Electric Fields and Potential

• Electric field – a force field that fills the space around every electric charge or charges

• Example:Electron

Proton

• An electric field has both magnitude and direction

• Therefore, an electric field can be represented using…

VECTORS!!!

Force Field LinesForce Field Lines

• The vectors are drawn by how they will affect a small positivepositive test charge

A positive charge in the presence of a negative charge

Two positive charges

Two negative charges

• Field is strongest where the force is the strongest – where the lines are the most concentrated (closest together)

Electric field lines indicate strength

A

So… the strength of the electric field, E, is given by

Electric Field = Force ÷ charge

E = F ÷ q

For example:

A 0.5 C charge experiences a force of 20 N when placed in an electric field.

What is the strength of the electric field, E?

E = F ÷ q =

20 N ÷ 0.5 C =

40 N/C

The electric field near a charged piece of plastic or styrofoam is around 1000 N/C.

The electric field in a television picture tube is around 10,000 N/C.

The electric field at the location of the electron in a Hydrogen atom is 500,000,000,000 N/C!

The further you go from an electric charge, the weaker the field becomes.

Electric ShieldingThere is no way to shield from

gravity, but there is a way to shield from an electric field….

Surround yourself or whatever you wish to shield with a conductor (even if it is more like a cage that a solid surface)

That’s why certain electric components are enclosed in metal boxes and even certain cables, like coaxial cables have a metal covering.

The covering shields them from all outside electrical activity.

Michael Faraday, 1791-1867Michael Faraday demonstrated

that the electrostatic charge only resides on the exterior of a charged conductor, and exterior charge has no influence on anything enclosed within a conductor.

This was one of many contributions he made to electromagnetic theory.

“Faraday Cage”

Electric ShieldingElectric ShieldingElectrons repel toward the outside of any conducting surfaceNet charge inside is zeroElectrons flow outward evenly, but pile up on sharp corners

Shielding is important in electronic devices such as televisions and computers

Person in a car hit by artificial lightning. The lightning strikes the car and jumps to the ground bypassing the front tire arcing from the axle to the ground.

Voltage

• Electric potential is commonly called VOLTAGE.

Rub a balloon on your hair and it becomes negatively charged, perhaps to several thousand volts. Does this mean that there’s a lot of electrical energy?Well, the charge transferred to the balloon is typically less than a millionth of a Coulomb.

There’s a LOT of difference between Voltage and Energy!

High Voltage does not necessarily mean that there’s a lot of useful energy or that something is dangerous.

Remember, one Coulomb is a HUGE amount of energy!

• Voltage = Energy / charge• Energy = Voltage x charge

• Energy = 3000 V x 0.000001 C

• Energy = 0.003 J

• That’s not much energy!

The Electric Potential (Voltage), V, changes as you move from one place to another in an

electric field

The change in Potential (“pressure”), called the “Potential Difference” is given by

V = Ed

For example, the potential difference between two locations separated by 3 meters in a 4000 N/C electric field is given byV = Ed = 4000 N/C x 3 m = 12,000 V3 meters

Electric Field

Accelerating Charges

A charge placed in an electric field will experience an electric force,

F = Eq

This force will make the charge accelerate according to Newton’s Second Law

F = ma

The Electric Field can also be determined by using Coulomb’s Law:

The Electric Field can also be determined by using Coulomb’s Law:

2

221

d

qk

qdqq

k

q

FE

• Static devices– Electroscope: the

separation of metal leaves indicates the presence of static charge

– Van de Graaff generator: charge is delivered by a rubber belt to a metal dome

– Electrophorus a device used to transfer electric charge

Capacitor – a device used to store electrical energy

Capacitors are Used in– camera flashes– defibrillators– Computers: tiny capacitors store

the 1’s and 0’s for the binary code– Many keyboards have a capacitor

beneath each key that records every key stroke.

– Virtually every electronic device

Vocabulary

• Capacitor

• Electric field

• Electric potential energy

• Electric potential

• Volt

• voltage