Transcript Electric Current and Curcuits

Electric Current and Circuits
17-2 and 17-4
Q: What part of the newspaper do electricians
read first?
A: Current Events!
Electrical Energy
• Electrical energy is the energy found in
electric charges (carried by electrons)
– As you already know, charged objects exert a
push or pull on other charged objects
• Electrical charges mostly flow through
wires
• This flow of charges is called an electric
current
Electric Current
• An electric current is the rate that the
charges pass a given point
– How many pass in a certain amount of time
– The higher the current, the higher the number
charges that pass that point each second
• Measured in amperes (usually called
amps)
• The symbol for ampere is A and the
symbol for current is I when writing
equations
Its electric (boogy woogy woogy)
• When a light switch is turned on, the
wires connect, creating an electric
field that moves at the speed of light
– Remember, an electric field is the place
around a charged object where an
electric force can act on another object
• The electric field causes any free
electron in the wire to move
• The energy of each electron is
transferred instantly to the next
electron
AC/DC (not the band)
• Two kinds of current
– Direct current (DC)
• Charges always flow in the same direction
• Usually found in electric objects that require a lower current
• Ex: batteries, cameras, stereos, etc…
– Alternating current (AC)
• Charges reverse from flowing in one direction, to flowing in
the other at certain intervals
• Usually found in electric objects that require a higher current
• Ex: outlets in your home
Voltage
• The potential difference between two points in a
curcuit
– Expressed in volts (V)
– How much work is needed to move a charge
between two points
• It is the amount of energy released as a charge moves
– The higher the voltage, the more work is needed to
move the charges, and the more energy is released
by each charge
• Voltage is like the hill you ride down on your bike
because the higher the hill, the faster the bike – the
higher the voltage, the more energy is released
Voltage and Current
• As long as there is voltage between two
points, charges will flow in the wire
• The size of the current depends on the
voltage
– The greater the voltage, the greater the
current
• A greater current means that more
charges move in the wire each second
Voltage and you
• Most US outlets supply a voltage of 120V,
so your appliances and electronics run on
120V
– An electric eel can create a voltage of more
than 600V
– That’s enough to power 5
appliance/electronics at the same time!!!!
Resistance – “Electric Friction”
• The opposition to the flow of electric
charge
– Expressed in ohms
• The symbol in equations is R
– Stops charges from moving
– The higher the resistance, the lower the
current
• So if voltage doesn’t change, as resistance goes
up, current goes down
– Depends on the object’s material, thickness,
length, and temperature
Resistance…
• And material
– Good conductors have low resistance
• Used to make wires
– Poor conductors (insulators) have high resistance
• The high resistance of the filament in a light bulb causes the light
bulb to heat up and give off light
• And thickness
– Thicker wire has lower resistance than a thin pipe because
there is more room for the charges to flow
Resistance…
• And length
– Shorter wire has lower resistance than a long pipe
because less work is needed to make the charges
travel
• And temperature
– Resistance of metals increases as temperature
increases
• Higher temperatures mean atoms move faster and they get
in the way of the flowing charges
– Materials that are cooled to very low
temperatures so that the resistance drops to 0
ohms are called superconductors
• Very little energy is wasted in them because there
is no resistance
Generating Electrical Energy
• You already know that energy cannot be
created or destroyed
– It can only be changed into other kinds of
energy
• Generators convert mechanical energy (kinetic and
potential energy) into electrical energy (light and
heat)
• Cells change chemical or radiant (sun’s) energy
into electrical energy
– Batteries are made up of one or two cells
• End of section 14-2
Electrical Circuits
• Roller coasters and electric circuits are
similar in that they both start and end at
the same place – they form a loop
• Electric circuits – complete, closed paths
through which electric charges flow
Parts of a circuit
• Energy source
– IE: battery
• Wires
– Usually copper surrounded by some type of insulation
• Load
– Ie: light bulbs, radios
– Connected to the energy source by wires
– Change electrical energy in other forms of energy
(thermal, light, mechanical, etc…)
One more part
• Circuits may also contain a switch
– Used to open or close the circuit
– When the switch is in the off position, the
circuit is open, so charges can not flow
through it
– When the switch is in the on position, the
circuit is closed, so charges can flow
Series circuits
• Circuit in which all parts are connected in
a single loop
– Ie: christmas lights
– Only one path for charges to flow
• If there is any break in the circuit, all charges will
stop flowing, none of the loads will work
– All of the loads (lightbulbs) share the same
current so they shine with the same
brightness
– If you add more lightbulbs, more resistance is
added, so the light will dim
Parallel circuits
• A circuit in which loads are connected side
by side
– Ie: your house
• If your home were wired in series circuits, turning
on the light in your room, would turn on everything
in your house
– Charges in parallel circuits have more than
one path that they can travel through
– Loads all use the same voltage