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W02D1 Electric Dipoles and Continuous Charge Distributions

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### Announcements

### Concept Question Electric Field of a Rod

### Concept Question Electric Field of a Rod: Answer

### Concept Question Electric Field of a Ring

### Concept Question Electric Field of a Ring: Answer

Math Review Tuesday Tues Feb 14 from 9-11 pm in 32-082

PS 1 due Tuesday Tues Feb 14 at 9 pm in boxes outside 32-082 or 26-152

W02D2 Reading Assignment Course Notes: Chapter Course Notes: Sections 4.1-4.2, 4.7

Make sure your clicker is registered

Nature Likes to Make Dipoles

http://youtu.be/EMj10YIjkaY

Concept Question: Dipole in Non-Uniform Field

A dipole sits in a non-uniform electric field E

Due to the electric field this dipole will feel:

- force but no torque
- no force but a torque
- both a force and a torque
- neither a force nor a torque

Concept Question Answer: Non-Uniform Field

Answer: 3. both force and torque

Because the field is non-uniform, the forces on the two equal but opposite point charges do not cancel.

As always, the dipole wants to rotate to align with the field – there is a torque on the dipole as well

Group Problem: Charge Densities

- A solid cylinder, of length L and radius R, is uniformly charged with total charge Q.
- What is the volume charge density ρ?
- What is the linear charge density λ?
- What is the relationship between these two densities ρ and λ?

Examples of Continuous Sources: Finite Line of Charge

E field on perpendicular bisector

Examples of Continuous Sources: Finite Line of Charge

E field off axis

Examples of Continuous Sources: Finite Line of Charge

Grass seeds of total E field

A rod of length L lies along the x-axis with its left end at the origin. The rod has a uniform charge density λ. Which of the following expressions best describes the electric field at the point P

A rod of length L lies along the x-axis with its left end at the origin. The rod has a uniform charge density λ. Which of the following expressions best describes the electric field at the point P

Group Problem: Line of Charge

Point P lies on perpendicular bisector of uniformly charged line of length L, a distance s away. The charge on the line is Q. Find an integral expression for the direction and magnitude of the electric field at P.

Hint on Line of Charge Group Problem

Typically give the integration variable (x’) a “primed” variable name. ALSO: Difficult integral (trig. sub.)

E Field from Line of Charge

Limits: s >> L (far away) and s << L (close)

Looks like the E field of a point charge if we are far away

Looks like E field of an infinite charged line if we are close

Examples of Continuous Sources: Ring of Charge

E field on the axis of the ring of charge

Examples of Continuous Sources: Ring of Charge

E field off axis and grass seeds plot

A uniformly charged ring of radius a has total charge Q. Which of the following expressions best describes the electric field at the point P located at the center of the ring?

A uniformly charged ring of radius a has total charge Q. Which of the following expressions best describes the electric field at the point P located at the center of the ring?

Demonstration Problem: Ring of Charge

Aring of radius a is uniformly charged with total charge Q. Find the direction and magnitude of the electric field at the point P lying a distance x from the center of the ring along the axis of symmetry of the ring.

Ring of Charge

3) Write Equation

Ring of Charge

4) Integrate

This particular problem is a very special case because

everything except dq is constant, and

Group Problem: Uniformly Charged Disk

P on axis of disk of charge, x from center

Radius R, charge density s.

Find E at P

Disk: Two Important Limits

Limits: x >> R (far) and x << R (close)

Looks like E of a point charge far away

Looks like E field of an infinite charged plane close up

Scaling: E for Plane is Constant

- Dipole: E falls off like 1/r3
- Point charge: E falls off like 1/r2
- Line of charge: E falls off like 1/r
- Plane of charge: E constant

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