There is not much content in the "buoyancy" section of Physics in junior high school, but the knowledge in this section is easy to expand and extend, and the questions about buoyancy are varied. If students only stay in the limited knowledge in textbooks, they are not good at expanding and sorting out, and they cannot adapt to the changes of topics, and the result will be lost in the sea of questions. This paper attempts to analyze the essence of buoyancy and summarize the calculation methods of buoyancy for teachers to guide students to review for reference.

First, the source of buoyancy

Imagine that a cube is submerged in water, as shown in Figure 1. Its six surfaces are all under water pressure, and the pressures on its left, right and front sides are equal in magnitude and opposite in direction, that is, the forces are balanced with each other; Only the upper and lower surfaces have different depths, and the water pressure is different, so the water pressure is not equal. The upward pressure F1 of water on the lower surface is greater than the downward pressure F2 of water on the upper surface, and the difference between the upward and downward pressures is the buoyancy of the liquid to the immersed object. The direction of buoyancy is always vertical and upward, which is opposite to the direction of gravity.

2. Calculation method of buoyancy

1. Twice-weighing difference method

From the above analysis, we know that the direction of buoyancy is vertical and upward, which is just opposite to the direction of gravity, so we first weigh the gravity F1 of the object in the air with a spring dynamometer, and then immerse the object in the liquid. At this time, the reading of the spring dynamometer is F2.

Example 1 A hook code weighing 2N, when it is hung under a spring dynamometer and immersed in water, the reading of the spring dynamometer is 1．2N, so the buoyancy of the hook code is

analytical

2. Two-force balance method

When an object is immersed in a liquid, it starts to move freely, and its motion state has only three possibilities: sinking, immobility or floating. When an object is immersed in a liquid and stands still, it is called suspension. When an object floats, a part of its volume finally comes out of the liquid surface and stands still on the liquid surface, which is called floating. The sinking object finally sinks to the bottom of the liquid. According to the force balance of objects, the buoyancy of suspended and floating objects is equal to gravity, that is, < P >, and the sunken objects are supported by the container wall, so < P >, that is, < P > Example 2 A ship sailed from the sea to the river. The following statement is correct: ()

A. The buoyancy of the ship in the river is large

B. The buoyancy of the ship in the sea is large

C. The buoyancy of the ship is unchanged

D. The buoyancy cannot be determined

According to the characteristics of the ship floating, the buoyancy is equal to the gravity of the object, so the correct answer is option C.

3. Archimedes principle method

The content of Archimedes principle is that an object immersed in a liquid is subjected to upward buoyancy, and the buoyancy is equal to the gravity of the liquid it displaces, that is,

Archimedes principle method is often used in combination with the two-force balance method.

example 3, as shown in figure 2, puts a ball into two overflow cups A and B filled with different liquids respectively. The mass of the overflow liquid in cup A is 4g, and that in cup B is 5g, so the mass of the ball is _ _ _ _ _ _ _ _ _ g..

Analysis According to Archimedes' principle, the buoyancy of the ball in Figure A is

that of Figure B is

, while the ball in Figure A sinks to the bottom of the liquid. According to the analysis of the two-force balance method, the ball in Figure B floats on the surface of the liquid, so it is

.

that is,

therefore.

4. Formula method

Known from Archimedes principle

This is a commonly used formula for finding buoyancy.

example 4 three balls with the same diameter, namely ball A, ball B and ball C, are put into the water respectively, and when they are finally balanced, as shown in Figure 3, the relationship of buoyancy is _ _ _ _ _ _ _.

The volume relation of three balls immersed in water is analyzed as follows, which is known from the buoyancy formula.

Buoyancy formula is an important tool to solve the problem of buoyancy. Combining with other methods, it can solve almost all the calculation problems of buoyancy. According to the formula method and the two-force balance method, the ups and downs conditions of solid objects in liquid are deduced.

According to the analysis of the two-force balance method, there are three possibilities for a solid object to be completely immersed in liquid: floating, standing and sinking:

sinking when it is in time; When suspended, suspended; When, float, and finally float. From the buoyancy formula, it is known that

and

because the object is immersed in liquid, it is.

all the above are combined: when in time, the object will sink; When, the object is in suspension; When moving, the object will move upwards and finally be in a floating state. This is an important conclusion, please see the following example.

the method of measuring human blood density in example 5 is: put copper sulfate solutions with different densities into several test tubes, and then drop a drop of blood into each test tube. If the blood is suspended in a test tube, the blood density will be _ _ _ _ _ _ (filled with more than, equal to or less than that of the copper sulfate solution in the test tube).

Analysis Because blood is suspended in copper sulfate solution in the test tube, ρ blood = ρ copper sulfate, and the density of blood is equal to that of copper sulfate solution in the test tube.

III. Comprehensive application

As long as the above methods are used flexibly, the comprehensive problems, that is, the combination of buoyancy and density, pressure, balance condition of two forces and lever, can be solved smoothly. Please look at the following examples.

Example 6 A student uses the same densitometer to measure and compare the densities of liquid A and liquid B. The measurement situation is shown in Figure 4, so the density relationship between liquid A and liquid B is ()

A.

B.

C.

D. It is not sure that both liquid A and liquid B can make the densitometer float on the liquid surface in the analytic diagram. From the two-force balance method, it is known that F floating A = F floating B = G, and because the volume of density meter immersed in liquid A is greater than that immersed in liquid B. Namely. From the formula method,, so there is

from the knowledge

so option C is correct.

example 7 figure 5 is a schematic sectional view of the structure of a water dispenser. the floating body a and the valve c in the water control tank are connected by a light rod. it is known that the water level of purified water in the bucket is 4cm high and the cross-sectional area of the valve c is 1cm2. regardless of the mass of the floating body a and the valve c itself, what is the buoyancy of the floating body a when maintaining a certain water level?

firstly, the force of the floating body A is analyzed, and the floating body A is balanced by buoyancy and the pressure of the valve C..

The pressure of valve C on floating body A and the supporting force of floating body A on valve C are a pair of mutual forces, which are equal in magnitude and opposite in direction, so the supporting force of floating body A on valve C is also equal to the buoyancy of floating body A, that is,

The force analysis of valve C shows that valve C is balanced by the pressure of water and the supporting force of floating body A regardless of gravity, so

Example 8 An ice floats on the water. If the ice contains stones or blocks of wood, how will the water level change?

analysis to judge the change of water level before and after ice melting, it is necessary to compare the size of V row before melting with that after melting, which is the volume of boiled water of wood blocks or stones after ice melting.

(1) If there are no impurities in the ice, let the mass of the ice be m ice, and the mass of the ice that becomes water after melting is m iced water. Because the ice floats before melting, it is

,

that is,

after melting, and

, so the water level does not change before and after melting.

(2) If the ice contains wood blocks, it can be known that the ice and wood blocks are floating before melting.

, that is,

After the ice melts, it is known from (1) that

because the wood blocks float, there is

, that is,

, so the water level has not changed before and after the ice melts.

(3) If the ice contains stones, the ice will be treated with ice cubes and stones before melting. Floating, then

, that is.

When the ice melts into water, because the stones sink, then

,

.

How to identify the circuit diagram originally published in Reference of Physics Teaching in Middle Schools in September, 25

Tang Xiaosheng of Samsung Wu Zhongliang Middle School in Lezhi County, Sichuan Province

Correctly identifying the circuit diagram is the most basic ability requirement of junior high school students, and some electrical calculations always need to be analyzed in advance, which accounts for a large proportion in the senior high school entrance examination. The difficulty and key point of the electrical finale in the senior high school entrance examination is often the problem of circuit identification. In order to pass the identification circuit, we must clear the obstacles in knowledge and method. Now several key issues are put forward and discussed with you in order to attract more attention.

First, the concept and characteristics of series and parallel circuits

Page 16 of the first volume of the eighth grade of the new textbook published by People's Education Press says: "Two small light bulbs are connected end to end and then connected to the circuit, we say that these two light bulbs are connected in series" and "The two ends of the two small light bulbs are connected to the circuit respectively, and then we say that these two light bulbs are connected in parallel", which is similar to page 57 of the second volume of the old textbook.

we can easily see the characteristics of series and parallel circuits from the above definitions of series and parallel circuits. The series circuit has only one path, and all electrical appliances are on and off, which affects each other. No matter where the switch is connected, it controls the whole circuit. The parallel circuit has two or more branches, and each electrical appliance works independently. The switch of the trunk circuit controls the whole circuit, and the switch of the branch circuit only controls the one where it is located.

Second, the judgment method of series and parallel circuits

1. Analyze the connection method of electrical appliances in the circuit by electrical appliance connection method. What is connected one by one is series, and what is connected in parallel between two points of the circuit is parallel.

2. The current method has no branch in the series circuit, and the main current is divided into several parts in the branch in the parallel circuit.

3. * * * Same connection point method In series, one electrical appliance has only one * * same connection point with another electrical appliance, while in parallel, one electrical appliance has two * * same connection points with another electrical appliance.

4. when analyzing the circuit connection, start from the positive pole of the power supply, follow the direction of current, and reach the negative pole of the power supply.

5. Because the connection of multiple electrical appliances in junior high school does not involve series connection, the final result of circuit connection can only be one of two basic circuit connection methods-series connection and parallel connection for junior high school students.

6. Because of the diversity of drawing methods of circuit diagram, students are not used to it and it is difficult to recognize it. At this time, we just need to sort out the original circuit diagram and draw it into a common standard form, and see the essence through the phenomenon. The standard form of Figure 1 is Figure 2, the standard form of Figure 3 is Figure 4, and the standard form of Figure 5 is Figure 6. In addition, it can be drawn as a standard form in Figures 7 and 8.

Figure 1 Figure 2

Figure 3 Figure 4

Figure 5 Figure 6

Figure 7 Figure 8

III, Special treatment of several common circuit components

When we analyze the circuit connection, we often focus on electrical appliances. Other components, such as switches, voltmeters and ammeters, have a great negative impact on our analysis of complex circuits. If some components can be removed from the circuit diagram and the original connection of electrical appliances can be guaranteed, the original circuit diagram can be simplified in order to uncover the truth of Lushan Mountain. To this end, we can completely simplify according to the characteristics of the removed components, and the methods are as follows:

1. If the switch is closed, draw a wire to connect the original switch, and if the switch is open, completely remove this path.

2. Voltmeter Because the resistance of the voltmeter is very large, it can be regarded as an open circuit at the place where the voltmeter is connected, just remove the voltmeter.

3. ammeter because the resistance of ammeter is very small, it can be regarded as a short circuit, and the place where ammeter is removed should be connected with wires.

the simplified diagram obtained by the above method shows the connection of electrical appliances in the original circuit. This step is best to draw the original circuit diagram with a pencil on the draft paper, and then add and delete it one by one from the positive pole of the power supply until the negative pole of the power supply. For example, we need to judge the connection of resistors R, R and R when both switches S and S are off in Figure 9. The simplified circuit diagrams of switches, voltmeters and ammeters are shown in Figures 1, 11 and 12 respectively. We can easily see from Figure 12 that the resistors R and R are in series.

Figure 9, Figure 1, Figure 11, Figure 12

IV. About the number of electric representations

First, make clear what physical quantities are measured by each electric meter, and which current is measured by the ammeter depends on which way it is connected in series, and whose voltage is measured by the voltmeter depends on whose terminals the voltmeter is directly connected to (Note: the power supply voltage is the total voltage, that is, the highest voltage, as in Figure 1, it cannot be said that the voltmeter is measured. Sometimes, for more complex circuits, it is necessary to simplify the circuit in combination with the above third item, and then restore the ammeter to the circuit one by one according to the original circuit diagram to judge, so as to reduce the difficulty.

v. changes of sliding rheostat

first, make clear the connection of sliding rheostat. Because the use of sliding rheostat is "one for each, one for each", it is the part between the slider and the lower terminal that is connected to the circuit. Many people can't understand the connection of sliding rheostat on the circuit diagram, but they don't understand the fact that the physical connection is exactly the same as the symbol of the component. In fact, when the slider slides, we can directly see whether this part has become longer or shorter on the circuit diagram, so as to determine whether the resistance value connected to the circuit has increased or decreased. Only when we really understand this problem. As shown in fig. 13, the part with diagonal lines on the element symbols of the sliding rheostat is the connected circuit part.

fig. 13, fig. 14, fig. 15 and fig. 16

The decomposition method of circuit analysis mentioned above only provides an auxiliary strategy for those who are at the end of their rope to turn difficult into easy, and its application may bring joy to the future. In fact, our analysis of the circuit is a complex synthesis, which requires the fluency and lightning of thinking in an all-round way, without being bound by the above methods, and only when necessary can we use them flexibly and comprehensively. We should pay attention to treating each method as a reactant participating in the chemical reaction. What we hope is that these reactants participate in the chemical reaction to generate new substances, undergo qualitative changes, and there are differences in divergence, otherwise it will be self-defeating and get twice the result with half the effort. Let's take a look at the circuit of the calculation problem in the 23 mid-term exam. In Figure 14 (Guangzhou Figure 8), it should be immediately seen that when S1 and S2 are disconnected, the resistors R1 and R2 are connected in series, the voltmeter measures the voltage of R2, and the ammeter measures the current passing through R1 or R2; When S1 and S2 are closed, the resistor R2 is short-circuited, and R1 and R3 are connected in parallel.