根据句意及汉语提示填空。

1.I was ___________ (想知道) whether you'd like to come to a party.

2.A car suddenly ___________ (出现)over the hill.

3.Our teeth have a hard ___________ (表面), so we can eat hard food.

4.Many old buildings ___________ (保留) in Beijing now.

5.We take care of animals and plants. In fact we ___________ (保护) ourselves.

什么是电流的热效应？