Question Video: Controlled Variables in Experiments Physics

Which of the following statements most correctly defines a controlled experimental variable? [A] A controlled experimental variable is a quantity that predictably changes in value during an experiment. [B] A controlled experimental variable is a quantity that cannot change in value. [C] A controlled experimental variable is a quantity that does not change in value during an experiment. [D] A controlled experimental variable is a quantity that may unpredictably change in value during an experiment.

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Video Transcript

Which of the following statements most correctly defines a controlled experimental variable? A) A controlled experimental variable is a quantity that predictably changes in value during an experiment. B) A controlled experimental variable is a quantity that cannot change in value. C) A controlled experimental variable is a quantity that does not change in value during an experiment. D) A controlled experimental variable is a quantity that may unpredictably change in value during an experiment.

All right, so of these four answer options, we want to pick which one most correctly defines a control experimental variable. Now importantly, we can start off by talking about what a controlled experimental variable is not. We know that in any given experiment, there’s some prediction that we want to test. To do this, there’s typically one factor in the experiment that we change on purpose to see how it affects some other factor. The factor or the variable that we change on purpose is called the independent experimental variable. And when we go ahead and change the independent variable, we look to see how that affects a factor called the dependent variable.

Now, we bring up independent and dependent experimental variables only to make the point that what we’re talking about here, a controlled experimental variable, is neither one of those. Rather, this is some other variable involved in an experimental context. To get a better feel for all this, let’s imagine applying these terms to a particular experiment. Let’s say we made the prediction that the farther we had to drive in order to get home, the longer our trip would take. If we set up an experiment to test this out, our independent variable could be the distance from home at which we start to drive. And the dependent variable would be how long it takes to get home.

But we can see there’re other variables involved besides just these. For example, what about the time of day that we try to get home? If we left at rush hour, it might take longer than if we left in the middle of the day. Likewise, variables like the weather and if there were any big events going on nearby or in town could also affect this outcome. So the time of day that we leave to make our transit home is an experimental variable. But it’s also something we can control. If we decide to begin our journey home at the same time each day, then we’re controlling this experimental variable of how the time of day affects the transit time. So then, a controlled experimental variable is something that could change. But we deliberately set up our experiment so that it doesn’t. Knowing all this, let’s look again through our four-answer options.

Option A says a controlled experimental variable is a quantity that predictably changes in value. But as we’ve seen, a controlled experimental variable is one whose value doesn’t change. The description in option A sounds more like the description of an independent experimental variable. So this won’t be our choice for the most correct description. Option B says that a controlled experimental variable is a quantity that cannot change in value. Now, this is interesting because it’s talking about a variable that even if we wanted to change it, we wouldn’t be able to. So really, option B is a better description of what’s called a universal constant. An example of a universal constant is the charge on an electron. This is something that we can’t change even if we want to. The description in option B is not the description of a variable. So we don’t choose this as our answer.

Option C is only slightly different from option B. It says that a controlled experimental variable is a quantity that does not change during an experiment. Reading this, we can be reminded of our car drive home experiment. We said that the time of day that we set out on this journey could change. But we can deliberately set up the experiment so that it doesn’t. Therefore, it becomes a controlled experimental variable. Option C describes exactly this, so this is probably going to be our answer.

Let’s look ahead the last option just to make sure. This says that a controlled experimental variable is a quantity that may unpredictably change in value during an experiment. Well, if a variable changed unpredictably, then it wouldn’t be a controlled variable. So we won’t choose option D for our answer. Option C then is indeed the answer we choose. A controlled experimental variable is a quantity that does not change in value during an experiment. And this typically happens, by the way, thanks to a deliberate experimental design on the part of the experimenter.

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