A student wants to measure the diameter of a straight thin copper wire. His experimental steps are as follows: A. Measure the length L1 of the thin copper wire with a scale; B. The thin copper wires are tightly arranged and wound on the pencil rod for several circles; C. Measure the total length L2 of the copper wire coil on the round pencil rod with a scale (as shown in the figure); D. Count the number of turns n of the thin copper wires on the pencil rod E. Divide the length of copper wire L1 by the number of turns of copper wire n to get the diameter of thin copper wire D; (1) In the above steps, the wrong and unnecessary steps are (fill in letters) (2) The error step is corrected to: ■ (3) This method can also be used as an example

A student wants to measure the diameter of a straight thin copper wire. His experimental steps are as follows: A. Measure the length L1 of the thin copper wire with a scale; B. The thin copper wires are tightly arranged and wound on the pencil rod for several circles; C. Measure the total length L2 of the copper wire coil on the round pencil rod with a scale (as shown in the figure); D. Count the number of turns n of the thin copper wires on the pencil rod E. Divide the length of copper wire L1 by the number of turns of copper wire n to get the diameter of thin copper wire D; (1) In the above steps, the wrong and unnecessary steps are (fill in letters) (2) The error step is corrected to: ■ (3) This method can also be used as an example

(1) Necessary step a, wrong step e
(2) Divide L2 by the number of turns n of the copper wire to get the diameter D of the fine copper wire
(3) This method can also be used to measure the thickness of a sheet of paper

How to measure the diameter of a thin copper wire

Wrap the copper wire tightly around the pencil for several more turns, 30 ~ 40 turns, then measure the length of the copper wire around the pencil, and then check the number of turns and divide it by one

Recall the following three experiments: (1) when measuring the diameter of the thin copper wire with a scale, first wind the thin copper wire tightly on the pencil for 50 turns, and then measure the total length of the coil with a scale, and then divide it by 50; (2) when measuring the mass of a pin, first measure the total mass of 100 pins, and then divide it by 100; (3) when studying the factors affecting the friction force, first maintain the pressure In the above three experiments, the relationship between the friction force and the roughness of the contact surface is studied, and then the relationship between the friction force and the pressure is studied by keeping the roughness of the contact surface the same______ The thinking method of the two experiments is the same, and the common point of the problems encountered in the experiments is that______ The common solution to the problem is______ ．

Because the diameter of the copper wire and the mass of the pin are very small, they can not be measured directly (less than the dividing value of the measuring tool), so it is necessary to measure the sum of several identical physical quantities and calculate the average value, so as to reduce the error. Therefore, the answers are: (1) and (2); the measured physical quantities are too small to be measured directly; measure the sum of several identical physical quantities and calculate the average value