About the test questions for the National Junior High School Physics Competition

(2) Current, voltage, resistance

1. Multiple choice questions:

. (04 Zhejiang Lishui Nature) The following figure is a schematic diagram of a sliding rheostat connected to the circuit. The resistance of the connected circuit cannot be changed by moving the slider (C)

. (04 Dalian, Liaoning) Every festival night, many tall buildings are decorated with thousands of light bulbs. The lit bulbs reflect the outline of the building, making it look beautiful and spectacular. These light bulbs are (A)

A. Parallel connection B. Series

C. It may be in series or in parallel D. Unable to judge

． (04 Taizhou) Among the following data, the one closest to the truth is (A)

A. The mass of an egg is 50g B. The walking speed of a person is 5m/s

C. The temperature of the bath water is 70℃ D. The current of a household desk lamp in normal operation is 1A

. (04 Chongqing) In the circuit shown in Figure 2, the voltage U remains unchanged and after the switch K is closed, the following

The correct statement is (A)

A. The ammeter reading increases

B. The ammeter reading decreases

C. The reading of the voltmeter increases

D. The voltmeter reading decreases

. (04 Chongqing) As shown in Figure 4, two light bulbs are connected in series in the circuit. After closing the switch, it is found that one of them emits light and the other does not. Which of the following statements is correct (B)

A. The filament of the non-luminous light bulb is broken

B. Comparing the two lamps, the filament resistance of the non-luminous bulb is smaller

C. Comparing the two lamps, the voltage across both ends of the non-luminous bulb is larger

D. Comparing the two lamps, the current through the non-luminous bulb is smaller

. (04 Guiyang Experimental Area) Among the following descriptions, which one is a characteristic of a parallel circuit (C)

A. The current is equal everywhere in the circuit B. Any disconnection in the circuit will cause an open circuit

C. The voltages on all branches are equal D. The sum of the voltages of each part of the circuit is equal to the total voltage

. (04 Ningde, Fujian) Methods such as "control variable method", "equivalent substitution method", "model method", and "analogy method" are often used in physics research. The following are several research examples in junior high school physics:

①When studying the action of several forces on an object, introduce the concept of resultant force

⑦Use light to express the propagation direction of light

③When studying electric current, combine it with the flow of water Comparison

④Use magnetic field lines to describe the magnetic field

Among the above examples, the same research method is used (C)

A. ①③ B． ②③ C． ②④ D． ①④

． (04 Chenzhou, Hunan) As shown in Figure (1), it is the reading of an ammeter connected to a certain circuit. The correct one is (B)

A. 0.32A B. 1.6A C. 0.26A D. 1.52A

． (04 Chenzhou, Hunan) As shown in Figure (3), when the switch s is closed and the slide P of the sliding rheostat moves to the right, the ammeter's indication will be (B)

A. Get bigger b. become smaller c． unchanged D. Unable to determine

1. Fill in the blanks (18 points for this major question)

1. A car is traveling at a constant speed of 36 kilometers/hour on a road parallel to the railway. A train is approaching at a speed of 54 kilometers/hour. The passengers in the car see the train passing by for 7 seconds. From this, We know that this train is ______ meters long.

2. When using a pressure thermos bottle, just press the piston down and the boiling water in the bottle will flow out from the water outlet. This is because pressing the piston _________ can force water out.

3． The simplest and easiest way to determine whether a lens is convex is to___________.

4． An object sliding down an incline at a constant speed experiences a force of ___________.

5. When selecting the metal wire to make the light bulb, the part of the metal wire welded in the glass must be the same as the _________ of the glass to ensure a good bond between them.

6. A classmate saw lightning in the distance and started counting time. After △t time, he heard thunder. If it is known that the propagation speed of light in the air is C and the propagation speed of sound in the air is V, if the three quantities C, V, and △t are used to represent the distance S from the thundering place to the classmate, then S = ________; because C》V, then the above distance can be approximately expressed as S≈___________.

7． Any two of the three small grass balls that are close to each other will attract each other. The electrification of these three small grass balls must be _________.

8. As shown in Figure 1, the power supply voltage is stable, and the power consumed by the three resistors is equal. If the resistance of R3 is increased, then the resistor that consumes more power than the original one is _________.

2. Multiple-choice questions (fill in the number of the correct answer in the brackets in the question, this question is worth 30 points)

1. When a motorcycle flies over a tall obstacle, in order to avoid overturning, the wheel that touches the ground first should be [ ]

A. Front wheel

B. Rear wheel

C. The front and rear wheels should touch the ground at the same time

D. The landing method is the same

2. A photo taken by the lake is shown in Figure 2. Because the lake is calm, the scenery on the shore and the reflection in the lake are very similar in the photos. Among the following methods, the method that cannot be used to distinguish the real scene from its reflection in the lake is [ ]

A. The reflection is slightly darker than the real scene

B. The reflection is slightly less clear than the real scene

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

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

3. The I-U graph of the change of current intensity with voltage of two resistors with different resistance values ??is shown in Figure 3. The correct conclusion drawn from the figure is [ ]

A. R1>R2.

B. The I-U graph of the total resistance after R1 and R2 are connected in series is in area II

C. The I-U graph of the total resistance after R1 and R2 are connected in parallel is in area III

D. The I-U graph of the total resistance after R1 and R2 are connected in parallel is in area I

4. A ball weighing 4 N is kicked out with a force of 50 N. The ball rolls to a stop 20 meters away. Then the amount of work done by the force of kicking the ball is [ ]

A. 1000 joules

B. 80 joules

C. 1080 joules

D. Insufficient conditions to determine

5. After the astronauts landed on the moon, they could still see a red sun in the sky when they looked up, but the other surrounding scenes should be [ ]

A. In a dark night, you can see the stars, but they don’t twinkle

B. In a dark night, you can see stars, and the stars are twinkling

C. It’s so bright that you can’t see the stars

D. A piece of sky blue, you can see the earth and stars

6. As shown in Figure 4, the Torricelli experimental device for measuring atmospheric pressure is placed on an electronic scale. The glass tube A is fixed by the bracket and is not in contact with the bottom of the mercury tank B. When the atmospheric pressure is 1.0×105 Pa, the reading of the electronic scale is Q. If the external atmospheric pressure drops, the reading of the electronic scale will be [ ]

A. Greater than Q

B. Less than Q

C. Equal to Q

D. Unable to determine

7. As shown in Figure 5, the experimental car moves at a constant speed on a horizontal road. Two objects with masses M and m are placed on a fixed smooth table in the car, and M>m.

When the car suddenly stopped, the two objects [ ]

A. Must collide

B. They must not collide

C. Not necessarily colliding

D. First they hit, then they separated

8. Four students made circuit breakers respectively to check whether the wires are connected or disconnected. It connects a coil, a small light bulb, an earphone, an ammeter and a dry battery in series, as shown in Figure 6. When in use, connect P and Q to both ends of the wire to be tested, and determine whether the small magnetic needle deflects, whether the small bulb emits light, whether the earphone makes a "click" sound, and whether the ammeter pointer deflects to determine the continuity in the wire. Which of these four checkers do you think is infeasible [ ]

9. As shown in Figure 7, the total resistance of the sliding varistor M is 10 ohms, and the total resistance of the sliding varistor N is 200 ohms. The resistance of resistor R is about 20 ohms. In order to slightly increase the reading of the voltmeter, you should [ ]

A. Move the slider of N a short distance to the left

B. Move the slider of M a short distance to the left

C. Move the slider of N a short distance to the right

D. Move the slider of M a short distance to the right

10. Porcelain bowls are commonly used to stew food in daily life. The bowl and the bottom of the pot are not in direct contact, as shown in Figure 8. When the water in the pot boils, the soup in the bowl must [ ]

A. It will boil, but slowly

B. Boils at the same time as the water in the pot

C. It will not boil and the temperature is 100℃

D. Will not boil, the temperature is lower than 100℃

11. In most parts of our country, the season when you can see "white mist" when you open the refrigerator is summer, and the season when you can see white breath exhaled from the mouth is winter. Among the conditions that produce these phenomena described below, the incorrect one is [ ]

A. Summer is humid and the air contains a lot of water vapor, which is one of the conditions for the former

B. Drought and heat in summer are one of the conditions that produce the former

C. The gas exhaled from the mouth contains a large amount of water vapor, which is one of the conditions for the latter

D. Dry and cold winter is one of the conditions that produce the latter

12. Pour a small bucket of hot water with temperature t2 into a large bucket of cold water with temperature t1. The temperature after mixing is t. The correct relationship among the following temperature relationships is [ ]

13. For a "220V 1000W" electric stove, I want to modify it to be used in a 110-volt circuit without adding other materials and equipment, and the power is still 1000 watts. The following measures are feasible [ ]

A. Cut the electric stove wire into two equal lengths, connect them in parallel and connect them to the 110 volt circuit

B. Cut the electric stove wire in half and connect it to the 110-volt circuit

C. Cut off 3/4 of the electric stove wire, and connect the remaining 1/4 to the 110-volt circuit

D. Cut off 1/4 of the electric stove wire and connect the remaining 3/4 to the 110-volt circuit

14. When the sensitive ammeter is moved, its pointer will swing left and right due to collision or vibration. In order to reduce this swing, the correct one of the following measures is [ ]

A. Connect the positive and negative terminals of the ammeter with wires

B. Try to fix the pointer of the ammeter on the dial

C. Remove the pointer of the ammeter and store it separately

D. Fix the ammeter with foam plastic

15. Five resistors all with resistance R are connected to form a quadrilateral ABCD as shown in Figure 9. The resistance value between any two nodes of A, B, C, and D cannot be [ ]

A. R B. R/S

C． R/2 D. 5R/8

3. Drawing questions (***12 points for this major question)

1. The cross-sectional schematic diagram of a bicycle pump is shown in Figure 10. There is an incomplete structure in the picture. Please complete the structure on the picture according to the principle and process of inflating.

Compared with the inflator without small tube B and only large tube A, the advantage of this type of pump with small tube B is ___________.

2. The sun and moon are in the positions shown in Figure 11. According to the law of straight line propagation of light, please draw on the diagram that when people on the earth are located in the _____________ area, they can see a total solar eclipse, the _____________ area can see a partial solar eclipse, and the ___________ area can see an annular solar eclipse.

IV. (6 points) In the experiment of measuring the density of salt water, the position of the weight and weight used when weighing the mass of salt is shown in a in Figure 12. Use a measuring cylinder to measure pure water as shown in b in Figure 12. Then the salt is completely dissolved in the water, and the position after the water level rises is shown as C in Figure 12. From this we know that the mass of salt is ____________ grams, the volume of pure water is ____________ cm3, and the volume of salt water is ____________ cm3. From this we know that the density of salt water is ________kg/m3.

5. (10 points) The lights will dim when household circuit power consumption peaks. This is due to the voltage drop across the lights. A classmate wanted to measure the actual voltage of the home circuit at this time, but he did not have an AC voltmeter at hand. It only had a watch, a soldering iron with a nameplate, and a running electricity meter with a nameplate on the switchboard in the circuit. He used these devices to measure the actual voltage across electrical appliances in household circuits. Please explain his method of measurement and calculation.

6. (12 points) A factory designed a reservoir (as shown in Figure 13). The liquid level height h1 of water source tank A remains unchanged. There is a small water outlet at the bottom of the tank with an area of is S1, and is connected to the lever BC through a piston with a cross-sectional area S2 under the hole. The lever can rotate up and down around end B. There is a hollow cylindrical float at the other end with a cross-sectional area of ??S3. BO is 1/3 of the total length of the lever. The original design intended that when the lever is horizontal, the float is immersed in the water to a depth of h2, and the piston can just Block the water outlet, but during use, it is found that when the piston is still a very small distance from the water outlet, the float no longer floats. At this time, the float is submerged in the water to a depth of h3. In order to make the piston automatically block the water outlet, the float has to be reduced. Heavy. Try to find the size of the float minus the weight G'. (The weight of the piston and connecting rod is not counted, and the buoyancy force on the lever is not counted)

7. (12 points) There are two kinds of light bulbs, marked "110V 60W" and "110V 25W" respectively. Connect them to the home In the lighting circuit,

(1) Can one of each be used in series? Please explain why.

(2) If two types of light bulbs are mixed and used in a household circuit and they can both emit light normally, how many should be used at least and how to connect them?

(3) What is the total power of the light bulb at this time?