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 GCSE     >Capacitor<     Coupling     Decoupling     Explosion     More Uses     Parallel     RC Timing     Series     Smoothing

# Capacitor

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GCSE    Capacitors    Questions 0 to 22   -->  View All

Capacitors store energy in the form of charge.
They are like electronic buckets filled with electrons or perhaps more like compressed gas cylinders.
The potential difference across a capacitor changes slowly unless large currents are flowing.

Nick Reeder's Capacitor Challenge

• Capacitors store energy in the form of charge.
• Charge is either more or fewer electrons than normal in uncharged material.
• Volts ( V ) push the charge around creating the excess or lack.
• Capacitance ( C ) is measured in Farads.
• Charge ( Q )  is measured in Coulombs.
• The charge Q   =   C V.
• Capacitance is defined as the number of coulombs stored per volt across the capacitor. C   =   Q / V
• The energy stored is measured in Joules.
• The energy stored = 0.5 C V2
• If the voltage is big, the stored energy can be enormous and lethal.

## Capacitor Construction

Two parallel metal or foil plates insulated from each other. Many capacitors have this structure rolled up into a tube to save space.

## Capacitor Symbols and Images

 Left to right ... Electrolytic - Connect the right way round to prevent explosion! Tuning - A variable capacitor used for tuning a radio transmitter. Small capacitors including two electrolytics. Look for the arrows with the minus signs.

## Capacitor Simulations

Click the switch to alternate between charging and discharging.

This animation has correct physics so it could be used to plot a realistic capacitor charging graph.

 On the animation page, click the Fill/Empty button to alternate between filling and emptying. It takes time to fill the tank / charge the capacitor The tank/capacitor fills fast at first and the fill rate slows down, the fuller it gets The tank/capacitor empties fast at first and the emptying rate slows down, the emptier it gets Eventually the battery/reservoir would be drained The water level/capacitor charge can not change suddenly If there were more pressure/voltage, the tank/capacitor would fill faster The tank fills until the water levels are equal. The capacitor charges until the voltages are equal.

## Capacitor Properties

The main characteristics or properties of capacitors are ...

• Capacitance is measured in Farads or more often pico, nano or microfarads.
• One Farad is an enormous capacitance and rarely found in real life.
• Capacitors store energy in the form of a stored electric charge.
• Charge is measured in Coulombs.
• Energy is measured in Joules.
• Q = CV where
• Q is the charge,
• C is the capacitance and
• V is the potential difference across the capacitor
• If you double the voltage, the stored charge will double.
• E = CV2 / 2 where
• E is the energy stored,
• C is the capacitance and
• V is the potential difference across the capacitor
• If you double the voltage, the energy stored increases by 22 (four times)

More details ...

• capacitors block direct current (DC)
• capacitors pass alternating current (AC)
• The breakdown voltage (in Volts). The voltage across the capacitor should never exceed this.
• The upper useful frequency properties.
• The DC leakage current (a particular problem with electrolytic capacitors).

YouTube Video: VERY Big Capacitor vs Watermelon

## Electrolytic and Tantalum Capacitors

Electrolytic capacitors use thin rolled up foil plates separated by a liquid or gel electrolyte. The insulation between the plates relies on a chemical reaction. If the capacitor is connected up the wrong way round, this chemistry fails and the capacitor works as a conductor instead. It gets hot and can explode!

• The negative pin is indicated by the arrows and minus signs.
• Electrolytic capacitors have a very large capacitance for their size.
• The working voltage is low. 12 to 160 volt ratings are common. The image above shows a 35 Volt capacitor.
• They have a significant DC leakage current, sufficient to upset some timing circuits
• They are not manufactured with good tolerance / accuracy and this can be up to 50% out
• Their capacitance is not stable and can change with time
• They are not suitable for high frequency radio applications because the coiled up foil roll acts as an inductor blocking high frequency performance
• They work well at audio and ultrasonic frequencies.

Their uses include ...

• DC power supply smoothing - This is a particular example of decoupling.
• Audio signal coupling - Block DC and pass AC.
• Audio signal decoupling - Remove unwanted AC signals.
• Timing in 555 or other timer circuits - RC resistor capacitor circuit

They can not be used ...

• In AC circuits where the polarity across the capacitor reverses
• In logic gate astable circuits because the polarity across the capacitor reverses

## Capacitor Labelling

Subject Name     Level     Topic Name     Question Heading     First Name Last Name Class ID     User ID

 CloseEscapeX Q: qNum of last_q     Q ID: Question ID         Score: num correct/num attempts         Date Done

Question Text

image url

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