题目

关于通电直导线在匀强磁场中所受的安培力，下列说法正确的是（ ） A．安培力的方向可以不垂直于直导线 B．安培力的方向总是垂直于磁场的方向 C．安培力的大小与通电导线和磁场方向的夹角无关 D．将直导线从中折成直角，安培力的大小一定变为原来的一半 答案：解：A、B、根据左手定则可知，安培力方向与磁场和电流组成的平面垂直，即与电流和磁场方向都垂直，故A错误，B正确； C、磁场与电流不垂直时，安培力的大小为F=BILsinθ，则安培力的大小与通电导线和磁场方向的夹角有关，故C错误； D、当电流方向与磁场的方向平行，所受安培力为0，将直导线从中阅读理解。     A plant (植物) makes its own food in its leaves, water comes to the leaves through the roots (根),air gets into the leaves through very small holes (孔), the green coloring in the leaves uses the water and air to make the food for the plant, it also needs sunshine because a plant can make food only when the sun is shining.     Animals and people could not live without green plants, they both eat plants, people and some animals also eat the meat of some animals and these animals eat plants.1. A plant gets food from________.A. the sun      B. its leaves     C. its roots      D. the air2. What does the green coloring use to make the food for a plant?A. Sunshine, water, air and the roots     B. Air, sunshine and waterC. Water, air and meat                D. Water, sunshine and animals3.A plant has very small holes in its leaves for_______ to come in.A. air         B. water        C. sunshine     D. green coloring4. A plant can make its food when it is _______.A. cloudy      B. rainy        C. sunny        D. windy5. People would die without________.A. animals     B. meat         C. plants        D. leaves