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VOLTAGE
Remember..
• One coulomb is the charge equal to
the charge of 6.25 X 1018 electrons
– Unit of charge (C)
• 1 ampere =1 coulomb per sec
– Unit of electric current (I)
• Voltage= the push/force behind
electrons
Voltage
• Voltage is the force that moves
electric charges in a circuit.
• Voltage drop/potential difference:
The difference in the amount of
voltage between two points in a
circuit
– Things that make voltage drop
include anything that uses energy
like LOADS or other RESISTORS
Electric potential (also
referred to as voltage)
• is the energy given to each electron
by the energy source. It is the
potential energy that the electron
leaving the energy source contains.
• This energy is then transferred to
one or more electric loads that
convert the energy to another form.
The electric potential is measured in
the SI unit called volts.
Voltage/potential
difference
• Measure in volts (v)
• Measured using a voltmeter
• A voltmeter is always connected in
PARALLEL around a LOAD, to
measure the change in voltage.
– Positive side of the voltmeter is
connected to the positive end of the
circuit and the negative side is
connected to the negative terminal
Circuit diagrams
• A voltmeter is the device that measures the
amount of potential energy used by an object
in a circuit
V
• The voltmeter has two outlets. The red outlet
is the positive end and the black outlet is the
negative end. To use a voltmeter, two wires
are connected from each of its plugs and the
wires are connected around the object that is
being tested.
Connect in parallel
Common Voltages
Source of
Electric
Potential
Tape playback
head
Human cell
Microphone
Photocell
Electrochemical
cell
Electric eel
Portable
generators
Wall outlets in
house
Generators in
power stations
Voltage (volts)
0.015
0.08
0.1
0.8
1.1 to 2.9
650
24, 120, 240
120, 240
550
Potential Energy
• Electric potential energy is the
electrical energy stored in a battery
• All forms of energy are measured
in joules (J).
• When describing energy of
electrons in a circuit, we talk about
the potential energy per coulomb
of charge.
Potential energy
• Think of electric potential as stored
energy that can be used to do work,
much like water that drives a water
wheel or a turbine in a generator.
Water in jug has
the potential to
do work….like
move the wheel!
• Top of the hill is the max potential
energy 100J
• As electrons flow through the
circuit energy is lost
Potential Difference
(voltage)
• The difference in potential energy
per unit of charge (coulomb)
measured at two points
– Ex. The difference of 25J/coulomb
= 25volts
Voltmeter
measures at 2
points to find
the difference in
voltage;
potential
difference
Potential Difference=
25V
Uses 25V
How much
work can be
done here
100V
Uses
25J/coulomb
vs.
How much
work can be
done here
75 V
Formula: Voltage
potential energy (J)
• Potential difference (v) = __________________________________________
Charge (C)
V= E/Q
Example 1
1. If a wall outlet provide 300J of electric energy
and there is 28C of charge, what is the
potential difference between the negative and
positive end of the socket?
G: E= 300J; Q=28C
R: V= ?
A: V=E/Q
=300J/28C
S: =10.7V
S: Therefore, the wall socket provide 10.7V.
Example2
2. If the potential difference across a
battery is 1.5V and provides 30J of
energy, what is the amount of charge?
G: E= 30J; V=1.5V
R: Q= ?
A: V=E/Q
Q= E/V
=30J/1.5V
S: =20C
S: Therefore, there are 20C of charge
Example 3
3. How many joules of energy are
produced when there is a potential
difference of 106V and 46C of charge?
G: V= 106V; Q=46C
R: E= ?
A: V=E/Q
E=QV
= (46C)(106V)
S: = 4876 J
S: Therefore, 4876 J are produced.
Overall….
• A current is the movement of electrons
• In order to move our electrons we need to
build up energy (through chemical reactions
that cause charge separation and the electric
field.)
• That build up is called potential energy and
voltage is the difference in energy/coulomb
• Voltage is measured in volts
• In order for a circuit to continue, electrons that
left the negative terminal must return to the
positive terminal of the power source to
complete the circuit