Urgent for junior high school physics questions! ! Thank you all.

Example 1(09 Shandong Plain) The microwave oven is a clean, hygienic, pollution-free and efficient household appliance. The nameplate of the microwave oven is shown in the following table. Put water with an initial temperature of 20℃ and a volume of 275ml into a microwave oven, and heat it at the maximum power for 65438±0min. Take it out and measure the water temperature at 40℃. If the specific heat capacity of water is 4.2× 103J/(kg? ℃)。 Q:

Type of electric shock protection Class I rated capacity 20L

The frequency of NN-S3 13WF power supply is 50Hz.

Rated voltage 220V, mass 12kg.

Maximum power1100 w.

(1) What is the current in the circuit when the microwave oven is heated?

(2) What is the electric energy consumed in the heating process?

(3) How much heat does water absorb when heating?

The answer (1) is based on P=UI, 1 100w/220V=5A.

(2) Method 1: According to W=Pt, the power consumption of microwave oven is: W = Pt =1100 w× 60s = 6.6×104j.

Method 2: according to W=Pt, the electric energy consumed by microwave oven is:

W = pt =1.1× h ≈ 0.0183kwh (0.0183 ∽ 0.02kwh gives full marks).

Method 3: w = uit = 220v× 5a× 60s = 6.6x104j.

(3) According to:1.0×103kg/m3× 275x10-6m3 = 0.275kg.

Q=cm(t-t0), heat absorbed by water:

Q suction =c water m water (t-t0)=4.2× 103J/(kg? ℃)×0.275kg×(40℃-20℃)= 2.3 1× 104j

Example 2 (Jining, Shandong, 09) Electric bicycle is a popular means of transportation (as shown in figure 1). It can ride by electric or pedal. During electric riding, the battery supplies power for the motor on the vehicle, and the motor provides power for the vehicle. The following table is the main technical parameters of electric bicycles.

The vehicle mass is 40kg.

Maximum speed ≤ 30km/h

Maximum noise ≤ 62db

Battery voltage 48V

Capacity 12A.h

The rated voltage of the motor is 48V.

Rated power 240 w

(1) Please observe the structure of electric bicycles carefully, and you will find that physical knowledge is used in many places. Please give an example.

Example: The seat of an electric bicycle is large and flat, which can relieve stress.

A:

(2) The total contact area between the tire and the ground during driving is 0.0 1m2. What is the pressure on the ground when you ride an electric bicycle on a straight road? (take g= 10N/kg)

(3) What is the working current of the electric bicycle at rated power?

(4) How much energy does the battery store when it is fully charged at one time?

(5) If 80% of the energy stored in the battery is used to overcome the resistance when the electric bicycle runs on a straight road, the average resistance of the electric bicycle is 40N. How far does the battery run continuously with full power?

(6) From the perspective of environmental protection, what are the advantages and disadvantages of electric bicycles compared with motorcycles? (one for each person)

Answer (1) For example, the wheels of electric bicycles may have uneven patterns to increase friction.

(2) The estimated mass is about 50kg, and the pressure on the ground is:

p = F/S =(40k g+50k g)× 10N/kg/0.0 1 m2 = 0.9× 105 pa

(3) Current at rated power: I=P/U=240W/48V=5A.

(4) The electric energy stored in the battery: w = UIT = 48V×12A× 3600s = 2073600J (5) Because when the electric bicycle is running, 80% of the energy stored in the battery is used to overcome the resistance and do work, that is:

2073600J× 80% = fs So, the distance that an electric bicycle can run continuously is:

S = m

(6) Advantages: no tail gas and low noise. Disadvantages: Generally, it takes about one year to replace the battery, and the discarded battery pollutes the environment.

Example 3(09 Taian) Taian Wuyue Special Purpose Vehicle Co., Ltd. is a large-scale special purpose vehicle production base.

In the vehicle type test, a special vehicle in this factory runs at a uniform speed of 5.6km on a straight road with a resistance of 3.0.

× 103N, and the fuel consumption is 1.5× 10-3m3 (assuming the fuel is completely burned). If the fuel density ρ = 0.8× 103kg/m3, heat it.

The value q = 4× 107j/kg, and find:

(1) Work done by special vehicle traction.

(2) Given the heat engine efficiency η = WQ (where W is the work done by the heat engine in a certain period of time and Q is the heat generated by the complete combustion of the fuel consumed during this period), what is the heat engine efficiency of this special vehicle?

Answer (1) Because the special vehicle runs at a constant speed, the traction force F = F = 3.0× 103N.

Traction work w = fs = 3.0×103n× 5.6×103m =1.68×107j.

(2) Fuel mass consumed: m = ρ V = 0.8×103kg/m3×1.5×10-3m3 =1.2kg.

Heat generated by complete combustion of fuel: q = MQ =1.2kg× 4×107j/kg = 4.8×107j.

Heat engine efficiency η = WQ =1.68×107j4.8×107j = 35%.

Standard inspection exercise

1. As shown in Figure 2, Xiao Ming, a student with a mass of 40kg, rides a bicycle with a mass of 10kg on a straight highway at a constant speed for 5 minutes 1500m.

(1) What is the speed of Xiaoming's cycling?

(2) If Xiaoming's resistance when riding a bike is 50N, the classmate will ride 5mg.

How many joules of work did the resistance do?

(3) When Xiaoming's total contact area with the ground is 20cm2, the classmate rides a bike.

What is the ground pressure when driving? (take g = 10n/kg)

2. As shown in Figure 3, the weight of the crown block is 2N, and the weight of the moving pulley is1n.. Under the pulling force f, the object A with a weight of 8N rises vertically by 0.2m in1s. If the rope weight and friction are not considered, find

(1) moving speed of free end of rope

(2) Mechanical efficiency of pulley block

(3) the magnitude of the pulling force f.

(4) Power of pulling force F

3. There is a simple way for the tank to cross the trench: install airbags on the tank. When encountering a trench, the airbag is put down. After inflating the airbag, the tank went through the trench as if it were leveling the ground. Let the tank mass be 4× 104 kg, and the crawler landing area be 5 m2. When the first half of the tank's track is pressed against the airbag, what is the pressure of the tank on the airbag? (Let the tank be symmetrical before and after, and g is 10 N/㎏)

4. As shown in Figure 4, the power supply voltage is constant, r1=12Ω, and the small bulb is marked with "6V 12W" (constant resistance). Ask questions: (draw the equivalent circuit diagram of each of the following questions)

(1) Only when Sl is closed, the current is expressed as 0.8A, so what is the power supply voltage?

(2) When Sl, S2 and S3 are all closed, move the slider P to the B end, and the ammeter

If the indication is 1.2A, what is the maximum resistance of the sliding rheostat?

5. In the circuit of Figure 5, r1=10Ω, R2 = 20ω. When the switch is closed, the ammeter indicates 0.3A.

Find the voltage across the (1) resistor R 1

(2) What is the current flowing through R2?

6. A solar water heater can produce 100 kg of water in 5 hours. If the temperature rises by 60℃, how much heat will water absorb?

Quantity? How many kilograms of liquefied gas do you need to heat water, excluding metering loss? (The specific heat capacity of water is 4.2× 103J/ (? ℃) ,

The calorific value of liquefied gas is 4.5× 107J/㎏).

7. The motor is a widely used power machine. From the point of view of energy conversion, it is mainly to convert electric energy into mechanical energy, and part of it is lost in the coil in the form of heat. There is an existing motor. When 220 volts is applied to the motor, the current passing through the motor coil is 50 amps. ..

Q: (1) What is the electric energy consumed by this motor per minute?

(2) Given the heat generated by the current passing through the conductor (where I, R and T respectively represent the current passing through the conductor, the resistance of the conductor and the energization time), if the resistance of the motor coil is 0.4 Ω, how much heat does the coil generate per minute?

(3) How much mechanical work does this motor do per minute?

8. Electric bicycles are increasingly favored by people, and many people use electric bicycles instead of motorcycles. Before using an electric bicycle, the battery on the bicycle should be charged. When riding, the battery provides power for the electric bicycle on the bicycle, and the motor provides power for the bicycle. The following table shows some main technical parameters of a certain brand of electric bicycle:

The maximum speed is ≤ 30km/h, and the continuous driving mileage on one charge is 50km.

The working voltage of the battery is 36V, and the charging time is 8h.

The power consumption is 0.6 kWh at one charge, and the motor efficiency is 75%.

The maximum ride noise with a mass of 40kg is ≤ 62dB.

If a person with a mass of 60kg rides an electric bicycle at a constant speed of 6m/s on a horizontal road, the resistance is 0.02 times of the total weight of people and vehicles, take g= 10N/kg and find:

(1) driving 10 minutes, the work done by the motor on the bicycle.

(2) Current passing through the motor coil.

(3) If the motorcycle consumes 2.5L oil per 100 kilometers, 4.65 yuan per liter of oil and 0.52 yuan per kilowatt-hour of lighting, try to compare which is more economical, electric bicycle or motorcycle.

9. When the car is running, the actual power of the engine is different due to the different road conditions and speed. Usually when driving, the actual power is less than its maximum power. The relevant data of a certain car are as follows.

(The calorific value of a certain grade gasoline is about 4.5× 107J/kg, and the density is about 07× 103kg/m3).

Q: (l) How many kilograms of gasoline does a car consume if it drives lh at a constant speed of 100km/h?

(2) If the efficiency of an automobile engine is 30%, what is the actual power of the engine when the automobile is driving at a constant speed of 100km/h?

10, solar water heaters are used by many families with the characteristics of energy saving and environmental protection, but if it is rainy,

The water in the solar water heater cannot reach the required temperature. For this reason, people have developed a water heater with both solar energy and electric energy. small

Lan Jia just installed such a water heater, and its volume is120L. May I ask:

(1) If this water heater is filled with water at 20℃, how much heat does it need to absorb to raise the water temperature by 40℃?

(2) Xiaolan learned from the manual that the rated voltage of the electric heater is 220V and the rated power is 2kW. If the water in the water tank is heated by an electric heater, the temperature of the full tank will still rise from 20℃ to 40℃. How long does it take for the electric heater to work normally? (Assuming that all the consumed electric energy is converted into the internal energy of water)

(3) If the electric heater works normally under the above conditions and it actually takes 2 hours, what is the efficiency of this electric heater?

1 1. With the development of society, the energy crisis has begun to plague mankind. People are looking for various possible future energy sources to maintain the sustainable development of human society. On May 24th, 2006, the European Union, the United States, China, Japan, South Korea, Russia and India initialled a series of agreements on the International Thermonuclear Experimental Reactor (ITER) at the headquarters of the European Union. This is a major attempt by mankind to develop new energy sources, and it is also the largest international scientific and technological cooperation project in which China participates on an equal footing.

Thermonuclear fusion is a kind of nuclear fusion that takes place under controlled conditions at a high temperature of hundreds of millions of degrees Celsius by using hydrogen isotopes deuterium and tritium. The nuclear energy released by the fusion reaction provides a lot of heat, just like creating an "artificial sun". The product after the reaction is helium, and there is no radioactive pollution.

The experimental reactor which is expected to be completed in 20 10 will be completed in 16 minutes. Heat 1. Generate 44 ×l0 12 J. According to media reports, the isotope of hydrogen extracted from 1L seawater releases energy in the complete fusion reaction, which is equivalent to the heat released by the complete combustion of 300L gasoline. Please estimate according to the data provided and answer the following questions:

(1) If the density of gasoline is about 0. What is the mass of 7× 103kg/m3 and 300L gasoline?

(2) If the calorific value of gasoline is about 5× 10J/kg, how much heat can be released by complete combustion of 300L gasoline?

(3) How many liters of hydrogen isotope should be extracted from seawater to generate the heat of1.44×1012j? If the efficiency of converting this heat into electric energy is 50%, how many hours can it supply an air conditioner with a power of 2kw to work normally?

(4) Do you know? There is also an "artificial sun" on the Science Island in Hefei, Anhui. This device, called the Tokamak, will be ignited in July this year. After reading the above materials and data, you will definitely have a lot of feelings. Talk about your understanding of "artificial sun".

12, Xiaogang's family bought a new induction cooker with rated voltage of 220V and rated power of 1000 W. Let the induction cooker work at rated voltage and heat the water in LO from 20℃ to1000℃. (excluding heat loss, c water = 4.2× 65400) C)) Q:

(1) How much heat does water need to absorb?

(2) What is the heating time?

13, in an extracurricular activity, a classmate made relevant observation and research on household appliances. He took the induction cooker as the observation research object and recorded relevant data, as shown in the following table:

Relevant records and activities of the observed object

Observe the nameplate of induction cooker: U quantity =220V P quantity = 1KW.

Pay attention to the nameplate of the electric energy meter: 220v10 (40) a 50hz 2000r/kwh.

Heat a pot of water with an induction cooker (water mass m=2kg, initial temperature t = 30℃), turn off other household appliances, only connect the induction cooker to the circuit to boil water and observe the watt-hour meter, and the dial will turn 150r within 5 minutes. At this time, the water temperature measured by thermometer is 50℃.

Please answer or ask the information this student got in the activity:

(1) Heat absorbed by water after 5min minutes of heating;

(2) The electric energy consumed by the induction cooker for 5 minutes;

(3) the power of the induction cooker;

(4) Please compare the solution of (3) with the relevant data of this classmate in the activity. What new problems have you found? And explain why.

14. After learning the knowledge of electricity, Wang Leyan modified the lighting circuit of the living room. The lighting circuit, which was originally composed of five light bulbs controlled by one switch, was divided into two groups, which were controlled by two switches respectively. All bulbs are lit when strong brightness is needed, only two bulbs are lit when it is not needed, and all bulbs are turned off during the day or when there is no one in the hall. After a long period of statistics, Wang Leyan got the following set of data:

The average daily turn-on time requires a strong brightness time, not the power of each bulb.

6h 2h 4h 60W

According to the statistics of Wang Leyan, please answer the following questions through calculation:

(1) How many kWh can be saved per month after the transformation (calculated by 30 days)?

(2) If water is heated by electric water heater, how much water of 20℃ can be heated to 60℃ for bathing by saving electricity every month? [The specific heat capacity of water is 4.2×103j/(kg℃)]

15, as shown in the figure, is the location of wind power generation in a certain area in western China. The air density in this area is 1.3㎏/m3, and a fan blade can form a circular surface with an area of 100m2 when rotating. The following table shows the energy obtained by wind turbines at different wind speeds.

Average wind speed (m/s) 5 10 15 20 25 30

Energy obtained in1s (×104j) 481216 2024.

(1) If the wind speed is 10 m/s within a certain period of time, the electric energy generated by the operation of a wind turbine 1 hour can supply the 10 "220V 100W "light bulb to work normally for 8 hours, so as to calculate the power generation efficiency of this windmill.

(2) If the annual average wind speed here is 15m/s, how many kilograms of coal can be completely burned by the wind energy obtained by 100 windmills in one year (calculated by working time of 4000h)? (The calorific value of coal is 3× 107J/)

(3) If the wind speed is 10 m/s, how many kilograms of airflow will impact the circular surface formed by the rotation of the rotor blades within 1min?

16, as shown in fig. 6, is a schematic diagram of a device for measuring the fuel quantity of an automobile fuel tank. The resistance of pressure sensor R will change with the change of pressure, and the pointer of oil meter (modified from ammeter) can indicate the amount of oil in the oil tank. As we all know, the relationship between resistance and pressure of pressure sensor R is shown in the following table.

Pressure part number 0 50/kloc-0 00/kloc-0 50 200 250 300 …

r/ω500 380 3 10 245 200 170 150…

If the upper surface area of pressure sensor R is 5cm2, the calorific value of gasoline is 4.6× 107J/㎏, and the density of gasoline is 0.7×.

103/m3, and the power supply voltage is 6V. Please answer:

(1) When the total weight of oil and fuel tank is 600N, what is the pressure on pressure sensor R?

(2) If the oil in the fuel tank is10, how much heat can be released when the gasoline is completely burned?

(3) If the mass of the empty fuel tank is 5.8㎏ and the pointer of the fuel gauge points to 2× 10-2m3, what is the current in the circuit?

17. In order to exercise, Xiaoming designed an electronic tensiometer by using his knowledge of physics. Fig. 7 is a schematic diagram. The right end of the hard spring is fixed with the metal slider P (the resistance of the spring is ignored, and the friction between P and R 1 is ignored). Constant resistance R0 = 5Ω, A and B are uniform resistance wires with a length of 5cm, resistance r1= 25Ω, power supply voltage U=3V, and ammeter range is 0 ~ 0.6a. ..

(1) Xiaoming connects RO in the circuit, with the purpose of _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.

(2) When the tab is not pulled, the slider P is at the A end, and the ammeter reading _ _ _ _ _ _ _ _ _ A after the switch is closed.

(3) As we all know, the relationship between the extension length δL of the spring and the pulling force F is shown in Figure B. The calculation shows that when the switch S is closed, when the ammeter pointer is 0.3A, what is the horizontal right pulling force acting on the pull ring?

18. Xiaoming's kitchen is equipped with gas stoves and electric cookers.

(1) When using a gas stove, open the valve of the gas storage tank, and the liquefied petroleum gas in the tank will vaporize due to the boiling point drop caused by _ _ _ _ _ _ _ _.

(2) Xiaoming cooks with a rice cooker with a resistance of 55 ohms. After electrifying, the current through the rice cooker is 4 amps. It took 20 minutes to cook a pot of rice. In this process, how much heat energy is generated by current passing through the rice cooker?

(3) Xiaoming made a simple induction cooker under the guidance of his teacher.

① The bottom of the simple induction cooker is a coil made of enameled wire, and some iron cores are inserted into the coil (as shown in Figure 8). The purpose of inserting the iron core is _ _ _ _ _ _ _ _ _ _ _.

② Xiaoming uses a simple induction cooker to heat 2kg of cold water at 20℃ to 60℃ (as shown in Figure B). If the efficiency of converting electric energy into heat energy by a simple induction cooker is 80%, how much coke does the current make through the simple induction cooker?

【 The specific heat of water is 4.2x 103 coke /(kg? ℃)]

19, (09 Zhengzhou, Henan Province) As shown in Figure 9, electric bicycles are favored by consumers for their portability, economy and environmental protection. The main technical parameters of electric bicycles are shown in the table below.

TDR0788G model

Brushless motor 48V400W

Lead-acid battery 48V 80Ah

The vehicle mass is 80Kg.

The outer packing size is 1860× 540× 1055.

(1) Please write down the energy conversion of electric bicycle during the following use:

① Electric bicycle charging:

② Driving process of electric bicycle:

(2) If the cyclist weighs 60Kg and the contact area between the two tires and the ground is 1.0× 10-2m2 when driving, what is the pressure of the bicycle on the ground?

(3) When the electric bicycle runs at a constant speed with rated power on a straight road section, the average resistance is 40N.

① What is the power of the bicycle at this time?

② If the driving speed is 7m/s, what is the work done by the power in 1S on the bicycle?

③ How efficient is the electric bicycle?

(4) Compared with internal combustion engine motorcycles, electric bicycles have many advantages in environmental protection and energy saving. Please write two.

20.(09 Mianyang, Sichuan) The temperature in the bacterial incubator used in modern biomedical research needs accurate measurement and control, and one of the methods of measurement and control is to use thermistor to detect the temperature. As shown in the figure 10 circuit, the thermistor Ro is placed in the bacterial incubator, and the rest is placed outside the incubator. In this way, the temperature in the box can be expressed by the readings of ammeter and voltmeter. It is known that the power supply voltage in this circuit is 12v, and the resistance of constant resistor R is 200Ω. The relationship between the resistance value of thermistor Ro and temperature is shown in figure B. Ask questions:

(1) How does the ammeter reading change when the temperature in the incubator decreases?

(2) When the temperature in the incubator is 40℃, what is the reading of the voltmeter?

(3) It is known that the range of ammeter is 0 ~30 mA and the range of voltmeter is 0~8 V, so this circuit can measure most.

What is high temperature? What is the electric power consumed by thermistor Ro at this time?

2 1, (09 Henan) induction cooker is a new type of cooker, which has the characteristics of fast heating, high efficiency, energy saving and environmental protection, and convenient use. Its working principle is that the induction coil is energized with alternating current to generate an alternating magnetic field, which generates eddy current in its wok, so that moving electrons frequently collide with metal positive ions, thus generating a lot of heat, which directly occurs in the wok itself, so the heat loss is small, the thermal efficiency is as high as 80%, and the heating is uniform. Xiaohua observed and studied the induction cooker in an extracurricular activity, and recorded the relevant data, as shown in the following table:

Relevant records and activities of the observed object

Observe the nameplate of induction cooker: U quantity =220V P quantity = 1kW.

Observe the nameplate of electric energy meter 220v 10 (40) A 50hz 2000r/kW? h

Heat a pot of water with an induction cooker (water mass m=2kg, initial temperature t = 30℃), turn off other household appliances, only connect the induction cooker to the circuit to boil water, and observe the electric energy meter. The dial rotates 150r within 5 minutes, and the water temperature measured by thermometer is 50℃.

Please answer or ask questions about the information he got during the activity:

(1) Heat absorbed by water after 5min minutes of heating;

(2) The electric energy consumed by the induction cooker for 5 minutes;

(3) the power of the induction cooker;

(4) Please compare the results of (3) with the relevant data in his activities. What new problems have you found? And explain why.

22.(009 Jiaxing, Zhejiang) May 20th is "China Student Nutrition Day", and students' dietary structure has attracted more and more attention from schools, families and society. In order to improve Xiaohong's breakfast nutrition, her mother bought a fully automatic soymilk machine. See 1 1 for the working schematic diagram of the soymilk machine:

(1) Calculate the maximum current when the soymilk machine works normally.

(2) When the motor works for 3 minutes and the resistance wire heats the juice 12 minutes, how much power does the soymilk machine consume?

23.(09 Nanchong, Sichuan) As shown in figure 12A, it is a schematic diagram of a product conveying and counting device on a production line. Where s is the laser source and R 1 is the photosensitive resistance (when there is light irradiation, the resistance is small; When there is no light, the resistance is large), R2 is a fixed protection resistance, and A and B are indirectly an "oscilloscope" (the access of the oscilloscope does not affect the circuit). The image of the voltage across the photoresistor changing with time can be displayed by an oscilloscope. The horizontal conveyor belt advances at a constant speed, and whenever the product passes between S and R 1 from the conveyor belt, the light directed to the photoresistor will be blocked by the product. If a uniform cubic product with a side length of 0. 1m and a mass of 0.6kg is transported, the image of voltage change with time displayed by the oscilloscope is shown in Figure (b). Please answer the following questions:

(1) What is the density of this product?

(2) What is the pressure of the product on the conveyor belt? (g = 10N/kg)

(3) It is known that the power supply voltage of the counter circuit is always 6V, and the resistance of the protection resistor R2 is 40Ω. What is the resistance of the photosensitive resistor in two states? What is the power consumption of photoresistor 1h?

24.(09 Qiandongnan Prefecture, Guizhou Province) On April 18, 2007, the new high-speed train "Harmony" EMU began to gallop on major railway trunk lines, thus opening the curtain of the sixth national railway speed increase. As shown in Figure 13, CRH2 EMU consists of five cars, with a total power of 6000kW, a maximum traction power of 4800kW and a maximum running speed of 200 km/h. ..

(1) What is the distance that a train set travels on a straight track for 3min at maximum speed and maximum traction power? What is the resistance of the train set in this process?

(2) In order to ensure the normal operation of the train set, the power consumption of the electric water heater for drinking water for passengers in each carriage should not exceed 2800W W ... How much heat can the electric water heater in the car generate at most in 5 minutes? How much 20℃ water can this heat raise to 70℃? 【 The specific heat capacity of water is 4.2× 103J/(kg? ℃)]

(3) Observe the shape of the front carefully, and please talk about the benefits of this design.

Reference answer:

1, (1) Xiaoming's cycling speed:

(2) How to overcome resistance by cycling for 5 minutes after 5 minutes:

(3) Xiao Ming's pressure on the ground when riding a bike:

Xiao Ming's pressure on the ground when riding a bike;

2. 0.4m/sec 88.89% 4.5 1.8

3、p= =8× 104Pa

4、

5 、( 1)u 1 = I 1r 1 = 0.3a× 10ω= 3V

(2)∵R 1, R2 in parallel ∴ U2 = u1= 3vi2 = U2/R2 = 3v20ω = 0.15a.

6、Q = cmδt = 4.2× 103j/(？ ℃)× 100kg×60℃= 2.52× 107J

M= Q absorption q = 2.52×107j4.5×107j/㎏ = 0.56.

7.( 1) Power consumption: w = UIT = 220v× 50a× 60s = 6.6x105j.

(2) heat generated: q = i2rt = (50a) 2× 0.4ω× 60s = 6.0x104j.

(3) Mechanical work done: W machine = w-q = 6.6×105j-6.0×104j = 6.0×105j.

8 、( 1)7.2× 104J (2)4.4A

(3) The power consumption cost per 100 kilometers of electric bicycle is 2×0.6×0.52=0.624 yuan.

The fuel consumption cost per 100 kilometers of motorcycle is 2.5×4.65= 1 1.55 yuan.

It can be seen that the cost of a 100-kilometer trip of an electric bicycle is much lower than that of a motorcycle. So it is more economical to use electric bicycles.

9 、( 1)4.2kg(2) 1.575× 104 w

10 、( 1) 1.008× 107j(2) 1.4h(3)70%

1 1, (1)2 10Kg, (2)1.05×10/0j or1×/kloc. You can give answers from the aspects of human sustainable development, environmental protection, understanding China's national strength and security, and you can get full marks if you answer one point. 12 、( 1)3.36× 106j(2)3.36× 103s

13, (1)1.68×105j (2) 2.7×105j (3) 900 w (4) The actual power of the induction cooker is less than the rated power, which is due to the local voltage of the transmission line and the two ends of the induction cooker.

14, (1)2 1.6 degrees (2)463Kg 15, (1)10% (2) 5.76×/kloc-0.

16, (1)1.2×106pa (2) 4.6×108j (3) 0.03a17, (/kloc-0.

18, (1) air pressure decreases, (2) 1.056× 106J (3)① magnetism increases.

19.( 1)① electrical energy is converted into chemical energy ② chemical energy is converted into electrical energy, and electrical energy is converted into mechanical energy.

20、

2 1 、( 1) 1.68× 105j(2)2.7× 105j(3)900 w

(4) The actual power of the induction cooker is less than the rated power. This shows that the voltage at both ends of the induction cooker is lower than the rated voltage, and the induction cooker is not working normally.

22. (1) from p = ui: I = = = 3.64a (one for formula, one for process and one for result).

(2) from p =: w = pt

Electric energy consumed during heating: w1= p1t1= 800 w× L2× 60s = 576,000 j.

Electric energy consumed when the motor works: w2 = p2t2 =150w× 3× 60s = 27000j.

Total power consumption: ws = w1+w2 = 576000j+27000j = 603000j.

23 、( 1)ρ= m/V = m/L3 = 0.6kg/(0. 1m)3 = 0.6× 103kg/m3

(2)P = F/S = mg/L2 = 0.6Kg× 10N/Kg/(0. 1m)2 = 0.6× 103 pa

(3) As can be seen from the figure, R 1 is in series with R2. The voltage of R 1 is 4V when there is no illumination, and 2V when there is illumination:

∴①I 1 = I2 =(u-u 1)/R2 =(6v-4v)/40ω= 0.05 a

r 1 = u 1/I 1 = 4V/0.05 a = 80ω

② When there is light, I1'= I2' = (u-u1')/R2 = (6v-2v)/40ω = 0.1a.

r 1 ' = u 1 '/I 1 ' = 2V/0. 1A = 20ω

According to the image, the time without illumination in 1h is t1= (0.6s-0.4s)/0.6s× 3600s =1200s.

The illumination time in 1h is T2 = 3600s- 1200s = 2400s.

∴w=w 1+w2=u 1i 1t 1+u2i2t2=4v×0.05a× 1200s+2v×0. 1a×2400s=240j+480j=720j

24.( 1) The distance traveled by the train group within 3min s = vt = 200km/h× (3h/60)10km.

Work done within 3min of traction w = Pt = 4800×1000 w× 3× 60s = 8.64×108j.

F = w/s = 8.64×108j/104m = 8.64×104n by W=Fs.

Because the train set moves in a straight line at a uniform speed, the resistance is equal to its traction of 8.64×104n n.

(2) If all the electric energy consumed by the electric water heater is converted into heat for 5 minutes, the maximum heat that the electric water heater can generate in a compartment for 5 minutes is Q = W = Pt = 2,800 w× 5× 60s = 8.4×105j.

m = q/[c(t-t0)]= 8.4× 105j/[4.2× 103j/(kg？ ℃)×(70℃-20℃)4 kg

(3)① The streamlined front can reduce the air resistance at high speed.

(2) The high air velocity in the upper part of the locomotive and the low air velocity in the lower part can reduce the pressure (pressure) on the track when the locomotive runs at high speed, thus reducing the resistance and noise.