"Resistor" Voltage Resistance PPT Teaching Courseware

"Resistor" Voltage Resistance PPT Teaching Courseware

Part One: Guidance Design

Learning point 1: Resistance: the resistance of a conductor to electric current

Experiment 1: Connect conductor R1 to the circuit as shown in Figure 16-3-10, close switch S, read the ammeter indication at this time, and record it as I1.

Experiment 2: Remove R1 in the circuit shown in the figure, connect it to R2, close the switch S, read the current reading of the ammeter, and record it as I2. The experiment found that I1≠I2.

Question 1: In physical experiments, quantities that are difficult to observe are often converted into quantities that are easily observed. This method is called the conversion method. In the above experiment, the resistance of the conductor to the current is converted into ___________ for measurement.

Question 2: Under the same voltage, different conductors impede current flow _________.

Learning point 2 Factors affecting the size of resistance

experimental design:

Question 1: (1) How to "display" the resistance of a conductor in an experiment? Please put a "√" in the () under the equipment you selected.

(2) After properly selecting the equipment provided on the experimental table, please complete the circuit diagram below.

Question 2: Since there are many factors that affect the resistance, the experimental process must use ____________ (fill in a research method). Please complete the form below. (Warm reminder: If you think the table is not enough, you can add it; if you think the table design is unreasonable, you can design it yourself)

Resistor PPT, Part 2: Application Examples

Type 1 Resistor

Example 1 Regarding the resistance of a conductor, which of the following statements is correct ( )

A． There is a current in a conductor, and the conductor has resistance.

B． The resistance of a conductor depends on the amount of current flowing through it

C. The resistance of a conductor is a property of the conductor itself and has nothing to do with the current passing through it

D. The greater the resistance of the conductor, the stronger the conductive ability of the conductor

[ Analysis When the current is flowing, its blocking effect on the current is not reflected, so options A and B are incorrect, and option C is correct. The greater the resistance of a conductor, the weaker its ability to conduct electricity.

[Error-prone warning]When understanding the physical meaning of resistance, do not think that since resistance is the obstruction of current by a conductor, when there is no current in the conductor, the conductor has no resistance. Resistance is a physical quantity that represents the resistance of a conductor to electric current. It is a property of the conductor itself. When the length, material, cross-sectional area and temperature of a conductor are determined, its resistance is constant. The size of the resistance has nothing to do with whether the conductor is connected to the circuit, whether there is voltage across the conductor, or whether there is current flowing through the conductor.

Type 2: Explore factors that affect conductor resistance

Example 2 When exploring "factors affecting the resistance of conductors", two students, Xiaobing and Xiaohong, made the following conjectures:

Conjecture ①: The resistance of the conductor may be related to the length of the conductor.

Conjecture ②: The resistance of a conductor may be related to the cross-sectional area of ​​the conductor.

Conjecture ③: The resistance of the conductor may be related to the material of the conductor.

The laboratory provided four resistance wires. The materials and specifications are as shown in the table below:

In order to verify the above conjecture, they designed the experimental circuit shown in Figure 16-3-3.

(1) In order to verify the conjecture ①, two resistance wires numbered ________ should be selected for the experiment; if two resistance wires numbered A and C are selected for the experiment, it is to verify the conjecture ________ (fill in the serial number); if Two resistance wires numbered A and D were selected for the experiment in order to verify the conjecture ________ (fill in the serial number). When two resistance wires A and D are connected between points M and N in the circuit, the current values ​​are different. From this, a preliminary conclusion can be drawn: when the length and cross-sectional area are the same, the resistance of the conductor is the same as the _______ of the conductor. _related.

(2) Some students believe that the change in resistance of the connected resistance wire can be judged based on the change in brightness of the light bulb. The teacher pointed out: This method is not advisable in this experiment. This is because when the power supply voltage is constant, the resistance value difference of the selected resistance wires is too small, and the brightness of the bulb changes ________ (please fill in "obvious" or "not obvious").

(3)The experimental methods used in this research experiment are ________. (optional letter)

A． Equivalent substitution method B. conversion method

C. Amplification method D. Control Variable

[ Analysis The size of the resistance is indirectly reflected by the brightness of the small light bulb or the size of the current representation. This method of studying the problem is the conversion method.

[Experimental Points]Three things to pay attention to, correctly answer the experimental questions on the factors that affect the resistance of the conductor

(1) When it comes to conjectures, pay attention to the fact that the conjecture must be comprehensive, that is, the factors that affect the resistance of the conductor include the material, length and cross-sectional area of ​​the conductor. (Generally, the temperature is considered to be constant)

(2) The two scientific research methods used in this experiment are the conversion method and the control variable method.

①Conversion method: When the power supply voltage is constant, the resistance of the conductor is judged by the size of the current indication number or the brightness of the bulb. The smaller the current indication number, the darker the bulb, and the greater the resistance of the conductor connected to the circuit.

②Controlled variable method: There are many factors that affect the resistance. When exploring the impact of a certain factor on the resistance, other factors need to be controlled unchanged to obtain the relationship between the resistance and this factor.

(3) When designing experiments or designing experimental steps according to requirements, pay attention to the correct use of the control variable method.

Type 3 Comprehensive comparison of conductor resistance

Example 3 Regarding resistance, which of the following statements is correct ( )

A． The resistance of an iron wire must be greater than that of a copper wire

B． If wires made of the same material have the same cross-sectional area, the resistance of the long wire will be greater at the same temperature.

C. Two wires of the same length, the resistance of the thin wire must be greater

D. Two wires of the same thickness, the longer wire must have greater resistance

[ Analysis Compare, so option A is wrong; the materials of the two wires are unknown and the lengths are the same. The resistance of the thin wire is not necessarily large, so option C is wrong. Similarly, for two wires of the same thickness, the resistance of the long wire is not necessarily large, so Option D is wrong.

[Method guidance]Comprehensive consideration, comprehensive analysis, and correct comparison of conductor resistances

(1) When the temperature remains constant, the factors that affect the resistance of a conductor include the material, length and cross-sectional area of ​​the conductor.

(2) When judging or comparing the size of resistance, the influence of various factors must be comprehensively considered. For example, when a conductor is evenly stretched to 2 times its original length, the length becomes 2 times the original, and the cross-sectional area becomes half of the original. Both the length and the cross-sectional area affect the size of the resistance.

Therefore, when doing specific analysis, we must consider it comprehensively and avoid focusing on one thing at the expense of the other and making a one-sided analysis.

(3) In junior high school physics, the effect of temperature on conductor resistance is generally not considered. Therefore, if temperature is not specifically mentioned in the question, there is no need to consider the effect of temperature on resistance.

Resistance PPT, Part 3: Class Feedback

1. Resistance is the _________ effect of the conductor on the current. The factors that determine the resistance include _________, _________ and ____________ of the conductor; the resistance of the conductor is also related to _________.

2. Complete the following unit conversions.

(1)0.5 MΩ=__________kΩ=__________Ω.

(2)560 kΩ=________MΩ=__________Ω.

3. As shown in Figure 14-1, A and B are two keys made of different metals. Connect them between A and B in the figure respectively. Close the switch. It is found that when key A is connected to the circuit, the light bulb is brighter, it can be seen that the resistance of Key A and Key B is ________ (optional "A" or "B"). When key A is not connected to the circuit, its resistance is ________ (optional "Unchanged" or "B") "get smaller", regardless of the effect of temperature on resistance).

[ Analysis , the resistance of key B is larger. The resistance of a conductor is a property of the conductor itself and has nothing to do with whether it is connected to a circuit.

4. As shown in Figure 14-2, the resistance of a conductor not only depends on factors such as the length, cross-sectional area and material of the conductor, but also is affected by temperature. As shown in the figure, when the tungsten filament is heated, it can be observed that the indication of the ammeter changes to ________, and the brightness of the small bulb changes to ________. This shows that when the temperature of a general metal conductor increases, its resistance changes to ________.

5. Normally, regarding the resistance of a piece of nichrome wire of uniform thickness, which of the following statements is correct ( )

A． The resistance of the alloy wire has nothing to do with the length of the alloy wire

B． The resistance of the alloy wire is related to the cross-sectional area of ​​the alloy wire

C. The greater the voltage across the alloy wire, the smaller the resistance of the alloy wire

D. The smaller the current passing through the alloy wire, the greater the resistance of the alloy wire

6. Regarding the resistance of a conductor, there are the following statements, the correct one is ( )

A． Two resistance wires made of the same material, the thinner one must have greater resistance

B． Two resistance wires with the same thickness, the longer one must have greater resistance

C. Two resistor wires of the same length have the same resistance

D. Copper wires and aluminum wires of the same length and thickness have unequal resistances

[ Analysis When the material and length are constant, the larger the conductor cross-sectional area, the smaller the conductor resistance, so the three options A, B, and C are incorrect. Copper wires and aluminum wires have the same length, same cross-sectional area, different materials, and different resistances. Option D is correct.

7． When exploring the factors that determine the size of resistance, three students in the research group, A, B, and C, made the following conjecture:

A: The resistance of a conductor is related to the length of the conductor.

B: The resistance of a conductor is related to the material of the conductor.

C: The resistance of a conductor is related to the cross-sectional area of ​​the conductor.

There are several kinds of resistance wires available in the laboratory, and the parameters are as follows.

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