A fundamental magnitude used in Physics and other sciences to describe mechanical movement is speed. Measuring it has been a recurring activity in experimental classes. I usually use a video camera and TRACKER software to study the movement of certain objects with my students. One difficulty we have experienced is: objects that move at relatively high speeds appear blurred in the video frames, which introduces uncertainties in the measurements made with the software. The most common methods and instruments for the study of objectsat relatively high speed are based in DOPPLER effect and optical sensors coupled with chronograph.
In the present INSTRUCTABLE I approach an alternative experimental method to measure the average speed of an object with the use of a solar panel and an oscilloscope. It is applicable in the lab lessons of the subject Physics (Classical Mechanics), in particular in the topic: Kinematics of the mechanical movement of translation. The proposed method and its experimental application is powerfully applicable to other experimental tasks within the Physical discipline for non-graduates and graduates ones. It maybe also used in other science courses where these contents are studied.
If you want to shorten the theoretical foundations and go directly to the experimental apparatus construction, how to perform the measurements, the materials needed and the images of my design, please go directly to step 6.
Step 1: Some Theory:
The "speed" is known as the distance traveled by an object in a certain time interval. It is a vector magnitude that also requires the direction in which position changes occur. We will talk in this INSTRUCTABLE to measure speed, but we will really be measuring average speed.
Step 2: Measuring Speed With a Solar Panel?
Solar panels are devices that operate under the principle of the photoelectric effect and whose main function is to circulate an electric current in the circuits in which they are used. For example, solar panels are used to operate certain types of watches, charge batteries of all kinds, also in AC generation systems for the public network and in homes. The applications are many, its price in the market is increasingly attractive and contributes to sustainable development which is great.
Due to the develoment of this tecnology has experienced we find it in many devices, for example, the one I show you was extracted from a cheap flashlight that I saved and now has a new use.
The principle is basic. Whe a light is projected over a panel, it causes a diference in electrical potencial(voltage) at its terminals. When a voltmeter is connected this is easily verifiable. This difference in potential is responsible for the circulation of an electric current when a consumer device is connected, for example, an electrical resistance. Depending on the "impedance" of the circuit and the characteristics of the panel, it will circulate more or less current. In relation to this current, a voltage drop will be experienced at the terminals of the solar panel once the consumer has been connected, but if the impedance remains constant, the voltage is also kept constant as long as the characteristics of the illumination also are. Voltmeters generally have a high impedance so they will affect very little the voltage that is measured with them. But what happens if the illumination changes?, so will the voltage and this is the variable that we will use.
• A solar panel when illuminated shows a voltage on its terminals that can be measured with a voltmeter.
• The voltage does not change if the impedance of the circuit and the characteristics of the illumination are kept constant (must be in the sensitive spectrum of the panel for the photoelectric effect to occur).
• Any change in the illumination will lead to a variation in the voltage, a variable that will be used later to obtain the velocity of the objects in the experiments.
Based on the previous precepts the following idea could be formulated:
The projected shadow of an object, moving on a solar panel will cause a decreasement on its terminal voltage. The time it takes for the decreasement can be used to calculate the average speed with which that object moves.
Step 3: Initial Experiment
In the previous video the principles on which the previous idea is based are experimentally shown.
The image shows the time that the voltage variation lasted which was plotted by an oscilloscope. By correctly configuring the trigger function you can obtain the graph to which we can measure the elapsed time during the variation. In the demonstration, the variation was approximately 29.60ms.
Actually, the blackboard draft in the experiment is not a point object, it has dimensions. The left end of the eraser begins to project its shadow on the solar panel and consequently begins to decrease the voltage to a minimum value. When the eraser moves away and the panel begins to be discovered again, an increase in voltage is seen. The total time measured corresponds to the time it took for the projection of the shadow to travel the entire panel. If we measure the length of the object (which should be equal to the projection of its shadow if we take certain cares) we add it with the length of the active zone of the panel and divide it between the time that the voltage variation lasted, then we will obtain the speed average of that object. When the length of the object to measure its speed is quantitatively higher than the active zone of the panel, the panel can be considered as a point object without introducing a notable error in the measurements (it means not adding its length to the object length).
Let's do some calculations (see pic)
Step 4: To Apply This Method Some Precautions Must Be Taken Into Account
• The solar panel must be illuminated by the light source provided in the experimental design, avoiding as far as possible other light sources affecting it.
• The light rays must strike perpendicular to the surface of the solar panel.
• The object must project a well-defined shadow.
• The surface of the panel and the plane containing the direction of movement must be parallel.
Step 5: A Typical Exercise
Determine the speed of a falling ball from 1m height , consider inicial velocity cero.
If the ball falls in free fall it is very simple: see pic
In real conditions the previous value may be lower due to the action of friction with the air. Let's determine it experimentally.
Step 6: Design, Construction and Execution of the Experiment:
• Stick a plastic tube to the active area of the solar panel.
• Solder new leads to the solar panel terminals so false contacts are avoided.
• Create a support for the solar panel-tube assembly so that it can be held horizontally.
• Place a flashlight or other light source on another support so that the projection of the emitted light hits the solar panel perpendicularly.
• Check with a multimeter that when a light hits on the solar panel, a constant voltage value greater than zero is recorded.
• Place the solar panel-tube assembly on the front of the lantern, leaving a greater clearance than the object whose speed you want to measure. Try to keep as far as possible the light source (flashlight) from the solar panel. If the light of the lantern is created by a single led, the better.
• Measure from the center of the solar panel and upwards a distance of one meter and mark it in a rod, wall or similar.
• Connect the probe of the oscilloscope to the terminals of the solar panel, respecting the polarity.
• Set the TRIGGER option correctly on the oscilloscope, so that all voltage variation can be recorded during the passage of the shadow on the panel. In my case the time divisions were in 5ms and the voltage divisions in the scale were 500mv. The line of zero voltages had to be adjusted downwards so that all the variation would fit. The trigger threshold was placed just below the initial constant voltage.
• Measure the length of the object and that of the active zone of the panel, add them and write it down for the calculation of speed.
• Drop the body from the height of 1m so that its shadow interrupts the beam of light projected by the lantern.
• Measure the time of the voltage variation with the oscilloscope cursors on the time scale.
• Divide the sum of the lengths previously made between the time measured in the oscilloscope.
• Compare the value with the theoretical calculations and arrive at conclusions (take into account possible factors that introduce errors in the measurement).
Results obtained: see pic
Step 7: Some Notes of the Experiment:
• The results obtained seem to be correct in correspondence with the theory.
• The object selected for this experiment is not ideal, I plan to repeat it with others that can project a better defined shadow and that are symmetrical to avoid possible rotations during the fall.
• It would have been ideal to position the panel-tube and the lantern on separate tables, leaving a free space down.
• The experiment should be repeated several times, trying to control the possible causes of errors in the measurements and statistical methods should be used to obtain more reliable results.
I´ll be waitting for your comments, questions and suggestions.
Good luck and see you in the next project.
This is an entry in the
Classroom Science Contest