Potential Difference & Capacitance

March 9th, 2009

Electrical Potential

The electrical potential energy associated with a charge particle, divided by the charge of the particle

V = PEelectric / q

Electrical Potential vs. Gravitational Potential

It might help you understand electrical potential to think about it like gravitational potential

When does an object have high gravitational potential? Where do objects like to come to rest? With high

potential energy or low potential energy?

It requires energy to move an object from an area of low potential energy to an area of high potential energy

Electrical PotentialThe same is true for charged particlesThink about a positive field, where will a

positive test charge have have the highest potential energy? Lowest potential energy?

HINT: Is this force attractive like gravity???

Think you got it? Try these!For each picture determine:

If work is being done on the charge Where the Electric potential is greatest

Potential Difference Think about this picture:

As the charge moves from point A to point B work is done, and the charge now has more potential energy

This can be represented by the equation below

The unit of potential difference is the Volt (V) which is a J/C

Potential Difference in a Uniform Electric FieldV = -Ed

The gap between electrodes in a spark plug is 0.060 cm. To produce an electric spark there must be an electric field of 3.0 x 106 V/m. What minimum potential difference must be applied to start a car?

Potential Difference & Batteries

The role of a battery is to provide that potential difference

Capacitors and Charge Storage

C = q/V

Capacitor: ability of a conductorsto store energy in the form of electrically separated charges.

Capacitors & Batteries Capacitors are similar to batteries because they store electrical

energy.

A battery can produce electrons, but a capacitor just stores them

Think of it like a water tower. A water tower "stores" water pressure -- when the water system

pumps produce more water than a town needs, the excess is stored in the water tower.

At times of high demand, the excess water flows out of the tower to keep the pressure up.

A capacitor stores electrons in the same way and can then release them later.

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 camera uses 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 contain.

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.

Keeping all your vocab straight… In a 12 V car battery the POTENTIAL DIFFERENCE between the terminals is 12 V

As a charge moves through the battery (from negative to positive) the field inside the battery does work to move the charge and the potential energy of the charge increases (from 0 to 12 volts)

Since a Volt is J/C a 1 C charge would now have 12J of electrical potential energy

The ELECTRICAL POTENTIAL ENERGY of a charge changes as it moves through the circuit Increases as it moves through the battery Decreases as it moves through external devices, they

consume the 12 J of energy so when the charge returns to the battery it now has 0 J of potential energy.

Battery

Stereo