Middle School Physics Choice Middle School Physics Choice

The following is only a personal opinion -- a quality: definitely not adulterated in order to change the quality, if it is said that the density is somewhat possible.

b Temperature: How is it possible ? The temperature is the same in the same environment, just like the iron or wood that has been left outside for a long time in winter, which has the highest temperature? The answer is that the temperature is the same, because FE conducts heat well, so the heat in the hand is directed away when you touch it, and it feels cold!

C: That's right! Because the stone does not exothermic q=qm

Q is the heat released, Q is the calorific value, M is the mass, the same mass, the calorific value is large, and the heat is more, so when the adulterated mass is the same as the original, the calorific value is smaller, and the heat is less.

d Specific heat capacity: You can give a certain amount of heat to the substance, and then measure the increased temperature, so as to obtain the specific heat capacity, if it is different from the raw coal, it means that there are impurities (but I think this reason is far-fetched, because although the specific heat capacity is the basic property of the substance, the material that can be had has a compatible specific heat capacity).

To sum up: what you want coal is heat, that is, the calorific value, so the calorific value is the fundamental problem. c！c！c！

c I am in the third year of junior high school, and this kind of question is generally selected calorific value.

The calorific value and specific heat capacity are difficult to measure, and the general laboratory conditions cannot reach them!

And the temperature is definitely not good, only to measure the quality, its essence is to calculate the density, the density of real and fake coal is different. Choose A

c The above is very clear, this question is actually very vague, such as what will happen to this black stone, there is no such introduction, there will be no such question in the general exam.

d, is the specific heat capacity, the specific heat capacity is a property of the substance, and has nothing to do with the temperature, mass, and amount of heat absorption of the substance.

So it can be used to identify substances.

Good luck with your studies, thank you.

First of all, don't think about it, go to the table first.

From the circuit diagram, when the sliding resistor slides to the right, the resistance value becomes larger.

As its resistance increases, so does its ratio of voltage in the circuit, but so does the total resistance in the circuit.

From this, it can be judged that when the total voltage remains unchanged and the total resistance becomes larger, the indicator of ammeter A becomes smaller, the indicator of V1 becomes smaller, and the indicator of V2 becomes larger.

Judging by myself, I chose from the options I don't know how to ask me.

It's not over.

Since the resistance is in series, it can be assumed that the value of the P resistance when it is not moved is Rx, and the absolute value of the current change is U Rx-U Ry, and then the total resistance wallpaper of R1 R2 and the total resistance wallpaper in the circuit can be calculated one by one.

The easiest way to represent all the data is to just represent it in concrete data.

For example, r1=100 r2x=100 r3=100 total voltage 600v

Then the current in the circuit is 2A R2 and the voltage occupied is 200V

After the change, R2 is R2Y=200, then the current in the circuit is R2, and the voltage occupied by R2 is 300V.

So it's clear.

This is called the landlord's formula.

The formula u i=r (this r refers to the invariant resistance in the circuit) works well.

That's it. The resistance on the left doesn't change, so that ratio doesn't change, of course.

Of course it's AD, since it's a free-fall motion, the object is only subject to gravity, so B won't be under pressure from A, and A is right and B is wrong.

Gravity can't disappear out of thin air, and it can't change, so C is wrong and D is right.

Answer: The correct answer to question 9 is d.

Analysis:1The tip of the needle points north, indicating that the tip of the needle is the north pole of the compass, so it is aerroneous.

2.The pointing of the tip of the needle is the tangential direction of the magnetic inductance line of the geomagnetic field at that point. Since the magnetic inductance line is a curve, b is wrong.

3.The tip of the needle can point north, which is affected by the geomagnetic field. Magnetic inductance lines are imaginary curves drawn to describe the direction and distribution of the magnetic field, and option C is wrong to say that the tip of the needle is affected by magnetic inductance lines.

Option: The direction of the needle tip is the geomagnetic field, and the direction of the point is correct.

8. The answer is d.

A false. Physics dictates that the direction in which a positive charge moves directionally is the direction of the current.

B is wrong, the rated voltage is large, and the rated power is not necessarily large. p=u r, and r is not determined.

C false. The resistance of a conductor is directly proportional to the length, inversely proportional to the cross-sectional area, and also related to the material9, the answer is false. North refers to the geographical north, which is the south pole of the geomagnetic field (b wrong) and the tip of the needle is the n pole.

The tip of the needle points north by the force of the geomagnetic field, c is wrong.

10. The answer is c.

11. When the slider P moves to the B terminal, the resistance of its access circuit increases. The total resistance r of the circuit increases. i=u r, when the u supply voltage does not change, the current i in the circuit decreases.

When the bulb is dimmed, the voltage at both ends of the bulb is the voltage representation UL=IRL, and when the RL is unchanged, UL becomes smaller. So the answer is d.

12. The answer is b.

Choose BA: If it sinks, then the upper surface of the object and the water surface should be more than 25cm apart, so it is not right.

b: The surface of the suspended or floating object is 5cm away from the water surface, if it floats, then the length of the object deep into the water is 15cm, and if the upper surface of the suspended object and the water surface are more than 25cm apart, it is true.

c, d: the weight does not clearly put forward the neutrality of the object, how can the buoyancy be found, excluded.

It must be suspended, when it is said to be stationary, the upper surface is 5cm, then the lower surface is 25cm, and you can see that it is suspended in the water by drawing a picture.

Answer C density: solid, liquid, gas, group A two solids, group B three are solids, can not distinguish the hardness: copper bars, pencil leads, and plastic rods are all soft objects, mercury or liquid, so wrong magnetic:

These objects are all non-magnetic in the absence of external influences, so they are not conductive: group A substances are all conductive; Group B substances are very weak in conductivity, which is what we usually call insulators.

The projector is a commonly used device in teaching, and the following description about the projector is correct (a, b, d).

I hope it helps you, and if you have any questions, you can ask them

I wish you progress in your studies and go to the next level! (*

The function of the plane mirror is to image and change the direction of light propagation, so A is correct Only when the screen is rough can diffuse reflection occur, so that students in all directions can see, so B is correct.

The screen is the light screen, and the image on the light screen is a real image, so C is wrong If the image on the screen becomes larger, and the lens is close to the projection, it is the object distance becomes smaller, the image distance becomes larger, and the image can become larger.

10. Answer: C is correct. In this experiment, the friction of object B should be read indirectly from the spring scale, because the spring scale is fixed on the wall, so the horizontal force given by the hand can only act on object A, (answers A and B are wrong) To accurately read the data from the spring scale, the direction of friction experienced by B should be to the right, so the answer D is wrong; Answer: The motion of object A in C has no effect on the amount of friction experienced by B.

There is also no effect on the spring scale reading, as object B remains stationary at all times when object A is moving to the right. The force it is subjected to, the two forces are balanced, and the friction force = the tensile force of the spring scale = the indication of the spring scale.

aDensity is the same, density is a property of a substance, for example, a block of ice, missing a corner, that is still ice.

Volume, mass, size, these are all changes with the external form of the substance, only the density does not change.

c (Note: This question is very controversial and rarely appears in different sources).