Attendance Quiz: Sec 21-3, 21-4

Two particles with charges 1 = +2 and 2 =+ 3 are separated by a distance of . What is the magnitude of the electric force that 1 exerts on 2? Express your answer in terms of , 0 , , .

What is the magnitude of the electric field at a field point a distance 2r from a point charge 1 = 4Q? Express your answer in terms of , 0 , , .

Coulombs Law

Objective

Calculate the net electric force on a point charge exerted by a system of point charges

Coulombs Law

k = constant = 1/(40)

0 = permittivity of free space

SI unit of electric force: Newton (N)

Physics 71: Newtons Law of gravity

(both are inverse-square laws)

What if there are more than 2 point charges?

Use principle of superposition of forces

Use vector addition (NOT scalar addition)

Example 1

Find the magnitude and direction of the net electric

force on charge A.

Example 2

Two identical particles with charge +3 are on the y-axis, one at the origin and the other at y=6 m(meters). A third charge 3 = +2 in on the x-axis at x=8 m.

What is the magnitude and direction of the net electric force exerted on 3? Answer must be in vector form.

Approach1. Make a sketch for the problem and draw the corresponding

FBD with proper labels. Identify and indicate all needed information in the FBD.

2. Obtain the force contribution one by one. Write the force vector in component form.

3. Magnitude is obtained using Coulombs Law.

Note: magnitude is always a positive value

4. Direction is obtained using trigonometry.

5. NET force vector obtained using superposition principle.

Try for yourself: Same as Example 2, except that one is at y=-6 m instead

of at the origin. So: Two identical particles with charge 3 are on the y-axis, one at y=-6 m and the other at y=6 m.

A third charge 3 = 2 in on the x-axis at x=8 m. What is the magnitude and direction of the net electric force exerted on 3? Answer must be in vector form.

HINT: Exploit the symmetry of the problem.

Objective

Calculate the net electric force on a point charge exerted by a system of point charges

Electric Field & Electric Forces

Objectives

Describe the electric field due to a point charge quantitatively and qualitatively

Establish the relationship between the electric field & the electric force on a test charge

Predict the trajectory of a massive point charge in a uniform electric field

Electric field

a vector

intermediary for the electric force

aura of electric charges

SI unit of electric field: Newton per Coulomb (N/C)

Vector fields IRL

Electric field of a point charge

inverse-square relation

Electric field of a point charge

E points away from a positive charge

E points toward a negative charge

Electric field of 2 point charges(opposite signs, same magnitude)

Electric field of 2 point charges(opposite signs, same magnitude)

-lines should be directed

outwards from a + charge

towards a charge

-lines should not intersect

-same # of lines from/to point

charge

Electric field of 2 point charges(same signs, same magnitude)

Sharks can locate prey that are completely hidden beneath

the ocean sand by sensing weak electric fields produced by

the muscle contractions in their prey.

Their sensitivity to electric fields are derived from jelly-filled

canals in their bodies.

Electric field & Electric force

When a point charge q is placed in a region with an electric field E it will experience an electric force

Motion trajectory prediction

What is the trajectory of an electron, initially at rest, in the region of uniform electric field?

What is the trajectory of an electron as it passes through the region of uniform electric field?

Motion trajectory prediction

Recall: Kinematic equations and Newtons 2nd Law

2, =

Get an equation for a parabola, like projectile trajectory.

Motion trajectory prediction

What is the trajectory of an electron as it passes through the region of uniform electric field?

Motion trajectory prediction

What is the trajectory of a proton as it passes through the region of uniform electric field?

Motion trajectory prediction

Motion trajectory predictionWhat is the trajectory of a proton as it passes through the region of uniform electric field?

Objectives

Describe the electric field due to a point charge quantitatively and qualitatively

Establish the relationship between the electric field & the electric force on a test charge

Predict the trajectory of a massive point charge in a uniform electric field

End.

Announcements

Attendance Quiz coverage for Tuesday, January 27:

21-5: Electric Field Calculations

UVLe: Physics 72 Caeso (2s AY 14-15)

Enrolment key: physics72

Submit 1st Requirement on January 27 (index cards, prereq check)