Why can an electrometer measure potential difference?

Electrometer is also called potential difference.

Meter or pointer electroscope.

It is a semi-quantitative measuring instrument commonly used in electrostatic experiments in secondary schools.

When A is charged, the charges are mainly distributed in the four tip parts of A, B, C and D, in which the charges carried by C and D act with repulsion, and the pointer is affected by an electric moment L1 that makes it open.

Since the center of gravity of the pointer is slightly below the axis of rotation o, when l1 opens the pointer, the gravitational force of the pointer produces a gravitational moment l2 that resets the pointerAs the pointer is deflected, L1 decreases (because the distance between C and D increases, the Coulomb force.

Smaller the arm becomes) and L2 gets bigger (as the gravitational arm increases).

When L1 is equal to L2, the pointer stops at a certain position (which is stable equilibrium), and the opening angle of the pointer.

For°. When the charge Q carried by A is larger, the charge carried by C and D is also larger, and L1 is larger, so it is also larger. Since q determines , the size of can represent the size of q. This is the reason why the electrometer can be used as an electroscope.

Voltage, also known as potential difference or potential difference.

It is a physical quantity that measures the difference in energy of a unit charge in an electrostatic field due to different electric potentials. Its magnitude is equal to the unit positive charge due to the electric field force.

The work done by the action to move from point A to point B, the direction of the voltage is specified as the direction from the high potential to the low potential.

SI system of units for voltage.

It is volts (V, referred to as volts), and commonly used units include millivolts (mV), microvolts (V), kilovolts (kV), etc. This concept is similar to the "water pressure" caused by the high and low water level.

It should be pointed out that the term "voltage" is generally only used in electrical circuits, while "potential difference" and "potential difference" are generally applied to all electrical phenomena.

Electrometer principle.

1. Clarify the structure of the electrometer: one plate A of the plate capacitor is grounded, the other plate B is connected with the metal ball, and the shell of the electrometer is grounded (note: the metal ball, rod, and platinum sheet are integrated and the connection with the shell is isolated with insulating rubber).

The purpose of this construction is that the A plate and the metal shell are equal due to the grounding potential, and the B plate and the metal ball are equal to the potential. That is to say, the potential between the plates AB is equal to the potential between the metal ball and the shell.

2. It is clear that the electroscope part is a tiny capacitor, and the ability to accommodate the amount of charge is very small, and the slight change of the amount of charge will cause obvious changes in the angle of the metal foil. The capacitor to be tested is relatively large capacitance, with a large ability to accommodate charge, and a small change in power is negligible.

3. It is clear that most of the charge to be measured is concentrated on plate B, and the charge on plate A and the shell are transmitted from the ground through the grounding wire.

4. When the capacitance of the capacitor to be tested changes, because the electroscope is a tiny capacitor, the allowable amount of charge is very small, so the amount of charge can be considered unchanged for the capacitor to be tested.

For example, the distance between the plates becomes smaller, the capacitance increases, and the charge does not change, which shows that the voltage between the plates decreases.

The angle of the metal foil is reduced. Explain why:

The distance between the plates becomes smaller, the coulomb attraction between the plates becomes stronger, the capacitance becomes larger, and the upper limit of the amount of charge accommodated becomes larger, and according to the interaction between the charges, the charge on the electroscope metal ball will have a trace amount of charge moved to plate B

The metal foil is less charged, the repulsion force becomes smaller, and the angle becomes smaller. But the small variables are for the amount of electricity to be measured.

Negligible.

Electrometer, also calledThe electric potential carries the differencegauge or pointerThe first morning electrical appliance was inspectedIt is a semi-quantitative measuring instrument commonly used in electrostatic experiments in middle schools. Electrometers can be used to check the type of charge carried by an object and to measure the amount of electricity.

Metal balls, metal rods, and pointers are equivalent to one electrode of a capacitor, and the metal shell is equivalent to an electrode of a capacitor, and they are insulated from each other. The size of the capacitance is determined by the geometric size of the metal shell and the length and position of the metal rod and pointer. Because the change in the deflection angle of the pointer has little effect on the capacitance of the electrometer, it can be approximated that the capacitance value of the electrometer does not change during the rotation of the pointer.

Electroscope.

Due to the shielding effect of B, the lower part of A is less affected by the external electric field. The upper end of a is exposed outside of b, so the external electric field can be induced by the upper end of a. When the charged body moves closer to the uncharged electrometer, due to electrostatic induction, a charge with a different name from the charged body appears at the upper part of A, and a charge with the same number as the charged body appears at the lower ends C and D of A.

And the needle opened. The more electricity the charged body carries and moves closer to it, the angle is opened.

The larger. When the charged body is removed, the pointer returns to its original position. We can use this induction method to test whether and how much an object is charged and to demonstrate the phenomenon of electrostatic induction.

Theoretically, the analysis is influential. Because when not grounded, the electric potential of b is 0; When not grounded, the electric potential is not 0In both cases, the boundary conditions are different.

In fact, the above analysis of the encyclopedia is also wrong. Because the electrostatic charge exists near B, if it can affect A, it must be able to affect B, and B is grounded, and this effect will still exist, so the charge distribution on the outer surface of B is complex.

As for how to do this, you need to solve a rather complex equation.

A and B, which make up the electrometer, are two conductors that are close to each other and insulated from each other. A and B form a capacitor, and A and B each are one pole of the capacitor. The capacitance C0 of the general electrometer is 9-11pf measured by the WQ-5 A type universal bridge.

The relationship between the charge of a q and the potential difference u between a and b is .

q=c*uu is large, q is large, and the pointer angle of the electrometer is also large. So, the size reflects the size of u. This is why electrometers are used to measure potential differences. Because electrometers are often used to measure the potential difference, they are also called potential difference meters.