Electric Circuits, Current, and Energy1
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We’ve just learned about electric circuits and what components and connections are necessary to
light a light bulb. In the following activities, we’ll discuss what happens to the energy from a
battery in an electric circuit and how this happens.
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In the previous activities, you observed that a light bulb lit
up when it was part of an electric circuit. In terms of energy,
we say that electrical energy is transferred from the battery to
the bulb, causing it to light up. This transfer of energy
represents one of the fundamental pillars of physics:
conservation of energy.
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In the following activities, we will investigate how
conservation of energy works in electric circuits and mechanism that is responsible for the
energy transfer.
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Physical Sciences Connection: Conservation of Energy and Energy Transfer (PS3.B)
Activity 1: Keeping track of the energy
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Initial thoughts: If you were to plug a 100-Watt light bulb
into a socket, what would you observe? What causes the
light bulb to light up?
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Discuss your thoughts with your group.
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Part 1: Energy and Source/Receiver Energy diagrams
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To better understand how energy is transferred between objects, we use a diagram to
visually describe the process. This is referred to as a Source/Receiver (S/R) diagram. It
describes how energy is transferred between two objects. The object that gives energy
away is the “Energy Source,” the object that receives this energy is the “Energy
Receiver” and an arrow between the two objects shows which way energy is transferred
as well as the type of energy.
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Some of the material here is taken from Physics and Everyday thinking by Fred Goldberg.
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Below is an example of a S/R diagram:
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This diagram one example of a way to illustrate the transfer of energy between objects!
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Energy Source
Energy Receiver
Type of Energy
Transferred
How does the
energy in the source
change?
How does the
energy in the receiver
change?
For the electric circuit with a battery and a light bulb, chemical energy inside the battery
is turned into the electrical energy. This electrical energy is transferred to the light bulb
which causes the filament inside to get extremely hot. This type of energy is called
thermal energy. In incandescent light bulbs, the filament temperature can be as high as
4000 oF! Once the filament reaches these temperatures, it glows, emitting light in the
process. The light emitted in this process consists of visible light (light that we can see)
and infrared light (light that we can feel).
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_______________________ Interaction
Using this information
above, fill the missing parts
in the S/R diagram for the
battery-lightbulb electric
circuit.
Energy Source
Energy Receiver
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Hint: the light that is
emitted from a light bulb
(referred to as light energy)
should not be included in
this diagram.
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Part 2: Input/Output Energy diagrams
S/R diagrams are useful when focusing on one particular transfer of energy between specific
objects of interest. However, in some situations S/R diagrams can quickly become very complex
when trying to show several different ways energy can be transferred. Thus, rather than showing
all the interacting objects in an energy diagram, sometimes it is convenient to focus on only one
of the interacting objects. In this case we only need to show the energy transferred into the object
(called the energy input), the energy transferred out from the object (called the energy output),
and the energy changes within the object.
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We will call this type of energy diagram an input-output energy diagram, or I/O energy diagram.
The general form is shown below:
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Consider the following circuit with a battery and a light
bulb:
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Draw the I/O diagram for the battery.
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Draw the I/O diagram for the light bulb.
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Part 3: Conservation of Energy
Using the same circuit above, a group of students were able to record the energies related
the light bulb. This is what they found:
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For the battery, they measured this:
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Light bulb
Energy Input
Electrical Energy
Energy Output
Visible Light Energy
Infrared Light Energy
Energy Changes in System
Thermal Energy
.71
.11
.56
.04
Battery
Energy Input
Energy Output
Infrared Light Energy
Electrical Energy
Energy Changes in System
Chemical Energy
Thermal Energy
.01
.68
-0.71
.02
How does the electrical energy transferred out from the battery compare to the electrical energy
transferred into the bulb, during the ten second period?
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Does this make sense?
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Conservation of Energy tells us that energy is not created or destroyed, but only
changes form. The equation that describes this concept for an object is:
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Input Energy=Energy Changes +Energy Output
In other words, the energy added to an object/system will change the energy of the
system (warm it up) and/or cause it to energy.
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Using this equation, is energy conserved for the battery?
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Using this equation, is energy conserved for the lightbulb?
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Activity 1 Recap Question:
Some people claim that electrical energy is “used up” in the bulb. Others claim that the energy is
not used up, but instead is transformed into a different type (or different types) of energy. Which
claim makes the most sense to you, and why?
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Activity 2: Current, Resistance, Voltage
In this activity, you will develop the tools necessary to form a mathematical model of
electric circuits using the concepts of current, resistance, and voltage.
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Initial Thoughts: Consider a battery, a light bulb, and some wires. How does electrical
energy go from the battery to the light bulb?
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Electric Current
Within metal wires there are electrically charged particles
(called electrons) that are freely moving in random directions.
This means that just as many electric charges move in one
direction as move in the opposite direction. In other words,
there is no net movement of the electric charges in any
particular direction. (This is illustrated to the left in the
blowup of a tiny section of the wire.) Materials with electrons
that can easily move are called conductors and those with
electrons that are fixed in placed are called insulators.
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However, when the circuit is connected together so the bulb
glows, the battery exerts a force on each and every electric
charge in the circuit. The amount of electric charge moving
past any point in the circuit in one second is defined as the
value of the electric current at that point (measured in units of
amps ).
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So, if the number of electric charges passing by a point in one
second increases, the value of the electric current will increase in a corresponding way. Since the
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electric force can be felt almost instantaneously by all charges in the circuit as soon as the circuit
is connected, an electric current exists almost simultaneously at all places in the circuit—inside
all wires, inside the filament of a bulb, inside the battery, and inside any other electric component
connected to the circuit.
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Battery Voltage
The strength of the push by the battery on the electric charges is called the battery voltage
(measured in the units of volts). For instance, a 9-Volt battery has six times greater push
than a 1.5-volt battery (9=1.5*6).
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This push/force from the battery gives a small amount energy to each electric charge, and
it is these electric charges that deliver this energy to the light bulb.
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Resistance
Elements in a circuit which resist the movement of electric charge, thereby, taking energy away
from them are said to have resistance. One way to better understand this is to compare resistance
to drinking a milkshake through a straw. We know from experience that it is more difficult to
drink a milkshake quickly through a narrow straw. In terms of resistance, we would say that a
narrow straw has a large resistance. Thus, to reduce the resistance (and more quickly consume
your milkshake before it melts), one would choose a wider straw or use multiple straws of the
same width.
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In the remainder of this activity you will develop and test ideas about the relationships between
the voltage of the battery, the amount of resistance of the bulb, and the value of the electric
current in the electric circuit. You will also investigate the relationship between the electric
current and the brightness of the bulb.
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Part 1: What is the relationship between the value of electric current and battery
voltage?
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Initial Thought: Do you think that the amount of electric current in a circuit depends on
the battery voltage? What about the bulb brightness?
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Curious about this question, a group of (highly motivated) students set out to discover the
answer. Here is what they measured:
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Battery
Electric
Light Bulb
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Voltage (volts) Current (amps/
Brightness
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1000)
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5
0.5
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1.5
7.5
1.125
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10
2
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2.5
12.5
3.125
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15
4.5
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3.5
17.5
6.125
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20
8
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4.5
22.5
10.125
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25
12.5
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What claim can you make about the relationship between battery voltage and current?
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What claim can you make about the relationship between brightness and current?
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Part 2: What is the relationship between the value of electric current and bulb
Bulb Filament
Resistance (Ohms)
Electric Current (amps/
1000)
14
107
16
94
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83
20
75
25
60
30
50
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38
50
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filament resistance?
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What claim can you make about the relationship between bulb filament resistance and current?
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It turns out that the proportional relationship between current and battery voltage and between
current and resistance can be combined into the following single equation:
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Current= Battery Voltage ÷ Resistance
This relationship is known as Ohm’s Law. It not only applies to the simple case of a single
battery and a single bulb, but it also applies to more complicated circuits.
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Summarizing Questions:
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1. Two students, Julia and Frank are discussing their answer to the
following question: In a circuit, what happens to the energy provided
by the battery?
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Julia says: “I don’t think energy is used up in the circuit. Instead it is
just changed into another type of energy.”
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Frank says: I don’t agree. Since the battery eventually dies, then I think energy must be
used up in the circuit.”
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What do you think? Do you agree with Julia, Frank, or with neither student? Using what
you know about electric circuits and energy, justify your answer.
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2. Jane has a circuit with a light bulb and a 1.5-volt battery. She claims that using a 9volt battery won’t change how bright the light from the bulb appears.
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Is she right? Use observations from the activity to justify your answer.
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3. Robert is trying to calculate the current in his circuit. He is using a 9-volt battery and
a light bulb with a filament resistance of 100 Ohms. He calculates that his current should
be 900 Amps. Is he correct? If not, help him find the correct value of the current.
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