Ch 21Electric Fields

Evidence of Electric Fields• Sailors in the time of

Columbus would occasionally witness the ends of their masts glowing. It would appear to be on fire but not burning. Sailors learned that this meant there was an approaching lightening storm.

Other Evidence of Electric Fields

21.1Creating and Measuring Electric Fields

Electric Fields

•Michael Faraday developed the concept of an electric field

•Faraday discovered that a charged object creates an electric field in all direction around it

Electric Fields

•Electric field (E): a vector quantity that relates the force exerted on a test charge to the size of the test charge

•Magnitude of an electric field is measured in newtons per coulomb (N/C)

'q

FE

Example

•A negative charge of 2.0 x 10 -8 C experiences a force of 0.060 N to the right in an electric field. What are the field magnitude and direction?

Electric Field Lines

•The direction of an electric field is in the direction of the force on a positive test charge▫Which direction would a positive charge go

if put near that charge▫Has to be a small charge so that it doesn’t

mess with the field of the charge being tested

Electric Field Lines

•Direction shown by direction of arrows•Strength shown by spacing between lines

Electric Field Lines

Electric Field Lines

•Note: field lines do not really exist

21.2 Application of Electric Fields

Electric Potential Energy

•Just like gravitational potential energy increases with distance, so essentially does electric potential

•It requires work to separate two charges and the work is stored as potential difference

'q

WV

Electric Potential Difference

•Unit is joules per coulomb•1 joule per coulomb is 1 volt

•Electric Potential Difference in a Uniform Field

'q

WV

EdV

Example

•Two parallel plates are given opposite charges. A voltmeter measures the electric potential difference to be 60.0 V. The plates are 3.0 cm apart. What is the magnitude of the electric field between them?

Example

•Two large, charged parallel plates are 4.0 cm apart. The magnitude of the electric field between the plates is 625 N/C.▫What is the electric potential difference

between the plates?▫What work will you do to move a charge

equal to that of one proton from the negative to the positive plate?

Storing Charges

• A device used to store charge is called a capacitor.▫A device designed to have a specific

capacitance Capacitance (C): the ratio of an object’s store

charge to its potential difference Measured in farads, F

V

qC

Example

•A sphere has an electric potential difference between it and Earth of 60.0 V when it has been charged to 3.0 x 10 -6 C. What is its capacitance?

Homework

•P502: 2, 6, 12, 23, 25, 32, 33