Download Voltage, Current, Resistance, Power, and Energy PPT

Voltage
Review
• What do like
charges do to each
other?
• What do opposite
charges do to each
other?
• If you want to get
like charges closer
together, what do
you have to do on
them?
• Consider the picture of
the slide. How do the
kids get energy to go
down the slide?
• They do work on
themselves climbing
the stairs. That work
done equals their gain
in Gravitational
Potential Energy.
• W=F∙d
To bring like
charges together…
WORK must be
done!
How can charges gain energy?
• Charges gain energy
when work is done
on them bringing
like charges closer
together.
• This work done
gives the charge
more Electric
Potential Energy or
EPE.
What energy can PE turn into?
When a charge gains EPE and is then released,
the EPE turns into KE. Conservation of energy!
The Math:
If you take EPE and
divide by the
amount of charge
the object has you
get a value called
Voltage.
Voltage is a measure
of the amount of
Energy per Charge.
How much energy
does 1C of charge
have as it exits a 6V
battery?
1.
2.
3.
4.
Voltage
Names for
Potential
Electric Potential
Electric Potential Difference (next
slide)
•
5. Voltage Drop
6. EMF = electro-motive force
7. Electric Pressure, (conceptual
name) because Voltage is the
pressure that causes flow.
Reason: EPE turns into KE when a
charge is released (flow).
Voltage
Electric potential
Energy is not
voltage, it is
energy.
Electric Potential Difference
• Will water flow from one container to the
other in the top right image?
• Will water flow in the bottom right image?
Why?
• For Electricity (electrons) to flow from one
source into another we need more than
just Voltage (electric pressure), we need a
DIFFERENCE in voltage.
• Example: If the water pressure in the
plumbing of your home is 60psi, why does
the water flow out of the faucet when you
open it?
• Would water flow out of the faucet if the
pressure at the opening was also 60psi?
• It takes a difference in voltage to produce
flow of electrons.
Voltage Drop
• Potential Difference can also be described as Voltage
Drop.
• When you go to work, you give some of your energy
to your employer and have less energy when you
leave work compared to when you arrived.
• A similar situation exists as charge (electrons) pass
through a device, the energy they possess is given to
the device leaving them with no energy as they exit.
• Recall V=PE/q, so the PE is transferred and converted
into new forms of energy as the electrons pass
through the device, dropping the voltage to zero as
charge reaches the end (exits the device).
• Important to note: charges are NOT consumed or
used up in this process, just the energy they carry is
transformed!
Ohm’s Law :
• Recall that Voltage is
conceptually the pressure that
produces flow of charge.
• Flow is electrically called
Current (the rate at which
electrons flow) and is defined
as:
• I= q/t, measured in Amperes.
1 Amp = 1 C of charge flowing
past a point every 1 second. (1 A
= 1 C/s)
• Resistance is a measure of how
easily charge moves through a
conducting material and is
measured in ohms (Ω).
• http://phet.colorado.edu/en/si
mulation/ohms-law
Putting them all
together…
(Ohm’s Law)
• We get the following
math relationship:
• I=V/R
• This reveals that: Current
is directly related to
Voltage and inversely
related to Resistance.
• So we have 2 equations
to solve for voltage and
current.
What causes Resistance
http://phet.colorado.edu/en/simulation/resistance-in-a-wire
Resistivity
The resistance of a wire
depends on its dimensions
and the resistivity of its
material:
• L = Length of resistor (m)
• A = cross-sectional Area of resistor (m2)
• ρ = Resistivity and depends on the
material used. See pg. 721, Table 22.1.
(Ω·m)
Checking Understanding
A battery is connected to a wire, and makes a current
in the wire. Which of the following changes would
increase the current?
(1) Increasing the length of the wire; (2) keeping the
wire the same length, but making it thicker; (3) using
a battery with a higher-rated voltage; (4) making the
wire into a coil, but keeping its dimensions the same;
(5) increasing the temperature of the wire.
A.
B.
C.
D.
E.
All of the above
1 and 5
1, 4, and 5
2 and 3
None of the above
Answer
A battery is connected to a wire, and makes a current
in the wire. Which of the following changes would
increase the current?
(1) Increasing the length of the wire; (2) keeping the
wire the same length, but making it thicker; (3) using a
battery with a higher-rated voltage; (4) making the wire
into a coil, but keeping its dimensions the same; (5)
increasing the temperature of the wire.
A.
B.
C.
D.
E.
All of the above
1 and 5
1, 4, and 5
2 and 3
None of the above
Electric Current
• Recall that metals are
good conductors which
means that electrons
move easily through them.
• Here are some visual
examples…
• Ben Franklin actually said
that it was the positive
charges that flow. Today,
we call this
CONVENTIONAL CURRENT.
• A schematic diagram is a Schematic
drawing of the path charge
takes in traveling through
different objects.
• The short and long parallel
lines on the left represent
2 batteries
• The zig zag line on the right
represents a resistor which
is anything that actually
uses electrical energy like a
hair dryer or light bulb.
• The line connecting them
represents wire.
Diagram
Voltmeter and
measuring volts
• Note that the black wire is
plugged into the COM port
and the red wire is in the V
port. Voltage is measured
by touching the test leads
to opposite ends of the
device you want to
measure.
• The meter wires are
connected PARALLEL to
the device being tested.
Ammeter and
measuring Amps
• Note the black wire is still in
the COM port but the red
wire moves to the A port,
specifically the 10A port.
• Current is measured by
producing ONE path for
electricity to flow through all
devices all at once.
• We say the meter is
connected in SERIES with the
device being tested. In Series
means one right after the
other or in line.
• Recall power is the
rate that energy is
being converted or
used (HP lab)
• P = Energy/time or
• Energy = P·t
• 1 Joule = 1 watt·1 sec.
• P = V· I
• 1 watt = 1volt·1amp
Electric
Power
Summary