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# Practice Test 3 - page 5 / 16

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make sure you refer specifically to what was done in the experiments you did in class and how that helped you change or enhance your own model or idea.

Note: Acceptable student responses to this question will vary depending on what model or ideas they had before being introduced to the blowing-through-straws analogy, and how they interpreted the analogy itself. Given below is an example of students using the analogy to move from an idea of the independence of resistance to variations in thickness, to a definite dependence.

When thinking about resistance of bulb filaments we first though that they would all be the same, since we thought that the battery supplied the same current to all bulbs and so the resistance to current flow in all circuits would be the same.

We then blew through a thin straw and a thicker straw of the same length and made two observations:

• i.

It was easier to blow through the thicker straw

• ii.

For the same strength blow, more air flowed through the thicker

straw than the thinner straw This meant the thinner straw had more resistance to air flow through it than the thicker straw.

This made us think that perhaps in a bulb it is easier to push the charges through a thicker filament than a thinner filament. Therefore, for the same push from a battery, there would be more current flowing through a thicker filament than a thinner filament. This would then also mean that the thicker filament must have less resistance to the flow of electricity than the thinner filament. Since more current also means a brighter bulb, this would mean that when connected to the same battery (or other power source) a bulb with a thicker filament would glow brighter than a bulb with a thinner filament of the same length.

5. Below is a circuit with three batteries, an ammeter and three bulbs. The ammeter reads 337.3 mA, and all three bulbs glow equally bright.

Imagine that the bulb on the left is removed from its socket; everything else remains the same—nothing else is changed. (In the picture below, the ammeter and bulbs are covered so you cannot tell what happens.)

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