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

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

teaching objectives.

1. Know how to measure resistance by voltammetry.

2. Be able to analyze the hazards of short circuit.

3. Be able to use Ohm’s law to perform simple calculations

There is a resistor with unknown resistance in the laboratory. How to measure its resistance?

Pass a current through the resistor, measure the current value and the voltage across the resistor, and apply Ohm's law to find the value of Rx. (This is an indirect measurement)

This method is called voltammetry

1. Use voltammetry to measure the resistance of a small light bulb when it is working

Experimental principle: Deformation formula according to Ohm's law: R=U/I

Experimental equipment: DC power supply (dry cell), ammeter, voltmeter, switch, small light bulb, wire, sliding rheostat

Design experimental circuit diagram:

The voltage across the bulb cannot exceed the voltage value marked on the bulb

Experimental steps:

1. Design and draw the circuit diagram.

2. Disconnect the switch and connect the circuit according to the circuit diagram. Before closing the switch, place the sliding blade of the sliding rheostat at the position with the highest resistance.

3. After checking that everything is correct, close the switch and connect the circuit.

4. Change the resistance value of the sliding rheostat several times, read out the two numbers respectively, and fill them in the record form one by one; turn off the switch immediately after the experiment.

5. Organize experimental equipment.

2. Measure the resistance of the small light bulb when it is working

Student Activities:

1. What is the difference between the measured resistance values ​​of light bulbs at lower voltages and higher voltages? Can the average value of the three resistances be used as the resistance of the small light bulb? Why?

The resistance is smaller at lower voltages and larger at higher voltages. Why?

cannot. Because the temperature of the filament rises when it emits light, the resistance varies greatly. The value measured each time is the resistance value of the filament at different temperatures, so the average value cannot be taken.

Discussion: If the resistance of an ordinary resistor is measured, can the experimental error be reduced by averaging multiple measurements?

2. (1) If the U-I diagram of a fixed-value resistor is drawn using the point drawing method, it should be a ______ line. Why?

Because when the resistance remains constant, the current is proportional to the voltage.

(2) Use the point drawing method to draw the U-I diagram of the small light bulb. Will it be a straight line? Why?

no.

Because the resistance of the filament increases with temperature, the relationship between current and voltage is not proportional.

The principle of measuring resistance by voltammetry: RX=UX/IX.

Note that the three quantities U, I, and R must be in the same section of the circuit. Since the ammeter and voltmeter have internal resistance, errors will inevitably occur.

1. Internal connection method of ammeter

IA=IX UV=UX+UA＞UX

R=UV／IA=RX+RA＞RX=UX/IX

Summary of the internal connection method: the current is the true value, the voltage is larger, the measured value is greater than the true value,

When RX ＞ ＞RA, Rmeasure≈RX

2. External ammeter connection method:

UV=UX IA=IX+IV ＞IX

R test = UV／IA = RX · RV／（RX+ RV）＜RX = UX/IX

Summary of external connection method: the voltage is the real value, the current is too large, and the measured value is smaller than the real value

When RX＜RV, Rmeasure≈RX

Measuring resistance using voltammetry

Precautions

1. When connecting the circuit, the switch should ____. Before closing the switch, the slider of the sliding rheostat should move to the ________ position.

2. The function of sliding rheostat

(1) Protection circuit;

(2) Change the current and voltage in the circuit to facilitate multiple measurements and average values ​​to reduce errors.

Experimental data recording

The above table is a student’s experimental record, then the resistance of the resistor being measured should be _________

Analyzing and comparing the above experimental data, what other conclusions can you draw?

When the resistance is held constant, the current and voltage are proportional.

Homework and preview

1. Homework 1.2 .3

Preview: What is electric energy and the use of electric energy meters?

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"Application of Ohm's Law" Exploring Ohm's Law PPT Courseware 3 1. Measure resistance by voltammetry 1. Principle: Ohm's law I=U/R deformation formula R=U/I 2. Equipment: power supply, switch, voltmeter, ammeter, sliding rheostat, small light bulb and some wires. 3. experiment..

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

"Application of Ohm's Law" Exploring Ohm's Law PPT Courseware 2 Learning Objectives 1. Learn to measure resistance using voltammetry. 2. Be able to use Ohm's law to analyze practical problems such as the hazards of short circuits. 3. By using Ohm's law measurement and analysis and discussion, we know that the resistance of the filament varies with...

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Update Time: 2024-07-23

This template belongs to Physics courseware Guangdong and Shanghai Edition Ninth Grade Physics Volume 1 industry PPT template

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