If the temperature goes above 40 °C, disconnect the circuit immediately.
Energy Transformations
Physical Science
ES 5-6
Light Bulbs
Motors and Generators
Solar Cell
Students may be assigned to groups of 2-3 per computer/probe. If sufficient quantity of probes is not available, instructor may consider assigning activities within the unit to groups and then allowing time for each group to share their activity and results. If availability of solar cells is limited, the Solar Cell component may be conducted as a whole-class based activity using an LCD projector and/or interactive white board.
Note: The activities using the batteries, wire, and bulbs need to monitored since heat from these can cause burns.
Do not apply more than 1.5 V to the paper clip. It will dangerously overheat and damage the sensor.
If the temperature goes above 40 °C, disconnect the circuit immediately.
Setup for the light, temperature and voltage probes is straightforward. In the "Light Bulbs" activity, caution students that the paperclip attached to the battery may become hot. Additionally, wires used to complete the circuits may also become hot. Circuits should be disconnected when data is not being collected. If you are using holiday lights consider having a parent or aide pre-cut and strip the wires. This can be a little tricky getting enough exposed wire to connect to the battery. (After holiday sales are a good time to get strings of bulbs cheap.) Make sure that you have access to both hot glue guns and glue. Having a "hot glue station" where all the hot glueing would take place may make things work more smoothly.
You will need a number of small motors, solar cells, and resistors to complete these activities. One source for these is www.kelvin.com.
Each activity in the unit may be completed within a 45-minute class period. Be sure to allow time for student debriefing, either as a whole-group activity or in small group conferencing. If time availability is limited, instructor may consider using one of the components of the unit as a demonstration or whole-class activity.
It will be helpful for students to have an understanding of circuits prior to completing the activities in this unit. Students should also have knowledge of how the poles of magnets interact with each other (opposites attracting and likes repelling).
Materials: Light Bulbs
For building a motor (Further Investigation):
This unit is motivated by the discovery questions:
Electrical energy is the energy carried by moving electrons in an electric conductor. You can't see it, but it is one of our most useful forms of energy because it is relatively easy to transmit and use. In the first of this 3-part lesson module, students engage in experiments to compare heat outputs of an LED bulb, a AA battery, and a holiday light bulb. Which bulb is most efficient? In the Solar Cell activity, students will attach voltage sensors to a solar cell to generate real-time graphs of voltage values over time. How does the distance from a light source affect the output of the solar cell? In the Motors and Generators activity, students experiment with very simple motors (cost is $1.00-$2.00 apiece) to figure out if electrical energy can be transformed into mechanical energy (and back again).
Safety: Given the target age group, teachers may want to ask parents to help with set-up and tear-down of the experiments. Specific safety guidelines are listed below:
Students of all ages tend to believe that energy is being used up by a device, and this misconception often remains entrenched well into the college years. Emphasize to students that the charge in a circuit is not used up or consumed. This energy is being transformed into non-electrical forms (heat, light, mechanical, etc.) The Law of Conservation of Energy states that energy may be transformed from one form to another, but is never created or destroyed. Another common misconception is that outlets (plugs) are creating the electrical energy used by electric devices, and transfer energy even when the device is turned off and there is an incomplete circuit. It's important for students to start building an accurate mental model of electric charge through a circuit. The charge that flows through a circuit originates in the wires of the circuit. We call metal wires "conducting materials" because they have an atomic structure that allows electrons to move freely and produce an electric current. Electric circuits require a complete loop through which the current can pass.
Content Support for Teachers
If you'd like a refresher on electricity or electric circuits, The Physics Classroom has a comprehensive free tutorial that features animations and question sets with answers. You can brush up your knowledge in a range of topics including electric potential, types of circuits, Ohm's Law and resistance, the meaning of "power", and more. Click here: http://www.physicsclassroom.com/class/circuits
What are some different sources of light?
Answers will vary, but may include light bulbs, fire, glow sticks, flashlights, and lanterns. Students may also mention the sun and the moon.
Where does the energy come from to produce these types of light?
Batteries: Chemical reactions within a battery cause electrons to build up. These electrons flow from the battery through the wires of a circuit, producing current. The chemical energy in the battery is converted to light energy. Electricity: Electrons flow through the circuit. Light that we see from the sun, stars, and moon come from different sources, the sun's light is produced during nuclear fusion (changing hydrogen to helium). The moon reflects the suns light.
What is mechanical energy?
Mechanical energy is the often defined in physics textbooks as "the ability to do work". This definition won't be very beneficial to students in this age bracket. It's better to discuss mechanical energy as the energy an object possesses due to its motion and position. It includes kinetic energy (energy of motion) and potential energy (often thought of energy of position).
How does a solar cell work?
A solar cell is made of two layers of a silicon-based material. One layer has extra electrons (negatively charged) and the other layer has fewer electrons (positively charged). Light energy causes the electrons in the negatively charged layer to move around and flow to the positively charged side. When electrons flow, this produces an electrical current.
What are some things that might make solar cells not work as a power source?
If there are clouds or you are in a region that has limited sunlight during certain times of the year, they might not be a good energy choice.
What components are needed to make an electrical circuit?
Power source, conducting wires, device(ex. light bulb). Teachers: A functioning circuit involves these fundamental components: 1) Source of electrical potential difference or voltage (battery or electrical outlet), 2) Conductive path that allows movement of charge (wires), and 3) Electrical resistance (light bulb, electric motor, heating element, etc.)
Does the amount of heat produced continue to increase the longer the battery is connected?
The amount of heat should eventually level off, though this might not be noticed depending on how long things are kept connected.
Why do you think that you are measuring the heat that is being produced?
Light and heat are both forms of energy, so by measuring heat you are measuring how much thermal energy is produced by the light bulb. Transformations of energy within a system usually result in some energy escaping into its surrounding environment. Some systems transfer less energy to their environment than others during these transformations, and we consider them to be more "energy efficient". As students perform this experiment, they will begin to get the connection between types of bulbs and efficiency. Incandescent bulbs are much less efficient than LED bulbs because their energy transformation produces more thermal energy that dissipates into the environment as heat.
What are you measuring when you measure voltage?
Voltage is a unit of electric charge. Voltage is the term used when describing the the electrical potential difference. This is different from concept of potential energy. The electric potential difference is the difference in electric potential (Volt) between the final and the initial location when work is done upon a charge to change its potential energy. Voltage can be a very difficult concept for students this age. Acceptable responses can range from "Volts measure charge" to "Voltage tells you how much electricity you have to work with."
What happens when you turn the shaft on the motor?
As the shaft spins, a coil of wire is rotated between the poles of two magnets. The magnetic field generates an electric current, and the electric current produces a magnetic field. The two motors you are using will have a magnetic attraction for each other, and you may test this by putting the motors side by side.
What is the purpose of the resistor?
The purpose of the resistor is to regulate the flow of electricity within a circuit. Without the resistor, electrical surges may be enough to blow the bulb or other device connected to the circuit.
Why do you use black paper to cover the solar cell?
Black reflects the light so that its energy can't be used by the solar cell. These surges may be caused by lightning strikes in real life.
What are some factors to consider when choosing a light bulb? Explain.
Answers will vary, but in general, students should include brightness required and efficiency desired.
Describe the pathway of energy transfer as your finger turns the shaft on a motor and a bulb lights up.
Mechanical energy from the fingers is transferred to the shaft. The turning shaft causes a coil of wire to spin between the poles of two magnets. This magnetic field produces an electric current. The electric current travels along the wires of the circuit and enters the bulb, causing the bulb to light.
The solar cells used to power homes and buildings are often very large and located on rooftops. Why are these factors important?
Solar cells must be large enough to collect energy to run the intended devices. Open rooftop areas provide better access to sunlight needed by the solar cell.
Why are solar cells made of dark materials?
Darker materials absorb more light than lighter materials.
Electrical current is produced through the flow of electrons. The flow of electrons produces an electrical current, and an electrical current produces a magnetic field. Energy may be transferred throughout a system. For example, chemical reactions may be used to produce an electrical current that allows a bulb to light. A spinning coil of wire passing through poles of two magnets produces a current that is in turn transformed to make a light bulb light. During these processes, no energy is created or destroyed; the energy is simply transformed from one form to another. Through these processes, not all of the energy will be transformed to the desired product; i.e. light bulbs give off some energy as heat. The wires in a circuit may become hot and give off some energy as heat. More efficient means of energy production allow a greater portion of the energy produced to go to the desired product.
Can electricity make heat but no light? What is your evidence?
Yes. The paperclip heated up but no light was produced.
Compare the heating by a regular flashlight and the heating by an LED flashlight. Is there a large difference?
Although the LED flashlight may produce some heat, it is generally less than a regular flashlight. LED flashlights are usually more efficient than regular flashlights.
Turn on the regular flashlight next to the LED flashlight. Which one is brighter?
LED flashlights are usually brighter than regular flashlights.
If the flashlights are similar in brightness, the one with much greater increase in temperature is making more heat for the same amount of light. That means that much of the electrical energy is used up making heat instead of light. Based on your measurement, which type of light is more efficient?
The light produced by an LED flashlight is generally more efficient than the light produced by a regular flashlight.
Can a motor be a generator? Explain.
A motor can be a generator. The primary function of a motor is to change electrical energy into mechanical energy. In a DC motor, a coil of wire rotates between the poles of two magnets. The magnetic field produces an electric current and the electric current produces a magnetic field. In a battery-operated car, batteries give power to a motor that causes the wheels to turn. In turn, the generator converts mechanical energy into electrical energy to re-charge the car's batteries.
Can a generator be a motor? Explain.
A generator can also act as a motor. The primary function of a generator is to change mechanical energy into electrical energy. An example is a battery-operated car. Batteries give power to a motor that causes the wheels to turn. In turn, the generator converts mechanical energy into electrical energy to re-charge the car's batteries.
Do you think some energy is wasted when you go from electrical energy to mechanical energy and back again? How can you tell?
During energy transformations, not all energy from a source goes into the desired action. For example, with light bulbs some of the chemical energy from the battery is converted to heat energy as well as light. When going from electrical energy to mechanical energy and then back again, some of the energy will be transferred as heat energy.
Every gasoline-powered car has an electric starter motor attached to the gasoline engine. The electric motor is powered by the battery. When you start the car, the motor turns the gasoline engine over (rotates it) until it starts up. Is the motor acting as a motor or a generator? Explain.
The motor is acting as a motor. Motors convert electrical energy into mechanical energy.
After the engine starts, the gasoline engine turns the motor, and the motor recharges the battery. In that situation is the motor acting as a motor or a generator? Explain.
The motor is acting as a generator. Generators convert mechanical energy into electrical energy.
How does the shading of part of a solar cell affect its voltage output?
Shading part of the solar cell should produce less voltage.
How does distance from a light affect the solar cell's output?
Greater distance between the light source and the solar cell should produce less voltage output.
How do you think distance affects output?
Greater distance between the light source and the solar cell produces less voltage output. As the distance from a light source increases, intensity of the light decreases. This means there is less light to move the electrons in the solar cell, and less voltage is produced.
Can a single solar cell run a motor?
Yes, a single solar cell can run a motor, but output may be limited. Solar buildings and houses require very large solar panels to generate a sufficient quantity of electricity.
Based on your experiments, would you say that solar cells are better for devices that require very little electrical energy or for devices that use more energy? Explain.
The size of the solar cell determines how much light may be used to produce an electrical current. Larger (or more) solar cells can produce greater amounts of energy. Devices that require very little electrical energy require smaller or fewer solar cells.
How can solar energy be converted into electrical energy?
Solar panels are made of two layers of silicon-based material. One layer has a negative charge (extra electrons) and the other layer has a positive charge (electron deficient). Light energy causes the extra electrons in the negative layer to move to the positively-charged layer. This results in electric current. The electrical energy is then converted into mechanical energy.
Students may wish to measure the light output of regular flashlight bulbs compared to LED bulbs.
Many regions have considered limiting production and/or availability of incandescent light bulbs in favor of compact fluorescent light bulbs. What are some energy costs associated with incandescent bulbs compared to compact fluorescent bulbs?
Students may wish to research and construct a working water wheel and measure voltage output. Can water generate enough voltage to light an LED?
Students may consider researching and constructing a solar-powered car. Some suggested materials include a solar phone charger and remote-controlled car.
Are some areas of the world better locations for solar energy? Students may wish to investigate the use of solar energy in different geographic regions.