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W02D1 Electric Dipoles and Continuous Charge Distributions

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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

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Electric Dipoles

Force and Torque on Dipole

Continuous Charge Distributions

http://youtu.be/EMj10YIjkaY

Demonstration: Dipole in a Van de Graaff Generator D22

E

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

E

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

Continuous Charge Distributions

Break distribution into parts:

V

E field at P due to Dq

Superposition:

- 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?

E field on perpendicular bisector

E field off axis

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

Concept Question Electric Field of a Rod

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

Concept Question Electric Field of a Rod: Answer

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.

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

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

E field on the axis of the 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?

Concept Question Electric Field of a 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?

Concept Question Electric Field of a Ring: Answer

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.

1) Think about it

Symmetry!

2) Define Variables

3) Write Equation

4) Integrate

This particular problem is a very special case because

everything except dq is constant, and

5) Clean Up

6) Check Limit

P on axis of disk of charge, x from center

Radius R, charge density s.

Find E at P

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

- 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