External resistance changes the power of the appliance
2 Change the power of the appliance as required by an external resistor R0. 2. Buy an electric blanket from the store. Sometimes the temperature is too high when using it. Now I want to use a switch and a resistor to convert it into two gears, so that the electric blanket The amount of heat released per unit time is either the original value or half of the original. The electric blanket only writes the rated voltage of 220V. Other electrical indicators are blurred. Please design a scheme to complete the modification. It is required to derive the calculation formula of the resistor used. (3) Indicate the problems that must be paid attention to during installation. Analysis: (1) The resistor used can only be connected in series with the electric blanket, and the circuit is as shown. (2) Let the size of the resistor used be R* first measure the resistance R of the original electric blanket, so that the power consumed on R is half of the rated power. According to P=U2/R, the voltage across R should be changed. Ur=U/ /2 because R is connected in series, the voltage across R' should be f/=U*U/ in the characteristics of the series circuit, R'R=U*Ur=(U*U/VU//) Thus, the speciality of R*=((2*1)R. (down to page 60) can be solved, which brings certain difficulties to correctly and accurately grasp the scoring standard in the batch work. The condition that the board moves forward to produce the maximum distance: /= +m)g is derived from a mathematical expression, but the analysis of its physical process is not detailed, it seems to give people a heavy mathematical operation skills, the feeling of light physical process analysis . This question was once the all-Russian high school students' Olympic competition (10th grade in 1979). Some of the school's candidates have been trained, and the scores are obviously different. A similar situation has appeared in the 2001 competition. For example, the first question is similar to the 16th question on page 14 of the "New Olympic Physics Competition Guide" edited by Fan Xiaohui. The second question of the initial test is a very common college entrance examination training question. I believe everyone has seen it in many materials. The fourth question of the retest is the same as the 49th question on page 467 of the "New Olympic Physics Competition Guide" edited by Fan Xiaohui. As a national competition selection test, it is not uncommon for many times to have the same problems as before, and it is not in line with our original intention of selecting high-quality talents. According to the experience in the examination and revision of the test paper, I feel that the answer is necessary to make appropriate modifications. 2 Discussion of the solution to this problem (1) If the person runs on the board, the board does not move according to vL 2as under the condition of the board length s, the larger a (ie, the larger), the larger v. For wood, the maximum friction that the ground can provide is WM+m)g, so the maximum speed appears in: /=KM+m(2) If the person runs on the board, the board retreats; The system is a research object, from the start of the person to grasping the baffle and stopping on the board. The momentum obtained by the system during this period is generated by the impulse of 'f; f / ground (. / 'ground t) Yes, when: the board retreats when / ground = W (M + m) g, and remains unchanged. If you want: the maximum, that is, the ground friction force needs the longest time; from the relative motion knowledge of the board and the person, the relative displacement of the two is the plate length 1 is constant; if the longer the time is desired, the two The smaller the relative speed, the better. The minimum value appears in the analysis of the above two possible situations. The maximum distance that the plank is required to move forward in the question should be the moment when the board just starts to slide, ie /ground = W (M+m)g, at this time: when the person runs to the P point, the speed is held by the board and the momentum is conserved: mv=when the two slide together on the ground, (: 2 (1) 1 (on page 57) should Install in a box that can dissipate heat and pay attention to insulation. 3 By externally connecting a resistor R, the power of the appliance is changed, and the loss of the entire circuit is reduced. Example 3. The soldering iron needs a certain warm-up time before use, so that it is connected to the power source even if it is not used for a while, but Waste of electricity can cause the tip to overheat and oxidize not to stick to tin. Therefore, sometimes the circuit is used, and when the welding is temporarily unnecessary, the electric key S is turned off to make the electric soldering iron in a preheating state. When welding is required, the closing of the electric key S can quickly reach the welding temperature. Now give a *220V, 25W* soldering iron preheating. If the resistance of the bulb filament is 800, the power consumed by the soldering iron is watts in the preheating state. The power consumed by the whole circuit is consumed when the soldering iron is working normally. A few percent of the power (the resistance of the soldering iron and the filament does not change with temperature), analysis: the meaning of the problem, the power supply voltage is unchanged, when the soldering iron is in the preheating state, S is disconnected, which is equivalent to the soldering iron and the bulb in series In the circuit, as shown. The resistance of the soldering iron is U2/P=(220V)/25W=1936. Therefore, the total resistance in the preheating state is R=Ri+R2 2736a. The current through the bulb and the soldering iron can be obtained according to Ohm's law, ie / =U/R=Because the soldering iron and the bulb form a series circuit, the current through them is 0.08A. According to P=12R, the electric power consumed by the soldering iron is P*=the same reason, the power consumed by the whole circuit at this time. It is P=/2 feet=(the power consumed at 0.08 is P()=25W, so the power consumed by the whole circuit at this time accounts for the percentage of power consumed by the soldering iron during normal operation. P/P0 can be seen from the above three examples. According to the actual needs to change the power of the electrical appliance, is achieved by connecting a resistor in series with the electrical appliance. The different requirements of the modification determine the size of the series resistor. To solve such problems, we should first determine the series resistance and then use the electrical circuit. The connection method; according to the characteristics of the series-parallel circuit and the value of the power required after the modification, the size of the resistor to be connected in series is obtained. (:2(1)2―
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