Capacitance The potential of a conductor and the charge on it are directly proportional to eachother

CAPACITANCE The Capacitance of a conductor is the ratio of the charge on the conductor to its potential, defined as: C = Q/V Where C is a constant The value of C depends on the size and shape of the conductor.

Symbol: C A capacitor is an electrical device used to store charge (Q) Unit: Farad (F)

Picture a water container. The amount of water the container can hold (its capacity) will depend on – among other things – how quickly the water level rises when water is poured in; if the water level rises rapidly it suggests the container must be quite narrow and therefore may not hold much water. We would deduce that the container had a small ‘capacitance’.

Capacitors!

A capacitor is similar to a battery but they work in different ways.

Inside the capacitor, the terminals connect to two metal plates separated by a non- conducting substance, or dielectric.

In theory, the dielectric can be any non- conductive substance. However, specific materials are used that best suit the capacitor's function. Mica, ceramic, cellulose, porcelain.

What happens??? The plate on the capacitor that attaches to the negative terminal of the battery accepts electrons that the battery is producing.electrons The plate on the capacitor that attaches to the positive terminal of the battery loses electrons to the battery.

Once it's charged, the capacitor has the same voltage as the batteryvoltage

Here you have a battery, a light bulb and a capacitor. If the capacitor is pretty big, what you will notice is that, when you connect the battery, the light bulb will light up as current flows from the battery to the capacitor to charge it up. The bulb will get progressively dimmer and finally go out once the capacitor reaches its capacity. If you then remove the battery and replace it with a wire, current will flow from one plate of the capacitor to the other. The bulb will light initially and then dim as the capacitor discharges, until it is completely out.light bulbbattery

To show that a Charged Capacitor stores Energy 1.Set up as shown. 2.Close the switch to charge the capacitor. 3.Remove the battery and connect the terminals together to ‘short’ the circuit. 4.The bulb will flash as the capacitor discharges, showing that it stores energy

Farads 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. For this reason, capacitors are typically measured in microfarads

An electrical capacitor can be compared to this water container and the rate at which the potential of the capacitor increases gives us an indication of how much charge the capacitor can hold; if putting a small charge on it raises its potential considerably, then its capacitance must be small. Remember ‘raises its potential considerably’ means that a lot more work needs to be done to bring further charge up to it.

So for example if a capacitor has a capacitance of 2 farads, then putting a charge of 6 coulombs on it will increase its potential by 3 volts (from C = Q/V, so V = Q/C).

However if the capacitor had a capacitance of 200 farads, putting a charge of 6 coulombs on it would only raise its potential by 0.03 volts.

Energy stored in a charged Capacitor The energy stored (W) in a charged capacitor is given by: W = ½ CV 2

Why should I care about Capacitors? Uses: Radio,

Capacitive Touch Screens One of the more futuristic applications of capacitors is the capacitive touch screen. These are glass screens that have a very thin, transparent metallic coating. A built-in electrode pattern charges the screen so when touched, a current is drawn to the finger and creates a voltage drop. This exact location of the voltage drop is picked up by a controller and transmitted to a computer. These touch screens are commonly found in interactive building directories and more recently in Apple's iPhone.touch screencomputeriPhone

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. That's why the electronic flash on a camerauses a capacitor -- the battery charges up the flash's capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly. This can make a large, charged capacitor extremely dangerous -- flash units and TVs have warnings about opening them up for this reason. They contain big capacitors that can, potentially, kill you with the charge they containcameraTVs

The value of Capacitance depends on the shape and size of the plates This can be given by the formula: C = Ɛ A / d Ɛ= permitvity of the dielectric A= area of the plates D= distance of the

Example The area of overlap of the plates of an air space capacitor is 20cm2. the distance between the plates is 1mm. Given Ɛ = 8.9x10-12 Fm-1, find the capacitance of the capacitor.

1.[2007] Calculate the energy stored in a 5 μF capacitor when a potential difference of 20 V is applied to it. 1.[2005] A capacitor of capacitance 100 μF is charged to a potential difference of 20 V. What is the energy stored in the capacitor?

1.[2002] How much energy is stored in a 100 μF capacitor when it is charged to a potential difference of 12 V?

A capacitor of capacitance 0.47uF carries a charge of 2.0 uC. Calulate the potential diff between the plates and the energy stored

Formula Manipulation A capacitor has a capacitance of 6.3uF. What is the charge on the plates when the energy stored is 0.44 mJ?

To Demonstrate the Factors affecting the Capacitance of a Parallel Plate Capacitor 1.Connect the two parallel plates to a digital multi-meter (DMM) set to read capacitance. Note the capacitance. 2.Increase the distance between them – note that the capacitance decreases. 3.Move one plate slightly to the side (decreasing the overlap area) – note that the capacitance decreases. 4.Place different slabs of insulating material between the plates – note that the capacitance is lowest when nothing (air) is between the plates

Ɛ= permittivity of the dielectric Ɛ= Ɛr x Ɛo Where Ɛr is relative permittivity And Ɛo is permittivity of free space