PrepTest 138, Section 4, Question 1
Jim's teacher asked him to determine whether a(36 more words not shown)
Jim's reasoning is questionable in(8 more words not shown)
- Airon sometimes fails(5 more words not shown)
- Biron is attracted(5 more words not shown)
- Cthe magnet needed(7 more words not shown)
- Dmagnets attract substances(3 more words not shown)
- Esome magnets attract(5 more words not shown)
Reading the argument
The magnet picked it up. Does that tell us what it is? Jim knows iron is attracted to magnets, but what if another substance is also attracted? His sample could stick to the magnet without containing any iron at all. Attraction identifies a group of possible materials; Jim hasn't established that iron is the only member.
What we’re looking for
That's the possibility I want the flaw answer to capture. We can accept exactly what Jim observed and still question his conclusion. He needs to rule out a magnetic material that isn't iron before using this test to identify the substance.
Working the answers
- A
This asks whether the test could miss iron that is present. Jim's sample was attracted, so that isn't the problem with his inference. We need to ask whether a positive result could occur without iron, not whether iron could sometimes produce a negative result.
- B
What else attracts iron doesn't tell us what this magnet has attracted. Iron could respond to several kinds of objects, and we'd still need to determine whether Jim's sample is iron or some other substance.
- C
The sample did become attached to the magnet. Whatever orientation was needed for that attraction, the observed attraction occurred. An orientation requirement doesn't explain why attraction would fail to identify the sample as iron.
- DCorrect
Now we have another way to get Jim's result. A sample containing some other material could attach to the magnet, too. That means the observation doesn't establish the presence of iron. D doesn't prove the sample lacks iron; it shows why Jim hasn't proved that it contains iron.
- E
The strength of attraction isn't the distinction Jim uses. He reasons from any attraction to the presence of iron. Whether another magnet would pull more strongly leaves the central question unanswered: can this magnet attract a substance that contains no iron?
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