"Application of Ohm's Law" Ohm's Law PPT Courseware 5

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"Application of Ohm's Law" Ohm's Law PPT Courseware 5

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Indirect measurement of resistance

Under the condition of only a voltmeter (or only an ammeter), with the help of a resistor of known resistance and applying Ohm's law, the resistance of an unknown resistor can be measured.

According to Ohm's law, the current passing through a conductor is equal to the ratio of _____ at both ends of the conductor to the _____ of the conductor. Therefore, when there is only a voltmeter, the current in the circuit cannot be measured directly. We can use a known resistance Resistance determines _____ in a circuit. Utilize the characteristic that _____ in series circuits are equal everywhere, establish the relationship between the current in the circuit and the current passing through the resistor to be measured, and determine the current on the resistor to be measured through the resistor with a known resistance.

Ammeter and voltmeter resistance

The resistance of the ammeter is very _____, so when the ammeter is connected in series in the circuit, it has a very small effect on the total resistance of the circuit; the resistance of the voltmeter is very _____, so when the voltmeter is connected in parallel, the total resistance of the circuit is also very small. Small

How to measure resistance

(1) Two-volt method: As shown in the figure, use voltmeter V1 to measure the voltage U1 across the known resistor R0, and use voltmeter V2 to measure the voltage U2 across the unknown resistor Rx, I1=U1/R0, I2=U2/ Rx, since the currents in the series circuits are equal, that is, U1/R0=U2/Rx, Rx=U2/U1R0.

(2) Double-amp method: As shown in the figure, use ammeter A1 to measure the current I1 passing through resistor R0, and use ammeter A2 to measure the current I2 passing through unknown resistor Rx. U1=I1R0, U2=I2Rx. Since each branch of the parallel circuit The voltages at both ends of the circuit are equal, that is, I1R0=I2Rx, Rx=I1/I2 R0

(3) Measure resistance with voltmeter and sliding rheostat: As shown in the figure, close the switch, first move the slider to the far left, record the indication U of the voltmeter, at this time U is the power supply voltage, and then move the slider to On the far right, write down the voltmeter indication U2 at this time, and the voltage at both ends of R0 U0=U-U2. Since the currents in the series circuit are equal, that is, U2/RX=U-U2/R0,RX=U2/U-U2 R0

[Example] There is a battery pack (voltage is constant), an ammeter, a switch, a resistor R0 with a known resistance, and several wires.

Use the above equipment to measure the resistance of the resistor Rx to be measured.

(1) Draw the experimental circuit diagram.

(2) Briefly write down the experimental steps and use letters to represent the measured physical quantities (current).

(3) Write the expression for the resistance of the resistor Rx to be measured.

Tip: The key to measuring resistance with a single meter is how to use the voltmeter to indirectly measure the current in the circuit when there is a voltmeter; when there is an ammeter but no voltmeter, how to indirectly obtain the voltage at both ends of the resistance to be measured with the help of existing equipment. That is, an ammeter connected in series with a fixed-value resistor is equivalent to a voltmeter, and a voltmeter connected in parallel with a fixed-value resistor is equivalent to an ammeter.

Analysis: The experimental equipment only provides an ammeter and no voltmeter, so the voltage at both ends of the conductor cannot be directly measured. If two resistors are connected in parallel, since the voltage at both ends of the resistor is equal, the voltage can be obtained by using the current value measured by the ammeter and the known resistance.

Method 1: (1) The circuit diagram is shown in Figure A.

(2) Steps: ① Close the switch S and read the current indication number I1; ② Open the switch S, change the ammeter to be connected in series with Rx (dashed line A), close the switch S and read the current indication number I2.

(3)Expression Rx=I1/I2 R0

1. Without an ammeter, a student used a voltmeter and a fixed-value resistor R0 with a known resistance to measure the resistance of the unknown resistor Rx. The circuit diagram in the figure that cannot measure the resistance of Rx is ()

2. If only a voltmeter and a sliding rheostat are provided, ______ (optional "yes" or "no") is used to measure the resistance of an unknown resistor. When measuring, the resistance to be measured should be connected to the sliding rheostat ______.

3. When the voltage across a conductor is 6 V, the current flowing through it is 0.2 A. If the voltage across the conductor is increased to 15 V, it is more appropriate to use a student experimental ammeter with a range of ______ to measure the current flowing through it. This is The resistance of the conductor is ______.

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Update Time: 2024-11-22

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