"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 target

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 calculation and analysis and discussion, we can know the law of the change of filament resistance with temperature and the danger of short circuit.

Core test points: "voltammetry" measurement of resistance, application of Ohm's law

Preview before class

1. In a student's experiment using a voltmeter and ammeter to measure resistance, which of the following precautions was unnecessary ()

A. When connecting a circuit, turn off the switch first

B． The wiring should start from the positive terminal of the power supply, and the switch should be close to the positive terminal of the power supply.

C. When the resistance to be measured cannot be estimated, the ammeter and voltmeter should first select a larger range.

D. When connecting the varistor to the circuit, the slider should be placed at the position with the maximum resistance of the varistor.

2. In the experiment, the voltage across an unknown resistor was measured to be 4.8V, and the current flowing through it was 320 mA. The resistance of this resistor is ______Ω

3 A short circuit means that the current ______ directly forms a loop. When a short circuit occurs, ______ in the circuit is small and ______ is large.

Knowledge point 1. Measure resistance by voltammetry

(1) Experimental principle: Deformation formula of Ohm’s law I=U/R R=U/I

It can be seen from the formula R=U/I that a voltmeter is used to measure the voltage across the small bulb, an ammeter is used to measure the current passing through the small bulb, and Ohm's law can be used to calculate the resistance of the bulb. This method is called voltammetry to measure resistance.

(2) Experimental equipment: power supply (dry battery pack), switch, ammeter, voltmeter, sliding rheostat, small light bulb and some wires.

(3)Experimental steps

① Design the experimental circuit diagram as shown below. Connect the circuit according to the circuit diagram: the switch must be in the off state, and the slider P must be moved to the maximum resistance end before the experiment, the appropriate range must be selected, and the positive and negative terminals must be connected correctly.

② Connect the circuit according to the circuit diagram: the switch must be in the off state, and the slider P must be moved to the maximum resistance end before the experiment, the appropriate range must be selected, and the positive and negative terminals must be connected correctly.

③After the inspection is correct, close the switch, adjust the slider P of the sliding rheostat, adjust the current in the circuit gradually from the smallest possible current to the normal lighting of the bulb, observe the brightness of the bulb and read the indications of the voltmeter and ammeter, and complete the experiment. The data is recorded in the table below.

④According to the four sets of recorded data, find the corresponding resistance values ​​of the four sets.

[Exercise 1] (2014·Changchun) In the experiment of measuring the resistance of a small light bulb when it lights normally, the power supply voltage is 3V and the rated voltage of the small light bulb is 2.5V. The picture shows part of the connected circuit. Please complete the following questions as required:

(1) Use stroke lines instead of wires to completely connect the circuit in Figure 1.

(2) Before closing the switch, the slider of the sliding rheostat should be placed at the ______ (optional "left" or "right") end.

(3) After connecting the circuit, close the switch. No matter how you move the slider, if the small bulb never lights up, the voltage indication is always 0, and the current indication changes, the cause of the failure may be.

(4) After troubleshooting, close the switch and move the slider. When the small bulb glows normally, the current indication is as shown in Figure 2. At this time, the resistance of the small bulb is _______Ω.

Knowledge point 2. Short circuit

There are two common situations of short circuit:

① The power supply is short-circuited, that is, the current flows directly from the positive pole through the wire back to the negative pole without passing through any electrical appliance, which can easily burn out the power supply.

② A short circuit of an electrical appliance is also called a partial circuit short circuit. That is, a wire is connected to both ends of an electrical appliance. If this electrical appliance is short-circuited, it is easy to burn other electrical appliances.

[Example 2] When the power plug has two situations A and B as shown in the figure, once it is inserted into the socket, the short circuit will be caused by _____ (optional "A" or "B")

[Exercise 2] In the circuit shown in the figure below, when _____ is closed, L1 and L2 are connected in parallel; when _____ is closed, the circuit will be short-circuited and the power supply will be damaged.

In-class testing

1. In the circuit as shown in the figure below, when the switch S is closed, it is found that neither light is on, the ammeter pointer is almost pointing at the zero mark, and the voltmeter pointer is obviously deflected. The fault in this circuit may be ( )

A.Lamp L2 is short-circuited B.Lamp L2 is open-circuited

C. Bulb L1 is open circuit D. Both bulbs are open circuit

2. For the circuit shown in the figure below, close the switch and observe that bulb L1 is on but L2 is not on. Adjust the rheostat slide P, the brightness of the bulb L1 changes, but the bulb L2 always does not light up. The reason for this phenomenon may be ( )

A. The L2 filament of the light bulb is broken B. The sliding varistor is shorted

C. The light bulb L2 is short-circuited D. Poor contact of sliding varistor

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