Electric charge Forces between charged objects The field model and the electric field

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

• Electric charge

• Forces between charged objects

• The field model and the electric field

• Forces and torques on charged objects in electric fields

Chapter 20Electric Forces and Fields

Topics:

Sample question:

In electrophoresis, what force causes DNA fragments to migrate through the gel? How can an investigator adjust the migration rate?

Slide 20-1


Using Mastering Physics to improve scores & learning• Start assignments early. That way you will have time to get

help if needed.

• Don’t work solutions online if possible,Print them out and work offline

• Keep a notebook of good solutions to MP problems

• Don’t use up your submissions. Once you use your last submission, you are done with that problem.

• Don’t use up more than 2-3 submissions tweaking an answer, even if you are sure it right. If you can’t get tweaking 2x, then get help, something is probably wrong

• If you find yourself with only one submission left, email Dr. Saul and request more

• Corollary: Don’t be stuck for more than half an hour. After that put the problem aside and seek help when you can.

• Recall that you have 3 one-week, no-penalty extensions to use


Using Mastering Physics to improve scores & learning• Working for time

• Practice doing complete solutions (as much as you can) in 30 minutes for 7-10 days

• Then reduce the time 2-5 minutes for the next 7-10 days

• The goal is to be able to write a complete SPS solution in 20-25 minutes on exams


Force and Motion overview• Force and motion models

• General Motion Model

• Constant Velocity Model

• Constant Acceleration Model

• General Force Model

• Net Force Model


Force and Motion overview

• General Motion Model


Force and Motion overview

• Constant Velocity and Constant Acceleration Models


General Force Model


Net Force Model

Note: Use force and acceleration vector components when using Newton 2


Key Points for Force DiagramsFree-Body Diagram (FBD) or Force Diagram:Shows what forces act on an object(s) • Represent object as a dot• Represent each force with an arrow • (direction and relative length should be

correct to within finger accuracy)• Label each force with a label that indicates the type of force, the object

exerting the force, and the object being acted on

• Fnet should be visible to the side or above/below• Include coordinate axes as dashed lines

System Schema(Used to show all force interactions in a system, can also be used to define

system when using Conservation of Momentum or Conservation of Energy)

• Write out the name of each object in the system and draw a solid line around it

• Show the 2-way interactions (Newton 3rd Law pairs) between objects as double-headed arrows

• Label each interaction with the type of interaction (since you can see the objects, you do not need to list them)

• Draw a dotted line around the object(s) of interest (you can have more than one object of interest) (Hint: if you draw a free-body diagram for an object, it is an object of interest)


Problem Solving Strategy (PSS)When solving problems, it is important to show how you reasoned from the information

given in the problem and key physics ideas to your final answer. The correct final answer with units is only worth 1-3 points. The remainder of the points (70-90% of credit) are awarded for the quality of your solution. You are expected to include the following to receive full credit:

Prepare• Identify the Physics: State explicitly which physics’ principle(s) apply to the

problem situation and that you will use to solve the problem• Drawing a Picture: Draw at least one picture to visualize the physics of the

problem and define your variables and constants. For motion problems this could be a motion diagram, motion graph, or pictorial diagram

• Collecting Necessary Information: State all the information given in the problem with correct units. Include preliminary calculations such as unit conversions

• Assume/Observe: State assumptions or observations that would be useful

Solve• Start with key equation(s) in symbol form• Solve for the unknown quantity in symbols before numeric calculations• Then substitute numbers with units and calculate the numeric answer

Assess• Check to see if your answer is reasonable• Does it answer the question that was asked• Does it have the right units?


Solving Problems - Prepare (also identify key physics)


Solving Problems (continued)


Dryer Sheet Problem - SPS Problem

Slide 20-15

You and a friend are doing the laundry when you unload the dryer and the discussion comes around to static electricity. Your friend wants to get some idea of the amount of charge that causes static cling. You immediately take two empty soda cans, which each have a mass of 120 grams, from the recycling bin. You tie the cans to the two ends of a string (one to each end) and hang the center of the string over a nail sticking out of the wall. Each can now hangs straight down 30 cm from the nail. You take your flannel shirt from the dryer and touch it to the cans, which are touching each other. The cans move apart until they hang stationary at an angle of 10º from the vertical. Assuming that there are equal amounts of charge on each can, you now calculate the amount of charge transferred from your shirt


Ski Ramp - SPS Problem

Slide 20-15

You are working over winter break to design a new skiing ride at Cliff’s Amusement Park. The ride consists of a ramp that acts like a beginners ski hill. The ramp has 2 parts. Both parts are inclined at 15 degrees to the horizontal. The skiers will start at the top of the upper part of the ramp and ski down both parts of the ramp.

The upper part of the ramp is made from a low-friction (friction is negligible) surface so that skiers pick up speed going down the upper part of the ramp. The skiers should be at 40.0 MPH at the end of the upper part of the ramp. Assume the skier has a mass m = 75 kg.

The lower part of the ramp, which is 1000. m long, has just enough friction force so that the riders maintain constant speed over this part of the ramp. At the end of the 2nd part of the ramp is a horizontal braking area.

As part of the design of the ramp, determine the following:•How long the upper part of the ramp has to be so that the skiers reach 40.0 MPH starting from rest.•What is the magnitude and direction of the friction force of the lower ramp on the skier so that skiers on the lower ramp stay at constant velocity.•How long would it take skiers to reach the bottom of the ramp


Dryer Sheet Problem - SPS Problem

Slide 20-15

You and a friend are doing the laundry when you unload the dryer and the discussion comes around to static electricity. Your friend wants to get some idea of the amount of charge that causes static cling. You immediately take two empty soda cans, which each have a mass of 120 grams, from the recycling bin. You tie the cans to the two ends of a string (one to each end) and hang the center of the string over a nail sticking out of the wall. Each can now hangs straight down 30 cm from the nail. You take your flannel shirt from the dryer and touch it to the cans, which are touching each other. The cans move apart until they hang stationary at an angle of 10º from the vertical. Assuming that there are equal amounts of charge on each can, you now calculate the amount of charge transferred from your shirt


Van de Graff Generator

Slide 20-3


How to make an object move in a circle

Slide 6-14

Consider coordinate system with radial, tangential, and z components

Consider Force Diagrams for

•Ball on String on table

•Ball with plastic circle

•Ball on string hanging

•Ball on string in vertical circle


Uniform Circular Motion

Slide 6-13


Forces in Circular Motion

Slide 6-21


Circular Motion

There is an acceleration because the velocity is

changing direction.

Slide 3-35


Circular Motion

Old vinyl records are 12" in diameter, and spin at 33⅓ rpm when played. What’s the acceleration of a point on the edge of the

record?

Two friends are comparing the acceleration of their vehicles. Josh owns a Ford Mustang, which he clocks as doing 0 to 60 mph in a time of 5.6 seconds. Josie has a Mini Cooper that she claims is capable of a higher acceleration. When Josh laughs at her, she proceeds to drive her car in a tight circle at 13 mph. Which car

experiences a higher acceleration?

Slide 3-36


For uniform circular motion, the acceleration

A. points toward the center of the circle. B. points away from the circle. C. is tangent to the circle. D. is zero.


For uniform circular motion, the acceleration

A.points toward the center of the circle. B. points away from the circle. C. is tangent to the circle. D.is zero.


AccelerationThe average acceleration of a moving object is

defined as the vector

As an object moves, its velocity vector can change in two possible ways.

1.The magnitude of the velocity can change, indicating a change in speed, or

2. The direction of the velocity can change, indicating that the object has changed direction.


Tactics: Finding the acceleration vector


Uniform Circular Motion

Electric charge Forces between charged objects The field model and the electric field

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