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# MIDTERM 1 UTC 4.132 Thu-Sep 27 , 7:00PM - 9:00PM - PowerPoint PPT Presentation

MIDTERM 1 UTC 4.132 Thu-Sep 27 , 7:00PM - 9:00PM. Bring pencils, calculators (memory cleared). Course Summary Unit 1 Provided. Question 1. Which one of these statements is true ? When near the center of the object, the electric field hardly changes with increasing distance from.

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### MIDTERM 1 UTC 4.132 Thu-Sep 27, 7:00PM - 9:00PM

Bring pencils, calculators (memory cleared)

Course Summary Unit 1 Provided

Which one of these statements is true?

When near the center of the object, the electric field hardly changes with increasing distance from

a uniformly charged long rod.

a uniformly charged ring.

a uniformly charged disk.

a point charge.

-Q

+Q

metal

plastic

A solid metal ball carrying negative excess charge is placed near a uniformly charged plastic ball.

Which one of the following statements is true?

The electric field inside both balls is zero.

The electric field inside the metal ball is zero, but it is nonzero inside the plastic ball.

The electric fields inside both objects are nonzero and are pointing toward each other.

The electric field inside the plastic ball is zero, but it is nonzero inside the metal ball.

1) Vector Arithmetic

Find = <observation loc.> - <source loc.>

2) Electric Force and Field of a Charged Particle

-Q

y

3) Dipoles

-Q

+Q

x

from – to +

Electric field of a dipole

Forces on a dipole

Electric field due to something else.

Calculate force on each Charge

4) Induced Dipoles

Know and gives

– +

Example: Force between charge and induced dipole

4) Polarization of Metals and Insulators

+

+

+

+

Metal

Metal

+

+

+

+

+

Insulator

+

Insulator

+

+

Neutral

Excess +

What is E inside metal in static equilibrium?

General Procedure for Calculating Electric Field of Distributed Charges

• Cut the charge distribution into pieces for which the field is known

• Write an expression for the electric field due to one piece

• (i) Choose origin

• (ii) Write an expression for DE and its components

• Add up the contributions of all the pieces

• (i) Try to integrate symbolically

• (ii) If impossible – integrate numerically

• Check the results:

• (i) Direction

• (ii) Units

• (iii) Special cases

Review Distributed Charges

6) Flux and Gauss’ Law

Examples: Spherical Shell, Long Rod, Large Sheet

Review Distributed Charges

6) Gauss’s Law and Metals

Use E=0 inside metals

Examples: Neutral Metal Solid with Void; Charge in Void

7) Superposition

Example: Large Sheet and Infinite Rod

X

X

X