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First, this week's teaching content:

Review of Physics Topics in Grade Three —— Special Training of Physics Calculation in Junior Middle School

Two. Highlights and difficulties

1. There are three main types of physics calculation questions for the senior high school entrance examination, namely:

A mechanics (including buoyancy and pressure, density, motion, mechanical work and mechanical energy, power, mechanical efficiency, etc.). ),

B. Heat (including calculation of specific heat capacity and heat),

C electricity (including the calculation of ohm's law, electric work, electric power and electric heating)

Three. Main knowledge points

When reviewing the senior high school entrance examination, the calculation problem is the most important knowledge point. The proportion of this part in the senior high school entrance examination paper is about 18% ~ 26%. So in a sense, this part of the content is the key to the success of students' senior high school entrance examination, and the calculation problem will not be too difficult. It is not too difficult for students to find out some internal laws of doing calculation problems according to certain methods.

(A) computational mechanics

The main formulas and problems are:

1. Speed formula: v =

2. Calculate the density: use the formula ρ = (or ρ =; Rho = rho liquid)

3. Pressure calculation: use the formula P=

4. Calculation of buoyancy: Use the formula = g-f' = ρ liquid g V row = G object.

5. To calculate the work, use the formula w = fs.

6. For the calculation of power, use the formula P = (= =).

7. To calculate the mechanical efficiency, use the formula: η = × 100%.

8. Lever balance condition: F 1L 1=F2L2.

(2) Thermal calculation problem

1.Q=Cm△t

(3) Electricity

1. ohm's law: I = u/r

2. Electric power calculation formula:

W=UIt=I2Rt=U2t/R=Pt

3. Electric power calculation: p = = ui = U2/r.

4. Joule's Law: Q = W = UIT = T = PT.

Method Guidance: There are mainly the following methods to solve physics problems in junior high schools:

A. Analysis method: Starting from the unknown quantity, select the formula containing the unknown quantity (called the original formula), if there is still unknown quantity in the original formula, select the formula for calculating this unknown quantity, if there is still unknown quantity in the previous formula, select the formula related to another unknown quantity until it is completely linked with this known quantity. After analysis, when solving the problem, it is carried out according to the inverse process of analysis.

B. comprehensive method: when solving problems by comprehensive method, the idea is to start with the known quantity and establish the relationship between the known quantity and the unknown quantity according to the meaning of the question. For more complicated problems. It is difficult to find the direct relationship between the known quantity and the unknown quantity. At this time, it is necessary to choose some suitable intermediate quantities and intermediate relations according to the meaning of the problem and the change of each step of the scene, and gradually complete the transition from known quantities to unknown quantities.

C. Positioning method: In the physics examination paper, the knowledge structure of force, heat and electricity and the corresponding formula structure are determined. For the students who want to take the exam, in order to improve the efficiency in a short time, we must first locate the examination questions, so as to find out the corresponding knowledge structure, and then apply the formula to solve the problem. It is necessary to ask students to remember physical formulas.

Typical example

During the holiday, Xiaolan participated in a family game with her parents. The activity requires that any two family members stand on the board as shown in figure 15, just to make the board horizontally balanced.

(1) If Xiaolan and Dad weigh 400N and 800N, respectively, and Xiaolan stands on the side 2m away from the central fulcrum, how far should Dad stand from the fulcrum to make the chessboard horizontally balanced?

(2) If Xiaolan and Dad have successfully stood on the board, and now they are walking towards each other at the same time, Xiaolan's speed is 0.5m/s, and how fast does Dad keep the horizontal balance of the board from being destroyed?

Figure 15

Analysis: This is a relatively simple question about mechanics, involving the conditions of lever balance.

And the application of speed. We should clearly analyze which point is the fulcrum of the lever balance condition.

Which point is the action point of power, which point is the action point of resistance, where is the direction of motion force, where is the direction of resistance, and what are the power arm and resistance arm respectively? Because the fulcrum of this problem is in the center of the board, the influence of the board's own gravity can be ignored, Xiaolan's gravity can be regarded as power, then Dad's gravity is resistance, and this problem can be solved by using the lever balance condition.

When solving the second problem, according to =, find out the ratio of the forces on the two arms = find out the speed of dad.

Answer: 1. According to the lever balance condition:

= You got L Dad =

2. According to: = AND = = GET = = SOV2 =× 0.5m/s = 0.25m/s

Note: When calculating this physical problem, it is not difficult to realize this part of the problem, but the idea and method of solving the problem is to learn and pay attention to applying it to solving practical physical problems. For example, the deformation of the formula and the application of the method in the second question are worth summarizing.

Example 2. Hang a cubic metal block on a spring dynamometer and weigh it. When the metal block is in the air, the reading of the spring dynamometer is 19.6N, and when the metal block is submerged in water, the reading of the spring dynamometer is 9.8nn. Try to find out:

(1) volume and density of metal block;

(2) When the metal block is placed in the center of the horizontal desktop, what is its pressure on the desktop?

Analysis: In this question, the buoyancy is calculated by weighing method first, then the V row is calculated according to the buoyancy formula = ρ liquid g V row deformation, and the V object = V row when submerged, and then the density of the object is calculated. When a metal block is placed on a horizontal desktop, its pressure on the horizontal desktop should be equal to the gravity of the object itself. By using the pressure formula, P = f/s, where F = g, s is the surface area of the cube S = A2A =

So this problem can be solved.

Answer: u/ρ = where g is known, V object = V row = =

Substitution: ρ = = 2.0× 103kg/m3.

V = =dm3 is from V = V row = =

(2) According to the formula P=F/S,

F=G

P = g/s

It's also VIII

∴

∴

Note: This topic involves pressure and simple application of pressure. It is not difficult to solve this problem, but the physical ideas and methods involved are the main problems we have to solve. We solved the problem by positioning.

Example 3. Xiaoli's height 160cm and mass are 56kg, and the supporting area of the sole is as shown in Figure 26 (in Figure 26, the outline of the sole contacting the ground is drawn with square paper, and the area of each cell is 5cm2. When calculating the area, anything smaller than a cell counts as a cell, and anything smaller than a cell counts as a cell. In the jump rope test of physical education extra test, Xiaoli jumped 1min for 50 times, and the average height of each jump was 4cm. Find: (g takes 10N/kg).

Figure 26

(1) What is the ground pressure when Xiaoli stands?

(2) In skipping rope, what is the average power for her to do work against gravity?

Analysis: This is a relatively basic question about pressure and pressure, and it is a major issue that our senior high school entrance examination candidates should learn. When calculating the pressure, apply the formula p = f/s. Because Xiaoli is standing on the ground here, her pressure on the ground should be equal to her own gravity. The key problem is the solution of stress area. We should adopt the method in physics textbook here, that is, for this shoe print problem, the number of full scales+the total number of unqualified scales ÷2. This problem is closely related to our physics textbook. The knowledge in the textbook is used, and the second question is about motivation. According to the formula P = W/T = GH/T, the height of each hop is 4cm, and the total height is 150 times of 1min, so it can be obtained.

Answer: (1) Xiaoli's gravity is g = mg = 56kg× 10n/kg = 560n.

The number of whole cells is 24 and the number of half cells is 8, so the total number of cells is recorded as: 24+8× 1/2 = 28.

Then the pressure area of Xiaoli's feet on the ground is 2× 28× 5cm2 = 2×140cm2 = 2.8×10-2m2.

Then the pressure is p = f/s = 560n/2.8×10-2m2 = 2×104m2.

(2) In skipping rope, Xiaoli's strength is p = w/t = gh/t = 560 n×150× 0.04m/60s = 56w.

Note: This question is written under the background of textbook knowledge. When we answer this question, we use analytical methods. This is also one of the main methods that students should try.

Example 4. Xiao Lin's family bought a new household electric kettle. The nameplate on the bottom of the kettle is marked with the words "220v-50h900w", and the transparent window on the side is clearly marked with the maximum capacity and minimum capacity of water "Max- 1.0L min-0.5L". Xiaolin was interested in the new electric kettle, so she started an experiment at home. He poured water into the kettle to the maximum capacity, and measured the initial temperature of the water at 20℃ with a thermometer. After electrifying for 6min, the water boiled (assuming that the air pressure at that time was standard atmospheric pressure). According to the above data, please calculate:

(1) How much coke does the electric kettle consume when heating?

(2) What is the heat absorbed by water when heating? (C water = 4.2×103j/(kg℃))

(3) Do you think the data obtained by comparing (1) and (2) are reasonable?

(4) If it is unreasonable, what do you think is the possible reason? (Write a possible reason)

Analysis: This is a physical problem that combines electricity and heat, and it is also a typical problem that is often encountered in the senior high school entrance examination in recent years. We can locate each physical problem first, and then find the relevant physical formula to solve it.

The first question is about the calculation of electric energy. The formula to be used is w = pt, which is calculated by the product of the known electric power and time in the topic;

The second problem is to investigate the calculation of heat in thermal knowledge; Use the formula: q = cm △ TM = ρ v.

The third problem is the processing and evaluation of experimental data, which seems to involve the content of thermal efficiency, but there may be other situations.

The fourth question is about the evaluation of this problem, that is, the cause of heat loss and the effect rate of heat.

Answer: (1)W amplifier = Pt = 900 W× 6× 60s = 324000J.

(2) The heat absorbed by water is q = cm △ t = 4.2×103j/(kg ℃×1.0 kg/dm3 )× (100-20)℃.

=3.36× 105J

(3)W release

(4) The reasons may be as follows: a. The water level is inaccurate (greater than the true value); B. At that time, the actual voltage was higher than the rated voltage of 220Vc, and the nominal value on the electric kettle was less than the actual value; D Kobayashi recorded less time than the actual time.

Note: the actual solution of this problem should use the positioning method. The answer to the question can be obtained by locating which formula first and then solving the required quantity in the formula. When solving the last two questions (3) and (4), we should first fully verify our answers to the first two questions. If our own practice is correct, we should think more about the actual reasons and finally find the answer.

Example 5. A student in an interest group found an interesting element in an electrical experiment. Because A and B are encapsulated in a transparent glass case, A and B are its two exposed terminals (as shown in Figure 27- A). They learned that B is a constant resistance, and the relationship between voltage and current is shown in the table: A is a conductor made of special metal wire, and its rated voltage is 6V. In order to further understand some characteristics of A, they connected the two ends of element ab to two points of a'b' in Figure 27- B, measured the voltage and current values of several groups of elements, and drew the curve as shown in Figure 27-C. Please answer according to the above information:

(1) Calculate the resistance of resistor B. ..

(2) Calculate the rated power of conductor A. ..

(3) Adjust the sliding rheostat so that the voltage is 4V, and calculate the heat generated by conductor A and the actual power of conductor B within 10s.

Voltage u (volt)

2.0

4.0

6.0

Current I(A)

0. 10

0.20

0.30

Figure 27

Analysis: This question is a typical one in the calculation of electricity. Because this problem involves some basic calculations in electricity, it is not difficult. It is better to do this problem with the above comprehensive method.

Of course, the first is positioning. The first question: ohm's law is used to find the resistance. It is not difficult to solve the problem by substituting the data in the table into ohm's law formula.

Second, to find the rated power of conductor A, use the formula P = UI. Just note that the UI in the formula refers to their rated voltage and rated current. This data should be found in the coordinate map. The rated voltage is 6V, and the corresponding current value can be solved by formula.

Question 3: On the basis of knowing the voltage, the voltage is 4V because two electrical appliances are connected in parallel. For the current of A, we can know that the actual power of B is calculated by P = U2/R by looking up the table. ..

Answer: (1) According to the data in the table, R = U/I = 2.0V/0.1A = 20Ω can be obtained from Ohm's law.

R = U/I = 4.0V/0.2A = 20ωR = U/I = 6.0V/0.30 a = 20ω.

That is, the resistance of b is 20 ω.

(2) As can be seen from the image, when the voltage is 6V and the corresponding current is 0.46A, then the rated power of A is P = UI.

=6V×0.46A=2.76W

(3) When the voltage is 4V, the current is 0.38A according to the data in the figure.

The actual power of a is p = UI = 4v× 0.38a =1.44w..

The power of b is p = U2/r =1.6w. ..

Note: In this problem, we first locate the knowledge system to which this topic belongs by positioning method, and then express the formulas involved in the required physical quantities by analytical method, in which the unknown quantities are solved from some information provided in the topic.

Example 6. As shown in fig. 28, it 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. It is known that the relationship between the resistance of the pressure sensor R and the pressure is shown in the following table (g is 10N/kg).

Pressure F/N

50

100

150

200

250

300

…

Resistance r/ω

500

380

3 10

245

200

170

150

…

If the upper surface area of the pressure sensor R is 5cm2, the calorific value of gasoline is 4.2× 107J/kg, and the density of gasoline is 0.7 1×.

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

Figure 28

(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?

Analysis: This is a subject that integrates mechanics, heat and electrical calculation. It is comprehensive and involves many knowledge points, but it is not difficult.

The first formula: p = f/s, and the pressure, that is, the total weight of oil and fuel tank, is obtained, and the stress area is the area of the upper surface of the sensor;

The second problem is that the mass of oil in the fuel tank is known, and the heat released by its combustion can be solved by the formula Q = QM.

The third problem is the current in the circuit, the power supply voltage is known, as long as we try to find the resistance in the circuit, but the resistance is related to the pressure in the circuit. When the amount of oil in the oil tank is 2× 10-2m3, we can calculate the mass of oil in the oil tank, thus calculating the gravity of the oil, calculating the pressure at this time and looking up the table to find out the resistance value, thus calculating the current.

Answer: (1) According to p = f/s, p = = 1.2× 106 Pa.

(2) according to Q = QM, q = QM = 4.2×107j/kg×10 kg = 4.2×108j.

(3) According to: m = ρ V = 0.7/kloc-0 /×103/m3× 2×10-2m3.

=14.2kg

G total = mg = (14.2kg+5.8kg) ×10n/kg = 200n.

Look-up table shows that the resistance is 200Ω and the power supply voltage is 6V, so it is obtained from Ohm's law:

I = U/R = 6V/200ω= 0.03 a

Note: This problem can be solved analytically. When solving the last problem, it is comprehensive and should be solved analytically.

Test answer

Station 1. A You can hear the flute of bilibili train in 3 seconds. If the train moves in a straight line at a constant speed of 6 1.2km/h, what is the running time of the train from bilibili to Station A?

The distance between Party A and Party B is 3000 meters. Someone starts from Party A, walks 10 minutes before 1600 meters, rests 10 minutes, and then walks 10 minutes before arriving at Party B. What is the average speed of this person from Party A to Party B?

3. The quality of the wooden mold used for iron castings is 18kg, and the density of the wooden mold is 0.6x103kg/m3. After casting, the weight of cast iron is 206kg, and the known density of cast iron is 7.0× 65,438+003 kg/m3. Q: Are there bubbles in cast iron? Why?

4. A cubic metal block with a side length of 0. 1 m and a mass of 10 kg is placed in the center of a horizontal desktop with an area of 1 m2. What is the pressure of the metal block on the desktop? If you press the metal block vertically with a force of 12 Newton, what is the pressure of the metal block on the desktop?

As shown in the figure, a tube with a cross-sectional area of 4 cm is fixed on the top of a cube box with a side length of 20 cm. Now, inject water into the box from above the pipe, so that the water level in the pipe is 30 cm higher than the top of the box.

Find: (1) the total weight of water in the tank and pipe; (2) the pressure of water on the bottom of the box; (3) The pressure of water on the top of the box.

6. Hang an object on the spring scale. When the object is completely immersed in water, the pointer of the spring scale is 9.8 Newton; If the volume of this object exposed to water is 4/5 of its total volume, the spring scale shows 19.6 Newton. Find the volume and density of an object.

7. When a cube with a volume of 0.00 1 m3 floats on the water, the volume exposed to the water is 2/5 of the total volume of the cube. If it is placed in a liquid and a downward pressure of 39.2 Newton is applied to it, the block will be completely immersed in the liquid. What are the densities of blocks and liquids?

8. Use a pole to carry heavy objects, the length of the pole is 1.8m, and buckets of 250 Newton and 200 Newton are hung at both ends. Where should the shoulder be placed to flatten the shoulder pole? (The gravity of the pole is not considered in the calculation)

9. How many volumes of water can a pump with a power of 4.9 kW pump pump into a pool with a height of 1 0 meter in1minute?

10. Use a rope to make the two crown blocks as shown in the figure into the most labor-saving pulley block. If the weight of each crown block is 100 Newton, the maximum bearing capacity of the rope is 200 Newton. (excluding friction)

(1) Draw the winding diagram of pulley block;

(2) How many objects can this pulley block lift at most?

(3) If a 500-Newton weight is lifted vertically by 0. 1 m within1s, what is the power of the pulling force?

(4) When lifting the load of 500-N, what is the mechanical efficiency of the pulley block?

1 1. One thermometer has an inaccurate scale. The temperature in ice-water mixture is 5℃, and the temperature in boiling water at 1 standard atmospheric pressure is 95℃. So when the temperature measured by this thermometer is 23℃, what should the actual temperature be?

12. Put an aluminum block with a volume of 40cm ~ 3 into a beaker filled with 0. 1kg water, immerse it in water, and then heat it with an alcohol lamp. After stopping heating, the thermometer measured the water temperature at 80℃. Q:

(1) How much heat is absorbed by water and aluminum blocks?

(2) 20% of the heat released by alcohol combustion during heating is absorbed by water and aluminum blocks. How much alcohol was burned?

(The specific heat of aluminum is 0.88× 103 coke/(kg℃), the density is 2.7× 103 kg/m3, and the combustion value of alcohol is 3.02× 107 coke /kg).

13. Mix 200 grams of 90℃ hot water with 400 grams of cold water at 30℃. What is the original temperature of cold water? (No heat loss during mixing)

14. The electric furnace is connected to a 220 volt power supply. The resistance of the electric furnace is 22 ohms. How much water can be burned from 20℃ to 100℃ in 0/0 minute? (70% of the heat generated by the resistance wire is absorbed by water)

15. The rated voltage of the bulb is 4 volts, and the current intensity is 0.5 amperes when it emits light normally. Now it is connected to a power supply with a voltage of 6 volts. Q:

(1) What is the resistance of this light bulb when it emits light normally?

(2) How much resistance should the bulb be connected in series to make it glow normally?

(3) How many hours does the electric energy of1kwh make this light bulb glow normally?

16. the electrical appliance marked "220V 800W" is connected in series with a rheostat, so that the electrical appliance can work between 220V and 200V. The power supply voltage of the circuit is 220 volts, so: (the resistance of the electrical appliance is constant)

(1) What is the resistance of the electrical appliance?

(2) What is a sliding rheostat with variable resistance range?

17. When resistors R 1 and R2 are connected in series in a circuit with a voltage of 12 volts, the current through R 1 is 0.3 A, and the voltage across R2 is 4.5 volts. If R 1 and R2 are connected in parallel to the same circuit, find:

What is the total resistance of (1) parallel circuit?

(2) What is the current intensity through R 1 and R2?

18. The bulb is connected in series with a rheostat and connected to a 54-volt power supply. The required voltage across the bulb is 36 volts, and the resistance of the bulb is r= 10 ohm. To make the voltage on the bulb exactly 36 volts, what is the resistance of the resistance wire connected to the circuit by the series rheostat?

19. When a lamp works normally, the voltage at both ends of the lamp is 24V, and the current flowing through the lamp is 0.5A. If a 36V power supply is connected, how much resistance does it need to be connected in series to make the lamp work normally?

20. the light bulb L 1 is marked with "80ω0. 1A" and the light bulb L2 is marked with "20ω0.2A". Connect them in parallel at both ends of a power supply, so that only one of the small bulbs can emit light normally without damaging the bulbs. Q:

(1) What is the power supply voltage?

(2) What is the mainstream at this time?

2 1. For the circuit shown in Figure 13, it is known that the power supply voltage is 6V, the resistance of R 1 is 2Ω, the resistance of R2 is 3Ω, and the resistance of R3 is 6Ω. When (1)S 1 and S2 are closed, find the instructions of the ammeter; (2) The total power of the circuit when both S1and S2 are disconnected.

Figure 13

22. A bulb and a rheostat are connected in series, connected to a 54-volt power supply. The required voltage at both ends of the bulb is 36 volts, and the bulb resistance r= 10 ohm. To make the voltage on the bulb exactly 36 volts, what is the resistance of the resistance wire connected to the circuit by the series rheostat?

23. The picture below shows the nameplate of the electric water bottle produced by Guangdong Shunde Weishide Electric Appliance Factory. Practice has proved that when water with a volume of 2L and a temperature of 20℃ is added to the kettle, the water can be heated to boiling after the electric kettle works normally for 25 minutes (when the air pressure is standard atmospheric pressure).

Q: (1) How much electricity is consumed in the process of boiling water in the electric kettle?

(2) If the power supply is used during peak hours, and the actual voltage of the power supply is only 198V, what is the actual power of the electric kettle at this time? (Assuming that the resistance of the electric kettle is constant)

electric kettle

model

WSD880

Power?Supply?

220V-50Hz

rated power

700 watts

capacity

5.5L

24. In the circuit shown in Figure 1 1, close the switch S and move the slider P of the sliding rheostat. When the voltmeter indicates 6V, the current indicates 0.5A；; When the voltmeter indicates 7.2V and the current indicates 0.3A, what is the resistance R0 and the power supply voltage?

As shown in the figure, the power supply voltage is constant. When S 1 is closed and S2 is open, the ammeter reads 0.4A and the voltmeter reads 2v. When S 1 is off and S2 is closed, the ammeter reads 0.2A and the voltmeter reads 4V.

Find the resistance of (1)R 1 and R2.

(2) Set the resistance value of the resistor R and the power supply voltage.

26. After all kinds of household appliances entered the ordinary family, Nan Nan counted the existing household appliances in the family and their rated power as follows: (rated voltage is 220 volts)

Name of household appliances

lamps and lanterns

Kitchen lamps

Kitchen electricity consumption

colour television

washing machine

fridge

iron

Rated power (Watt)

200

250

90

225

125

700

500

(1) The watt-hour meter installed at Nannan's classmate's home is shown in the figure. Ask her what is the maximum power that the electrical appliances in the home circuit can withstand? Write out the calculation process simply.

(2) Nannan's family wants to buy another 900-watt electric water heater. Please analyze what may happen after adding this new device. Explain the reason by calculation. What should I pay attention to when using electrical appliances in her home?

27. As shown in the figure, the power supply voltage is 18.5V, and the constant resistance r0 =10Ω. When S 1 and s 2 are closed and the slider P is placed at the right end of the sliding rheostat, the ammeter reading is 2.22A When S 1 and S2 are disconnected and the slider P is placed at the midpoint of the sliding rheostat, the ammeter reading is 0.22.

(1) Maximum resistance of sliding rheostat.

(2) The rated power of the bulb and its resistance value.

Figure 1 1

28. The drinking machine is a common household appliance, and its working principle can be simplified as the circuit shown in the figure, in which S is the temperature control switch and the heating plate. When the water dispenser is in the heating state, the water is rapidly heated. When the predetermined temperature is reached, the switch S automatically switches to another gear, and the water dispenser is in the heat preservation state.

(1) Try to determine the working state of the water dispenser when the temperature control switch S is connected to A or B respectively.

(2) If the power of the heating plate is 550W when the water dispenser is heated and 88W when it is kept warm, find the resistance value of the resistor (regardless of the influence of temperature on the resistance value).

29. As shown in the figure, r1= 20Ω, R2 = 40ω, and the power supply voltage and filament resistance remain unchanged.

(1) When both switches S 1 and S2 are closed, the indicator of ammeter A 1 is 0.6A, and the light bulb just emits normally. What is the required power supply voltage and rated voltage for small bulbs?

(2) When the switch S 1 and S2 are turned off, the indicator of ammeter A2 is 0.2A What is the resistance and actual power of the small bulb?

(3) What is the rated power of the small light bulb?

Test answer

1.60 seconds 2. 6000 m/s 3. The causes of bubbles.

4.9800 Pa, 1 1000 Pa

5.79.6 Newton, 196 Pa, 1 16.4 Pa.

6.0.0025 m3, 1400 kg/m3

7.600 kg/m3, 4,600 kg/m3

8. 0.8m away from one end of 250 cows.

9.3 m 3

10. The figure is slightly 900 N, 60 W, 83.3%.

1 1.20℃

12.30902.3 joules, 5. 1 g

13.0℃

14.2.75 kg

15.8 ohm, 4 ohm, 500 hours

16. U2 = u1= I1r1= 0.4×10 = 4 (volts).

I2= U2/R2 =4/20=0.2 (Ampere)

I= I 1+I2 =0.4+0.2=0.6 (ampere)

17.( 1)9.4ω(2)0.44 a 0.6A

18.5 euro

19.30Ω

20.( 1)4V (2)0.25

2 1. solution: when S 1 and S2 are closed, the total parallel current of R2 and R3 is I=U/ R2+ U/ R3=6/3+6/6A=3A.

When S 1 and S2 are disconnected, the total series power of R 1 and R2 is p = U2/(R1+R2) = 36/(2+3) w = 7.2w.

A: At present, the indication number is 3A, and the total power is 7.2W.

22.5 euros

23. solution: (1) w = pt = 700w× 25× 60s =1.05x106j.

(2) Let the rated power be p = U2/r. .................

The actual power is p real number = u real number 2/r..........................②.

①/② = P/P Real = U2/U Real 2

Solution: p real = pu real 2/U2 = 700W× (198V) 2/(220v) 2 = 567W.

24.6 9

25.( 1)5ω20ω(2) 10ω，6V

26.( 1)P Da = UI = 220× 10 = 2200(W)

(2)P = P 1+P2+P3+P4+P5+P6+P7 = 200+250+90+225+ 125+700+500 = 2090(W)

Because the maximum allowable load of the watt-hour meter is P =2200W, if a 900W electric water heater is added, when all the appliances work together, P =2090+900=2990W is much larger than the maximum load, which will overload the watt-hour meter, so it may damage the watt-hour meter and even cause a fire. The appliances in her house can't be used at the same time, so they should be used separately, or an electric water heater should be added after the line is reformed.

27. Solution: (1) When S 1 and S2 are closed and the slider P is placed at the right end of the sliding rheostat, the current flows through R0.

The current flowing through the sliding rheostat I = 2.22a-1.85a = 0.37a.

Maximum resistance of sliding rheostat:

(2) When S 1 and S2 are both disconnected and the slider P is placed at the midpoint of the sliding rheostat, the rheostat is connected in series with the bulb.

Total resistance in the circuit:

Bulb resistance:

Rated power of light bulb:

28.( 1)S is connected to a for insulation, and s is connected to b for heating.

(2) Work when heating, and connect in series when disconnecting heat preservation.

29.( 1)U total = 12V UL= 12V.

(2) RL = 20ω p real =0.8W (3)P quantity = 7.2W