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Electrical Energy
& Power
Big ideas
• Energy
• Power
• Potential energy
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•
•
•
Voltage
Charge
Current
Resistance
Calculating power from a turbine
What is energy?
When scientists talk about
ENERGY, they are referring
to the ability to do work.
When scientists talk about
WORK, they are referring to
a force over applied over a
distance.
ENERGY:
ability to change the
motion of an object
Quantifying energy
The unit of measure for energy is joules, J.
Energy is calculated by
multiplying force times distance
It takes about one joule to lift one
apple one meter high on Earth.
A joule is the same as
newtons x meters
Abbreviated: J = Nm
What is power?
When scientists talk
about POWER, they
are referring to the rate
at which energy is
converted from one
form to another.
POWER =
𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑒𝑛𝑒𝑟𝑔𝑦
𝑡𝑖𝑚𝑒 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙
Quantifying power
The unit of measure for power is watts, W.
Power is calculated by
dividing energy by time
A 60-W lightbulb converts 60 J
of electric energy to light and
thermal energy every second.
A watt is the same as
a joule per second
Abbreviated: 𝑊 =
𝐽
𝑠
What is potential energy?
When scientists talk about
POTENTIAL ENERGY,
they are referring to an
object’s potential to do work.
That is, the object is not doing
work yet, but could. It is
stored energy
POTENTIAL ENERGY:
stored energy
Look for potential energy…
Chemical bonds
• In fuels
• In food
Gravitational field
• Raising an object
above the ground
Elastic
• Compressed spring
• Stretched rubber band
Electrical field
• Moving “like charges”
closer together
Big ideas
• Energy
• Power
• Potential energy
•
•
•
•
Voltage
Charge
Current
Resistance
Calculating power from a turbine
Opposites attract…
You have probably heard
that “opposites attract
and likes repel”.
When scientists refer to
“opposites” and “likes”,
they are referring to
ELECTRIC CHARGE.
Quantifying charge
The unit of measure for charge is coulombs, C.
The charge on one electron is
1.6 x 10-19 C
A coulomb a LOT of charge
When you rub a balloon on
your head, you transfer
~1/1,000,000 C
Electrical potential energy
If you can force like charges
together, they behave like
compressed microscopic
springs.
The closer you push the charges
together, the harder they push
against each other.
Electrical energy can be stored
this way.
Quantifying electrical potential energy
The unit of measure
for electrical
potential energy or,
more commonly,
voltage is volts,V.
Voltage is calculated by
dividing potential energy of concentrated
charges by the amount of charge
A 9-V battery can provide 9 joules
of energy to every coulomb of
charge in the circuit.
A volt is the same as
a joule per coulomb
Abbreviated: 𝑉 =
𝐽
𝐶
What is current?
When scientists talk about
current, they are referring
to the rate at which
something flows by.
When they talk about
ELECTRIC CURRENT,
they mean the rate at
which charge flows by.
Current:
rate at which charge
passes through a circuit
Quantifying current
The unit of measure for current is amps, A.
Current is the rate at which
charge flows by a particular point.
The wires in your house can
safely carry up to 15 amps
An amp is the same as
a coulomb per second
Abbreviated: 𝐴 =
𝐶
𝑠
Difference between potential and current
Consider flow of water
About the same
gravitational potential
A romantic spot for a
swim with your special
someone
A disastrous spot for a
swim with anyone
Danger?
A Van der Graaf generator can
build up a voltage of 20,000 J/C.
It’s shocking to touch but not
typically dangerous.
A car battery maintains a potential
difference of just 12 J/C and will
seriously harm or kill you if you
complete the circuit with your
body.
Check this from UC Denver Med School
The difference
is the current!
Relationship between voltage and current
If you have a higher
voltage, it means that
you have more energy
per charge.
If you have higher
energy per charge,
charges will be pushed
faster, i.e., current will
be higher.
voltage
voltage
voltage
current

current
= ?? current
Relationship between voltage and current
voltage
= ?? current
The rate at which
charge flows through a
circuit will depend on
how hard it is to get
through the circuit.
voltage
Examples:
We call that ‘hardness’
RESISTANCE
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= resistance x current
Motors
Speakers
Lights
Heating elements
Fans
What is resistance?
When scientists refer
to RESISTANCE, they
are referring to how
hard it is for current
to pass through an
element in a circuit.
Resistance:
ratio of how hard
charge is pushed
(voltage) to how much
gets through (current)
Quantifying resistance
The unit of measure for resistance is ohms, .
Resistance is calculated by
dividing voltage by current.
𝑉
𝑅=
An ohm is the same as
a volt per amp
𝐼
Dry skin has a resistance of
100,000 . Wet skin has a
resistance of 1,000 
Abbreviated:  =
𝑉
𝐴
𝐽
= 𝐶𝐶
𝑠
Big ideas
• Energy
• Power
• Potential energy
•
•
•
•
Voltage
Charge
Current
Resistance
Calculating power from a turbine
Calculate power from pieces on a circuit
We measured
voltage and we
know resistance.
How do we
calculate power?
𝑝𝑜𝑤𝑒𝑟 = 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 x 𝑐𝑢𝑟𝑟𝑒𝑛𝑡
𝑣𝑜𝑙𝑡𝑎𝑔𝑒
Recall that 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = 𝑐𝑢𝑟𝑟𝑒𝑛𝑡
𝑣𝑜𝑙𝑡𝑎𝑔𝑒
With a tiny bit of algebra, 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 = 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒
So, 𝑝𝑜𝑤𝑒𝑟 = 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 x
Therefore,
𝑣𝑜𝑙𝑡𝑎𝑔𝑒
𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑝𝑜𝑤𝑒𝑟 =
𝑣𝑜𝑙𝑡𝑎𝑔𝑒 2
𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒
Example
Calculate the power of a
turbine that produces an
average voltage of 4 V in
a circuit that has a 2 
resistor.
Given:
𝑉 = 4𝑉
Unknown:
𝑃 =?
𝑃=
𝑉2
𝑅
Substitute:
𝑃=
(4 𝑉)2
2
Solve:
𝑃 =8𝑊
Equation:
𝑅 = 2
Calculate energy from turbine
We calculated
power. How much
energy did the
turbine produce?
𝑒𝑛𝑒𝑟𝑔𝑦
𝑝𝑜𝑤𝑒𝑟 =
𝑡𝑖𝑚𝑒
With a tiny bit of algebra,
𝑒𝑛𝑒𝑟𝑔𝑦 = (𝑝𝑜𝑤𝑒𝑟)(𝑡𝑖𝑚𝑒)
Example
Calculate the amount of
electrical energy
produced by a turbine
with an average power of
6 W over 60 seconds.
Given:
𝑃 =6𝑊
Unknown:
𝐸 =?
Equation:
𝐸 = 𝑃𝑡
Substitute:
𝐸 = (6 𝑊)(60 𝑠)
Solve:
𝐸 = 360 𝐽
𝑡 = 60 𝑠