Lab Letter #2
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Week6-ElectricEnergyPotential.pdf
Week 6?!
Week 7 Feb. 12 - 15
NO SCHOOL
Mastering Physics (MP) HW 2/6 due midnight
Canvas Picture Upload HW 2/6 due midnight*
Reading Assign. (RA) 2/9
due midnight Video Example (1/qtr - youtube) due midnight**
Quiz 3 2/7
Reading Assign. (RA) 2/7
due midnight
**Goal is Friday - hard cut off Saturday
Week 6 Feb. 5 - 8
Reading Assign. (RA) 2/16
Video Example (1/qtr - youtube)
**
Mastering Physics (MP) HW 2/13
Canvas Picture Upload HW 2/13
Reading Assign. (RA) 2/14
Let’s go back to energy...
On an individual whiteboard:
● Write the names of the four types of energy we learned about last quarter.
On an individual whiteboard:
● Write the equation to calculate ○ Kinetic energy ○ Gravitational energy ○ Spring Energy
And those Bar Charts... The bar chart for a process where energy is conserved is shown. What is wrong with the bar chart?
Activity - Electric Energy
Week 7 Feb. 12 - 15
NO SCHOOL
Mastering Physics (MP) HW 2/6 due midnight
Canvas Picture Upload HW 2/6 due midnight*
Reading Assign. (RA) 2/9
due midnight Video Example (1/qtr - youtube) due midnight**
Quiz 3 2/7
Reading Assign. (RA) 2/7
due midnight
**Goal is Friday - hard cut off Saturday
Week 6 Feb. 5 - 8
Reading Assign. (RA) 2/16
Video Example (1/qtr - youtube)
**
Mastering Physics (MP) HW 2/13
Canvas Picture Upload HW 2/13
Reading Assign. (RA) 2/14Lab Letter 2/12
BA +q -q +x
+y
(A) (B) (C) (D)
+x+x+x+x
Uelec Uelec Uelec Uelec
Charge A is fixed in place and charge B is free to move along the x-axis. How will the electric energy of charge B vary as it moves along the x-axis?
(A) (B)
(C) (D)
Charges A and B are fixed on the x-axis, while charge C can move. The graph shows the electric energy of charge C as it moves along the x-axis. Which is the correct combination of signs for all three charges?
+x
Uelec
BA + - +x+
C
BA + - +x
C -
BA + - +x+
C
BA - +x
C --
Which graph shows the electric energy as a function of position for a proton placed on the gray line?
(A) (B) (C)
+x
Uelec
+x
Uelec
+x
Uelec
Last Quarter
How did we relate force and energy?
Last Quarter
How did we relate force and energy?
Work!
Calculate the work done by one positive charge pushing another positive charge from ri = r to rf = ∞
+ +
r = 0 ri
r
+ +
r = 0 rf
r
Hint: What is the direction of the force? The displacement? Hint: How do we calculate electric force?
Practice Problem 1 - Electric Energy of Point Charges Find the electric energy for only the B and C charges in the figure, where d = 3 cm, q = 2 nC, and θ = 70º.
1. Draw the picture including all the labels. 2. Write an expression for the electric energy, Uelec. 3. Assess your answer.
+2q + -
+ +q
-4qd
A
B Cθ θ
Uelec = -9.6x10 -6 J
Practice Problem 2 - Electric Energy of Point Charges
Find the total electric energy for the 3 point charges in the figure, where d = 3 cm, q = 2 nC, and θ = 70º.
1. Draw the picture including all the labels. 2. Label the unknown distances between the charges, rAB
and rAC 3. Write an expression for the total electric energy, Uelec,
in terms of the electric energy between each pair of charges, UAB, UAC, and UBC.
+2q + -
+ +q
-4qd
A
B Cθ θ
Practice Problem 2 - Electric Energy of Point Charges
Find the total electric energy for the 3 point charges in the figure, where d = 3 cm, q = 2 nC, and θ = 70º.
4. Use our equation for Uelec of two point charges to write each Uelec term in terms of qA, qB, qC, and rAB, rAC, and d.
5. Use trig to calculate rAB and rAC. 6. Calculate the total electric energy. 7. Assess your answer in at least two ways.
+2q + -
+ +q
-4qd
A
B Cθ θ
Uelec = -1.1x10 -5 J
Week 7 Feb. 12 - 15
NO SCHOOL
Mastering Physics (MP) HW 2/6 due midnight
Canvas Picture Upload HW 2/6 due midnight*
Reading Assign. (RA) 2/9
due midnight Video Example (1/qtr - youtube) due midnight**
Quiz 3 2/7
Reading Assign. (RA) 2/7
due midnight
**Goal is Friday - hard cut off Saturday
Week 6 Feb. 5 - 8
Reading Assign. (RA) 2/16
Video Example (1/qtr - youtube)
**
Mastering Physics (MP) HW 2/13
Canvas Picture Upload HW 2/13
Reading Assign. (RA) 2/14Lab Letter 2/12
0 ft
-100ft
-200ft -300ft
-400ft
100ft
200ft
300ft
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500ft
Lab Letter!!
0 ft
-100ft
-200ft -300ft
-400ft
100ft
200ft
300ft
400ft
500ft
Sketch a graph of elevation vs position along the red line.
Energy per kg - the “gy” thing
We can create a similar idea fro electric energy that doesn’t depend on a second charge.
Energy per Coulomb Uelec = kqsource qprobe r
the “kq/r” thing
Electric Potential! (energy per cat)
Use symbol capital V (with wings!) Units of Joules/Coulomb (J/C)
● also referred to as a Volt (V)
Caution! There are lots of the letter v running around now; velocity, electric potential, the unit symbol for the volt. The context is important!
The charges have the same magnitude. Rank the the electric potential at points 1-4 in the figure.
1 2 3
4(A) 2 > 4 > 1 = 3 (B) 3 > 2 > 4 > 1
(C) 2 > 4 > 3 = 1
(D) 3 > 2 = 4 > 1
(E) 3 = 1 > 4 =2
In which region(s), 1, 2, or 3, along the gray line is the electric potential zero?
1 2 3
(A) 1
(B) 2
(C) 3
(D) 1 & 2
(E) 2 & 3
