Investigating Capacitors - page 3
Keywords: Investigating Capacitors coursework GCSE physics
By slashwk on 23/11/2006 17:28:09
Level: GCSE Key Stage 4 (Years 10-11)
Page Number: 3 of 9 pages: 1 2 3 4 5 6 7 8 9
at work in a capacitor.
When this electrostatic effect increases the imbalance of electrons between the two plates:
• The electrostatic field grows stronger.
• The amount of energy stored in the dielectric increases.
• The capacitor is said to be charging.
When this electrostatic effect decreases the imbalance of electrons between the two plates:
• The electrostatic field grows weaker.
• The amount of energy stored in the dielectric decreases.
• The capacitor is said to be discharging
The unit of capacitance is a farad. A 1-farad capacitor can store one coulomb of charge at 1 volt. A coulomb is 6.25e18 (6.25 * 10^18, or 6.25 billion billion) electrons. One amp represents a rate of electron flow of 1 coulomb of electrons per second, so a 1-farad capacitor can hold 1 amp-second of electrons at 1 volt.
A 1-farad capacitor would typically be pretty big. It might be as big as a can of tuna or a 1-liter soda bottle, depending on the voltage it can handle. So you typically see capacitors measured in microfarads (millionths of a farad).
To get some perspective on how big a farad is, think about this:
• A typical alkaline AA battery holds about 2.8 amp-hours.
• That means that an AA battery can produce 2.8 amps for an hour at 1.5 volts (about 4.2 watt-hours -- an AA battery can light a 4-watt bulb for a little more than an hour).
• Let's call it 1 volt to make the math easier. To store one AA battery's energy in a capacitor, you would need 3,600 * 2.8 = 10,080 farads to hold it, because an amp-hour is 3,600 amp-seconds.
If it takes something the size of a can of tuna to hold a farad, then 10,080 farads is going to take up a LOT more space than a single AA battery! Obviously, it is impractical to use capacitors to store any significant amount of power unless you do it at a high voltage.
The standard units of Capacitance:
farad: F
microfarad: µF (1 µF = 10-6 F)
nanofarad: nF (1 nF = 10-9 F)
picofarad: pF (1 pF = 10-12 F)
The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge itself. That's why the electronic flash on a camera uses a capacitor - the battery charges up
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