"Measurement of Current" Current and Circuit PPT Courseware 8

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"Measurement of Current" Current and Circuit PPT Courseware 8

Experience: the strength of the current

Activity preparation: a small light bulb, switch, wires, and two dry batteries.

Activity 1: Connect a small light bulb to the circuit using a dry battery. Close the switch and observe the brightness of the bulb. Turn off the switch and connect two dry batteries to the circuit. Are light bulbs the same brightness?

Activity 2: Adjust the switch that controls the electric fan at home so that the switch rotates to different gears. Does the electric fan rotate at the same speed?

Reference answer:

Activity 1: Not the same

Activity 2: Not the same

1. The strength of the current:

(1) means: ________, symbol ________.

(2)Unit conversion:

1 A=________mA=________μA.

2. Measurement of current - ammeter:

(1)Two measuring ranges: ________, ________.

(2) Use.

①_______________ before use.

②_______________ with the electrical appliance being tested.

③Enter from ____________ and exit from _______________.

④The measured current cannot exceed _______________.

⑤Cannot be connected directly to _______________.

(3) Reading.

①Clear the ______________ and _______________ of the ammeter.

②Observe the _______________ of the deflection of the hands.

Explore 1 The strength of the current

Combined with "experience", think about the following questions:

1. It can be seen from "experience" that the brighter the light bulb, the faster the electric fan rotates, indicating that the current passing through is more______. The darker the light bulb, the slower the electric fan rotates, indicating the more current passing through.

Reference answer: strong or weak

2. Understand some common currents:

(1)The current in the flashlight is approximately __________mA=________A.

(2) The current of household energy-saving lamps is approximately ____________A.

(3)The current of a household refrigerator is approximately ____________A.

(4)The current of a household air conditioner is approximately _______________A.

(5)The lightning current is approximately ____________________A.

Reference answer: (1)200　0.2　(2)0.1　(3)1　(4)5　(5)2×105

Exploration 2 Ammeter

Observe the ammeter as shown in the figure and think about the following questions:

1. Understand the external structure:

(1)The symbol of the ammeter: ________.

(2) This ammeter has _____ terminals, of which _____ terminals are shared.

(3) The ammeter has ________ ranges.

① When using the two terminals "-" and "0.6", the measuring range is ________ and the graduation value is ________.

②When using the two terminals "-" and "3", the measuring range is ________ and the graduation value is ________.

Reference answers: (1) (2)三 “-” (3)2 ①0～0.6 A

0.02 A　②0～3 A　0.1 A

2. Use of ammeter:

(1) Comparing the two pictures A and B, the correct use of ammeter is __________.

This shows that the ammeter must be ____________ with the electrical appliance being measured.

(2) Comparing the deflections of the pointers in Figures A and B, the correct connection is ____________, which means that the ammeter must allow the current to flow in from the ________ terminal of the ammeter and out from the ________ terminal; otherwise, the ammeter pointer will move toward _ _____ deflects, cannot read, and easily damages the ammeter.

(3) Analyze the two pictures A and B. In picture A, the ammeter has a selected range of ___________, and in picture B, the ammeter has a selected range of ___________, so it must be ensured that the measured current cannot exceed ____________________; before measurement, _____________ Measure the size of the current and select the appropriate range.

When the measured current cannot be estimated, first select the ________ range, and use the ________ method to determine the current size and select the appropriate range.

【Micro Dial】

(1) Before using the ammeter, pay attention to two points:

① Clarify the measuring range and graduation value;

② Check whether the pointer is pointing at the zero scale line. If not, adjust it to zero.

(2) Select the measuring range by trial touch method: After selecting the large range of the ammeter and connecting it to the circuit, close the switch and quickly open it, and select the measuring range according to the instantaneous pointer deflection angle.

(3) When selecting a range, ensure that the deflection amplitude of the pointer reaches more than half of the dial. Use a small range instead of a large range to improve the accuracy of readings.

[Knowledge Expansion] Working Principle of Ammeter

The ammeter is made based on the magnetic field force exerted by the current-carrying conductor in the magnetic field. There is a permanent magnet inside the ammeter, which generates a magnetic field between the poles. There is a coil in the magnetic field. There is a hairspring spring at each end of the coil. Each spring is connected to a terminal of the ammeter. A rotating shaft is connected between the spring and the coil. The rotating shafts are opposite to each other. On the front end of the ammeter, there is a pointer. When there is a current passing through, the current passes through the magnetic field along the spring and the rotating shaft, and the current passes through the magnetic induction lines, so it is affected by the magnetic field force, causing the coil to deflect, driving the rotating shaft and pointer to deflect. Since the magnitude of the magnetic field force increases as the current increases, the magnitude of the current can be observed through the deflection of the pointer.

Test point 1: Use of ammeter

[Demonstration question 1] Use an ammeter to measure the current in the light bulb L1. Among the four circuits shown in the figure, the correct connection method is ()

[Problem Solving Guide] (1) How to determine the object measured by the ammeter?

(2) What are the main aspects to judge whether the ammeter connection is correct?

Tips: (1) The ammeter is connected in series with any electrical appliance to measure the current passing through that electrical appliance. (2) Three aspects: ① Whether it is connected in series; ② Whether the current is positive in and out; ③ Whether the range selection is appropriate.

【Exquisite analysis】Choose C. This question tests the use of ammeter. To determine whether the ammeter connection in the circuit diagram is correct, first observe whether the ammeter is connected in series with the circuit or conductor of the current to be measured; secondly, analyze whether the "+" and "-" terminal connections are correct. In Figure A, the ammeter is connected to the main circuit and does not measure the current through L1, so Figure A does not comply. In Picture B, the "+" and "-" terminals of the ammeter are connected reversely, so Picture B does not match. In Figure D, the ammeter is connected in parallel with L1 and L2 in the circuit, and is directly connected to the two poles of the power supply, causing a short circuit, which makes the ammeter easy to burn out, so Figure D does not comply. So choose C.

[Golden Key to Passing Customs] Overview of the use of ammeter

(1) Look at the connection: the current "+" goes in and "-" goes out.

(2) Look at the connection method: connected in series in the circuit, and cannot be directly connected to the two poles of the power supply.

(3) Look at the measuring range: If you can use a small range, you don’t need a large range, but the measured current cannot exceed the selected range.

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Update Time: 2024-06-28

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