Вопрос школьника по предмету Физика
Что общего у явлений конвекции и теплопередачи, и вообще что общего у всех явлений переноса
Ответ учителя по предмету Физика
Для теплопроводности необходим контакт тел, между которыми будет происходить теплопередача. При этом температура тел должна быть разной, т. е. они не должны находится в состоянии теплового равновесия.В основе теплопроводности лежит молекулярный механизм: молекулы с большей кинетической энергией передают ее молекулам с меньшей кинетической энергией. Т. е. более быстрые молекулы толкают более медленные, при этом их скорость выравнивается.
При теплопроводности теплопередача происходит за счет передачи энергии, но не переноса вещества. При конвекциитеплопередача осуществляется с помощью переноса вещества.
Поэтому конвекция не может происходить в твердых веществах. Она происходит только в газах и жидкостях. Теплопроводность может происходить и в твердых телах, и в жидкостях, и в газах.
Подробнее — на Znanija.com — https://znanija.com/task/29566832#readmore
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Mellish school is in the slums of Brooklyn for pupils between twelve and fifteen years of age. There are about 1500 pupils there, and 90 teachers.
It’s a problem school, perhaps one of the most difficult schools in America. The white teachers can’t walk to the school because they may be attacked. They all drive to school, and their cars have special locks. There’s always a black policeman at the door of the school. He has a radio contact with the school guards. Pupils must have a written permission from the teacher if they want to go to the toilet. There’s a guard outside the washroom. Only one child can go into the washroom at a time, and he can’t stay there long.
The children who live in that district go to that school. They are all black children. However, why do the teachers work there?
“It’s very hard of course,” says Jane Flinch, a forty- five-year-old teacher. Most children have problems at home. Some of them are beaten by their parents. Some of them sleep at the lessons because they had to work at night. Some of the children get sick at the lessons because they didn’t have breakfast or even a meal for a day or two.
It’s the love of the job that keeps the teachers in that school. They teach and do social work. They feel that they are doing something good for the children who need help.
“There’s also the problem of violence,” says one of the teachers. “Sometimes when I go into a class, I can feel that some of the children are like a dynamite. However, I feel quite safe. I am sure that other pupils will help me.” There is very little money for teaching in that school. There is no money to buy food for the children.
The principal of the school has been there for many years. A lot of his happy optimism has gone. “No, I’m not optimistic about the future. Look around you,” he says and points out of the window at the high wall round the school. ”It’s not a very bright sight: buildings have holes instead of the windows, shops are closed, groups of people are sitting on the steps of the houses. They have nowhere to go and nothing to do.”
“We need money,” he says, “money is the only way to help this school and these people, and, of course, jobs for the people.”