The accumulated charge on two plates of a capacitor

A, Charging and discharging of capacitor

The graph is capacitor charge and discharge experiment circuit, including large capacity C (stored charge) not charging capacitor, E dc power supply for internal resistance is small, the HL for small bulbs.

1. The capacitor charging

The switch is "S" in contact "1" to capacitor charging power supply. Observations are: at the beginning of the bulb is bright, then gradually dim, from current meter can observe the change of the charging current from big to small, from the voltmeter can be observed on both ends of the capacitor voltage changes from small to large. After a period of time, the light bulb, ammeter pointer back to zero, the voltmeter is shown in voltage value close to the electromotive force, namely, this shows that the capacitor has been full of charge.

Think about:

The basic structure of the capacitor is sandwiched between two metal plates, a layer of insulating medium in essence it is cannot conduct electricity, so why it can store charge again?

The graph is capacitor charging schematic diagram, at the moment the power is connected, power supply to the negative electrons flow of negative plate of the capacitor, but due to the insulating medium between the two plates is not conductive, so these negative charges can accumulate on the negative plate, and as a result of these negative electron repulsion, of dielectric and insulating medium near the negative plate capacitor side then form a positively charged layer, then form a negative charge layer near the capacitor is plate side. The electrons in the positive plate is negative charge in the dielectric exclusion to the positive, so, the capacitor is on the plate then accumulated positive charge.

With the steady accumulation of positive and negative charges, capacitors are polar plate potential increased, and the power of positive electrode potential difference gradually decreases, and when the potential to achieve equal charge no longer move, the charging current is zero, the accumulated charge on the capacitor two plate is no longer increasing, and the charge will be stored in the capacitor. Power supply to the negative electrons flow of negative plate of the capacitor, the electrons in the positive plate is negative charge in the dielectric exclusion to the positive, thereby formed the current.

2. The capacitor discharge

Circuit is shown in figure 3-7, after the capacitor charging, the switch is placed in the "2", can observe the small bulbs flashed and went out. This is caused by capacitor discharge when the capacitor is equivalent to an equivalent power. In capacitor under the action of electric field force between two plates, negative plate negative charge is removed and the positive plate of the positive charge neutralization, at the ends of the capacitor voltage is decreased, too, until completely and charge on the plates. This is a capacitor voltage between the two plates is zero, circuit of current is zero.

Think about:

(1) the capacitor charge and discharge current through the capacitor dielectric actual?(2) when the capacitor coupled with ac voltage, circuit of current flow through? Why is that?

(1) the capacitor charge and discharge current didn't pass the dielectric capacitor.

(2) when the capacitor coupled with ac voltage circuit of current flow, but not through the capacitor current, when the ac voltage rises, the capacitor charge storage charge, form the charging current, when the ac voltage capacitor discharge release charge, discharge current, voltage, rising alternates charging capacitor discharge, make a capacitor with the current in the circuit. Look from the capacitor at both ends, the current continuity seems to remain, so usually have that image to alternating current (ac) "through" the capacitor.

Second, the capacitor easy detection

/ > using the capacitor charging and discharging principle, can use a multimeter is roughly judge quality stand or fall of high-capacity capacitor. Then how should do?

Detection: put a multimeter in ohms range 1 k file, two pens and ends respectively in the capacitor. If measured components for electrolytic capacitor, the attention should be paid to its polarity (capacitor polarity usually directly mark on the surface of capacitor), black pens and capacitor anode, red pens and capacitor anode.

Judgment method of quality:

(1) if the capacitance is large enough and capacitor quality is very good, multimeter something will turn to the right first, soon after the left back to the in situ;(2) if the capacitor leakage power big very big, the multimeter pointer back to less than in situ, but stay on a certain scale, the reading is the capacitor leakage resistance. This value is generally should be more than a few hundred to several thousand Europe;(3) if the clock deflection to the ohm zero deflection back no longer, after that the capacitor inside has a short circuit (4) if the clock has no deflection is the capacitor inside may be break, or capacitance is small, the charge and discharge current is very small, not enough to make the clock deflection.

Three, the capacitor in the electric field can

After power capacitor can make little light bulb glow, capacitor in the release of energy. Actually when capacitor charging, the positive and negative charge accumulation on the two plates, an electric field is formed between the plates, the capacitor stored in the storage charge as well as the energy. Theoretical analysis and experiments show that the electric field can be stored in the capacitor charging available under type said: 112

Type in the

The WC?QUC?CUC22 - the electric field can be stored in the capacitor, the unit is J;C - the capacitance of the capacitor, the unit is F;- capacitor voltage between the two plate, the unit is V.Q - charge, the unit is C.

On type shows that the electric field energy stored in the capacitor is proportional to the capacitance of capacitor (when the capacitor capacitance is big, can store more energy), so the capacitance reflects the capacitor's ability to store electric energy. When the capacitor charging, on both ends of the capacitor voltage increases, the capacitor will absorb energy from the voltage and storage; Ends, and when the capacitor discharge voltage decreases, it kept the original store electric energy released, visible, only with the power capacitor, energy conversion, it itself does not consume energy, so is a kind of energy storage capacitor component. The change of the voltage on both ends of the capacitor, the change of electric field energy reflects the capacitor. Electric field energy accumulation and release in the capacitor are a process of gradual change, it can only change from a stable state to another steady state. Therefore, at the ends of the capacitor voltage will never mutations, also only is a process of gradual change.

Four, transition process of RC circuit

Capacitor charge and discharge, the change from a stable state to another steady state must pass through the physical process is called the transition process. On both ends of the capacitor charging, the voltage increases gradually, charging current gradually reduce; When discharge, both ends of the voltage decreases, discharge current and decreases. Charge and discharge time used in the steady-state value related to the size of the R and C. The product of R and C is called the RC time constant of the current, with tau said. Namely RC

Time constant of the unit (s) is the tau, the greater the charge and discharge, the slower, the longer the transition process. Conversely, the smaller the tau, the short transition process. In practical application, when the transition after (3 ~ 5) tau time can think the basic end of the transition process, has entered a steady state.

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