1. Analyze the floating and sinking state of the ball in water and alcohol.
2. Find the density of the ball
Solution: Assuming that the ball does not float in water or alcohol, or floats or floats, it should have:
Because it contradicts the known conditions of the topic, it is impossible;
Assuming that the ball sinks into water and alcohol, there should be:
It is impossible because it contradicts the known conditions of the topic.
From the above analysis, it can be seen that when the ball is in water and alcohol, it can only drift and sink.
Because of this, the ball must float in water and sink in alcohol.
(2) Because the ball floats in the water,
∴ ①
Through the known conditions:
Get ②
Substitute Type ② into Type ①:
Figure 1
2. A cube with a volume of 1.0× 10-3m3, as shown in figure 1, is placed in a container filled with water, and the height above the water surface after standing is 5× 10-2m, and the bottom area of the container is 0.04m2(g is/kloc-0.
It is found that: ① buoyancy of wood block;
② Gravity of wood block;
(3) After the wood blocks are put into use, the pressure at the bottom of the container increases.
(4) If the wooden block is put into the liquid and the exposed liquid level is 4 cm, find the density of the liquid.
Solution: ① 5n; ②5N； ③ 125 pa； ④0.83× 103 kg/m3
Figure 2
3. In the circuit shown in Figure 2, the power supply voltage is constant, and the resistance of R 1 is 10 ohm. When the switch S is closed, the slider of the sliding rheostat slides to point B, the ratio of voltmeter to indicator is 5:7, and the sum of power consumed by R2 and R is four times that of R 1. Find:
① the ratio of current passing through R 1 and R2.
② Maximum resistance Rab of sliding rheostat
(3) the power consumption ratio of R 1 when the sliding vane of sliding rheostat is at two points A and B..
Solution: ①1:1; ②25 euros; ③4: 1
Figure 3
4. As shown in Figure 3, the object A with a weight of 600N is pulled by the pulley block to move at a uniform speed in the horizontal direction. For 40 seconds, the object moves 8 meters, and the work done by relay F is 1280 Joule. Q:
(1) the magnitude of tension f and the power of tension.
② If the weight of the moving pulley is 12 N (excluding rope weight and friction), calculate the mechanical efficiency of the pulley block.
Solution: ① 80n, 32w, ② 92.5%
Figure 4
V In the circuit shown in Figure 4, R 1 and R2 are constant resistors, and the lamp L is marked with the words "16V4W", and the power supply voltage is unchanged.
(1) Close the switches S, S 1, S2, and the ammeter indicates 0.35a. At this time, the lamp L can just emit light normally, and find the size of the resistance R 1;
② Close switch S, switch S 1 and switch S2. The indication of voltmeter is 3.2 volts higher than that in ①. Find the electric power consumed by resistor R2 at this time (filament resistance remains unchanged).
Solution: ① 160 Euro; ②0.64 w；
Figure 5
6. In the circuit shown in Figure 5, the supply voltage is kept constant and the switch S is closed. It is known that when the slider P slides to the A end and the B end respectively, the ratio of the electric power consumed by the resistor R 1: P2 is 9: 4. When the slider is at terminal A, the ammeter shows 3 amps, and when the slider P is at terminal B, the voltmeter shows 6 volts. Find:
(1) the resistance of resistor R 1 and the maximum resistance R2 of sliding rheostat.
② How many volts is the power supply voltage?
Solution: ①R 1=6 ohms, R2=3 ohms; ② 18V；
Rated voltage: 220V, functional steaming and ironing.
Rated frequency 50HZ steam drop type
Rated power 1000 w mass1.3kg.
Floor area 160cm2, self-supporting.
17. (Zhenjiang, 2007) As shown in the figure, it is the physical drawing and nameplate of a household steam electric iron. When in use, water drops in the water tank fall into the heating steam chamber of the bottom plate, and the steam chamber quickly vaporizes to generate downward steam.
(1) inject 0.3kg of water into the hot metal ladle,
Put it flat on a horizontal table,
Question: The pressure of iron on clothes.
(2) preheating before steaming with an iron to raise the temperature of the metal bottom plate from 20℃
When the temperature reaches 220℃, the temperature control switch is automatically turned off. It is known that the mass of metal is 1kg, and the specific heat capacity of metal bottom plate is 0.46×103j/(kg℃). How much heat does the metal soleplate absorb during the preheating process of the steam iron?
(3) In the process of steam ironing, water falls into the steam room and takes away heat due to the heating and vaporization of water. When the temperature of the metal soleplate drops to 190℃, the temperature control switch automatically closes and the electric iron works normally. When heated to 220℃, the temperature control switch automatically closes to stop heating, and so on. If the working state of the steam electric iron changes as shown in the figure, it will be ironed 12min.
18. (Guangzhou housing market, 2007) Xiaoxing designed an automatic control device for her bath electric water heater. As shown in the figure, R 1 is a thermistor (placed in a water heater), and its resistance changes with temperature as shown in Table 1. Table 2 is the nameplate of this electric water heater. It is known that the resistance R2 of relay coil is10Ω, and the left power supply voltage U0 is 6V. When the current in the relay coil R2 increases to a certain value, the armature of the relay is attracted and the circuit of the electric water heater is closed. Please answer when you start work.
(1) What is the resistance of the electric water heater when it works normally?
(2) Analysis table 1. How does the resistance of thermistor change with temperature?
(3) If the temperature in the electric water heater is controlled between 30℃ and 60℃, and the armature is sucked down, what is the working current of the relay coil when the electric water heater circuit is disconnected?
(4) When the electric water heater works normally, how long does it take to heat the whole box of water from 30℃ to 60℃? (excluding heat loss)
19. (Linxi City, 2007) The data of Huayang 1870 series solar water heaters are as follows:
Vacuum tube diameter (mm) 70× 1800 specification (branch pipe) 15
Overall dimensions (mm)1840×1597×1765 insulation layer (mm)60
Water tank capacity (L L) 150 lighting area (m2) 2. 1
(1) It is understood that the sunny place in our city is flat.
An hour of square meters is too much.
The daily average solar radiation energy is 3×106j if.
The time for receiving solar energy is 8 hours.
Calculate, the water heater can convert 50% of the received solar energy into the internal energy of water, so how many degrees Celsius can the water heater heat a box of water with an initial temperature of 20℃ in a normal day?
(2) If an electric water heater marked "220V 1500W" is used at rated voltage, how long will it take to raise the same temperature for the same box of water?
(3) If the electricity price is 0.54 yuan/kWh, how much electricity can be saved every month (calculated by 30 days)?
20. (Xiamen, 2007) There is an automatic electric kettle in the office. The chart shows its principle and parameters.
A is an automatic temperature control switch, which automatically disconnects the circuit when the water temperature reaches the set temperature;
When the temperature is lower than the set temperature, the circuit is automatically connected. The specific heat capacity of water is called
4.2× 103J/(kg？ C), please make the following calculation through analysis:
(1) 90% of the heat released by the electric thermos is absorbed by water, so cook a bottle.
How long should the electric thermos work normally with water at 20℃ (standard atmospheric pressure)?
(2) When the yellow light is on, is the electric thermos in a heat preservation state or a heating state? Thermal insulation element
R what is the resistance at work? (Note: The power consumption of the indicator lamp can be ignored, and the heat preservation work is carried out.
Rate refers to the power consumed by the whole circuit)
Model p2 155-b
Power supply ad220v 50hz
Heating power 800 w
Insulation power 40W
Capacity 2.0L
Fuel gasoline
The fuel tank has a capacity of 50 liters.
Gasoline density kg/m3 0.7× 103
Gasoline calorific value j/kg 4.6× 107
Complete combustion products of fuel
The heat generated is converted into
The percentage of mechanical energy is 40%
Comprehensive problems of mechanics and heat
2 1. A car is driving at a constant speed of 70km on a straight expressway.
The vehicle speed is 72km/h and the fuel consumption is 6L. Please refer to the relevant letters provided in the table below.
Interest, calculate how many kilowatts this car drives.
No.93 gasoline for mopeds
The density of fuel gasoline is 0.7× 103kg/m3.
The fuel tank capacity is 7L, and the calorific value is 4.6× 107J/kg.
The power of 6KW is 4.70 yuan/L.
22. (Taiyuan, 2007) Teacher Wang's home is about 9 kilometers away from the school. In order to facilitate commuting, he bought a moped, and the fuel used was 93 # gasoline. See the table for reference figures, in which the calorific value of gasoline indicates that the heat released per kilogram of gasoline is 4.6× 107J J. If the average speed of Mr. Wang riding a bicycle is 36km/h, please calculate the distance from his home to school (excluding energy loss).
(1) approximate time required;
(2) Motorcycles need to consume the quality of gasoline;
(3) Fuel cost required
The average speed is 90 km/h.
Isokinetic oyster sauce 9L/ 100 km
The oil price is 4.80 yuan/liter.
The density of gasoline is 0.75× 103 kg/m3.
23. (Shenyang, 2006) On Sunday, my father drove Komori to the bookstore to buy popular science books. After coming back, Komori asked his father some questions about cars and took notes. The contents are shown in the following table. Please calculate according to the contents recorded by Komori:
(1) If it takes 20 minutes for Komori to go to the book city one way, the distance between Komori's house and the book city is probably more.
Less than a kilometer?
(2) How many kilograms of gasoline was used for the round trip? How much is gasoline?
24. A car is driving at a constant speed of 1.8km on a straight expressway. The speedometer of the car is shown in the figure.
Show that in some processes;
(1) How many kilometers per hour? How many meters per second?
(2) If the engine power of a car is 40kw, what is the resistance of the car when driving?
(3) If a car consumes 0.25kg of gasoline, what is its engine efficiency? (The calorific value of gasoline is
Q=4.6× 107 Joule/kg)
25. (Baotou City, 2007) As shown in the figure, it is the schematic diagram of the circuit for automatically measuring the temperature and air pressure in the pot (some circuits in the pot are in the dotted box). The power supply voltage is constant at 4.5V
(1) If steel instruments with a total mass of 5Kg and an initial temperature of 15℃ are disinfected, the disinfection temperature is.
How much heat does the instrument absorb when it reaches 1 15℃? 【C？ Steel =0.46× 103 J/( kg? ℃)]
(2) placing the pressure sensor R 1 (variable resistor whose resistance changes with pressure) in a tank,
Its compression area is 2cm? 2. If the maximum surface pressure of R 1 is 70N, it is allowed in the pot.
What is the highest air pressure?
(3)A 1 is an instrument (in essence, an ammeter) that displays the pressure in the pot. When the air pressure in the pot is not high,
At 1 standard atmospheric pressure, the display air pressure values of A 1 are all 0, and the resistance value of R 1 is 200 Ω. How many milliamperes should be marked on the dial of ammeter A 1?
(4) Thermal sensor R2 (a variable resistor whose resistance changes with temperature) is placed in the pot, and A2 is an instrument for displaying temperature (essentially an ammeter); The relationship between the resistance of R2 and the temperature is shown in the following table: When the ammeter A2 indicates 60mA, what is the temperature in the pot?
Temperature t/℃ ...
Resistance R2/ω12010100 90 80 70 60 50 ...
Review of physical calculation in junior high school
I. Electrical Calculation
1 has a small bulb with a rated voltage of 2.4V, and the resistance of the small bulb is 8 Ω. If we only have a 3V power supply, how much resistance do we need in series to make the small bulb work normally?
2. As shown in the circuit, R2 = 20ω, the total resistance in the circuit is12Ω, and the ammeter indicates 0.3A Please calculate.
Resistance value of (1) resistor r1(30 Ω)
(2) Power supply voltage (6V)
(3) Current through R 1 (0.2A)
3. As shown in the figure, the small bulb L of "6V 3W" is connected in series with a resistor with rated power of 10W, and the power supply voltage is 10V, which remains unchanged. Turn on the switch S, the ammeter and voltage indicator are 0.20A and 2.4V respectively, regardless of the change of resistance and small bulb resistance with temperature; Q:
(1) Resistance value of resistor R (38 Ω)
(2) Power consumed by the whole circuit (2W)
4. As shown in the figure, the power supply voltage is 6V, and r1= R2 = 30Ω is calculated.
(1) When the switch is off, the total resistance in the circuit and the voltage across R2; (60ω3V)
(2) When the switch is closed, the reading of ammeter and the voltage across R2. (0.2A 0)
5. In the circuit shown in the figure, R 1 is a sliding rheostat, R0 and R2 are both fixed-value resistors, and the voltage at both ends of the power supply is unchanged, so the slider position of R 1 is changed, and the indicated values of two voltmeters are plotted in the coordinate system shown in Figure B, and the resistance value of R0 (2 ω) is calculated according to the above information.
6. Xiaoming experimented with the light bulb L 1 labeled "6V 6W" and the light bulb L2 labeled "6V 3W" (1). When L 1 emits light normally, what is the current through L 1? (` 1A)(2) As shown in Figure A, A and B are the curves of the relationship between the current passing through L 1 and L2 and the voltage at both ends respectively. Now, as shown in Figure B, connect two lamps into the circuit, so that one of the bulbs can emit light normally. What is the total power consumed by the circuit? (4W)
7. The soldering iron is used to weld components. Sometimes, in order to save electricity and prevent the soldering head from being too hot to get tin, the circuit has the following characteristics by controlling the switch: the soldering iron is in a heat preservation state when it is not soldered temporarily, and can reach the temperature quickly when it needs to be soldered. It is known that the power supply voltage is 220V, and the internal circuit diagram of the soldering iron is shown in the figure. When the switch S is connected to A, the electric power P 1 of the soldering iron is 100W, and the switch is turned on.
(1) Where is the switch connected when the soldering iron is kept warm? (point b)
(2) If P 1=4P2, what is the resistance of the resistance wire R 1 and R2? (484Ω 1452Ω)
8. After learning the knowledge of electricity, Xiaohong drew the circuit diagram of the hair dryer at home. She thinks that when the switch S 1 is closed, the hair dryer will blow cold air, and when S 1 and S2 are closed at the same time, the hair dryer will blow hot air. Xiao fen found some problems when reading her circuit diagram: if S2 is only turned off, the motor will not work when the heating wire R works, which will lead to overheating and damage of the plastic shell of the hair dryer. Xiao fen believes that as long as one of the switches is moved to the main road, the motor must be working when the heating wire R works, thus ensuring that the plastic shell will not be damaged due to operational errors. Please analyze and answer the following questions:
(1) Do you think Xiao fen should move the turnout to the main road to meet the safety requirements?
(2) According to the relevant data provided on the nameplate of the hair dryer, calculate how much electricity is consumed by blowing hot air for one minute when the hair dryer works normally. What is the electric power of the heating wire R? What is the current through the heating wire R?
(27600J 440W 2A)
9. There is a certain distance between the school playground and the school electronic control room. A iodine tungsten lamp with the specification of "220V 1KW" is installed in the playground. It is known that the voltage of the electric control room is constant at 220V, the distance between the electric control room and iodine tungsten lamp is 100m, and the conductor used is a brand of copper-core rubber insulated wire. The following table shows the safe current carrying capacity of copper-core rubber insulated conductors with different cross-sectional areas of this brand at room temperature (that is, the maximum safe current when electrified for a long time). (1) The required copper wire conforms to the principles of applicability and economy. Please explain what kind of copper conductor is used in the wire by calculation. (4.55A Look-up table shows that copper-core rubber insulated wire with cross-sectional area of 1mm2 should be selected.)
(2) If the resistance per meter of copper conductor used is 0.017Ω, how much electric power will be lost on the transmission line when iodine tungsten lamp is used? (The total resistance in the circuit is 51.8Ω, the total current is 4.25A, and the line power loss is 6 1.4W).
(3) How much electricity does lighting in iodine tungsten lamp consume in one hour? (0.874 kWh)
Conductor cross-sectional area s/mm2 0.51.1.52.54610.
Safety bearing capacity I/A 48 12
20 25 32 50
As shown, 10 is a simple schematic diagram of an electric pressure cooker. A is a closed pot body with a pot cover area of 400cm2.
R 1 is the heating resistance at work, L is the indicator lamp, and its resistance is 20Ω, R2 is the insulation resistance, and S 1 is the power supply.
Off, S2 is the pressure switch. After S 1 is turned off, the circuit passes through R 1 to heat the electric pressure cooker. When the air pressure in the cooker is high,
When the pressure reaches the set value, the water begins to boil, the bottom of the pot moves down, the pressure switch is disconnected from contact A, and the pressure switch is connected with contact A..
Connect point B to control voltage and temperature, and the total power of insulation circuit is 20W.
Table 1 Relationship between boiling point t of water and air pressure p
T/℃.. 105 1 10 1 12. . 1 14. 1 16. 1 18 120
p/kpa 10 1 12 1 143 154 163 175 187 199
Table 2 nameplate of electric pressure cooker
Rated voltage, heating power, pressure setting value
220v1000w175kpa
(1) When the pot is 12 1kp, what is the pressure of the gas in the pot on the lid? (4.84× 103)(2) What is the resistance of insulation resistance R2? (2400Ω)
(3) It is known that the mass of water in the pot is m, the initial temperature is T0 and the specific heat capacity is c. In order to find out the efficiency of the electric pressure cooker from the beginning of heating to the pot pressure reaching the set value, it is only necessary to measure the closed switch S 1 and put it into the circuit; T, we can deduce the expression of efficiency η = (expressed by known quantity) [indicator light is on; cm( 1 16℃-T0)/ 1000 w×t)]
Two. Mechanical calculation
165438+
12. The taxi driver saw the sign as shown in the figure at the airport expressway entrance. How long does it take for the driver to drive from the entrance to the airport without violating traffic regulations? (0.3 hours)
13. As shown in the figure, there is a crown block at the top of the flagpole. The flag raiser lifts a flag with a mass of 2kg by 8m vertically with a force of 25N, and calculates (1) the work done by the flag raiser in pulling the flag (200J).
(2) Mechanical efficiency of crown block (80%)
14. As shown in the figure, an object with a weight of 350N is lifted by the pulley block at a uniform speed, and the object rises vertically within 10s1m..
The force that people pull the rope is 250N N N. What is the force that people pull the rope to do work? Mechanical efficiency of pulley block? If you use this
What is the pulling force used by the pulley block to lift a 350N object? (50W70%; 250 Newton)
15. A 4t truck runs at a constant speed of 90km/h on the straight road, and the resistance of the car is 0.069 times its weight.
Find (1) the driving force of the automobile (6.9× 104J)
(2) If the automobile engine efficiency is 30%, how much gasoline (gasoline calorific value is 4.6× 107J/kg)(20kg) will be consumed when driving at a constant speed 100km?
(3) The car has four wheels, and the contact area between each wheel and the ground is about 250cm2. What is the pressure of the car on the ground? (4× 105pa)
16. As shown in the figure, a pulley with a mass of 60kg and a bottom area of 0. 1m2 moves in a straight line at a uniform speed under the tension of 25N through the pulley block. It is known that the sliding friction of the block is 0. 1 multiple block weight. (g = 10N/kg)。 Q:
(1) Make a schematic diagram of the pressure of the object on the horizontal ground on the map; (2 points)
(2) What is the gravity of the object? (2 points)
(3) What is the pressure on the horizontal ground when the object is at rest? (2 points)
(4) What is the mechanical efficiency of the pulley block? (2 points)
17. The inclined plane is a simple machine, and Panshan Highway can be regarded as an inclined plane. A bus is driving on Panshan Highway. If the mass of the bus is 7000kg, the traction force will remain unchanged during the upward driving, and it will increase by 300 m m when the bus travels 2000 m. During this period, how much work has been done to find the traction force (1)? ( 6× 10? 7J)(2) Mechanical efficiency of Panshan Highway? (35%)
18. When a car passed the Jiahe Bridge, the traffic police stopped it. The driver's reaction time is 0.5s, and the reaction distance is12.5m. After parking, it is found that the total mass of the car is 10t, and the total contact area between the wheels and the ground is 0. 1m2. According to the speed limit of this section, the pavement pressure shall not exceed 7. 106pa overload)
19. In the hot summer afternoon, accompanied by soft music, hardworking sanitation workers drove sprinklers at a constant speed of 8 km/h on the beautiful binjiang road. If this road is 2 kilometers long,
(1) How long does it take for the sprinkler to cross this section? (0.25 hours)
(2) If the volume of the sprinkler tank is 7m3, how many kilograms of water can the sprinkler tank hold at most? (7×103kg)
20. There is a uniform block of wood with a bottom area of 0.3m2 and a horizontal volume of 0.2m3. Xiaoxing pulls the wood block at a certain distance with a horizontal pulling force F within 1min. The work done by the pulling force F is 7.2× 103J, and it is known that the resistance of the wood block during the movement is 0.2 times of the weight of the wood block (ρ wood = 0.6× 6538). 4000pa) (2) The moving speed of the wood block (0.5m/s)
Three. mechanical-electrical integration
2 1. Students developed an electronic scale for measuring hand pressure. The principle of the force measuring part is shown in Figure A, which mainly consists of a hand pressure plate, a lever OAB, a pressure sensor R (a rheostat whose resistance changes with pressure) and an instrument A (an ammeter in essence) for displaying pressure, where OA∶OB= 1∶2, and the pressure sensor R is known. After the power supply is turned on, the voltage across the pressure sensor is 4.8V, excluding the mass of the hand pressure plate and lever. Q:
(1) If the pressure on the hand pressure plate is 400N, what is the pressure on the pressure sensor? At this point, what is the reading of the ammeter? (800N0.024A)
(2) If the maximum pressure on the surface of the pressure sensor R is 4× 106pa and the contact area between the pressure bar and the pressure sensor is 3cm2, what is the maximum pressure on the surface of the pressure sensor R? ( 1200N)
(3) the range of electronic scales. (0~~600N)