"Ohm's Law" PPT courseware 4

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"Ohm's Law" PPT courseware 4

review

In the first section on the relationship between current, voltage and resistance we learned:

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

Please fill in the blanks in the form with the correct data:

Summarize

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

When the voltage across a conductor is constant, the current in the conductor is inversely proportional to the resistance of the conductor.

1. Ohm’s Law

1. Contents of 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. Mathematical expression of Ohm’s law:

I =U/R

3. Use international units in the formula (units must be unified)

Voltage: U --Unit: Volt (V)

Resistance: R --Unit: Ohm (Ω)

Current: I --Unit: Ampere (A)

4. Deformation formula: U=I R ​​R=U/ I

5. Note: Ohm's law reflects the relationship between I, U, and R in the same circuit at the same time.

2. Apply Ohm’s Law

1. General steps for calculations using formulas:

(1) Read and review the questions (pay attention to the content of known quantities);

(2) Draw a circuit diagram based on the meaning of the question;

(3) Mark the symbols and numerical values ​​of known quantities and the symbols of unknown quantities on the diagram;

(4) Use physical formulas for calculation (the writing format must be complete and standardized).

2. Example question analysis

Example 1 A car lamp has a filament resistance of 30 Ω and is connected to both ends of a 12 V power supply. Find the current flowing through the lamp.

Problem solving steps

(1) Draw a circuit diagram;

(2) List the known conditions and required quantities;

(3) Solve I.

Example 2 In the circuit as shown in the figure, adjust the sliding rheostat R' to make the light bulb emit light normally, and use an ammeter to measure the current value passing through it to be 0.6 A. It is known that the resistance of the bulb when it is normally emitting light is 20 Ω. Find the voltage across the bulb.

Solution: IL=I=0.6 A

UL=ILRL =0.6 A×20 Ω =12 V

Class summary

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

Deformation formula: U=I R ​​R=U/ I

Class exercises

1. (2013 Chongqing) Scientists have strong curiosity and persistent spirit of inquiry. German physicist _____ was the first to summarize the quantitative relationship between current, voltage and resistance in a conductor through experiments, namely: _____. In order to commemorate his outstanding contributions, people named the unit _____ after him.

2. Regarding the relationship between current, voltage and resistance, which of the following statements is correct ( )

A. The greater the resistance of a conductor, the smaller the current flowing through the conductor

B． The greater the resistance of a conductor, the greater the current through the conductor

C. The greater the voltage across a conductor, the greater the resistance of the conductor and the greater the current through the conductor

D. When the voltage across the conductor is constant, the smaller the resistance of the conductor, the greater the current flowing through the conductor.

3. When a voltage of 4 V is applied to both ends of a nickel-chromium alloy wire, the current flowing through it is 0.2 A, then its resistance is _____Ω. If the voltage at both ends of the alloy wire increases to 16 V, its resistance is _____Ω. If you want to use an ammeter to measure its current, you should use an ammeter with a range of _____A.

4. The ratio of the two resistors A and B is 2:1. When they are connected in series in the circuit, the ratio of the voltages at both ends of them is ()

A. 1:2 b. 2:1

C. 1:1D． Can't compare

Homework assignment

1. Textbook: P79 Questions 1 and 2

2. "Synchronous Training": Corresponding exercises in this section

3. The resistance of an electric iron is 80Ω and it is connected to a voltage of 220V. What is the current flowing through it?

4. The resistance of a fixed-value resistor is 10 Ω, and the current flowing through it during use is 200mA. What is the voltage applied to both ends of the fixed-value resistor?

5. The voltage at both ends of a small light bulb is 2.5V when it is working. The current measured by an ammeter is 300mA. What is the resistance of this light bulb when it is working?

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Update Time: 2024-09-18

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