"Ohm's Law" Ohm's Law PPT download (Lesson 1)

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"Ohm's Law" Ohm's Law PPT download (Lesson 1)

Part One: Knowledge Management

1. Ohm's law

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

Formula: I=U/R. I—current—ampere (A), U—voltage—volt (V), R—resistance—ohm (Ω).

Note: The three quantities of current, voltage and resistance in Ohm's law are for the same section of conductor (or the same section of circuit) at the same moment (or the same period of time), that is, Ohm's law has identity and simultaneity.

2. Application of Ohm's Law

Calculate current: I=U/R.

Calculate the voltage: U = IR.

Calculate the resistance: R=U/I. This is the definition of resistance. The resistance of a conductor is a property of the conductor itself. It is determined by the material, length and cross-sectional area of ​​the conductor. It has nothing to do with voltage or current. When the voltage U=0 at both ends of the conductor, the resistance R≠0 .

Ohm's Law PPT, Part 2: Classification and Research

Type One: Understanding Ohm’s Law

[2017•Yunnan Qujing]Which of the following statements about Ohm’s law I=U/R is wrong (C)

A． It can be seen from I=U/R that when the conductor resistance R is constant, the current I in the conductor is proportional to the voltage U at both ends.

B． It can be seen from I=U/R that when the voltage U at both ends of the conductor is constant, the current I in the conductor is inversely proportional to the resistance R

C. From the deformation of I=U/R, we can get R=U/I, which means that the resistance of a conductor is directly proportional to the voltage across it and inversely proportional to the current passing through it.

D. From the deformation of I=U/R, we can get R=U/I, which means that the resistance R of the conductor is numerically equal to the ratio of U and I. However, since the resistance is an attribute of the conductor itself, its size has nothing to do with U and I.

[Method Summary] (1) The current in a conductor and the voltage at both ends of the conductor are for the same conductor. They are the current and voltage of the same conductor measured with an ammeter and a voltmeter. The current in one conductor cannot be compared with the current in another conductor. Comparing the voltage across a conductor.

(2) When describing, we must pay attention to the conditions for the establishment of these two relationships: the prerequisite for "current is proportional to voltage" is that the resistance is constant; the prerequisite for "current is inversely proportional to resistance" is that the voltage is constant.

Type 2: Simple application of Ohm’s law formula and deformation formula

[2017•Mianyang, Sichuan]The standards for drinking and driving and drunk driving stipulated in our country are as follows. As shown in Figure A, a breath alcohol detector is used for on-site testing of drivers. It is used to detect the breath alcohol concentration of drivers. The relationship between the alcohol concentration in the breath and the alcohol concentration in the blood is blood alcohol concentration X = breath alcohol concentration y × 2 200.

The alcohol tester consists of an alcohol gas sensor (equivalent to a rheostat that changes with the breath alcohol concentration), a fixed-value resistor and a voltmeter. Figure B is its schematic diagram. The power supply voltage is 1.5 V, and the fixed-value resistor R1 = 40 Ω, the relationship between the resistance value R2 of the sensor and the breath alcohol concentration y is shown in Figure C. During the test, press the switch. After the instrument becomes normal, the subject holds the air tube tightly in his mouth and blows for 4 seconds, and the tester displays the result.

(1) A driver was tested and the voltage indicated was 0.5 V. What type of driver test result is this?

(2) If the subject blows air without holding the blow tube tightly in his mouth during the test, please analyze the impact on the test results based on the relationship between fluid pressure and flow rate.

Ohm's Law PPT, Part 3: Assessment in class

1. [2018•Beijing] Regarding the understanding of Ohm’s law, which of the following statements is correct (C)

A． The formula R=UI shows that resistance is directly proportional to voltage and inversely proportional to current.

B． The formula I=UR shows that the voltage across a conductor is proportional to the current passing through it.

C. For a fixed-value resistor, the ratio of the voltage across it to the current flowing through it remains constant.

D. It can be seen from U=IR that the voltage across the conductor is proportional to the resistance of the conductor.

[Analysis] Resistance is a property of the conductor itself. It is only related to the material, length, cross-sectional area and temperature of the conductor. It has nothing to do with the voltage at both ends of the conductor and the current passing through it. The formula R=UI is just a method of calculating resistance. Therefore, statement A is wrong; the formula I=UR shows that: when the resistance is constant, the current in the conductor is proportional to the voltage at both ends of the conductor, so statement B is wrong; for the same fixed-value resistor, when the resistance is constant, the voltage at both ends of the conductor increases Several times, the current passing through it also increases several times, and the ratio of voltage to current remains unchanged, so statement C is correct; the voltage at both ends of the conductor depends on the power supply, and it cannot be said that the voltage at both ends of the conductor is proportional to the resistance of the conductor, so statement D mistake.

2. [2019•Ningxia]As shown in the figure, the ratio of the filament resistances of lamps L1 and L2 is 1:2, then the ratio of the readings of ammeters A1 and A2 is (　B　)

A． 2:3 B． 3:1

C. 1:2 D. 2:1

[Analysis] In the circuit, two lamps are connected in parallel. Ammeter A2 measures the current through lamp L2, and ammeter A1 measures the total current of the circuit: the voltages of each branch of the parallel circuit are equal, so the voltages of the two lamps are equal. According to I=UR, in the voltage When equal, the current and resistance are inversely proportional, so the ratio of the currents passing through L1 and L2 is I1:I2=R2:R1=2:1; assuming the current passing through L2 is I, then the current passing through L1 is 2I. According to the parallel circuit According to the law of current, the indication of ammeter A1 is I+2I=3I, so the ratio of the indications of ammeter A1 and A2 is 3I:I=3:1.

3. [2017•Guangxi Hechi]In the circuit as shown in the figure, the power supply voltage is 6 V, R1=3 Ω, and R2=6 Ω. Close switch S. Which of the following statements is correct (C)

A． The current through R1 is 1 A

B． The voltage across R1 is 2 V

C. The current in the main circuit is 3 A

D. The total resistance of the circuit is 9 Ω

Ohm's Law PPT, Part 4: Layered Operations

1. [2019•Shandong Linyi] Regarding the relationship between current, voltage and resistance, which of the following statements is correct (B)

A． When the voltage is constant, the resistance of a conductor is inversely proportional to the current flowing through the conductor.

B． When the resistance is constant, the current through the conductor is proportional to the voltage across the conductor.

C. When the current is constant, the resistance of the conductor is proportional to the voltage across the conductor.

D. The voltage across a conductor is directly proportional to the current through the conductor and the resistance of the conductor.

2. [2019•Hubei Suizhou]As shown in the figure, R1 and R2 are connected in parallel in a circuit with a power supply voltage of 3 V. When only switch S1 is closed, the ammeter reads 0.3 A; switches S1 and S1 are closed at the same time. At S2, the ammeter indicates 0.5 A, then (B)

A． The resistance of R1 is 60 Ω

B． The resistance of R2 is 15 Ω

C. The current through R1 is 0.5 A

D. The current through R2 is 0.3 A

3. [2018•Chengdu, Sichuan]As shown in the figure, the power supply voltage is constant, R1=20 Ω, switch S1 is closed, switch S2 is opened, and the current value is 0.3 A. If S2 is closed again, it is found that the current indication has changed by 0.15 A, then the resistance of R2 is 40 Ω.

4. [2018•Sichuan]When a voltage of 4 V is applied to both ends of a conductor, the current passing through it is 0.5 A, and the resistance of this conductor is 8 Ω; when the current passing through it is 1.5 A, the resistance at both ends of it The voltage is 12 V.

5. [2018•Hebei]In the circuit as shown in the figure, the resistance of the fixed-value resistor R is 10 Ω, and the resistance of the small light bulb is 5 Ω. After closing the switch, the ammeter indicates 0.6 A . beg:

(1)Power supply voltage.

(2) The current passing through the small light bulb.

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Update Time: 2024-07-30

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