Caleb designs an experiment to determine if objects are each electrically charged or not. He takes a piece of wool and a plastic stick and rubs them together until the stick is charged. Then he takes the charged stick and moves it towards the object he wants to test to see if it attracts or repels that object.
What recommendations would you have for Caleb to improve his experiment and why would those be an improvement over his current design?
What important physics idea is this question targeting?
This examines whether or not students realize that some neutral objects can be attracted to a charged object (C)
This targets whether or not students can accurately control important variables when designing an experiment to test a claim. (V)
This reveals whether students know the difference between conductors and insulators when determining if an object is charged. (I)
Student A's response:
I would recommend a few things to Caleb: first, he should have a clear method of how close he is going to hold the rod to the objects. And he should repeat the experiment more than once to make sure that there were not any errors in setting up the experiment. Also, he should try multiple objects so that he can determine what factors influence whether or not an object is charged, like different kinds of paper, more than one object made out of metal and more than one kind of ball.
Which answer best describes what student A's response indicates about their physics understanding?
Student A has strong ideas about experimental design, however they have not considered that attraction between the rod and the object does not necessarily means that an object is charged. (C)
Student A has suggested some strong recommendations for the experiment. However, their ideas about investigating what factors influence an object's charge doesn't really address the main question about figuring out if objects are charged or not. (I)
Student A has not fully followed a scientific method in developing their recommendations. (V)
Which of the following options would best support Student A in being able to correctly respond to this question?
I would start by having the student charge a balloon by rubbing it on their hair or a sweater, and then discuss why this sticks to a wall even though the wall is neutral. We could then brainstorm ways to test objects that are attracted to the rod to see if they really are polarized or have a net charge. (C)
I would have Student A reflect back on Caleb's question, "determine if a set of objects are each electrically charged or not," and then look through their recommendations to select the ones that will directly help him answer that question and which ones will not. (I)
I would suggest that Student A try out the experiment they have proposed to see if it works. (V)
Student B's response:
Since Caleb doesn't know the charge on the rod, he cannot determine what charge the objects have. I recommend that he would need to either look up this information, or come up with a way to test to figure out what charge the rod is.
Which answer best describes what student B's response indicates about their physics understanding?
This response reveals a student who is thinking carefully, but is focused on the wrong goal — they are helping Caleb to identify the sign of charge, not to determine whether the object is charged at all. They are also missing the conceptual importance of repulsion as evidence of charge, not just attraction.
This response shows that the student is thinking critically about experimental design and understands that you need to know the charge on one object in order to figure out the charge on another. However, they should determine an actual method to figure out the charge on the rod because you cannot always look up information in science.
This student is thinking scientifically and questioning assumptions in the experimental setup, which is a valuable skill. They are clearly trying to be precise about how we know what we know in science.
Which of the following options would best support Student B in being able to correctly respond to this question?
I would affirm that this would be important if Caleb needs to know what type of charge an object has, and then ask them what they think might happen if the rod was negatively charged an approached a piece of neutral metal. We could then talk about how the electrons would respond and what would happen. (C)
I would ask them to try to determine a clear method to figure out what kind of charge the plastic rod has. One way is to take something, like scotch tape, that we already know has a particular kind of charge to it. (I)
I would have them talk with others to compare experimental design ideas before settling on the best approach. (V)