1. Physics 4 – Feb 9, 2017
P3 Challenge –What is the gravitational potential at a distance from Earth’s center equal to 3 Earth radii? (Me = 5.97 x 1024 kg, Re = x 106 m)
Get out #1-8 for
HWK check

2. Objectives/Agenda/Assignment
Ch 10.1 Fields - electrical
Assignment: p412 #9-21 (not due until Feb 21)
Time to work on IA projects today and Friday.
IA class reports on Tues and Thurs next week.
Following week: Review and electricity test Feb 2
Agenda:
Homework Review
More on electrical fields
Time to work on either electrical fields assignment or IA project reports

3. IA Reports
Tuesday and Thursday we will have reports for your IA projects. You should be done collecting data and know what kinds of conclusions you have.
Prepare a 10 min presentation for your project to explain what you have done, what data you collected, how you analyzed your data, and what conclusions you made.
You may either prepare a PowerPoint or display a digital copy of your IA paper and walk us through that.
Be prepared to give feedback, ask questions for clarification, etc.. I will do the same.
You will have time after getting feedback to make changes/adjustments to your project before uploading your project by March 1.
Sign up for a slot on either Tuesday or Thursday next week. 7-8 per day.

4. Electric fields
The same relationships hold for electric fields as the ones we derived for gravitational fields
𝑭 𝑬 =𝒌 𝑸𝒒 𝒓 𝟐
E=𝒌 𝑸 𝒓 𝟐 Unit = N/C
and F = qE

5. Electric Field Diagrams
Field diagrams are similar except that there are positive and negative charges.
Field lines are pointed towards negative charges just like gravitational field lines, but point away from positive charges.

6. Work from infinity…and beyond
Consider a positive point charge, q located an infinite distance away from a positive charge Q.
As q is moved from infinity to a position r away from M, the work done by the electrical force is given by ∞ 𝒓 𝒌 𝑸𝒒 𝒓 𝒓 𝒅𝒓 (FE is positive because it is a repulsive force.)
To evaluate this, k, Q, and q are all constants, so they move out front. The integral of 1/x2 dx is -1/x. and we’ll evaluate it from  to r
𝑾=𝒌𝑸𝒒(−𝟏/𝒓 | ∞ 𝒓 ) = 𝐤𝐐𝐪(−𝟏/𝒓 | ∞ 𝒓 ) =𝐤𝐐𝐪(− 𝟏 𝒓 −− 𝟏 ∞ ) =− 𝒌𝑸𝒒 𝒓
Work done BY electrical force is negative. (Corresponding work done by an outside agent is postive.)

7. Electrical potential energy
Every conservative force has a corresponding potential energy defined such that the potential energy is the opposite of the work done by the conservative force.
Predicts: UE = 𝒌𝑸𝒒 𝒓
Note: Same result. Take away: Electrical potential energy is a positive quantity for positive charges and a negative quantity for negative charges. It is also a scalar.
Creates a electrical potential energy well around negative charges and electrical potential energy hills around positive charges.

8. Electrical potential
The electrical potential at point P in an electrical field is the work done per unit charge in bringing a small test charge q from infinity to point P.
So just divide the potential energy by q. You get V.
V = 𝒌𝑸 𝒓 Unit is J/C.
Notice there are two kinds of lines here. Radial field lines and circular equipotential lines. These two kinds of lines are perpendicular to one another.

9. Summary of important formulas
Quantity
Equation
Relating variables
Force (Coulombs Law) (Vector)
(Negative means attractive force)
𝐹 𝐸 =𝑘 𝑄𝑞 𝑟 2
𝐹 𝐸 =𝑞E
Electrical field (Vector)
𝐸=𝐺 𝑄 𝑟 2
Parallel plates: 𝑉=𝐸𝑑
Work done by electrical force for positive charge ( to r) (Scalar)
𝑊=− 𝑘𝑄𝑞 𝑟
Work done by external force ( to r)
(Work done to move charge to r) (Scalar)
𝑊= 𝑘𝑄𝑞 𝑟
Electrical potential energy (Scalar)
𝐸 𝑝,𝑒 = 𝑘𝑄𝑞 𝑟 = 𝑈 𝐸
𝑊=∆ 𝐸 𝑃 =𝑞∆ 𝑉
Electrical potential (Scalar)
𝑉 = 𝑘𝑄 𝑟

10. Exit slip and homework Exit Slip –
What’s due? (homework for a homework check next class)
Ch 10.1 p412 #9-21 (not due until Feb 21)
What’s next? (What to read to prepare for the next class)
Start studying for Electrical Test on Feb 23
IA project reports