"Exploring Ohm's Law" PPT courseware 4

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"Exploring Ohm's Law" PPT courseware 4

Thinking: Is there any way to make the light bulb in the circuit darker?

1. Can save one dry battery

2. You can add a resistor to the circuit

The same light bulbs but different brightness levels indicate that the current flowing through them is different; so what factors are related to the current?

Conjectures and Assumptions

1. It is related to voltage; the greater the voltage, the greater the current.

2. It is related to resistance; the greater the resistance, the smaller the current.

What is the relationship between the three? Is our guess correct? If we know the voltage and resistance in a certain circuit, can we find the current flowing through it?

design experiment

Explore the relationship between the current flowing through an electrical appliance, its resistance, and the voltage across it.

Use the control variable method.

specific method:

a. Keep __ unchanged, change __, and study the relationship between current and voltage.

B. Keep __ unchanged, change __, and study the relationship between current and resistance.

experiment one. When the resistance is 5Ω, the relationship between current and voltage

Conclusion 1 Note: Simulation experiments with moving slides can also be performed

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

Conclusion 2 Note: Simulation experiments with moving slides can also be performed

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

2. Ohm’s Law

1. Content: The current in a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor. This law is called Ohm's law.

2.Formula:

I＝U/R Unit: I（A） U（V） R（Ω）

Note: The current in the conductor and the voltage across the conductor here are for the same conductor. It cannot be said that the current in one conductor is proportional to the voltage across the other conductor, nor can it be said that the current of one conductor is proportional to the resistance of the other conductor. Inversely proportional.

Pay attention to the understanding of this relationship

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

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

1. It cannot be said the other way around. When the resistance is constant, the voltage is proportional to the current. There is a causal relationship here. Voltage is the cause and current is the effect. Because there is voltage at both ends of the conductor, there is current in the conductor. It is not because there is current in the conductor first and then voltage is added. The causal relationship cannot be reversed.

2. It cannot be said that the resistance of a conductor is inversely proportional to the current flowing through it when the voltage is constant. Because resistance is a property of the conductor itself, its size is determined by the conductor itself and has nothing to do with the current passing through the conductor.

3. Classroom exercises

1. Based on the fact that the formula R=U/I is similar in form to the formula I=U/R, some people say that the resistance R is proportional to the voltage and inversely proportional to the current. Do you think this statement is correct? Why?

Answer: No. Because resistance is a property of the conductor itself, it is only related to the material, length, cross-sectional area and temperature of the conductor, and has nothing to do with current or voltage.

2. When the voltage applied to both ends of a conductor is 4V, the current passing through it is 0.25A, then the resistance of the conductor is ______Ω; when the voltage applied to both ends is doubled, the resistance of the conductor is _ _____ Ω, the current passing through the conductor is _______A.

Example (1)

The voltage of the lathe lighting is 36V, and the resistance of the filament is 32 Ω when it is working normally. What is the current passing through the filament?

(1) Draw the circuit diagram

(2) List the known conditions

(3) Solve:

Solution: According to I = U/R=36v/32 Ω=1.13A

Answer: When the lighting lamp is working normally, the current passing through the filament is about 1.13A.

Example (2)

Known: I=0.3A, R=8.3Ω

Request: U.

Solution: According to I=U/R, we get U=IR and substitute the data into it, we can get

U=0.3A×8.3Ω=2.49V.

Answer: When the bulb is working normally, the voltage across the bulb is 2.49V.

Operation

(1), Self-evaluation and assignments 1, 2, 3, 4 on page 39 of the textbook.

(2) How to use the knowledge and instruments you have learned to measure an unknown resistance value? Please tell me the experimental plan.

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Update Time: 2024-10-05

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