Electric Fields,CircuitsElectric Fields,Circuits Electric Fields;

Emf,Currents,Potential Difference and Multiloop

CircuitsLecture 21

Thursday: 1 April 2004

Coulomb’s LawCoulomb’s Law

another. one repel charges like and

attract oppositesby determined isDirection

C/mN1099.84

1

4

1

229

0

221

0

r

qqF

What is the force on q0 due to q if q= 2 C , qo = 3 C and the distance between them is 3 meters?

How about if qo = 3 C ?

How about if qo = 1.5 C? …..

Calculating the force exerted by q on any given charge you might place at the location of qo gets repetitive. The only thing that changes is the value of qo.

Recall the Coulomb Force Problem on Two Charges

20

todue on 4

10 r

qqF o

qq

q qo

The Electric Field

• We define the electric field associated with a charge or charge distribution to be the electrostatic force exerted per unit of charge on which the force acts.

• If we know E, the force, F, on any charge, qo, is then given by:

• We can calculate E once and then get F for any charge easily.

qoqq qo

EF

o

qqq q

oF

E

The Electric FieldThe Electric Field• From a point charge q, the force on q0 is,

• Then, at q0’s location, the field associated with q is

• Field is a vector. The direction is away from a positive q and toward a negative q. It is also always in the direction of F on a positive charge.

0q

FE

20

04

1

r

qqF

200 4

1

r

q

q

FE

q qo

EF

oq

ExampleExample

ExampleExampleT qE

T mg

T

T

qE

mg

qE

mg

sin

cos

sin

cos

tan