High marks before 2, physics examination papers of grade three in various places.

The Second National Junior High School Applied Physics Competition in p>1993

1. Multiple choice questions (***39 points, 3 points for each small question) Only one of the answers listed in the following questions is correct. Fill in the letters in front of the correct answers in the brackets after the questions.

1. Birds falling on high-voltage lines will not be electrocuted, because ()

A. The cuticle on the bird's p>A． is insulated.

B. Birds are more tolerant of current.

C. The bird's feet are on the same wire, and no current flows through the bird's body.

D. the high-voltage line has a rubber sheath.

2. People often say that the well water is warm in winter and cool in summer, because ()

A. The temperature in the well changes little in winter and summer, while the ground temperature changes greatly in winter and summer. The so-called "warm in winter and cool in summer" is the comparison between the well water temperature and the ground temperature.

B. Well water has more heat energy under the action of geothermal energy.

C. Well water is far from the ground and is not affected by air flow, so it has more heat energy.

D. Well water is exposed to air. In summer, the temperature is high, which evaporates quickly and absorbs more heat, so the temperature is low.

3. Oily vegetable soup is not easy to cool, mainly because ()

A. Oil layer hinders the heat radiation of soup.

B. heat exchange between oil layer and water in soup is not easy.

C. The thermal conductivity of oil is worse than that of water.

D. The oil layer covers the noodle soup, which hinders the evaporation of water.

4. In summer, there are often water drops on the tap water pipe, which is because ()

A. In summer, the temperature of tap water is high and it evaporates quickly, thus forming water drops on the pipe wall.

B. There is a lot of water vapor in the air in summer, and it condenses into water droplets on the pipe wall when it meets cold water pipes.

C. In summer, the air pressure is low, the pressure difference between inside and outside the pipe is large, and a small amount of water seeps through the micropores in the pipe wall.

D. In summer, the water consumption is large, so the water plant needs to pressurize the tap water, and the pressure difference between the inside and outside of the pipe increases, so a small amount of water seeps through the micropores in the pipe wall.

5. The backs of many refrigerators are painted black because ()

A. Black objects have strong heat radiation ability.

B. Black paint has higher strength.

C. Black objects have strong heat absorption ability, which is beneficial to the cooling of objects in the refrigerator.

D. Black objects are not easy to absorb dust.

6. In order to adapt to the ironing of different materials, the household electric iron has designed a multi-gear switch to adjust the temperature. When in use, the iron can be heated to the required temperature by turning the knob. Fig. 1 is a circuit diagram of an electric iron. Rotating the multi-gear switch can change the connection between 1, 2, 3 and 4. Now put the switch in the highest temperature range. At this time, which picture in Figure 2 is the connection between 1, 2, 3 and 4? ()

7. The measuring cylinder is made thin and high (as shown in Figure 3), not thick and short, mainly because ()

A. In the experiment, the thin and high measuring cylinder is easy to operate.

B. The measuring cylinder with thin height can make a relatively large base and increase the stability.

C. Compared with the thick and short measuring cylinder, the corresponding scale interval is larger, which is convenient for accurate reading.

D. There is more liquid in the thick and short measuring cylinder, and the wall of the cylinder is under greater pressure, so thicker glass is needed, which is inconvenient for reading.

8. Figure 4 shows Wang Xiaogang's glasses. Judging from the glasses, his eyes ()

A. are farsighted.

B. is nearsighted.

C. The eyesight is normal, and the glasses are sunglasses.

D. One eye has normal vision and the other is nearsighted.

9. There are two types of electric kettle sold in the market. One is as shown in Figure 5(a), and the electric heating element is in the base below the kettle bottom; The other is as shown in fig. 5(b), where the electric heating element is above the bottom of the pot and submerged in water. If the power consumption of the two kettles is the same and the amount of water in the kettle is the same, then ()

A. (a) The electric heating element of the kettle is under the kettle bottom, which is beneficial to the convection of water, and (a) the water in the kettle boils first.

B. (a) The electric heating element of the kettle is placed in the base, so the heat loss is small.

C. (b) The electric heating element in the pot is surrounded by water, so the heat loss is small, and (b) the water temperature in the pot rises rapidly.

D. (b) The electric heating element of the kettle is small, and the utilization rate of heat energy is low.

1. The picture shows a photo taken by the lake. Because the lake is calm, the scenery on the shore is very similar to the reflection in the lake in the photo. Which of the following methods can't be used to correctly distinguish the real scene from its reflection in the lake? ()

A. The reflection is slightly darker than the real scene.

B. The definition of the reflection is slightly worse than that of the real scene.

C. The left and right positions of the characters in the reflection are just the opposite to the real positions when taking pictures.

D. The height of the figure in the reflection is slightly larger than the real figure.

11. When black-and-white photos are processed in a darkroom, the liquid injection of the thermometer used is blue, not red. The following statement is incorrect: ()

A. The darkroom safety light is red, so everything looks red. The red liquid column of the thermometer is not easy to see in this environment.

B. The blue liquid column of the thermometer is black under red light.

C. the blue liquid column can be seen more clearly under red light.

D. the red liquid column reflects white light under red light.

12. There are two small bulbs, only marked with rated voltage, both of which are 6.3 volts. Connected in series to the power supply, the brightness is different. The following six sentences should be used when answering which bulb has a higher rated power:

(1) The brighter bulb has a higher resistance;

(2) we can know from the relation P=I2R;

(3) The currents flowing through the two bulbs are the same when connected in series;

(5) When working at the same voltage, the electric power of the bulb is inversely proportional to its resistance;

(6) Therefore, the rated power of dark bulbs is higher when connected in series.

which of the following order can make the problem clear? ()

A. (2) (1) (3) (4) (5) (6)

B. (2) (3) (L) (4) (5) (6)

C. (4) (5) (2) When asking this question, a classmate used the following seven sentences:

(1) Put the densitometer in milk and it will float on the liquid surface:

(2) When the densitometer floats on the liquid surface of any liquid, the gravity of the liquid it displaces is equal to that of itself;

(3) The density of milk is greater than that of water;

(4) The gravity of a certain volume of milk is greater than that of the same volume of water;

(5) Sufficient buoyancy can be obtained when the densimeter displaces a small volume of milk;

(6) The position of densimeter floating in milk is higher than that in water;

(7) Densitometer A should be selected.

Some of these words are superfluous. Which of the following statements can make the problem concise and accurate? ()

A. (1) (2) (3) (4) (6) (7) B. (2) (3) (5) (6) (7)

C. (2) (3) (5) (6) D. (2. If a resistance wire and a bulb are connected in series to the U power supply, in order to make the bulb glow normally, the resistance value of this resistance wire should be _ _ _ _ _ _ _ Ohm, and it should be able to withstand at least _ _ _ _ _ _ _ A current.

2. The train must whistle before entering the tunnel. If the train speed is 8 km/h, the speed of sound in the air is 34 m/s, and the driver hears the echo reflected from the cliff at the tunnel entrance 2 seconds after whistling, then the distance between the train and the tunnel entrance when whistling is _ _ _ _ _ _ _ _ _ _.

3. There is a thermometer that hasn't been shaken after use, and the reading stays at 38.2℃, which was mistakenly used to measure the patient's temperature. The actual temperature of the patient is 37.8℃, and the thermometer reading is _ _ _ _ _ _℃; If the patient's actual temperature is 38.5℃, the thermometer reading is _ _ _ _ _ _℃.

4. millimeter water column is commonly used as the pressure unit of gas in urban gas supply industry. The nameplate of JMB-3 gas meter is shown in the following table ("pressure" in the nameplate refers to pressure). When the gas pressure is greater than _ _ _ _ _ Pa and less than _ _ _ _ _ Pa, this gas meter can work normally.

5. The teacher used the projector to project the transparent film written before class on the screen and got a clear image (Figure 9). When he put two bar magnets on the projector again, put a glass plate on the magnets, and demonstrate the distribution of magnetic induction lines with iron powder, the image of magnetic induction lines on the screen is not clear, so he should move the lens of the projector to _ _ _ _ _.

6. In winter, after plugging in the electric heater, Li Ming found that the indoor lights were dimmed. He used a voltmeter to measure the power supply voltage. As a result, the voltage was 21 volts when the heater was not connected, and the voltage dropped to 2 volts after the electric heater marked "22V1W" was connected. Through calculation, it can be known that the resistance in the power supply circuit is about _ _ _ _ _ _ _ _.

7. Jia Ming plans to use the electric energy meter to measure the actual electric power consumed by the TV. He disconnected all other electrical appliances connected to the watt-hour meter, only turned on the TV, and measured that the number of revolutions of the watt-hour meter was n in t seconds. If the nameplate of the watt-hour meter indicates that the number of revolutions per kilowatt-hour is n, the electric power of the TV set is watts.

3. Question and answer questions (5 points)

1. Give you a lens, how to judge whether it is a convex lens in the simplest way?

2. Name two simple methods to estimate the focal length of convex lens.

4. (5 points) TV stations transmit radio signals to high-altitude communication satellites, which then transmit the signals to other areas on the ground, thus realizing satellite TV transmission. In the debugging process, the TV station itself also needs to receive the signal returned by the satellite and compare it with the signal transmitted by the cable in the studio. At this time, it is found that the changes of characters' movements on the pictures from satellites always occur later than the corresponding changes on the pictures from cables.

1. explain the reasons for this time delay.

2. It is known that the height of communication satellite is about 36, kilometers, and the speed of radio signal propagation is the same as the speed of light. Estimate the time of picture delay.

v. (7 points) Figure 1 shows an automatic device for drinking water for livestock under design. The bottom cover A usually supports the water outlet of the water tank. Once the water level in the drinking water tank drops, the buoyancy of the float decreases, and the water flows from the small tank into the drinking water tank. In the design, the maximum water level of the water tank is 6 cm, the diameter of the water tank outlet is 6 cm, the total mass of the bottom cover A and the vertical bar B is 42 g, the mass of the float D is 58 g, and the volume is 2 decimeters 3, and the mass of the cross bar C can be ignored. The calculation shows that this automatic device can't work normally when the water tank is full of water.

6. (7 points) There are two uses of bottle cap screwdriver made of hard steel sheet. As shown in figure 11. A and B are the two points where the screwdriver touches the bottle cap when the cap is opened. Among the contact points between the bottle cap and the bottle mouth, D and E are the farthest and nearest points from the hand, respectively. When opening the lid, the force F exerted by the hand on the screwdriver acts on the G point, and the resistance of the bottle mouth to the bottle cap can be considered as acting on the center C point of the bottle cap (not marked in the figure). If the bottle cap does not bend obviously during opening.

1. point out the fulcrum of the lever in the case of 11(a) and (b) in the figure respectively.

2. Which of the two usages of Figure (a) and Figure (b) is more labor-saving? Why?

7. (8 points) The table frame of some household glass coffee tables is attached to the desktop with four plastic suction cups (as shown in Figure 12). If the diameter of each sucker is 4 cm, calculate that when the mass of the table frame does not exceed much, the table frame can be lifted by lifting the desktop. In fact, the weight of the table frame can always be less than the above calculated value. Please analyze the reasons.

8. (8 points) The lights in the corridor are always on all night, so they are easily damaged. Connect it in series with a resistor and then connect it to the circuit. Although the brightness is slightly reduced, the service life can be greatly extended. Now, in order to make the power consumption of "22V25W" light bulb in actual use about 8% of the rated power, how much resistance should it be connected in series? How much electric power does this resistor consume?

answers to the p>1993 national junior middle school applied physics competition

1.c2.a3.d4.b5.a6.a7.c8.a9.C1.d11.d12.d13.b

2.1..176; 11.36 2．362.2 3．38.2； 38.5 4．49；

3. 1. It is the convex lens that can make an object into an enlarged image. 2. Method 1: let the sun shine vertically on the mirror, adjust the light spot where the sun converges to the minimum, and measure the distance from the converging light spot to the mirror as the focal length. Method 2: read a book as a magnifying glass. Adjust the magnified virtual image of the font until it just becomes blurred, and measure the distance from the lens to the font as the focal length.

4. 1. It takes some time for electromagnetic waves to travel back and forth from TV stations to satellites.

v. as shown in fig. 13, The forces acting on the bottom cover, vertical rod and float are: upward buoyancy:

F float = ρ VG = 13× 2× 1-3× 9.8 = 19.6 (cattle)

downward total gravity:

G total =(.42+.58)×9.8=9.8.

6. 1. Because the bottle cap is not obviously bent during opening, the screwdriver and the bottle cap can be regarded as a whole, with the fulcrum in Figure (a) being D and the fulcrum in Figure (b) being E..

2. The resistance in the two drawings is the same, and so is the resistance arm, which is the distance from the center to the GD＞GE of the bottle cap. In Figure (a), the power arm is GD, and in Figure (b), the power arm is GE, GD > GE. According to the principle of lever balance, Figure (a) is more labor-saving.

7. The table frame is lifted by the atmospheric pressure on the four suction cups. Assuming that the air in the suction cup is completely exhausted, the atmospheric pressure

f = 4ps = 4× 1.1× 15pa× 3.14× .22m2 = 57N

In fact, the air in the suction cup can't be completely exhausted, so the actual mass is always less than the above calculated value.

VIII. P Real =P Amount × 8% = 25× .8 = 2 (watts)

Power consumed by resistance P Resistance =U Resistance I=23.2×.116=2.36 (watts).