"Application of Ohm's Law in Series and Parallel Circuits" Ohm's Law PPT Courseware 3

Description

"Application of Ohm's Law in Series and Parallel Circuits" Ohm's Law PPT Courseware 3

1. Review

1. Ohm's law

The current in a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor.

2. Current and voltage rules in series circuits:

(1) The currents everywhere in the series circuit are equal; I=I1=I2=…=In

(2) The total voltage in the series circuit is equal to the sum of the voltages of each part of the circuit. U=U1+U2+…+Un

3. Current and voltage rules in parallel circuits:

(1) The trunk current in a parallel circuit is equal to the sum of the branch currents; I=I1+I2+…+In

(2) The voltages at both ends of each branch in a parallel circuit are equal. U=U1=U2=…=Un

Derivation of series and parallel resistance formulas

1. Series circuit

series connection

I=I1=I2 U=U1+U2

U=IR--IR=I1R1+I2R2--R=R1+R2

The total resistance of a series circuit is equal to the sum of the resistances of all conductors. That is: R total=R1＋R2+···＋Rn

If there are n resistors with resistance R in series, the total resistance is R series = nR

2. Parallel circuit

Parallel connection--U=U1=U2--I=I1+I2--I=U/R--U/R=U/R1+U/R2--1/R=1/R1+1/R2

The reciprocal of the total resistance of a parallel circuit is equal to the sum of the reciprocals of the resistances of the parallel conductors.

If there are n resistors all with resistance R connected in parallel, the total resistance is R total = R/n

The essence of resistors in series and parallel:

The total resistance of a series circuit is greater than the resistance of its parts because resistance in series increases the length of the conductor.

The total resistance of the parallel circuit is less than the resistance of each branch, because the resistance in parallel is equivalent to increasing the cross-sectional area of ​​the conductor.

The larger the resistor is, the larger it is.

The smaller the resistance is, the smaller it is.

2. Use Ohm’s Law to solve problems

1. There are two resistors, R1=3 ohms and R2=6 ohms. If they are connected in series and connected to a power supply with a voltage of 6 volts, the ratio of the voltages across the two resistors is U1:U2=______. If they are connected in parallel Then connected to the same power supply, the ratio of the currents passing through them is I1:I2=______.

2. Two fixed value resistors R1: R2 = 2: 3, connect them in parallel to a power supply, the ratio of the currents passing through them I1: I2 is _____, and the ratio of the voltages across them U1: U2 is _____.

3. If two resistors with exactly the same resistance value are connected in parallel and the total resistance is 10 ohms, then the total resistance of connecting them in series is [ ]

A. 5 euros B. 10 euros C． 20 euros D. 40 euros

4. There are 4 resistors, the resistance values ​​are all R. If they are connected in parallel, the total resistance is [ ]

A. 4RB. R/4　C. 4/R　D. 2R

Example question analysis

Example 1 As shown in the figure, it is known that I=4.5 A, I2=0.9 A, and the power supply voltage is 36 V. How big is the resistance R1 of the lamp L1?

Analysis:

∵ Lamp L1 and L2 are connected in parallel

∴ I = I1 + I2

I1 = I - I2 = 4.5 A -0.9 A = 3.6 A

And U1 = U2 =U= 36 V

∴R1=U1/I1=36 V/3.6 A= 10 Ω

Experimental steps

(1) Connect the circuit according to the circuit diagram and set the voltmeter pointer to zero;

(2) Close switch S and measure the voltage across Rx to be Ux;

(3) Open the switch S, connect the voltmeter to both ends of R0, close the switch and measure the voltage U0 across R0;

(4) Turn off the switch and tidy up the experimental equipment.

Class summary

Steps to solve circuit calculation problems

(1) Analyze the connection methods between resistors in each circuit state according to the meaning of the question, and draw the equivalent circuit diagram.

(2) By reviewing the questions, clarify the known conditions and unknown quantities given in the questions, and mark the symbols, values ​​and units of the known quantities and the symbols of the unknown quantities on the circuit diagram.

(3) Each step of the solution process must include three steps: writing the formula - substituting values ​​and units - obtaining the result.

1. In the circuit as shown in the figure, the power supply voltage remains unchanged, R3 = 5 Ω, when the switches S1 and S2 are both open, the ammeter indicates 0.6A, and the voltage indicates 6V. When S1 and S2 are both closed. , the current expressed number is 1A, what are the resistance values ​​of R1 and R2?

2. There are two light bulbs connected in series in the circuit. The power supply voltage is 12V. The voltage across bulb L1 is 9V. The total resistance in the circuit is 30 ohms. Find:

(1) The voltage across the bulb L2

(2) Current in the circuit

(3) Resistance of two light bulbs

3. In the circuit as shown in the figure, R1=7Ω, R2=3Ω, when S is closed, the voltmeter indicates 21V, and when S is disconnected, the voltmeter indicates 14V. Find the resistance of R3.

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Update Time: 2024-07-29

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