The puzzle of the physics interest group, the ups and downs of the physics interest group students o

The third point is correct.

Separately, this device is unobstructed, and at first glance it looks like it can drain water smoothly, but in fact, it will not drain any water at all. Why? Analyze the principle of this lower siphon, the left and right sides of the lower siphon are also under the pressure of the air, and the two sides are balanced, if the water level on the left increases, the water flows out from the left through the lower siphon to the right.

But here there are two siphon structures, the middle part forms a closed air cell, when the water level on the left increases, ready to flow out to the right, the right side is an air cell, blocked by the second siphon, the air can not be discharged to the left or to the right, as a result the water cannot flow to the middle part, the water level of the middle part does not increase, and it can no longer flow out to the right. Therefore, the method of treatment is to destroy the airtight air chamber in the middle, or cancel the first-stage water seal. The easiest way to do this is to remove the diaphragm at E, or to drill a small hole in the pipe at E.

The second method is to drill a small hole in the middle of the cover plate B of the downspout A, so that the air can escape, because there is a water seal at E, the same odor cannot come out of A. If construction has already been carried out, then the second option will be the easiest. Usually we see that the water supply system will be equipped with a valve somewhere, and the operator will open the valve at the right time to let the air out, until the air is emptied, and there is a continuous spray of water, this is the reason.

Opinion 1 is correct, but if the water at E is deeper than the water at C, the odor in D will come out of A.

(1) The buoyancy is getting bigger and bigger, and the pressure is increasing.

2）800g

f=g, so it is 4n; The bigger, the bigger, the floating. (I'm sorry, that's all I know, don't I know).

1) The heat absorbed by water: q c water m water δt water = 4200

2) No, because the heat absorbed by water is not equal to the heat released by alcohol (a part of the hand is not absorbed by water, and the loose preparation is resistant to the air to imitate the fierce spring and is absorbed by the cup).

(1) The magnitude of the current representation number.

2）①；A, C.

3) The conductivity of the saline solution is related to the distance between two points in the solution;

4) Control variable method or conversion method (other reasonable can be).

1) The power supply is simple and resistant to change to E 2

2) Judge the internal and external connection method of the electric divination and call the current meter, and make corresponding adjustments.

1) Scale; Spring-loaded dynamometer (balance).

2) the diameter of the suction cup d (the semi-detectable diameter r of the suction cup); The gravitational force g (mass m of fine sand) on fine sand

3)4g/πd 2

or 4 mg d2

mg/πr 2

4) The air in the suction cup is not completely squeezed out; Inaccurate readings using a spring dynamometer (balance) that has failed to ignite; The contact surfaces are not cleaned; The suction cup is not in close contact with the glass plate; There is an air leak, etc. (score points if you answer one point).