I went on vacation with my family last summer. “Wake up! It's time to go fishing,” My mom woke me up. After having breakfast we got on a boat. Getting up at 6 a.m. was really early and I was not happy, but later, I had fun.

    My uncle caught (抓住) a big fish first. And my mom caught one, too. Then I caught a Black Drum. Next, my dad caught a big Sheepshead. The captain said no one caught a big fish like this one. We were all happy when we heard this. Later we tried to catch small sharks (鲨鱼). My grandpa caught one, but it snapped the line (咬断绳子). My uncle caught one, but it also snapped the line. My dad caught one at last. It didn't look so scary but was a little funny.

    Then our fishing activity was over, and we went back with all the fishes. We were tired but happy!

如图甲所示的电路中，电阻R₁的阻值为20Ω，R₂为滑动变阻器，其规格能够满足电路要求．电流表A、A₁的表盘均如图乙所示，电源电压为12V，且保持不变．闭合开关S，在保持电路元件正常使用的情况下，求：

①电流表A₁的示数；

②电路中的最大电功率为多少？此时通过R₂的电流I₂是多少？

③若R₂为一个规格为“30Ω 2A”的滑动变阻器，电流表A的示数变化的最大值△I是多少？

④联系家庭电路，并结合以上计算结果，类比说一说对合理使用家用电器的启示．