About Capacitors

Ohm's Law

Ohm's Law is the linear proportionality between current and voltage that occurs for most conductors of electricity. A graph of voltage against current is a straight line. The gradient is the resistance.

The most well known form of Ohm's law is V=IR, where V is the voltage, I is the current and R is the resistance. However there is another form of Ohm's law which often used by physicists that operates on a microscopic level, relating the current density J to the conductivity σ and the electric field, E.

To see how consider, the volume of material with faces of area A a distance I apart. With an e.m.f. V across the faces of the material:

the current is proportional to the voltage V
the current is proportional to the surface area A
and the current is inversely proportional to the distance I.
The current is therefore,

I = VAσ/I.

R = I/Aσ

The proportionality constant σ is the conductivity of the material.

V/I = |E|, and J = I/A in the direction of E so in general we have the constitutive relation.

J = σE

Power Rating for various Appliances in the Home

Electrical appliances used around the home are rated in Watts and electricity is sold by the kilowatt-hour. This is equivalent energy used by a device rated at 1000 W running for 1 hour, or 3.6 x 106 J. This is also the same energy as a Board of Trade Unit (BTU) (Not to be confused with the (British Thermal Unit) b.t.u. which describes the power of heating and cooling systems.)

To give some idea of how much energy electrical items consume the table below lists various electrical appliances and their power ratings.

The actual cost of running all this equipment depends on how much a kilowatt of power costs and how long you use each appliance. However, it is clear that the most power hungry appliances are those used for heating things. What is surprising, is the high power consumption of LCD televisions in comparison to traditional CRTs.

Super Capacitors

Super Capacitors are capacitors which have the ability to store large amounts of charge, and therefore energy, in a very small volume. Energy storage is by means of static charge rather than of an electro-chemical process that is inherent to the battery. Applying a voltage differential on the positive and negative plates charges the super Capacitor. This concept is similar to an electrical charge that builds up when walking on a carpet. The super Capacitor was first conceived in 1957 but now research is focused on using these as a light weight power sources as an alternative for batteries. The super Capacitor crosses into battery technology by using special electrodes and some electrolyte.

Super Capacitors could find applications such as temporary back-up power supplies in the electrical power grid or providing the initial burst of energy to get electric cars moving.

A case when a circuit tester

When a circuit tester, such as an analog meter set to measure resistance, is connected to a 10 microfarad (µF) electrolytic capacitor, a current will flow, but only for a moment. You can confirm that the meter's needle moves off of zero, but returns to zero right away.

When you connect the meter's probes to the capacitor in reverse, you will note that current once again flows for a moment. Once again, when the capacitor has fully charged, the current stops flowing. So the capacitor can be used as a filter that blocks DC current. (A "DC cut" filter.)

However, in the case of alternating current, the current will be allowed to pass. Alternating current is similar to repeatedly switching the test meter's probes back and forth on the capacitor. Current flows every time the probes are switched.

Brief summary of Capacitors

The capacitor's function is to store electricity, or electrical energy.
The capacitor also functions as a filter, passing alternating current (AC), and blocking direct current (DC).
This symbol is used to indicate a capacitor in a circuit diagram.

The capacitor is constructed with two electrode plates facing eachother, but separated by an insulator.

When DC voltage is applied to the capacitor, an electric charge is stored on each electrode. While the capacitor is charging up, current flows. The current will stop flowing when the capacitor has fully charged.

Pay attention of the breakdown voltage

When using a capacitor, you must pay attention to the maximum voltage which can be used. This is the "breakdown voltage." The breakdown voltage depends on the kind of capacitor being used. You must be especially careful with electrolytic capacitors because the breakdown voltage is comparatively low. The breakdown voltage of electrolytic capacitors is displayed as Working Voltage.

The breakdown voltage is the voltage that when exceeded will cause the dielectric (insulator) inside the capacitor to break down and conduct. When this happens, the failure can be catastrophic.

An important property of a dielectric

An important property of a dielectric is its ability to support an electrostatic field while dissipating minimal energy in the form of heat. The lower the dielectric loss (the proportion of energy lost as heat), the more effective is a dielectric material. Another consideration is the dielectric constant , the extent to which a substance concentrates the electrostatic lines of flux. Substances with a low dielectric constant include a perfect vacuum, dry air, and most pure, dry gases such as helium and nitrogen. Materials with moderate dielectric constants include ceramics, distilled water, paper, mica, polyethylene, and glass. Metal oxides, in general, have high dielectric constants.

The Dielectric Material

A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic field s. If the flow of current between opposite electric charge poles is kept to a minimum while the electrostatic lines of flux are not impeded or interrupted, an electrostatic field can store energy. This property is useful in capacitor s, especially at radio frequencies. Dielectric materials are also used in the construction of radio-frequency transmission lines.

In practice, most dielectric materials are solid. Examples include porcelain (ceramic), mica, glass, plastics, and the oxides of various metals. Some liquids and gases can serve as good dielectric materials. Dry air is an excellent dielectric, and is used in variable capacitors and some types of transmission lines. Distilled water is a fair dielectric. A vacuum is an exceptionally efficient dielectric.

Types of electronic oscillator

There are two main types of electronic oscillator: the harmonic oscillator and the relaxation oscillator.

Harmonic oscillator

The harmonic, or linear, oscillator produces a sinusoidal output. The basic form of a harmonic oscillator is an electronic amplifier with the output attached to an electronic filter, and the output of the filter attached to the input of the amplifier, in a feedback loop. When the power supply to the amplifier is first switched on, the amplifier's output consists only of noise. The noise travels around the loop, being filtered and re-amplified until it increasingly resembles the desired signal.

A piezoelectric crystal (commonly quartz) may take the place of the filter to stabilise the frequency of oscillation, this is called a crystal oscillator.

Relaxation oscillator

A relaxation oscillator produces a nonsinusoidal output, such as a square wave or sawtooth. The oscillator contains a nonlinear component such as a transistor that periodically discharges the energy stored in a capacitor or inductor, causing abrupt changes in the output waveform.

Square-wave relaxation oscillators are used to provide the clock signal for sequential logic circuits such as timers and counters, although crystal oscillators are often preferred for their greater stability. Triangle-wave or sawtooth oscillators are used in the time base circuits that generate the horizontal deflection signals for cathode ray tubes in analogue oscilloscopes and television sets. In function generators, this triangle wave may then be further shaped into a close approximation of a sine wave.

What does 'open circuit' mean?

"Open circuit" means no connection. It is usually used to describe a break in some part of a circuit which could be deliberate (such as a switch in the open or off position) or a fault (such as a broken wire or burnt out component).