In many English homes people eat four meals a day: breakfast, lunch, tea and dinner. People have breakfast at any time from seven to nine in the morning. They eat porridge，eggs or bread. English people drink tea or coffee at breakfast. Lunch comes at one o'clock. Afternoon tea is from four to five in the afternoon and dinner is about half past seven. First they have soups, and then they have meat or fish with vegetables. After that they eat some other things, like bananas，apples or oranges. But not all English people eat like that. Some of them have their dinner in the middle of the day. Their meals are breakfast, dinner, tea and supper and all these meals are very necessary.

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如图所示，在平行金属带电极板MN电场中将电荷量为﹣4×10^﹣6C的点电荷从A点移到M板，电场力做负功8×10^﹣4 J，把该点电荷从A点移到N板，电场力做正功为4×10^﹣4J，N板接地，设地面为零势面．则