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A torch draws 0.38 A from a dry cell for 20 hours…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A torch draws from a dry cell for hours. At the anode, zinc becomes . At the cathode, each takes one electron. Find the mass of zinc used and the mass of manganese dioxide reduced. Use and .
Need a reminder? The key ideaShow
One ampere-hour is . A lead atom gives 2 electrons in the lead storage battery, a zinc atom gives 2 in the dry cell and an iron atom gives 2 when it rusts. Energy is . A fuel cell converts the energy of directly into electricity. Zinc, with a lower potential than iron, protects it.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Charge. .
Step 2. Electrons. .
Step 3. Zinc. Two electrons for each atom: .
Step 4. Manganese dioxide. One electron for each: .
Answer: of zinc and of manganese dioxide.
Check. There is more by mass than zinc, because each unit takes only one electron and is heavier. The cathode paste is made larger for this reason.
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