一盏白炽灯，没有通电时，灯丝的电阻______零（填“等于”或“不等于”），白炽灯正常发光时电阻为R1，不发光时的电阻为R2，则R1与R2的大小关系是：R1______R2．
导体的电阻与电压电流没有关系，当电路中没有电流时，导体仍有电阻．白炽灯的灯丝在正常发光和不发光时，材料、长度、横截面积不变，但白炽灯正常发光时温度比不发光时高，所以电阻大．
故答案：不等于、大于．
| 完形填空。 | |||
| The USA is a very big country to visit, so you'd better choose carefully the places to see and the time to go. Bring a good map, 1 you may want to travel around. New York and Washington D.C. are good 2 to visit in May or October. It's not too hot then. In winter there's a lot of snow. The best plan is to 3 in New England in September. The weather starts to get cooler and the trees start to change color. It's a good idea 4 bring your camera because you may want to 5 photos of the autumn leaves. 4,000 kilometers away in Los Angeles, California, the weather is fine all year. It's nice to see the sun in December! 6 your swimsuit because you might want to swim in the sea. In the northwest, it isn't very cold, but there's 7 rain, so bring an umbrella. It's 8 to visit Alaska in July and August but it might be cool in the evening. So remember to 9 some warm clothes. But don't come in winter. It'll be dark all day and freezing cold. In Texas and the southeast, there are storms from time to time in summer and fall. It's usually very hot and sunny compared to many 10 places. So when's the best time to visit the USA? Any time you like! | |||
|