Physics questions for junior high school 3 grades, 3 physics questions for junior high school

Because electric energy is more convenient to use, electric energy can be easily converted into power, heat energy and applied in production and life, (our current life is almost inseparable from electricity).

Hydropower plants use the gravitational potential energy of water, and seem to be of no other use than to generate electricity.

Nuclear power plants mainly use nuclear energy, which is not easy to control and is extremely polluting, just look at Hiroshima

Because electrical energy is convenient for transportation. And electricity is green energy. Electricity can also be easily converted into other energy.

Today's era is the era of electrification, electrical energy is widely used in our lives, and electrical energy is not just seen in nature, so it is necessary to use solar energy, wind energy or kinetic energy to convert.

It is pollution-free, easy to transport over long distances, and easy to convert with other forms of energy.

1.The speed of electrical energy transfer is fast.

2.Low loss of electrical energy transmission.

3.It can be transmitted over long distances.

4.Energy can be allocated according to the user's needs.

Textbooks.

Today's machines work on electricity.

Other energy sources cannot be used directly.

It can only be used if it is converted into electrical energy.

Electricity is currently the most convenient energy source to transport.

Categories: Education, Science, >> Learning Aid.

Problem description:1Household thermos flasks, used in the following situations, the best effect is ( ).

a.Fill it with boiling water for best results.

b.Fill up the boiling water quickly and leave a small amount of air for the best effect.

c.Fill only half a bottle of boiling water for best results.

d.Fill only half a bottle of boiling water for best results.

2.At both ends of a lever, two black and white balloons filled with air and of equal volume are hung, and the lever is just balanced. If the device is exposed to the sun, after a period of time ( ).

a.Leverage is still balanced.

b.The white end of the ball falls, and the end of the black ball rises.

c.The white ball ends rise, and the black ball ends fall.

d.All three scenarios are possible.

3.Place a glass of water and a cup of peanut oil at a low temperature slightly below their melting point. In the process of their solidification, the water starts to solidify from and the oil starts to solidify from . (fill in the top, middle, or bottom).

I want explanations, not answers!

Analysis: 1Because the thermal conductivity of air is very poor compared to water, leaving a little air can prevent the water from coming into direct contact with the stopper, and the heat dissipation is very small.

Of course, there should not be too little water, the less water it has, the less heat capacity it has, and the less water it has, the greater the temperature drops.

2.Black absorbs more heat than white under the same conditions, so its temperature rises a little higher, so that its expanded volume is larger, and it is more buoyant by the air, and its pull on the lever becomes smaller (the pull on the lever is equal to the gravitational force of the balloon minus the buoyancy of the air on it), so the force multiplier arm on the side of the black ball is smaller, so the black ball will rise.

3.When the water solidifies and freezes, it becomes less dense and will float upward, so it will start to solidify from above. Peanut oil, on the other hand, becomes denser and will sink as it solidifies, so it will begin to solidify from below.

Solution: When it reaches position 1, R2 and R3 are short-circuited, and by P=U2 R, when the voltage is constant, the greater the resistance, the smaller the electrical power, and the resistance is the smallest when the switch is connected to 1, so the heating power is the largest, and the heating power is 90W

At this time, R1 U 2 P1 (36V) 2 90W=refers to position 2, and the heating power P is 72W from 1 analysis, so the circuit current I p U=72W 36V=2A

The voltage at both ends of R1 is U1=IR1=2A

Therefore, the voltage at both ends of R2 is U2=U-U1=

When it is pointed to position 3, the heating power p = 45w is analyzed by 1, so the circuit current i = p u = 45w 36v =

The question is a bit cumbersome to calculate, and the score of 20 is a bit low.

As can be seen from the figure, S is at 1 and there is only one resistor R1, that is, the power is 90Wp1=U R1 can be calculated as Ri=

S in series at 2, R1, R2, power 72W

P2=U (R1+R2) R1+R2=18 R2=S in 3, series, power 45W

p3=u²/(r1+r2+r3) r1+r2+r3= r3=

The resistance values are clear.

2: When S points to position 2, what is the voltage across R2?

Current i=U (R1+R2)=Voltage at both ends U2=IR2=3When S points to position 3, what is the current in the circuit?

i1=u/r1+r2+r3=

Hehe, it's over, it's quite detailed, the touch sensor is the role of the switch, don't care.

This question is a test of the calculated deformation of electrical power. Actually, it's not that hard.

At the moment when the resistance decreases or increases, the current through the branch increases or decreases, while the current of the other branches does not change, so that the main circuit current changes, the voltage does not change, and the total resistance decreases or increases according to Ohm's law.

Copper = volume of the statue v = m wax p wax.

The mass of the copper required m copper = p copper * v =

2.The volume of the bottle v = m water p water = 1000 ml mercury density p mercury =

m mercury = p mercury * v mercury = 13600g =

First empty: 15000n

Second empty: 15000n

Third empty: 15000n

Fourth empty: 15000n

In all four cases, the vertical direction of the aircraft is in equilibrium, which is equal to the gravity of the aircraft.

There is a helicopter with mass of 15000 N when it is parked in the air, and this force is 15000 N, when it flies horizontally, the lifting force is 15000 N when it rises at a constant speed, and when it descends at a constant speed, the lifting force is 15000 N (g takes 10 N kg).

In all four cases, the helicopter is in equilibrium, and the lifting force is the weight of the aircraft itself.

All are 15000n

Because it is at rest or moving at a constant speed, the force experienced is a balanced force.

And the object itself is subject to gravity, and it has to be subjected to a lifting force in the opposite direction equal to the magnitude of gravity, so 1500 10 15000

The buoyancy is the water density * g * v =

Density: Because the buoyancy of 600N is just balanced, the spring countergravity is 600N, the mass is 60kg, and the density is 60 cubic meters.

Safety is not sinkable, the limit is just submerged, and the mattress is completely submerged, if the buoyancy is 5 * 600, it is 3000, the mattress itself weighs 600, and the remaining 2400 are manned, 4 of each 500n will be on the line, and 5 will be sunk.

1.Take any one of the bulbs from the dust of the world, and if the other no longer glows, connect them in series; If still glowing, in parallel.

2.I won't draw the picture. After the two bulbs are connected in parallel, they are connected with the electric bell in series, and the two switches are connected in series with the two lights respectively.

The battery pack ——— the green light ——— the electric bell ——— a switch - Lu Qi ——— the battery pack.

— The red light ——— another switch is guessed as ——— as a megalome.

That's what it means, and we'll see.

When the two lamp strips are lit, remove one bulb to see if the remaining bulb is bright or dark (string) Before the light line (and) Note: The bulb cannot be removed It can break a sleepy One is also effective.

The square at the beginning of 90mv - the square at the end of v = 2as (a is the acceleration) solution a =

After another 50 seconds, v2=

Because V2 is less than 0, the speed of the car is already 0 after 50s, so the square at the end of V = 2as

The dry band is solved s=90m

You won't ask me again without a reed, ok?

First, find the acceleration a=(64-36) and 140=

Then find the time used to reduce the speed to 0 t=6 <50, so when the car has stopped at 30 seconds, you can find the distance of the forward circle only s=

1. The third step is: fill the empty beverage bottle with milk, measure the total gravity as g3, and find the mass of the milk as m=(g3-g1) g, and the density p=m v=[(g3-g1) g] [g2-g1) g]=(g3-g1) (g2-g1).

2. Density p=(g3-g1) (g2-g1)3,After measuring the density of milk as cubic cm, and knowing that the density of milk is cubic cm, you can determine whether it is mixed with water by comparing the two densities.

Please click here, you can ask if you don't know.

F pressure = g = mg = 180kg 10n kg = 1800n f pull = f =

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 pressure of the object at rest on the horizontal plane is equal to the weight of the object: f g mg = 180 * 10 = 1800N

Because of the uniform linear motion, the tensile force is equal to the resistance: f pulls g*