Moving Charges Worksheet

Scribe for Tuesday, December 16, 2008 since Eric was kidnapped by Santa Claus and his evil green elves and hasn't scribed since.


AGENDA:

* Corrected Moving Charges Worksheet #1-10
* Read Magnetic Formulae handout
* Chapter 27 Review #1-7 for homework

The Moving Charges Worksheet was difficult to do at first, which is why Ms.K suggested on doing the questions in reverse order. Then it’s just a matter of knowing how to use your formulas correctly. Also, visualizing the problem (i.e. drawing a picture) helps a lot too.


MOVING CHARGES WORKSHEET

10. An electron is moving east to west at 5.0 x 10^5 m/s through a magnetic field. A force of 4.0 x 10-6 N north is acting on the electron. What is the magnitude and direction of the magnetic field?

F = Bqv
(4.0 x 10^6) = B (1.6 x 10^-19) (5.0 x 10^8)
B = 5 x 10^7 (into page)


9. What is the magnitude and direction of a magnetic force on a proton moving horizontally to the north at 8.6 x 10^4 m/s, as it enters a magnetic field of 1.2 T pointing vertically upward?

F = Bqv = (1.2 N/(c*m/s)) (1.6 x 10^19 C) (8.6 x 10^4 m/s) = 1.7 x 10^014 [right]


8. An electron experiences the greatest force by a magnetic field as it travels at 2.1 x 10^5 m/s when it is moving south. The force is 5.6 x 10^-13 N and up. Find the magnitude and direction of the magnetic field.

F = Bqv
(5.6 x 10^-13)/ [(1.6 x 10^-29) (2.1 x 10^5)] = B
B = 16.7 T [left]


7. A 10.0 m long high tension power line carries a current of 20.0 A perpendicular to the earth’s magnetic field of 5.5 x 10^-5 T. What is the magnetic force experienced by the power line?

F = BIL = (5.5 x 10^-5 T) (20.0 A) (10.0 m) = 0.011 N


6. A wire connecting a taillight to a motorcycle battery is 0.50 m long and is perpendicular to the Earth’s magnetic field. It experiences a force of 6.0 x 10^-5 N when carrying a current of 1.5 A. What is the magnitude of the Earth’s magnetic field at that location?

F = BIL
6.0 x 10^-5 N = B (1.5 A) (0.50 m)
B = 8 x 10^-5 T


5. A 15 cm length of wire carries a current of 20.0 A. It is perpendicular to a uniform magnetic field. If it experiences a magnetic force of 0.40 N, what is the magnetic field intensity?

B = F/(IL) = 0.4 N / (20.0 A x 0.15 m) = 0.13 T


4. Using the Millikan apparatus, a sphere of mass 3.2 x 10^-15 kg is held motionless. The plates produce an electric field strength of 19 600 N/C.


a) What forces are acting on the sphere?
electric force, gravitational force

b) What type of charge must be on the sphere?
positive

c) Determine the amount of charge on the sphere.
F_g = F_e
mg = Eq
q = mg/E = (3.2 x 10^-15 kg) (9.8 N/kg) / (19 600 N/C) = 1.6 x 10^-18 C

d) How many electrons does this correspond to?
(1.6 x 10^-18 C) x (1 electron)/(1.6 x 10^-19 C) = 10 electrons


3. The electric field between these charged parallel plates is 23 800 N/C. The drop’s mass is 2.4 x 10^-14 kg. What force would be experienced by the drop if it had the charge of three excess electrons?


F_e = Eq = (23 800 N/C) (3 x 1.6 x 10^-19 C) = 1.14 x 10^-14 N [up]
F_g = mg = (2.4 x 10^-14 kg) (9.8 N/kg) = 2.35 x 10^-13 N [down]
F_net = 2.35 x 10^-13 N - 1.14 x 10^-14 N = 2.24 x 10^-13 [down]


2. The plates are 5.0 cm apart and produce an electric field strength of 1200 N/C in the region between them. An electron enters the electric field at 1.00 x 10^2 m/s. With what velocity does the electron strike the positive plate? (Ignore gravity.)


F_e = qE = (-1.6 x 10^-19 C) (1200 N/C) = 1.92 x 10^-15 N
ma = qE
a = qE/m = (1.92 x 10^-15 N) / (9.11 x 10^-3 kg) = 2.11 x 10^14 m/s^2
v_f^2 = v_i^2 + 2ad
v_f = [(1.00 x 10^2 m/s)^2 + 2 (2.11 x 10^14 m/s^2) (0.05 m)]^0.5 = 4.6 x 10^6 m/s


1. This oil drop of mass 4.5 x 10^-15 kg has had two electrons removed. The electric field strength is 3.0 x 10^4 N/C.


a) What will happen to the drop?
F_g = mg = (4.5 x 10^-15 kg) (9.8 N/kg) = 4.4 x 10^-14 N
F_e = Eq = (3.0 x 10^4 N/C) (2 x 1.6 x 10^-19 C) = 9.5 x 10^-15 N
F_g > F_e, so drop will fall

b) What is the total force experienced by the drop?
F_net = 4.4 x 10^-14 N + 9.5 x 10^-15 N = 3.45 x 10^-14 N [down]

Next scribe is DANA.

Charges, Energy, Voltage Lab

Answers to 33-2 Electrical Fields and Potential are found on slides 1 and 2.
Answers to Electric Potential are found on slides 3 to 7.

After reviewing our answers to those worksheets, we did a lab entitled Charges, Energy, Voltage from Chapter 21 of the green book.

In the lab, we developed a model that showed the different amounts of charges at different energy levels. We erected a ruler in a Playdoh substance and made markings at 3 cm, 6 cm, 9 cm, and 12 cm. The 3 cm resembled 3 V or 3 J/C, the 6 cm resembled 6 V or 6 J/C, etc. We taped 4 pennies at the 3 cm marking, 3 pennies at the 6 cm marking, 2 pennies at the 9 cm marking, and a penny at the 12 cm marking. Each penny resembled a Coulomb.

A lab worksheet was handed out and has to be completed.

Next scribe is Vieteran.

Kepler's Three Laws Videotape

No one was selected for scribe in physics, so I'm assuming today's scribe post is up for grabs.

I am zeph. zeph is scribe. Today's scribe post has been zephed!

Answers to "8.1 Motion in the Heavens and on Earth: Kepler's Laws of Planetary Motion" and "Kepler's Three Laws Videotape" are given below.

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8.1 MOTION IN THE HEAVENS AND ON EARTH

Kepler's Laws of Planetary Motion

Tycho Brahe studied the motion of the planets in order to be able to PREDICT astronomical events. He believed that EARTH was the centre of the universe. Johannes Kepler believed that THE SUN was the centre of the universe. He analyzed Brahe's data, and developed THREE laws of planetary motion. One law says that the paths of the planets are ELLIPSES with THE SUN located at one focus. Another law states that an imaginary line extending from the sun to a planet will sweep out equal AREAS in equal amounts of time. According to this law, planets move FASTEST when closest to the sun and move SLOWEST when farthest from the sun. Kepler's last law states that the ratio of the squares of the PERIODS of any two planets in orbit around the sun is equal to the ratio of the CUBES of their distances from THE SUN. This law can be states as an equation, (T_A/T_B)^2 = (R_A/R_B)^2. To use this law to calculate the period of a satellite, you must know the RADIUS of its orbit and the PERIOD and RADIUS of the orbit of another satellite.

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KEPLER'S THREE LAWS VIDEOTAPE

Answer the following questions based on the videotape entitled "Kepler's Three Laws".

1. What planetary model did Johannes Kepler believe in?
Copernican

2. What planet's orbit did Kepler analyze?
Mars

3. How far off from a circle was the oribt of Mars?
8 min arc or 1/4th of the apparent width of the moon

4. What type of curve can explain the orbit of Mars?
ellipse

5. In an ellipse, is the total distance from 1 pin to the string and then to the other pin a constant?
yes

6. The name given to a single point in an ellipse is the __________.
focus point

7. Focus is the Latin word for __________.
fireplace

8. How would you describe Kepler's childhood?
poverty and illness

9. What journey did Kepler undertake in 1600 and for what purpose?
went to see Tycho Brahe in Denmark to learn more astronomy

10. What was a main interest of Tycho Brahe?
astronomy

11. How did Kepler obtain the careful observations that Brahe made during his lifetime?
stole it

12. Was the Copernican model of the universe easy to justify scientifically?
no

13. What were some of the problems that faced Kepler in discovering the secrets of the sky?
planets are in motion in its orbit and rotates on axis

14. How many pages of calculations did Kepler have?
about 900 pages

15. What does "in opposition" mean according to the positions of Mars, Earth, and sun?
Mars is in the same position as the Earth and the sun

16. Can any circle viewed obliquely be termed an ellispe?
yes

17. In ancient history, which group of people studied conic sections?
Greeks

18. Is a parabola a popular shape in our world today?
yes

19. In your own words, describe he term eccentricity?
how flat an ellipse is

20. State Kepler's 3 laws.
a) 1st: Each planet moves in an ellipse with the sun as its focus.
b) 2nd: A line from the sun to each planet sweeps out equal areas in equal times.
c) 3rd: T^2 directly proportional to R^3

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HOUSEKEEPING
Homework:
* 8.1 Motion in the Heavens and on Earth Study Guide
* Exploration of Space Project due Thursday, November 27.
Next scribe is ERICT.

Work and Energy Study Guide

Previous scribe was abducted by aliens and hasn't scribed ever since, so I inherited his scribing powers under the "first come, first serve" rule.

The study guide is straightforward. Basic stuff. Grade 12 physics students can do this in their sleep.


10.1 WORK AND ENERGY STUDY GUIDE

Note: Capitalized words are the answers to fill-in-the-blanks.

Work
Work is the product of the FORCE exerted on an object and the DISTANCE the object moves in the DIRECTION of the force. The equation used to calculate work is W = Fd. In this equation, W stands for WORK, F stands for FORCE, and d stands for DISTANCE. Work has no direction, so it is a scalar quantity. The SI unit of work is the JOULE. When a force of one NEWTON moves an object a distance of one METRE, one JOULE of work is done. Work is done on an object only if the object MOVES. Work is done only if the FORCE and the DISTANCE are in the same direction.

Work and Direction of Force
If a force is exerted IN THE DIRECTION OF the motion, work is done. If a force is exerted PERPENDICULAR to the motion, no work is done. If a force is exerted at another angle to the motion, only the component of the force IN THE DIRECTION OF the motion does work. The magnitude of this component is found by multiplying the force applied by the COSINE of the angle between the force and the DIRECTION OF THE MOTION. When friction opposes motion, the work done by friction is NEGATIVE. When work is done on an object, ENERGY is transferred. Work is the transfer of energy as the result of MOTION. This transfer can be POSITIVE or NEGATIVE.

Power
Power is the RATE of doing work, or the RATE at which ENERGY is transferred. The equation used to calculate power is P = W/t. In this equation, P stands for POWER, W stands for WORK, and t stands for TIME. The unit of power is the WATT. One JOULE of energy transferred in one second equals one watt. This is a very small unit, so power is often measured in KILOWATTS.

1. Symbol for kinetic energy: K
2. Calculation of kinetic energy: mv^2/2
3. Symbol for work: W
4. Calculation of work: Fd
5. Statement that the work done on an object is equal to the object’s change in kinetic energy: Delta K = W
6. Equivalent to 1 kg*m^2/s^2: 1 J

7. Through the process of doing work, energy can move between the environment and the system as the result of FORCES.
8. If the environment does work on the system, the quantity of work is POSITIVE.
9. If the environment does work on the system, the energy of the system INCREASES.
10. If the system does work on the environment, the energy of the system DECREASES.
11. In the equation W = Fd, Fd holds only for CONSTANT forces exerted in the direction of displacement.
12. In the equation W = Fd cos theta, angle theta is the angle between F and the X-AXIS.


13. W > 0: B, E, F
14. W = 0: A, D
15. W < 0: C


16. What was the magnitude of the force acting on the crate? 30.0 N
17. How far did the crate move horizontally? 0.50 m
18. What does the area under the curve of this graph represent? work
19. How much work was done in moving the crate 0.1 m? 3.0 J

20. Rate of doing work: power
21. Unit of power: watt
22. Symbol for power: P
23. Calculation of power: W/t
24. 1000 watts: kW

Next scribe is KAMIL.

Derivation for Circular Motion Formulas

Deriving a formula for velocity, acceleration, and the centripetal force when in a circular motion...


LEGEND
v = velocity (or speed, if direction is not indicated)
d = displacement (or distance, if direction is not indicated)
t = time (interval)
R = radius
a = acceleration
a_c = centripetal acceleration
F = force
F_net = net force
F_c = centripetal force
m = mass


1.
The distance an object moves in a circular motion is the circumference of the circular motion, which is equal to 2*pi*R. Using the definition of circumference (c=π*2*r) and the definition of velocity (v=d/t), we can derive this formula: v=2*π*R/t.

2.
Draw the circular motion, two radii, and two velocity vectors. Add the two radii vectors to get the net radius. Add the two velocity vectors to get the net velocity.

3.
Since the change in radii over the radius equals the change in velocities over a velocity (ΔR/R = Δv/v), using the definition of velocity (v=ΔR/Δt) and acceleration (a=Δv/Δt), we can derive this formula: a=v^2/R.

4.

Using the acceleration formula that we recently derived (a=v^2/R), we can use substitute velocity with v=2*π*R/t to get a more fancy-looking formula for centripetal acceleration (a=4*π^2*R/T).

5.
Using Newton's Second Law of Motion (F_net=m*a), we can substitute acceleration with a=v^2/R to get the formula for centripetal force (F_c=m*v^2/R), which can be derived even further using v=2*π*R/t to get F_c=4*π^2*R/t^2.

• Centripetal Force lab is due and was handed in today.
• Centripetal Acceleration and Centripetal Force assignments is due tomorrow.
• Next scribe is ERIC.

First Week of Autumn

A dinosaur from the future came in to our class one day and ate the previous scribe. Then it ran away. Since the previous scribe is unable to scribe and I'm due for scribing anyways (yes, scribing is worth marks), I'm taking advantage of this blog drought as I do this post over the weekend. Here's an abbreviated overview of what we did Friday, September 26 to Monday, September 22nd.

FRIDAY, SEPTEMBER 26

• Redo Coefficient of Sliding Friction Lab
• Chapter 9 Study Guide (Impulse and Change in Momentum and The Conservation of Momentum) Worksheet (homework)
  
• Chapter 7 Momentum Worksheet (homework)
  

THURSDAY, SEPTEMBER 25

• Sang along to the M Times V song
• Watched Momentum Video (same-same guy) (5 points and hand-in)
  

WEDNESDAY, SEPTEMBER 24

• Dynamics Test

TUESDAY, SEPTEMBER 23

• Corrected Forces Review and Forces Evaluation: Using Concepts

MONDAY, SEPTEMBER 22

• Forces Review, Forces Enrichment, Forces Evaluation: Using Concepts, Frictional Forces, and Advanced Forces was assigned last Friday. These should be done by now.
• 5.2 Using Newton's Law: Read pp. 93-103. Questions #16, 18, 19, 22, 23 (pp. 123-126).
• 6.3 Application of Vectors: Read pp. 123-126. Questions #31, 35 (p. 131)

5.2 Using Newton’s Law Read pp. 98-103 Questions #16, 18, 19, 22, 23 (pp. 106-107)

16. You are driving a 2500.0-kg at a constant speed of 14.0 m/s along an icy, but straight and level road. While approaching a traffic light, it turns red. You slam on the brakes. Your wheels lock, the tires begin skidding, and the car slides to a halt in a distance of 25.0 m. What is the coefficient of sliding friction (μ) between your tires and the icy roadbed?

18. A 4500-kg helicopter accelerates upward at 2 m/s2. What lift force is exerted by the air on the propellers?

19. The maximum force a grocery bag can withstand and not rip is 250 N. If 20 kg of groceries are lifted from the floor to the table with an acceleration of 5 m/s2, will the bag hold?

22. A sled of mass 50 kg is pulled along snow-covered, flat ground. The static friction coefficient is 0.30, and the sliding friction coefficient is 0.10.
a. What does the sled weigh?
b. What force will be needed to start the sled moving?
c. What force is needed to keep the sled moving at a constant velocity?
d. Once moving, what total force must be applied to the sled to accelerate it 3.0 m/s2?

23. A force of 40 N accelerates a 5.0-kg block at 6.0 m/s2 along a horizontal surface.
a. How large is the frictional force?
b. What is the coefficient of friction?


6.3 Application of Vectors Read pp. 123-126 Questions #31, 35 (p. 131)

31. A street lamp weighs 150 N. It is supported equally by two wires that form an angle of 120° with each other.
a. What is the tension of each of these wires?
b. If the angle between the wires is reduced to 90.0°, what new force does each wire exert?

35. You slide a 325-N trunk up a 20.0° inclined plane with a constant velocity by exerting a force of 211 N parallel to the inclined plane.
a. What is the component of the trunk’s weight parallel to the plane?
b. What is the sum of your applied force, friction, and the parallel component of the trunk’s weight? Why?
c. What is the size and direction of the friction force?
d. What is the coefficient of friction?

Passing the baton to benofschool.

Shelly and erict, you guys forgot to label your labels on your previous scribe posts. ;)