"Application of Ohm's Law" Ohm's Law PPT Courseware 2

Description

"Application of Ohm's Law" Ohm's Law PPT Courseware 2

The relationship between current, voltage and resistance:

When the resistance R is constant, the current I is proportional to the voltage U.

When the voltage U is constant, the current I is inversely proportional to the resistance R.

Ohm's law content:

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

Explanation: Ohm's law reveals the relationship between current and voltage; current and resistance.

The three physical quantities correspond to the same moment and the same conductor (or the same section of circuit).

The formula for Ohm's law:

I＝U/R

Unit: I(A) U(V) R(Ω)

Transformation formula:

U=IR R=U/I

Application Law: Apply the relationship between current and voltage resistance in a circuit

Example 1:

①Draw a picture according to the meaning of the question;

② Mark the relevant physical quantities on the diagram;

③When answering the question, write down: the known and solution processes and results (all physical quantities must be in unified international units)

④After completing the questions, analyze their practical significance.

Example 2: In an experiment, the voltage of an unknown resistor was measured to be 4.8V, and the current flowing through it was 320mA. Find the resistance of the resistor.

Notice:

(1) Of the three physical quantities in the Ohm's law formula, only by knowing two of them can the third one be found.

(2) The three physical quantities in the formula must be for the same conductor at the same time.

(3) If U and R are found, the formula should be transformed into U=IR, R=U/I

Solution: R=U/I=4.8V/0.32A=15 

Method of measuring resistance: From the formula R=U/I, we can find that to find the resistance of a conductor, we only need to use a voltmeter and an ammeter to measure the voltage and current of the conductor respectively, and then we can find the resistance of the conductor. . This method of measuring resistance is called "voltammetry".

Understanding the formula R=U/I: It is just a calculation formula, indicating that the resistance of a conductor is numerically equal to the ratio of the voltage at both ends of the conductor to the current flowing through the conductor. It cannot be simply understood as a direct ratio or an inverse ratio (it has no physical meaning!!).

Example 3: There is an indicator light with a resistance of 6.3Ω and a current of 0.45A that can illuminate normally. How much voltage should be applied?

Understanding of the formula U=IR: It is just a numerical relationship. Voltage is not directly proportional to current and resistance. In fact, voltage is the cause of current generation. When no voltage is applied to both ends of a conductor, the current is zero, but the resistance of the conductor is not zero.

Solution: U=IR= 0.45A× 6.3Ω=2.835V

Challenge yourself

1. There is a resistor. When a voltage of 4V is applied to both ends of the resistor, the current through the resistor is 2A. If the voltage is changed to 10V, what is the current through the resistor?

Problem breakdown:

① There is a resistor. When a voltage of 4V is applied to both ends of it, the current through the resistor is 2A. What is the value of this resistor?

Solution: R=U/I=4V/2A=2 

② Change the voltage of the resistor in ① to 10 V. What should its current be?

How else can you answer it?

Solution: I=U/R=10V/2Ω=5A

2. When a certain voltage U is applied to both ends of a 10Ω resistor, the current flowing through it is 2A. If this voltage is applied to both ends of a 20Ω resistor, how much should the current be?

Problem breakdown:

① When a certain voltage U is applied to both ends of a 10Ω resistor, the current passing through it is 2A. What should the voltage U be?

Solution: U=IR= 2A×10Ω=20V

② If the voltage in ① is applied to the 20Ω resistor, what should its current be?

Solution: I=U/R=20V/20 Ω=1A

How else can you answer it?

Application 1. Given voltage and resistance, find the current

The voltage of the lathe lighting is 36V, and the resistance of the filament is 30Ω when it works normally. Find the current through the filament.

Problem solving steps:

(1) Draw a circuit diagram

(2) List the known conditions and required quantities

(3) Solve

Application 2: Given resistance and current, find voltage

In the circuit as shown in the figure, adjust the sliding rheostat R' to make the bulb light normally, and use an ammeter to measure the current value passing through it to be 0.6A. It is known that the resistance of the bulb when it is normally emitting light is 20Ω. Find the voltage across the bulb.

Solution: From I=U/R we get:

U=IR=0.6A×20Ω=12V

Answer: The voltage across the bulb is 12V.

summary

Calculation steps using Ohm's law:

(1) Draw the equivalent circuit diagram

(2) Mark the known conditions on the diagram, and use the same unit (including directly known quantities and indirect known quantities using the current, voltage, and resistance relationships of series and parallel circuits) and the required quantity.

(3) According to Ohm's law, the indirect known quantities are marked for calculation based on the current, voltage, and resistance relationships of series and parallel circuits (the same conductor at the same time)

Format:

(1) "Poetry line" format, that is, left-end alignment

(2) Write formulas, generate data, and get results ("three steps")

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Update Time: 2024-07-30

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