First, the experimental purpose

(1) Master the principle and method of force sensor.

(2)? Understand the properties of liquid surface and measure the surface tension coefficient of liquid.

Second, the experimental principle

Liquid tends to shrink its surface as much as possible, as if the surface of liquid is a stretched rubber film. This force that contracts the liquid surface along the surface is called surface tension.

Common methods for measuring surface tension coefficient: pull-off method, capillary elevation method, drop hammer measurement method, etc. This experiment adopts pull-off method.

If the thin steel plate is immersed in liquid, the liquid level near the surface of the steel plate will be higher than other places, as shown in figure 1.

The tangential force Ft caused by liquid surface contraction is called surface tension, and the angle φ is called contact angle.

When the steel sheet is slowly pulled out, the contact angle φ gradually decreases and tends to zero, so the Ft direction is vertical downward. What is the condition of force balance before the steel plate leaves the liquid? =? Mg? +? Ft？ ( 1)。

Where f is the external force applied when the steel sheet is pulled out of the liquid surface, and mg is the total weight of the steel sheet and the liquid attached to it.

The surface tension Ft is proportional to the circumference 2(l+d) of the contact surface, so there is Ft? =? 2σ(l+d), where the proportional coefficient σ is called the surface tension coefficient, which is numerically equal to the force acting on the liquid surface per unit length.

Substituting Ft into equation (1) gives δ=F-mg/2( 1+d) (2).

When using ring wire instead of thin steel sheet to do this experiment, let the inner diameter and outer diameter of the ring be D 1 and D2, respectively. When it is pulled down from the liquid surface, the tension difference of the sensor is f? =? f–mg？ =π(D 1+D2)σ, where δ = f/π (D 1+D 2)? (3)。

As long as the force f and the inner and outer diameters of the ring are measured and substituted into the formula (3), the surface tension coefficient σ of the liquid can be calculated.

The unit of each quantity in the formula is unified as the international unit.

Third, the experimental steps

( 1)? Start and warm up.

(2)? Clean glassware and rings.

(3)? Put the liquid to be tested into glassware and put it on the lifting table.

(4)? Hang the weight plate on the force sensor and calibrate the force sensor.

(5)? Hang the lifting ring and measure the surface tension coefficient of the liquid. When the lower edge of the ring is completely immersed in the liquid, turn the nut of the lifting table to lower the liquid level.

Write down the voltmeter reading U 1 at the moment when the lifting ring breaks the liquid level, and the voltmeter reading U2 at the moment after the break. Then f=(U 1-U2)/B

Extended data:

The equipment needed for the experiment is: DH4607 liquid surface tension coefficient tester; Force sensitive sensor; 0.0005kg weight (7 pieces); Tweezers; Weight plate; Ring; Glass plate.

The pull-off method used in the experiment is a direct measurement method, and the force is usually measured by the elastic deformation (elongation or torsion) of the object.

The environment of molecules on the surface of liquid is different from that of molecules in liquid. Every molecule in the liquid is surrounded by other molecules of the same kind, and the resultant force of the surrounding molecules is zero.

Because there are few molecules in the meteorological layer above the liquid, the upward attraction of each molecule in the surface layer is less than the downward attraction, and the resultant force is not zero.

This force is perpendicular to the surface of the liquid and points to the inside of the liquid. Therefore, molecules tend to squeeze into the liquid from the liquid surface, so that the liquid surface naturally shrinks until it reaches a dynamic balance.