Beg. Power supply voltage. Fixed value resistance R 1, resistance value r 1, power supply voltage
Current change = 0.5a-0.3a = 0.2a.
So u/RC-u/Rb = 0.2a.
Because Rc= 1/2Rb
So u = 0.2a * Rb = 0.2a * 200Ω = 40v.
2. According to the resistance formula of parallel circuit
R sum = r1* Rb/r1+Rb = 40v/0.3a.
Bring 200 euros into
You can get it.
R1= 400Ω The resistance of the resistor R2 is10Ω. When resistors R 1 and R2 are connected across the power supply in some way, the voltage across R2 is 9V. When the resistors R 1 and R2 are connected to both ends of the same power supply in another way, the voltage across R2 is 3 volts. Q:
1. supply voltage
2. Resistance R 1) Power supply voltage =9V.
2) Because the parallel voltage of R 1 is U 1= power supply voltage-the voltage across R2 =9-3=6U.
Current R 1 = current r2i = 3/10 = 0.3a.
= = =>R1= u1/I = 6/0.3 = 20i = 6/0.3 = 20ω constant power supply voltage (220V). Connect the "220V, 900W" bulb in series with the sliding rheostat. When the power of the bulb is 400W, what is the power of the sliding rheostat?
Reference answer: P=U2/R indicates that the resistance of P*R bulb is constant under U= root sign. The voltage of electric lamp is proportional to the arithmetic square root of electric power. Then the actual voltage of small bulb is ureal/220 = 400/900 = 2/3 ureal = 2/3 * 220v, and the voltage of sliding rheostat is UR = 1/3.
Ohm's Law
I. Multiple choice questions
1.r 1 and R2 are two resistors with different resistance values, r 1 < R2. After the four connection modes as shown in Figure 8-78 are respectively connected to the two poles of the same power supply, the minimum current in the circuit is
[] (Nanjing Senior High School Entrance Examination)
2. For two resistors whose total resistance is10Ω after series connection, the maximum total resistance can be achieved after parallel connection.
A.2.4 euros
B.2.5 euros
5 euro
65438 Deutsche Mark +0.25 Euro
3. Connect a resistance wire with a resistance value of R to the power supply, and measure the current intensity in the circuit with an ammeter as 1.2 amps. Fold this resistance wire in half and connect it to the same power supply. In order to measure the current intensity in the circuit, then
A the range of ammeter "0 ~ 0.6a" can be used.
B only "0 ~ 3a" range of ammeter can be used.
When the current intensity exceeds 3A, the ammeter cannot be used.
D. insufficient conditions to judge
4. Two resistors are connected in series, and the voltage ratio is 3∶4. If these two resistors are connected in parallel on the same power supply, the current ratio is.
Answer 3: 4
B 4: 3
C. 1∶ 1
D.3∶7
5. As shown in Figure 8-79, the power supply voltage is constant. In order to maximize the reading of ammeter, the following practices are correct.
A.s 1 and S2 are disconnected.
B.s 1 open, S2 closed.
C.s 1 off, S2 on.
D.s 1 and S2 are closed.
(Shanxi senior high school entrance examination)
6. In the circuit shown in Figure 8-80, a resistance wire with uniform thickness is changed into a ring, and A, B, C and D are the quartering points on the ring. When the switch S is closed, the slider slides from B to D through C, and the current representation will change from small to large.
B. First get bigger and then get smaller
C. From big to small
D. First it gets smaller, then it gets bigger.
7. Total resistance of three parallel resistors
A. it must be greater than any resistance value.
B it must be less than any resistance value.
C must be equal to the sum of the reciprocal of each resistance.
It must be greater than the minimum resistance value and less than the maximum resistance value.
(Senior High School Entrance Examination in Henan Province)
8. As shown in Figure 8-8 1, the voltage of the power supply is constant and the resistance in the circuit is R, which is the voltmeter.
The reading is u. If the voltmeter reads U, the following methods are required.
A connect a 3R resistor in series between alternating current.
B, connect a resistor with a resistance value of 3R in series between CDs.
C connect a resistor with a resistance of 3R in parallel between CDs.
D, connect a resistor with a resistance of in series between DB.
9. As shown in Figure 8-82, the research shows that the relationship between current, voltage and resistance can be divided into two steps: "keeping resistance unchanged" and "keeping voltage unchanged". In the step of "keeping the resistance constant", the experiment requires a. keeping the position of the R' slider unchanged.
B. keep the voltage on r constant.
C, keep r unchanged, and adjust the slider of R' to different positions.
D, keeping the current in the circuit unchanged.
10. The resistance Rx of one resistor is unclear. Now we need to measure its resistance. Some students use the existing equipment: a voltmeter and an ammeter, a resistor R0 with known resistance, a battery pack, two switches and several wires, which are connected into four circuits as shown in Figure 8-83. Among them, the resistance Rx cannot be measured.
Second, multiple choice questions
1 1. Regarding the relationship between current, voltage and resistance, the following statement is correct.
A when the voltage is constant, the resistance of the conductor is inversely proportional to the current passing through the conductor.
B when the resistance is constant, the current passing through the conductor is directly proportional to the voltage across the conductor.
C when the current is constant, the voltage across the conductor is directly proportional to the resistance of the conductor.
The current in a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor.
12. The conductor AC made of copper consists of two sections of AB and BC with equal length, and the cross-sectional area of AB section is twice that of BC section. When it is powered on,
A.RAB resistance QBC
C. What is the current that flows when the C.UAB bulb emits light normally?
〈3〉 1KW？ 6? 1h power supply can supply light bulbs for a long time and work normally?
< 4 > if the actual voltage is 200V, what is the actual power of the bulb? How does a light bulb shine?
Solution: identify the invariants and variables in the problem and choose the appropriate formula for calculation.
Question 3: Electrothermal calculation
Example 1. Two resistors are connected in series in the circuit, r1= 4Ω, R2 = 6ω, and the power supply voltage is 10V. So what is the heat generated by the current passing through each resistor in 1min? How much heat does the total * * * generate?
Example 2: When the coil resistance of a motor is 0.3 Ω, and the circuit of 12V is connected, the current passing through the motor is 0.5A What is the work done by the current within 5min and the heat generated by the current?
Example 3: Two identical resistance wires, the resistance of which is R, are connected in series and connected to the power supply, and a pot of water can be boiled in 20 minutes; If they are connected in parallel and connected to the same power supply (assuming the power supply voltage is constant), how long will this pot of water boil?
Solution: First, distinguish the circuits (pure resistance circuits or impure resistance circuits) and select the correct formula for calculation.
Exercise: 1 The electric fan connected to the 220 volt circuit works normally. The current flowing through the fan motor is 0.455A, and the coil resistance of the fan motor is 5 Ω. Try to find out the answer.
(1) What is the input power of the electric fan?
(2) How much electricity does the electric fan consume per minute?
(3) How much electric heat does the fan motor generate per minute? How much mechanical energy is generated?
There are two heating wires in the water heater. One of them is electrified, and the water in the water heater boils after 15 minutes, and the other one is electrified alone, and the water in the water heater boils after 30 minutes. If two heating wires are connected in series and parallel respectively, how long will it take to boil after being electrified? (assuming that the resistance of the heating wire is constant).
Question 4: Household electricity consumption.
The power of refrigerators in Xiao Xin is 0.2kW, and only the refrigerators automatically work intermittently during the whole two days when the whole family goes out on weekends. The electric energy before and after these two days is shown in Figure 4. How much electricity does the refrigerator consume these two days? What are the actual working hours?
Example 2: The electric energy meter of Xiao Wang's house is marked with the words "220V 10A", and the total power of its original electrical appliances is 1500W. I bought a new electric water heater recently. The following table shows some data on the nameplate. (Two decimal places are reserved for the calculation result)
(1) What is the rated current of the electric water heater?
(2) How many ohms is the resistance of the electric water heater when it works normally?
(3) After the electric water heater is connected to the home circuit, can the electrical appliances in his home be used at the same time by calculating?
Exercise: The water dispenser is a common household appliance, and its working principle can be simplified as the circuit shown in Figure 17, where 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).
Question 5. Change the circuit problem
For example 1, as shown in figure 10- 13, the resistance r 1 = 10ω, R2 = 20ω, and the power supply voltage u = 6V. Try to solve according to the following requirements:
(1) When both switches S 1 and S2 are off and S3 is closed, look for the ammeter indication and the voltage of R 1;
(2) When S 1 and S2 are closed and S3 is open, find the current representation;
(3) Under what circumstances will the ammeter burn out?
Example 2. In the circuit shown in the figure, the power supply voltage U remains constant, the power supply voltage is 12 volts, the resistance of the constant resistor R 1 is 4 ohms, and the maximum resistance of the sliding rheostat is 8 ohms. When the slider P of the sliding rheostat slides from the A end to the B end, what is the change range of the ammeter and voltage representation? Solution: Because the on-off of the switch and the movement of the sliding rheostat slider change the structure of the circuit, the current and voltage values in the circuit will change, which is the so-called circuit change problem. The key to solve the problem of variable circuit is to turn dynamic circuit into static circuit, that is, draw the equivalent circuit diagram after each change, indicate the known and unknown quantities, and then solve it according to relevant formulas and laws.
Exercise: 1 As shown in figure 140, R 1=20 = 20 Ohms, R2 = 40 Ohms, and the power supply voltage remains unchanged. (1) When both switches S 1 and S2 are closed, the indication of ammeter A 1 is 0.6 amp.
(2) When the switches S 1 and S2 are turned off, the indicator of ammeter A2 is 0.2 A, and the actual power of the small bulb is calculated;
(3) What is the rated power of the small light bulb?
2. In the circuit shown in Figure 15, the power supply voltage U=6V is constant and the resistance R 1 is 5 ohms. (1) When the switch S 1 is closed and S2 is turned off, and the slider P of the sliding rheostat slides to the B terminal, measure the voltage at both ends of R 1 =1.2v, and calculate the maximum resistance of the sliding rheostat. (2) When the switches S 1 and S2 are closed and the slider Pa of the sliding rheostat slides to the A end, if R2 is known to be 7.5 ohms, what is the current in the main circuit at this time?
Question 6: Circuit protection problem
Example 1. In the experiment of resistance measurement by voltammetry, the indication of voltmeter is 4.8V and that of ammeter is 0.5A. If the reading of voltmeter is 12V after changing the power supply voltage, can ammeter with the range of 0 ~ 0.6a be used before and after? If not, what should I do?
Example 2: As shown in figure 1 1, the power supply voltage and bulb resistance of the circuit are unchanged, R 1=5, the lamp L is marked with the words "8V 6.4W", the ammeter has a range of 0 ~ 3A, and the sliding rheostat is marked with the words "2A".
(1) When only S 1 is closed, the ammeter pointer is 0.6A, and the power supply voltage is found.
(2) When S 1, S2 and S3 are closed and the slider moves to the B end, the ammeter indicates I =2A, and the maximum resistance of the rheostat is found.
(3) If the range of voltmeter is 0 ~ 3v, and the range of ammeter is 0 ~ 3a, and only S2 is closed, find the resistance range of rheostat connected to the circuit under the condition of ensuring the safety of the circuit.
Solution: Some circuit protection problems are often encountered in electrical calculation problems, which often involve the protection of ammeter, voltmeter, sliding rheostat, light bulb and so on. When encountering this kind of problem, making full use of the laws of electricity and the inequalities in mathematics will generally solve the problem.
Exercise: 1 As shown in Figure 25, the power supply voltage remains unchanged. When the slider P of the sliding rheostat R0 moves after the switch S is closed, the indication of the voltmeter changes from 0 to 4 volts, and the indication of the ammeter changes from 0.5 to 1 amp. Find the resistance of (1)R; ? 6? 9? 6? 9 (2) the maximum resistance of r0; ? 6? 9? 6? 9(3) Power supply voltage.
2. As shown in the figure, the power supply voltage is 4.5V, the resistance R 1 = 5 ohms, the maximum resistance of rheostat R2 is 20 ohms, the range of ammeter is 0 ~ 0.6 A, and the range of charging meter is 0 ~ 3 V. What is the resistance range of rheostat connected to the circuit to protect ammeter from damage?
Question seven, open question
For example, 1, as shown in figure 12, R 1 is constant resistance 12, the power supply voltage is 9V, the current indicator is 0.5A after the switch is closed, and it is energized for 5min. Please calculate four electrical physical quantities related to resistor R2 according to these conditions.
Example 2, the circuit shown in figure 13, R 1 is connected in parallel with R2, and it is known that R 1 = 10ω, and the current passing through R 1 is I 1=3A. Please add a condition to find the electric power of resistor R2 (requirements: the condition cannot be repeated, the data is customized, and a brief calculation process is written).
(1) additional condition calculation, 2) additional condition calculation
Exercise: 1 Electric kettle is a common tool for boiling water at home, which has the advantages of convenience and environmental protection. The following table is the data collected by Xiaohong about boiling a pot of water. Please answer as required. (Set the resistance value unchanged, and keep two decimal places in the calculation result. )
(1) How many calories does it take to boil this pot of water?
(2) Calculate three electrical physical quantities related to this question with what you have learned. It is required to write a short text description, calculation process and results.
(3) Xiaohong found through calculation and analysis that the actual heating time of boiling a pot of water is longer than the theoretical calculation time. What do you think is the reason for this phenomenon? Please analyze it briefly.
Mass of water m/kg 0.5 Nameplate rated voltage of electric kettle U/V 220
Initial temperature of water t0/℃
Specific heat capacity of water c/[j/(kg? 6? 1℃)] 4.2× 103 actual voltage U/V200
Atmospheric pressure value a standard atmospheric pressure actual heating time t/min 9
Fourth, the standard test:
1. The electric energy meter marked "220V3A" is selected for the household circuit, and three "220V60W" lamps and one "220V75W" TV set are normally used. On holiday nights, we want to install colored string lights marked "220V 15W" to add festive atmosphere. How many lamps can I install at most? (in two ways)
2. A teaching building has 12 classrooms, and each classroom is equipped with 6 "220V 60W" lamps. If the power supply voltage of the lighting circuit is normal, what is the main current intensity of this building when only two lights are turned on in each classroom? If the lights in every classroom are on, what is the main current of this building?
There is a DC car. When connected to a circuit with a voltage of 0.2V, the motor does not rotate, and the measured current flowing through the motor is 0.4A If connected to a circuit with a voltage of 2.0V, the motor works normally, and the working current is1.0A. What is the output power of the motor when it works normally? What is the heating power of the motor if the rotor suddenly gets stuck when the motor is working normally?
4. A small motor with an armature resistance of 20 ohms is connected to a power supply with a voltage of 120V. What is the maximum output power of the motor when the electric field intensity of the input motor is large? What is the maximum output power?
5. In the illustrated circuit, the power supply voltage is 12V, and the maximum resistance of the sliding rheostat is R2. When the switch S is closed, when the slider P of the sliding rheostat moves from the A end to the B end, the current representation number changes from 1.5A to 0.5a. Find: (1) the maximum resistance of the sliding rheostat; (2) the range of voltage representation; ? 6? 9? 6? 9(3) The heat generated by R 1 when the slider P of the sliding rheostat slides to the B terminal and is electrified for 2 minutes.
6. In the circuit shown in the figure, the power supply voltage is constant, and the lamp L is marked with the words "6 volts and 3 watts". When the key K is closed, the lamp L normally emits light, and the ammeter indicates 0.6 ampere. Find: (1) the current flowing through the lamp L? 6? 9? 6? 9(2) Resistance value of resistor R2. 6? 9? 6? 9(3) Work done by current passing through resistor R2 in 10 second.
7. As shown in Figure 5, the power supply voltage is constant, and the words "6V 3W" and "6V 6W" are respectively marked on the lamps L 1 and L2, and the words "12V" are marked on the lamp L3, so other handwriting is illegible. When S 1 is off and S and S2 are on, one of the lights can keep normal lighting for a long time; When S2 is off and S and S 1 are closed, the ammeter indicates 0.3A, and the power supply voltage and rated power of lamp L3 are found.
8. A "220V1000W" electric heater, energized at rated voltage 15 minutes.
What is the heat generated by (1)?
(2) If the household watt-hour meter is marked with the words "3000r/KWh", how many times has the dial of the watt-hour meter turned during this period?
(3) If 30% of this heat is absorbed by 2.5 kilograms of water at 30℃, how high will the water temperature rise at 1 standard atmospheric pressure? 【 The specific heat of water is 4.2× 103 coke /(kg? 6? 1℃)]
9. A classmate investigated the working condition of the water dispenser at home: the water dispenser has two alternate working states of heating and heat preservation after being filled with water and electrified; The following data comes from the instruction manual of the machine: the rated voltage of the heater is 220V, the rated power is 500W, and the volume of the water container in the machine is 5dm3. The investigation shows that during normal heating, water is not taken from the water dispenser, and the water temperature starts to heat again from 85℃, and stops heating when the water temperature rises to 95℃ after 8 minutes. The simplified circuit of the machine is shown in the figure.
(1) Analysis: When the temperature control switch S is turned off, is the water dispenser in a heating state or a heat preservation state?
(2) Find out the resistance value of the heater resistor R2 and the current during normal heating.
(How much electricity is consumed in the heating process of 8min? How much has the internal energy of water increased in the machine?
Answer:
Question 1:
For example, 1, (1) parallel circuit current relationship, then the current passes through the resistor R2.
(2) The voltage of each branch of the parallel circuit is equal, so the power supply voltage
(3) Starting from Ohm's Law, then
Example 2, according to the meaning of the question, after closing the switch S, the voltmeter indicates 6V and the current indicates 0. 14A, then:
Power supply voltage U=6V Current in the circuit IR=I=0. 14A
The voltage across the resistor r ur = u-ul = 6V-2.5V = 3.5V..
According to ohm's law:
(2) Electric power of small light bulb:
Exercise: 1, 40ω 20ω 0.9A2, 6V 6Ω 6W.