Unit 10: Electric Fields

Unit 10: Electric FieldsUnit 10: Electric Fields

An electric force is a field force.

Recall the difference between a fieldforce and a contact force.

◦Field forces act through space, producing aneffect even when there is no physicalcontact between the objects involved.

Unit 10: Electric FieldsUnit 10: Electric Fields

Strength of an electric field and itseffect on q0:

Unit 10: Electric FieldsUnit 10: Electric Fields

Strength of an electric field (caused byQ) and its effect on q0:

Unit 10: Electric FieldsUnit 10: Electric Fields

Electric field is a vector quantity

It has units of N / C

We consider point charges like q0 incases like this because if q0 is too largethen it will cause redistribution ofcharges in Q

Unit 10: Electric FieldsUnit 10: Electric Fields

Electric field strength depends oncharge and distance

Unit 10: Electric FieldsUnit 10: Electric Fields

Unit 10: Electric FieldsUnit 10: Electric Fields

Unit 10: Electric FieldsUnit 10: Electric Fields

Electric field is a vector

If q is positive, the field due to thischarge is directed outward radiallyfrom q.

If q is negative, the field is directedtoward q.

Unit 10: Electric FieldsUnit 10: Electric Fields

Example:

A charge q1 = +7.00 μC is at the originand a charge q2 = -5.00 μC is on the x-axis 0.300 m from the origin. Find theelectric field strength at point P, whichis on the y-axis 0.400 m from the origin.

(2.71x105 N/C at an angle of 66.6˚)

Unit 10: Electric FieldsUnit 10: Electric Fields

Electric field is a vector

If q is positive, the field due to thischarge is directed outward radiallyfrom q.

If q is negative, the field is directedtoward q.

Unit 10: Electric FieldsUnit 10: Electric Fields

A nice way to visualize electric fieldpatterns is to use lines in the directionof the field.

Unit 10: Electric FieldsUnit 10: Electric Fields

The number of fieldlines is proportionalto the electric fieldstrength.

The strength isstrongest when thelines are closesttogether.

Unit 10: Electric FieldsUnit 10: Electric Fields

Examples:

Sketch electric field diagrams for two chargesof equal magnitude but one is positive and oneis negative.

Sketch electric field diagrams for two chargesof equal magnitude but both are positive.

Sketch electric field diagrams for two chargesof where one charge is 3 times the magnitudeof the other and both are negative.