Answer:
1) 1 and 2
2) 1, 2, and 3
3) None
Explanation:
For any globe to light up, it must be in a closed loop with the battery.
1) Closing switch x will put globe 1 in a complete circuit with the battery, so globe 1 will light up. Globe 2 is already in a complete circuit, so it will also light up.
2) Closing switches x and y will put globes 1 and 3 in a complete circuit with the battery, so globes 1 and 3 will light up. Globe 2 is already in a complete circuit, so it will also light up.
3) This is a tricky question. Closing switches x and y will put globes 1 and 3 in a complete circuit, as we know from Question 2. However, closing switch z will create a short circuit, a path of least resistance across the battery terminals. All of the current will flow around the outside loop, and none of the current will flow through the globes. Therefore, none of the globes will light up.
