Download IB 10.1 Fields Feb 9 Agenda

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 =
6.38 x 106 m)
Get out #1-8 for
HWK check
Objectives/Agenda/Assignment

Objective:

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

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.
Electric fields

The same relationships hold for electric fields as the ones we
derived for gravitational fields

𝑭𝑬 = 𝒌

E= 𝒌

and F = qE
𝑸
𝒓𝟐
𝑸𝒒
𝒓𝟐
Unit = N/C
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.
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
𝒓 𝑸𝒒
electrical force is given by ∞ 𝒌 𝒓𝒓 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(− 𝒓 − − ∞ ) =−

Work done BY electrical force is negative. (Corresponding work done by an outside
agent is postive.)
𝟏
𝟏
𝒌𝑸𝒒
𝒓
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 =

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.
𝒌𝑸𝒒
𝒓
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.

𝒌𝑸
𝒓

V=
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.
Summary of important formulas
Quantity
Force (Coulombs Law) (Vector)
(Negative means attractive force)
Electrical field (Vector)
Work done by electrical force for positive
charge ( to r) (Scalar)
Work done by external force ( to r)
(Work done to move charge to r) (Scalar)
Electrical potential energy (Scalar)
Electrical potential (Scalar)
Equation
𝑄𝑞
𝐹𝐸 = 𝑘 2
𝑟
𝑄
𝐸=𝐺 2
𝑟
𝑘𝑄𝑞
𝑊=−
𝑟
𝑘𝑄𝑞
𝑊=
𝑟
𝑘𝑄𝑞
𝐸𝑝,𝑒 =
= 𝑈𝐸
𝑟
𝑘𝑄
𝑉=
𝑟
Relating variables
𝐹𝐸 = 𝑞E
Parallel plates: 𝑉 = 𝐸𝑑
𝑊 = ∆𝐸𝑃 = 𝑞∆𝑉
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