"Measurement of Resistance" Ohm's Law PPT teaching courseware

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"Measurement of Resistance" Ohm's Law PPT teaching courseware

Part One: Knowledge Management

1. Measure resistance by voltammetry

Equipment: ammeter, voltmeter, sliding rheostat, switch, power supply, resistance to be measured and wires.

Measurement principle: R=U/I

Measurement method: The circuit is as shown in the figure. Use a voltmeter to measure the voltage U at both ends of the resistor Rx to be measured, and use an ammeter to measure the current I passing through Rx. Then Rx = UI.

There are two ways to measure resistance by voltammetry: internal connection method and external connection method, as shown in Figures (a) and (b). When the resistance of the resistor Rx to be measured is much smaller than the internal resistance of the voltmeter, use the method in Figure (a) to measure, and the error is small; when the resistance of Rx is much greater than the internal resistance of the ammeter, use the method in Figure (b) Measurement error is small.

2. Other methods of resistance measurement

1. resistance method

Equipment: voltmeter, fixed-value resistor R0 with known resistance, resistor Rx with unknown resistance, switch, power supply and wires.

Method 1. Change the meter method: measure the resistance by moving the position of the voltmeter.

Method 2. Switch on-off method: Measure the resistance by closing or opening certain switches and changing the connection status of the circuit.

The circuit is as shown in the figure. Close switch S1 and use a voltmeter to measure the voltage U1 across Rx. Then close S2 again and short-circuit R0 to measure the total voltage U across the circuit.

2. security method

Equipment: an ammeter, a fixed value resistor R0 with a known resistance, a switch, a power supply, the resistance to be measured Rx and wires.

Method 1. Change the ammeter method: measure the resistance by changing the position of the ammeter. The circuit is as shown in the figure: first connect the ammeter in series with Rx to measure the current I1 passing through Rx, and then connect the ammeter in series with R0 to measure the current I2 passing through R0.

Method 2. Switch on-off method

Short circuit method: The circuit is as shown in the figure. First close switch S1, measure the current I1 after Rx and R0 are connected in series, then close S2, short-circuit R0, and measure the current I2 through Rx.

Resistance measurement PPT, part 2: classification and exploration

One type: voltammetric resistance measurement

[2018•Jingmen, Hubei]There are two new dry batteries, voltmeters, ammeters, sliding rheostat, switches and several wires. In the experiment using "voltammetry" to measure the resistance of fixed-value resistor R1:

(1) Please follow the circuit diagram shown in Figure A and connect the circuit in Figure B completely.

(2) In order to protect the circuit before the experiment, the slider of the sliding varistor should be placed at terminal B (optional "A" or "B").

(3) Close the switch and no matter how you move the slider P, you will find that the ammeter always shows no reading but the voltmeter does. The reason may be B (fill in the symbol).

A． Sliding rheostat open circuit B. R1 open circuit

C. R1 short circuit

(4) After troubleshooting, when the voltage indication is 2.7 V and the current indication is as shown in Figure C, the current through R1 is 0.3 A. Its resistance is 9 Ω.

(5) This experiment requires multiple measurements and averaging in order to reduce the error______.

[Method summary] (1) When connecting the sliding rheostat, select the binding posts "one up and one down".

(2) In order to protect the circuit, the sliding piece of the sliding rheostat should be placed at the maximum resistance point.

(3) Determine the fault based on the ammeter and voltmeter.

(4) Correct readings are evaluated using Ohm's law.

(5) The experiment requires multiple measurements and averaging to reduce errors.

Type 2: Special method of measuring resistance

[2019•Gansu Wuwei]The picture shows the experimental circuit diagram for measuring resistance by voltammetry.

(1) It can be seen from the figure that when the sliding piece P moves to the b end, the resistance value of the sliding varistor connected to the circuit becomes larger, the voltage indication number becomes smaller, and the current indication number becomes smaller (optional "increase", "get smaller" or "constant").

(2) A student performed standard operations and measured the data in the table below. The purpose of measuring multiple times is to take the average value and reduce the error. In the table, a is 10.6 Ω, and the measured resistance value is 10.5 Ω. (Results are rounded to one decimal place)

(3) How to measure the unknown resistance Rx using a voltmeter and the known resistance R0 without an ammeter? As shown in the figure is the experimental circuit diagram designed by a student. The power supply voltage remains unchanged during the entire experiment. Then when the switch S is closed

Resistance measurement PPT, the third part: evaluation in class

1. [2017•Heilongjiang Harbin]In the experiment of measuring resistance by voltammetry, when connecting the circuit, the switch should be in the off state; if the ammeter and voltmeter indicate as shown in the figure, then The resistance of the resistor being measured is 5 Ω.

[Analysis] To protect the circuit, the switch should be turned off when the circuit is connected.

It can be seen from the figure that the ammeter selects the 0~0.6 A range, the graduation value is 0.02 A, and the indication is 0.28 A; the voltmeter selects the 0~3 V range, the graduation value is 0.1 V, and the indication is 1.4 V; from I= UR can be obtained, the resistance value of the resistor under test is: R=UI=1.4 V0.28 A=5 Ω.

2. [2018•Hunan Yiyang]Picture A is a circuit diagram for measuring the resistance of a small light bulb. The voltage of the bulb when it is normally emitting light is 2.5 V.

(1) When the small bulb emits light normally, the ammeter's indication is as shown in Figure B. At this time, the ammeter's indication is 0.2 A, and the resistance of the small bulb is 12.5 Ω.

(2) If the switch is closed, the pointers of the ammeter and voltmeter will deflect, but the small bulb will not light up. The possible reason is that the resistance of the sliding rheostat connected to the circuit is too large (or the power supply voltage is too small) (just name one).

(3) If an electric meter fails, but a known fixed-value resistor R0 is available, the resistance of the small light bulb cannot be measured in the following circuit D.

Resistance measurement PPT, part 4: layered operations

1. The figure shows the indications of the ammeter and voltmeter when measuring resistance using the "volt-ampere method". The ammeter in the figure shows 0.2A, and the resistance of the resistor to be measured is 12 Ω.

2. [2019•Shandong Linyi]When student Xiaohu measured the resistance of a 2.5 V small light bulb, the connected circuit was as shown in the figure:

(1) When the teacher checked the connected circuit, he found that one wire was connected incorrectly. Please put an "×" on the wrong wire. If you do not correct the circuit and close the switch, the bulb will not light up and the voltmeter will show that the pointer exceeds the range. phenomenon. (Fill in both blanks to observe the phenomenon)

(2) After Xiaohu corrected the error and performed the correct operation, the measured data is as shown in the table. Then calculate from the table that the resistances of the three small bulbs are not equal. You think the reason for this result may be that the resistance of the lamp changes with the change of temperature.

Number of experiments 1 2 3

Voltage U/V 2.0 2.5 2.8

Current I/A 0.20 0.24 0.25

3. [2018•Ningxia]As shown in the picture, Xiao Ming performed the experiment of "voltammetric measurement of resistance":

(1) Please help Xiao Ming to completely connect the physical circuit B according to the circuit diagram A (the wires cannot cross).

(2) After the circuit is connected, when Xiao Ming is ready to close the switch and start the experiment, the slider of the sliding rheostat should slide to terminal B (optional "A" or "B").

(3) Xiao Ming closes the switch and adjusts the sliding rheostat. He finds that the ammeter has no reading, but the voltmeter has a reading and no change. The circuit fault may be that the resistor Rx is open.

(4) After Xiao Ming eliminates the fault, he adjusts the sliding rheostat. When the voltmeter reads 2.6 V and the ammeter reads as shown in Figure C, the resistance of the fixed-value resistor is 10Ω.

(5) Xiao Ming also wants to explore the relationship between current and resistance. Please help Xiao Ming design a data recording form.

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Update Time: 2024-08-16

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