
object will have an opposite
charge
What do you know about
voltage?
 Similar to potential energy
 The difference in positive and negative charges
determines the force
 Called potential difference
 Unit of measure is the volt (V)
 Household electricity is 120 volts
 Batteries range from 1.5 V- 12 V
 Nerve cells have a concentration of ions inside and
outside the cell which determine the charge
 Charges flow from one cell to another to create the
impulse and send the message
What is a current?
The flow of charges
through a wire or object
to a position of lower
potential energy
Si units are amperes or
amps
What is resistance?
The opposition to the flow of charges, due to the
internal friction of the material
Most metals, particularly copper, usually have low
resistance so they carry current easily
Some metals have high resistance, so friction
causes them to heat up and glow
(tungsten wire filaments in light bulbs, toaster
wires,
Equation for resistance if R=V/ I or
resistance =voltage/current (ohms=volts/amps)
Si unit of measure is Ohms (Ω).
What happens with resistance in
wires with area and length
Resistance in wires decreases with area
Resistance in wires increases with
length
Example 1: fire hose is thick(area) so
that the resistance to flow is less
Hoses are connected together to make a
longer hose so that the length of the hose
can be kept as short as possible
minimizing the resistance in hose to the
flow of water
If you use a thicker speaker wire what
will happen to the sound?
• The thicker the wire the louder and the
better the sound quality will be because
you now have low resistance in the
speaker wires
• If you use longer speaker wires, what
will happen to the sound?
• The speakers will play softer because
have increased resistance when
increased length.
Ohm’s law
• Describes the relationship
between volts, amps and
resistance
• 3 variations of the resistance
equation
R=V/I or ohms=volts/amps
I=V/R or amps=volts/ohms
V=IR or volts=amps x ohms
What are conductors?
Have very low resistance,
which allows for the flow
of electrons
Metals are best
conductors because they
share all electrons
equally.
What is a superconductor?
At extremely
low
temperatures (almost
absolute zero)
Have no resistance so
carry large amounts of
current very efficiently
http://www.fys.uio.no/super/levitation/
What are insulators?
Have very high resistance so do not allow
electrons to flow
Used for protection against electrical
shock
Examples :
Plastic
Rubber coating on wires or tools
What is a semiconductor
Metalloids that in a pure state will not
carry a flow of electrons
Adding other materials will allow a
current flow,
Used to regulate exactly how much
current will flow across the wire
Computer chips and other electrical
components are made of semiconductors
Examples: silicon, germanium
What is grounding
Safety feature when insulation may not be
totally effective
A separate wire provides a safe pathway
for the current to flow into the ground
Used for power/telephone poles,
household wiring, lightning rods,
automotive electrical systems
Most electrical appliances have three
prongs:
2 flat ones for carrying the current and one
round prong which is the ground
What is an electrical circuit?
Method of creating a pathway for
current to flow
What are the 4 types of
electrical circuits?
closed circuit
2. Open circuit
3. Series circuit
4. Parallel circuit
1.
Closed circuit
One continuous
loop or path
for current to
flow: no gaps
Open circuit
•An incomplete path,
so current will not
flow
•Flipping a switch to
“off” breaks the
path and stops
electricity
Series circuit
All the devices on the pathway are
in a row
So electricity must flow through all
of them.
If one device fails or is turned
off, all the others will be off,
because there is only one pathway
for current
Example: cheap christmas lights
Parallel circuits
Each device is on a separate or
parallel pathway
If one device goes off, others stay
on, because there are many
pathways for current to flow
Example: household current, higher
quality Christmas lights
What is electrical power?
• The rate at which electricity is used
• Measures how much work the
electrical device did.
• Si units is watts
Equation for power
power= current x voltage
p=I x V
or watts= amps x volts
How do you determine the cost of
electricity?
• The amount of electricity used per
hour
• Si units is kilowatt/hour
• Power companies charge a certain
amount per kilowatt-hour used.
• Rates vary depending on time of
year, time of day(costs more when
use is highest-like summertime or
early evening)
What are the 2 types of
batteries?
Dry cell
Wet cell
Dry cell
• Has a thick paste or gel inside
that reacts with a metal
electrode to generate the flow
of electrons
• Alkaline batteries use a
strong base
• Once chemical reaction is
complete, the battery goes
Wet Cells
• Have a liquid (acid) inside which
reacts with the metal electrodes
• Examples: car, motorcycles,
golf carts and other vehicles
• When using jumper cables, or a
battery charger, must follow
proper order of connecting
cables, or can explode
Electrical safety devices
• Fuses: has a thin strip of metal
inside that will heat up and melt if
too much current is used. This
breaks the circuit and turns off the
device until a repair is made
Circuit Breakers: uses magnets
or bimetallic strips to shut off
power in case of an overload.
Acts like a switch and can be
reset over and over. Used in
household wiring,
Some power outlets have their
own circuit breakers,
particularly in bathrooms or
near sinks as an extra
precaution.