ACT Science : How to find experimental design in physics
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Example Question #61 : Physics
Experiment 1
A scientist develops the following setup, shown in Figure 1 below, to study the charges of radioactive particles. A radioactive sample is placed into a lead box that has an open column such that the particles can only exit from one direction. A detector is placed in front of the opening. A metric ruler (centimeters (cm)), is aligned on the detector such that zero is directly in front of the opening of the column, with the positive values extending to the left and the negative values to the right. On the left side of the experimental setup, there is a device that generates a magnetic field that attracts positively charged particles and repels negatively charged particles.
Figure 1.
The device detects particles in three different places: alpha, α; beta, β; and gamma, γ; as labeled in Figure 1. The paths these particles take from the source of radioactivity are shown.
Experiment 2
A different scientist finds the following data, shown in Table 1, about the energies of the α, β, and γ particles by observing what kinds of materials through which the particles can pass. This scientist assumes that the ability of particles to pass through thicker and denser barriers is indicative of higher energy. Table 1 summarizes whether or not each type of particle was detected when each of the following barriers is placed between the radioactivity source and the detector. The paper and aluminum foil are both 1 millimeters thick, and the concrete wall is 1 meter thick.
Why did the scientist in Experiment 2 include a test with no barrier?
She forgot to use the barrier for the first data set.
She wanted to see if air alone could stop the particles.
She wanted to see if all the types of particles were dangerous.
She needed a data set with no barrier to use as a control, or standard of comparison, for the other three data sets.
She needed a data set with no barrier to use as a control, or standard of comparison, for the other three data sets.
All experiments need a control to indicate that observations are indeed caused by the manipulated variable. The "no barrier" condition ensures that particles are observed without the use of any barrier, and that the barriers that are observed to deflect the particle are truly deflecting them.
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