"Application of Ohm's Law in Series and Parallel Circuits" Ohm's Law PPT

"Application of Ohm's Law in Series and Parallel Circuits" Ohm's Law PPT

Part One: Guidance Design

Learning point 1: Series connection of resistors

Question: When multiple resistors are connected in series, their equivalent resistance is called the total resistance of the series circuit. Please prove: As shown in Figure 17-4-7, if two resistors R1 and R2 are connected in series, the relationship between the total resistance R and R1 and R2 is: R=R1+R2.

Solution: Assume the voltage across resistor R1 is U1, the voltage across resistor R2 is U2, the total voltage across resistors R1 and R2 is U, and the total resistance of resistors R1 and R2 is R; since resistors R1 and R2 are connected in series, assume that in the circuit The current is I. Then U1=I1R1, U2=I2R2, U=IR, because U=U1+U2, that is, IR=I1R1+I2R2, because I=I1=I2, so R=R1+R2.

Learning point 2: Parallel connection of resistors

Question: The "equivalence method" is a method commonly used in physics. As shown in Figure 17-4-8, if two resistors in Figure A are connected in parallel and connected to the circuit, the effect will be the same as the effect of one resistor in Figure B connected to the circuit. Please use the rules of voltage and current of the parallel circuit and Ohm's law derives the relationship between R and R1 and R2.

Application of Ohm's Law in Series and Parallel Circuits PPT, Part 2: Summary of Frequent Examinations

Type 1 Resistance relationship between series and parallel circuits

Typical Example 1 (Multiple Choice) When a student is doing an experiment, he needs a 10 Ω resistor, but he only has a few 5 Ω and 20 Ω resistors on hand, then he should ( )

A． Take two 5 Ω resistors and connect them in parallel

B． Take two 5 Ω resistors in series

C. Take two 20 Ω resistors and connect them in parallel

D. Take a resistor of 20 Ω and a resistor of 5 Ω and connect them in parallel.

Type 2 Analysis of changes in electric meter caused by slide movement

Typical Example 2 [Ya'an High School Entrance Examination]In the circuit shown in Figure 17-4-12, the power supply voltage remains unchanged and R1 is a fixed-value resistor. After the switch S is closed, when the slide P of the sliding rheostat moves to the right, which of the following statements is correct ( )

A． The ammeter's indication becomes smaller, and the ratio of the voltmeter's indication to the ammeter's indication becomes larger.

B． The ammeter's indication becomes larger, but the ratio of the voltmeter's indication to the ammeter's indication remains unchanged.

C. The ammeter's indication becomes smaller, but the ratio of the voltmeter's indication to the ammeter's indication remains unchanged.

D. The reading of the voltmeter becomes larger, and the ratio of the reading of the voltmeter to the reading of the ammeter becomes larger.

Type 3 Analysis of changes in electric meter values ​​caused by switch opening and closing

Typical Example 3 [Binzhou High School Entrance Examination] In the circuit shown in Figure 17-4-13, the power supply voltage is constant, switch S1 is closed and S2 is open. To increase the readings of both the voltmeter and the ammeter, the following operations must be possible ()

A． The slider P of the sliding rheostat moves to the right

B． The slider P of the sliding rheostat moves to the left

C. The slider P of the sliding rheostat does not move, and S1 changes from closed to open.

D. The slider P of the sliding rheostat does not move, and S2 changes from open to closed.

[Commentary by Famous Teachers] There are generally three steps to solve such problems: ① First analyze the connection method of the circuit and the measurement objects of each meter; ② Analyze the changes in the circuit caused by the movement of the slide or the closing and opening of the switch, Make it clear whether it is a change in the connection method or a change in the connected resistance; ③ Further analyze how the electrical meter changes.

Type 4 Comprehensive calculation of circuit images

Typical Example 4 [Quanzhou High School Entrance Examination] In the circuit shown in Figure 17-4-14A, the power supply voltage remains unchanged, R0 is a fixed value resistor, and R is a resistance box. After closing the switch S, adjust the resistance value of the resistance box R, and obtain the relationship between the voltage representation number U and the resistance value R of the resistance box as shown in Figure B. Then it can be found that the power supply voltage is _______V and the resistance value of R0 is _______Ω .

[Commentary by Famous Teachers]In the calculation of Ohm's law, equations (groups) are often listed for solution based on the constant power supply voltage or the laws of current and voltage. Finding out the implicit conditions of "unchanged" or "equal" is the key to solving the problem.

Application of Ohm's Law in Series and Parallel Circuits PPT, Part 3: Class Feedback A

1.In a series circuit, the total resistance of the series circuit is equal to ____________________. If the resistance values ​​of the two resistors connected in series are R1 and R2 respectively, and the total resistance after the series connection is R, then R = ________. The series connection of resistors is equivalent to increasing the length of the resistor, so the total resistance of the series resistors is ________ (optional "larger" or "smaller") than any of the partial resistors.

2. Resistors R1 = 10 Ω and R2 = 20 Ω. If they are connected in series in a circuit with a voltage of 3 V, the equivalent resistance after series connection is ________, and the current through R1 is ________.

3. In a parallel circuit, the reciprocal of the total resistance of the parallel circuit is equal to ____________________. If the values ​​of the two resistors connected in parallel are R1 and R2 respectively, and the total resistance after parallel connection is R, then 1/R = ________. The parallel connection of resistors is equivalent to increasing the cross-sectional area of ​​the resistors, so the total resistance of the parallel resistors is _________(optional "larger" or "smaller") than any of the partial resistors.

4. Resistors R1 = 15 Ω and R2 = 30 Ω. If they are connected in parallel at both ends of the power supply with a voltage of 4.5 V, the equivalent resistance after parallel connection is ________, and the current through R2 is ________.

5. When a student is doing an experiment, he only has a few 10 Ω resistors at hand. If he needs a 40 Ω resistor, the method he should take is ____________________. The basis for this method is ____________________.

6． When the following sets of resistors are connected in parallel, the one with the smallest equivalent resistance is ()

A． 30Ω and 15Ω

B． 20Ω and 25Ω

C. 10Ω and 35Ω

D. 5Ω and 40Ω

7． As shown in Figure 19-1, resistors R1 and R2 are connected in series on a power supply with a voltage of 6 V. It is known that the resistance of resistor R1 is 8 Ω, and the current through resistor R2 is 0.25 A. Find the resistance of resistor R2.

Application of Ohm's Law in Series and Parallel Circuits PPT, Part 4: Class Feedback B

1. Resistors R1 = 10 Ω and R2 = 20 Ω. If they are connected in series in a circuit with a voltage of 15 V, then the equivalent resistance R after R1 and R2 are connected in series is ________Ω. The voltage across the resistor R1 is ________. The resistance The voltage across R2 is ________. It can be seen that the series circuit has a voltage dividing effect, and the greater the resistance, the more voltage is divided. That is, the series circuit has a voltage dividing effect, and the divided voltage is proportional to the resistance. For R1 and R2, the formula can be expressed as U1/U2=R1/R2=________, U1/U=R1/R=________, U2/U=R2/R=________.

2. Resistors R1 = 10 Ω and R2 = 15 Ω. If they are connected in series in a circuit with a voltage of 5 V, the ratio of the currents passing through R1 and R2 is ________, the ratio of the voltages across R1 and R2 is ________, and the ratio of the voltages across R1 and R2 is ________. The ratio of terminal voltage to supply voltage is ________.

3. Resistors R1 = 15 Ω, R2 = 30 Ω, connect them in parallel in a circuit with a voltage of 3 V, then the equivalent resistance R after R1 and R2 are connected in parallel is ________Ω, then the current through resistor R1 is ________A, and through the resistor The current in R2 is ________A. It can be seen that the parallel circuit has a shunting effect, and the greater the resistance, the smaller the current divided. That is, the parallel circuit has a shunt effect, and the shunt is inversely proportional to the resistance. For R1 and R2, the formula can be expressed as I1/I2=R2/R1=________, I1/I=R/R1=________, I2/I=R/R2=________.

4. Resistors R1 = 15 Ω and R2 = 30 Ω. If they are connected in parallel in a circuit with a voltage of 4.5 V, then the ratio of the voltages at both ends of R1 and R2 is ________, the current through R2 is ________, and the ratio of the currents through them for________.

5. There is a light bulb with a filament resistance of 7.5 Ω when it emits light normally, and a voltage of 2.5 V when it emits light normally. If we only have a power supply with a voltage of 6 V, how big a resistor needs to be connected in series to make the light bulb glow normally? (Required to draw a schematic diagram of the circuit design and use the ratio method to calculate)

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