Charge

1. Describe the nature of charge.

• Like repel, opposites attract
• Freely moves in conductors, not free in insulators
• Conserved
• Quantized
• Analogous to mass in many equations

2. State the SI unit for charge. The Coulomb. 1 Coulomb is a very large amount of charge.
3. Explain what is meant by the term static charge.
4. State that moving flow of charge is called a current.
5. Explain the difference in charging an object by induction and charging it by conduction.
6. Compare the electrostatic forces of Coulomb's Law to the gravity forces of Newton's law of gravity.

   Coulomb's Law	Newton's Law of Gravity
   F = [ k (q1) q2 ] / r2	F = [ G (m1) m2 ] / r2
   valid for point charge	valid for point mass
   F = magnitude of a member of an action/reaction pair of forces	F = magnitude of a member of an action/reaction pair of forces
   F is inversely proportional to r2	F is inversely proportional to r2

    

7. Calculate electrostatic forces using Coulomb's law.

   F = [ k (q₁) q₂ ] / r²

   k   = 9.0 (10^9) N m^2 / C^2

   G = 6.7 (10^-11) m^3/ kg / s^2

    

Homefun (formative/summative assessment): Read sections , problems 42, 46, 47 page 559

Formative Assessment: Physics Investigation

Follow up Questions

 

E-Field

8. State and explain the mathematical definition of E-field.

E = F/Q

E = the electrostatic force per unit of charge

E is a vector

The direction of E is shown for a positive charge

9. Describe how rays (field lines) can be used to represent force fields.

   the direction of the rays represent the direction of the field

   the spacing of the rays can be used for representing the magnitude of the field. close spacing = high field strength.

   the direction and magnitude of the field is directly related to the magnitude and direction of the forces created on an object by the field.

10. Compare E-field to g-field using ray sketches.

    charge or mass at a point

    charge or mass uniformly distributed in an infinite plane (flat Earth model)

11. Compare E-field to g-field using the equations for E-field and g-field created by a point source.

12. Compare E-field to g-field using sketches of field strength from a point source vs. distance.

Relevance: A broadcast signal electronically fed into an antennae will cause electrons to move back and forth in the antennae, creating a moving E-field in the process. This in turn creates a force on electrons in an antennae at a distance, causing them to move up and down in a similar fashion. Electronic equipment attached to the distant antennae can then detect play the broadcast. This is the basis of all TV, radio, wifi, and other wireless systems.

Homefun (formative/summative assessment) . problems 67, 68, 69 page 585

Review

Formative Assessments:

1. Work review problems at the board

2. Work practice problems.

Metacognition Problem Solving Question: Can I still work the problems done in class, several hours or days later? Some amount of repetition on the exact same problems is necessary to lock in learning. It is often better to thoroughly understand a single example of a problem type than to work example after example understanding none of them completely.

Relevance: Good test preparation is essential to performance in physics class.

Homefun (formative/summative assessment): turn in on the day stapled to the back of the test.

Summative Assessment: Unit exam objectives 1-23