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SAM Teachers Guide Electric Current Overview Students explore the role of electron voltage and density and electric current. They compare the movement of electrons in a conductor and an insulator. They derive Ohmʹs Law from their observations. They apply their understanding to circuits, the working of an incandescent light, and a fuel cell. Learning Objectives: Students will be able to: • Explain how voltage is the driving force behind electron movement. • Define electric current as the number of electrons flowing through a wire over a given period of time. • Explain how conductivity and resistivity relate to electric current. • Infer the relationships between current, voltage and resistance as described in Ohm’s Law (I = V / R). • Explain how electricity can be converted to other forms of energy. Possible student pre/misconceptions • Electrons in a circuit come from somewhere else, rather than the circuit itself. • Electrons travel very quickly around a wire. This explains why a light bulb turns on as soon as someone turns on a light switch. • Charge is used up as it flows through a circuit. • The charge that flows through a circuit originates in the battery. Models to Highlight and Possible Discussion Questions After completion of Part 1 of the activity: Models to highlight: o Page 2 – Voltage Model o Link to Other SAM Activity: Atoms and Conservation of Energy. Discuss voltage by referring back to gravitational potential energy. o Page 3 – Electron Density and Flow Speed o Highlight the relationship between electron density and current of two materials. o Link to other SAM activity: Review the Structure of an atom, the parts and the charges. o Page 4 – Bottom Model of Conductivity and Resistivity o Review the differences between conductors and insulators. Use model to highlight the effect of temperature on current. o Emphasize that idea that the electrons are randomly flowing in all directions not in a directed flow as was model on page 3. o Page 5 – Ohm’s Law Models o Use both models to highlight the relationship between current, voltage and resistance as shown in Ohm’s Law. Possible Discussion Questions: • What is the difference between current and voltage? • What is the relationship between current and resistance? • What is moving as an electric current flows? • How many electrons are flowing through a typical wire? Refer to the caption on p.3. Discuss why you might receive an electric shock. • Do the electrons themselves flow from a power plant to your house? Why or why not? What does get passed that distance? • What are some real world applications that are rooted in the relationship between temperature on insulators and conductors? [Importance in designing space travel materials and for aircraft, satellite, and military applications.] • What effect does an increase in temperature have on the ability of an insulator to insulate? On a conductor to conduct? • Lab / Demonstration Ideas: o Use a circuitry board to connect a battery and a light o Set up circuits and vary the wire (diameter, type of metal, etc.) o For those who have access: CPO 13A and/or CASTLE Activities. After completion of Part 2 of the activity: Models to highlight: o Page 6 – Circuit Model o Review the flow of electrons through a circuit and how the law of conservation of energy plays a role. o Link to Other SAM Activity: Electrostatics. Review attraction / repulsion of charges and how this plays a role in a circuit. o Page 8 – Incandescent Light Bulb Model o Highlight the concept of energy conversion using this model and the role of resistance in the incandescence of the light bulb. o Link to Other SAM Activity: Atoms and Conservation of Energy. Review the idea of energy transfer vs. energy conversion to a different form. This also ties into the Law of Conservation of Energy. Possible Discussion Questions: • How does an incandescent light bulb work? • What effect does adding more batteries have on the total current in a circuit? • How would the knowledge of hydrogen fuel cells help us in the future? • Can you explain how the brightness of the bulbs would change if you add or subtract parallel branches? Connections to Other SAM Activities: The focus of this activity is to understand the motion of electric charge. This activity is supported by Electrostatics where static charges create fields of positive and negative charge. Electricity shows the motion of these charged particles. Heat and Temperature helps students explore how current can be transformed into heat and light.Similiarly, Atoms, Energetic States, and Photons explores atoms in their excited states how they can emit photons. The Electricity unit supports Diffusion, Osmosis and Active Transport, ATP‐
Biological Energy, and Photobiology. In each of these units, a build up of electric potential can be converted into chemical energy if current is allowed to flow back through the membrane. In Chemical Reactions and Energy, electricity is one of the forms of energy that can be created if electrons in a chemical reaction are forced through a wire. Activity Answer Guide Page 1: As the voltage is increased, the electrons move
more rapidly with the increase force acting on
them. This, in turn, increases the current or flow
of electrons.
Introduction
Page 2:
1. In the model above, describe the
relationship between voltage and kinetic
energy as the electron moves.
(a)
2. Complete the analogy: If gravitational
potential energy is the work done by gravity
on a mass moving a certain distance then
electric potential energy is...
3. The current passing through a material is:
(b)
4. Explain how the graph and model support
the multiple choice answer.
Lower resistance means that electrons are
flowing more freely. That correlates with an
increase in electric current.
Electric potential energy is work required to
move an electron from one position to another.
Voltage is the electric potential energy.
Page 6:
Page 3:
1. If there were no battery describe how the
model above would change.
1. Does a material with a higher density of
electrons or a lower density produce a
greater current under the same voltage?
(a)
The battery supplies the energy to create the
electric potential difference. If there were no
battery, there would be no electron flow.
2. Copper and iron have a lot of electrons
that can flow freely. When a voltage is
applied would that produce a lot of current or
a little current? Explain why.
When a voltage is applied a lot of current (in
contrast to a material with fewer free electrons)
would be generated.
Page 4:
1. Electric conductivity measures:
(c)
2. Describe why the current measurements
are so low for an insulator.
Insulators block or slow the flow of electric
current. The electrons cannot move freely so
current measurements are low.
Page 5:
1. The current passing through a material is:
(a)
2. Explain how the graph and model support
the multiple choice answer.
Page 7:
1. Imagine that the wire to the green resistor
was broken at point A. Describe what you
think would happen to the movement of the
electrons.
With the wire no longer connected at point A
electrons would no longer flow to the green
resistor. The electrons would instead only flow
through the red resistor to complete only one
branch of the circuit.
Page 8:
1. The light bulb gave off light under what
conditions?
(c)
2. Based on your experience with the model,
explain why a light bulb that uses AA
batteries won't work if you put in AAA
batteries.
The voltage of the batteries is the same but the
amount of current generated would differ. A
smaller battery may not generate enough
electricity to meet the demands of the resistor
and effectively power the light bulb.
Page 9:
No questions.
Page 10:
1. Compared to conductive wires, insulators
are considered to have:
(a)
2. If two batteries were serially used in a
circuit instead of one, would the electric
current be different? Why or why not.
Two batteries in series would increase the
voltage or electric potential. Referring to Ohm’s
law, if I = V/R, increasing the voltage would
increase the flow of current through the circuit.
This is true if the resistance is kept the same.
3. According to Ohm's Law, in any electrical
circuit, the current increases: (Choose all
that apply.)
(a) (d)
4. What causes the electric energy to change
to light energy in a light bulb?
The motion of the electrons through the resistor
increases temperature. When temperature
increases enough due to the filament’s
resistance, visible photons of light are released.
SAM HOMEWORK QUESTIONS:
Electric Current
Directions: After completing the unit, answer the following questions to review.
1. Electric current can be defined as the flow of electric charges. How can you define
voltage?
2. When there is a very low density of electrons in a wire, would you expect a large current
or a small one? Why?
3. What is the difference between a conductor and an insulator? Give an example of each.
4. Ohm’s law describes the relationship between current, voltage and resistivity. What is
the equation for Ohm’s law? Write out the mathematical relationship and then explain it
in words.
EQUATION:
Explanation:
5. Below is a picture of a simple parallel circuit.
a) What is represented by V? ____________________
b) Use an arrow to draw in the flow of electrons through this circuit.
c) Why is I = I1 + I2? _____________________________________
6. Explain how the presence of a resistor in a circuit can convert electricity into light?
*Hint: Think back to the model on page 8 of the unit that discussed Incandescence.*