魔法物理初三-（Magic physics III -）

1、魔法物理初三8-2（Magic physics III 8-2）Two Ohms law *Walk into the physical worldBlack clouds and dust cannot hide the light of scientific truthIn 1827, issued a mathematical exposition ohm galvanic circuit, theoretically, Ohms law, Ohm thought research results will be acknowledged by the academic circles

2、will ask him to teach. But he was wrong. The book attracted a lot of irony and slander, a university professor who look down upon him the middle school teachers. The Germans Bauer the attack he said: look at the world with the devout vision who dont read this book, because it is purely an incredible

3、 deception, its sole purpose is to violate the dignity of nature. All this makes the ohm is very sad, he wrote to a friend: the birth of galvanic circuit has brought me great pain, I really complain about it because the people in the court Born Under A Bad Sign, xueshiqianbao, they cannot understand

4、 its mothers true feelings Of course, there are many people who published the ohm and Ohm chemistry and physics magazine editor Schweig (i.e. galvanometer inventor) write to Ohm said: please believe that behind the clouds and dust in the light of truth will eventually be transmitted from and smiling

5、 to dispel it. ohm resigned from the post in Cologne again, when a few years private teacher, until seven or eight years later, with the development of circuit, people gradually realized the importance of Ohms law, Ohms reputation is also greatly improved the.1841 of the Royal Society awarded him th

6、e Copley medal in 1842, was hired as a foreign member, in 1845 was admitted to the Bavaria Academy of Sciences. To commemorate him, will be the unit of resistance is named after his name ohms.Entering the physical world, G can explain the main points1. Ohm law(1) the content of Ohms Law: the current

7、 in a conductor is proportional to the voltage at the ends of the conductor, inversely proportional to the resistance of the conductor(2) the expression of Ohms Law: I=U/R. type U voltage at both ends of the conductor said, in units of volts (V); R said the conductor resistance, ohms (omega); I said

8、 the current in a conductor, the unit is the ampere (A).2. correctly understand Ohms law(1) the I, U and R in the formula represent the current, voltage and resistance of the same conductor or the same circuit. When the value is calculated, it corresponds to the Mi Ichii and must be unified with the

9、 international unit(2) the deformation formula of U=IR product of voltage at both ends of the said conductor is equal to the current through it and its resistance, can not think of the voltage and current is proportional with the resistance is proportional to the current, because no resistance effec

10、t on voltage; deformation formula of R=U/I said the resistance of a conductor is equal to the ratio of the current voltage and the two ends of the conductor and the in the numerical, for the same conductor, U/I is a fixed value, can not be understood as resistance and voltage proportional to the cur

11、rent is inversely proportional to.Special tips:1. note Ohms law applies only to pure resistance circuit2., when using I=U/R, we should keep the same (circuit) and simultaneous nature of I, U and RKnowledge, key points, interpretation, Q, problem solving techniques, techniques1. a small bulb is conne

12、cted to a circuit whose voltage is 1.5V, and the current through the small bulb is 300mA(1) small resistance bulb filament for _,(2) the charge in the 1min through a small bulb is _C.Analysis: (1) according to Ohms law, I=U/R gets R=U/I=1.5V/0.3A=5 Omega(2) according to the definition of current, I=

13、Q/t gets Q=It=0.3A * 60C=18C.Answer: (1) 5 omega; (2) 18CMethod and technique: when calculating electricity, each physical quantity should be unified, such as current 300mA is 0.3A, time 1min is 60s.2. as shown in figure circuit, slide rheostat P marked 2A 24, closing the switch S, when the slide P

14、is located in the right side, said the current number of 0.5A, if the current meter is connected to a large range, the resistance of sliding rheostat circuit access number shall not be less than Omega?Analysis: (1) when the P on the right side, even into the sliding rheostat circuit resistance is 24

15、 ohms, namely R1=24, the circuit of the current I1=0.5A, according to Ohms law, the voltage at both ends of the sliding rheostat, namely the power supply voltage of 24 U=I1*R1=0.5A * =12V.(2) the ammeter changes to a great extent, and the measuring range is 03A, but the maximum current allowed in th

16、e circuit is 2A, and the supply voltage is constant. The minimum resistance of the sliding rheostat connected to the circuit is R2, and the value is R2=U/I2=12V/2A=6The resistance must not be less than 6 ohmsMethod skills:1. understand the meaning of sliding rheostat nameplate, mining hidden conditi

17、ons, such as: R1=24, I2=2A.2. generally, the voltage of the supply may be considered constant3., the problem can also be solved by the scaling method, such as: I1/I2 = R2/R1.Problem solving methods, skills, integrated application development3. the voltage of the lighting circuit is 220V; for the hum

18、an body, in general, the voltage below 36V is safe; when the current in the body is equal to 1mA, it causes the sensation of hemp;When the current in the body is equal to 30mA, there is a danger of life. According to the above data, it is estimated that the ordinary body resistance is closest to (th

19、e)A.1 * 10 OmegaB.1 * 10 OmegaC.1 * 10 OmegaD.1 * 10 OmegaResolved: this question relates to current lighting voltage, safe voltage, current, caused by the feeling of hemp of life, people and power contact is the security problem of these data. So the main line relates to is hidden according to peop

20、le with power contact is safe, looking for the relationship between voltage and current and the resistance of human body.From a voltage point of view, a voltage below 36V is a safe voltage, and 220V is a dangerous voltageFrom the current perspective, the human body in the current is up to 30mA, ther

21、e was a danger that the 30mA current is a dangerous current, current less than 30mA current. When the body is safe when the current is 1mA, just cause trouble feeling for people without danger.Taken into consideration, when people touch 220V voltage, people are dangerous, at this time the current in

22、 the body must be greater than 30mA; when the contact is lower than the 36V voltage, there is no danger, at this time the current in the human body is less than 30mA.List quantitative relationships:220V/R (person) 30mA,Get R (person) 7.3 * 10 Omega36V/R (person) 1.2 * 10 Omega1.2 * 10 Omega R ( 7.3

23、* 10),Therefore, R (human) is closest to 10, so choose B.Answer BThinking interaction:Student: why cant the ammeter be connected directly to both ends of the power supply? And the voltmeter is?Teacher: in order to protect the current meter and a voltage meter, through which the current is not too la

24、rge, excessive current is in danger of being burned. The characteristics of current meter is very little resistance, according to Ohms law I=U/R, the current in the circuit will be great, will burn out the current meter and power supply, and absolutely not allowed the ammeter power poles; resistance

25、 voltage meter is very large, the voltmeter in parallel at both ends of a circuit or a power supply, a voltage meter branch equivalent circuit, so the voltage meter can be directly connected with the power poles, in no more than the range, can not be burnt out.4. as shown in Fig. , the power supply

26、voltage remains constant, when the closing switch S and the slider P move to the a terminalA. current meter, the number of voltage increasesThe B. current indicates a larger number and a smaller number of voltagesThe C. current indicates a smaller number and the voltage indicates a larger numberD. c

27、urrent meter, the number of voltage representations are smallerAnalysis: by R and can be a sliding rheostat in series, current meter circuit current series, voltage meter measuring voltage at both ends of the sliding rheostat, when the sliding plate moves, to the end of a Rap decreases and the total

28、 resistance of the circuit decreases, according to I=U (total) /R (total), I, said that the current number of large; according to UR=IR, the increase of I, R the same, so UR increased. According to the Uap=U-UR, U constant, the increase of UR, so Uap becomes small, which indicates the number of volt

29、age decreases.Answer BMethod skills:1., analyze the circuit structure first, judge the measuring object of the meter, then analyze the reasoning according to the change of the resistance, the current and voltage in the circuit2. in this question, Uap=I, Rap, I increase, Rap smaller, and therefore ca

30、n not directly judge the changes in Uap, often using indirect methodComprehensive application development option 1., according to Ohms law formula, I=U/Rs deformation type, R=U/I and the nature of resistance, we can seeWhen A.U becomes larger, U/I becomes smallerWhen B.U becomes larger, U/I becomes

31、largerC.U changes, U/I remains unchangedD.U=0, U/I=0 C C! 2. (Dalian? 2004) a student on the relationship between electric current and resistance, according to the collected data to draw an image as shown in figure , the following conclusion is consistent with the image ()When the A. resistance is c

32、onstant, the current increases with the increase of the voltageWhen the B. resistance is constant, the voltage increases with the increase of the currentWhen the C. voltage is constant, the current decreases as the resistance increasesWhen the D. voltage is constant, the resistance decreases with th

33、e increase of the current. C, C, t 3., the voltage at the ends of a conductor is increased from 2V to 4V, and the resistance of the conductor is (0.1A) by the current of the conductorA.8.B.20.C.28.D. cannot determine the voltage at the end of a conductor of B B 4. When the voltage is U, the current

34、is I. If the voltage at both ends of the conductor is 3U, the current I of the conductor and the resistance R of the conductor are respectively ()A.I =I, R=3U/IB.I =3I, R=3U/IC.I =I, R=U/ID.I =3I, R=U/I D D 4 X /? choice? fill in L 1. there is a resistance for the 100 resistor, as both ends of appli

35、ed voltage is 20V, through which the current is _A; if both ends of the voltage is zero, the resistance is _ 0.2 ohm. 1002.; a current sheet resistance is 0.03 ohms, the maximum range is 0 3A, if it is directly with the 1.5V battery connected with a current of up to _A,所以电流表_直接接在电源两极间(填能或不能). 50;不能

36、3.(襄樊?2004)如图所示的电路中，A1和A2是两个完全相同的电流表，均有00.6A03A两个量程.当开关S闭合后，A1使用00.6A量程，A2使用03A量程，两表指针偏转的角度相同，则R1R2为_. 41 4.如图所示，甲为_表，乙为_表.闭合开关S，当滑片P向_滑动时，甲表读数会增大，乙表的读数会_,电阻R0将_(填：变大变小不变)，并在图中标出电表的正负接线柱. . 电流；电压；左；变大；不变；标电表正负接线柱略 m 5.如图所示的电路，如果原来正常发光的小灯泡的灯丝突然 Burning off the table number _ in V, the number of _ sho

37、wn in table A (fill in change).Increase; reduce to zero? C N? e? Fill in the blanks? To explore the topic? A classmate of the circuit in figure 1. as shown in figure the circuit of inquiry experiment. The resistance of R1 remained unchanged during the experiment, the maximum resistance of the slidin

38、g rheostat R2 is 20 ohm. Three times to change the position of P sliding vane rheostat, get the experimental data in the table below:Please answer the questions：(1) the circuit is _ contact circuit, voltage meter measuring the voltage of V _;(2) the number of comparisons are shown in table A1 and A2 current, we can come to a conclusion: _;(3) comparison of three voltage indicates the number of available. Conclusion: _;(4) comparing the current table A1 (or A2) and voltage meter shows the number of V1, we can come to a conc