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Physics 2

0. Look around your house for examples, what are the applications of the following topics of Physics 2:
  1. Magnetic and electric fields
    Example: electric motors used in washing machines have electric and magnetic fields
  2. Capacitors
    Example: Cellphones contain capacitors
  3. Switches
  4. Resistors
  5. Electromagnetic waves
  6. DC Circuits
  7. AC Circuits
  8. Lenses
Show answer
List all appliances and think about how each appliance works. Everybody uses physics and engineering everyday !
1. What is the SI unit for electric charge? Show answer
'C' for Coulombs
2. What is the charge of 1 electron? Show answer
Use google..
1.602*10-16C
3. Sketch the electric field lines for (a) neutral particle, (b) positive charge and (c) negative charge: Show answer
(a) A neutral charge does not have an electric field
(b) For a positive charge, electric field lines point outward ad
(b) For a negative charge, electric field lines point inward a
4. Sketch and solve for resultant electric field force acting on a particle, q1, which is near two charged particles, q2 and q3 as shown below? Assume the following:
(a) Particle q1 is positive whilst particles q2 and q3 are negative;
(b) Particle q1 is negative whilst particles q2 and q3 are positive;
(c) Particle q1 is positive whilst particles q2 and q3 are positive;
particles and charge
Hint : Sketch free-body diagrams of electric field forces on particle q1 alone for all three cases separately. YES, free body diagrams apply to electric fields as well - wherever there are forces, free-body diagrams are there. Show answer
This is a vector addition problem, review vector addition from Physics 1 before attempting the problem. Just like contact forces, electric field force is a vector (it has magnitude and direction).
Method #1:
  1. Draw free-body diagram of q1
  2. Calculate F21 and F31
  3. Apply law of cosines to solve for the resultant force

Method #2:
  1. Draw free-body diagram of q1
  2. Calculate F21 and F31
  3. Calculate the x and y components of F21 and F31. Keep track of signs in each case.
  4. Add the x and y components of F21 and F31 to find the result force

Free-body diagrams:
Part a
Note that the free body diagram is just showing particle q1 and forces that act on q1. part b
Forces F21 and F31 in part (b) are equal in magnitude and direction to those in part (a).
part c
Forces F21 and F31 in part (c) are equal in magnitude, but opposite in direction to those in part (a) and (b).
Symbolic Solution:
It should be noted that the magnitude of the resultant force is the same in all 3 cases. Direction changes for part (c) only.
Calculate F21 and F31 using the following equation: numerical
Review the formula for solving electric field force, the denominator is the distance between particles. Do you understand how to solve for distance between q1 and q2? numerical
The angle between F21 and the horizontal is determined by taking inverse tangent. Remember SOH CAH TOA
numerical
This is called 'Cosine Rule', read more about it here.
5. One important application of electric fields is electrostatic spray painting of cars. Assume that at Joe Bob's Body Shop near your house, paint particles are charged to -2C whilst the car is positively charged. It so happens that you are hired as an intern by Mr J. Bob. As part of your job, you explain and demonstrate how electrostatic spray painting works to potential customers. Answer the following questions to demonstrate you are the most qualified intern for the job:
  1. Draw electric field lines of one paint particle
  2. Calculate the electric field strength at a distance of 2 cm away from the center of the paint particle.
    Hint: assume that each paint particle is a point charge
  3. One advantage of electrostatic painting is that particles cover the car body uniformly because unlike charges repel. Calculate the force between two particles that are 2 cm apart.
    Hint: assume that all paint particles have the same charge
  4. Sketch the equipotentials of one paint particle
Show answer
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6. You decide to reject your internship offer from Joe Bob's Auto Body because eventually you want to be a medical doctor/nurse. Instead, you accept an internship offer to work at a local children's hospital. During your new intern orientation you are told to always wear your work shoes while you are in the operating room. The reason for wearing those 'special shoes' is to conduct any electrostatic charge you may acquire to the hospital floor. Electrostatic charges can cause fires (especially if you work with flammable anaesthetic drugs) or malfunction of equipment in operating rooms. Answer the following questions to demonstrate you are a prepared intern:
  1. On your first day at work, you were excited so you kept rubbing your hands together. By rubbing your hands, you picked up a charge of +q Coulombs, explain what happens to the electrostatic charge if you do not wear your 'special shoes'.
  2. Explain what happens to the electrostatic charge if you wear your 'special shoes'.
Show answer
I highly recommend you read Preventing preventing preventing fires and explosions in the operating room by Noyce L. Griffin
7. What is the SI unit for capacitance? Show answer
'F' for Farads
8. As a curious physics student, you decide to have hands-on experience with capacitors. You head over to a local RadioShark store to buy a 1000 micro-Farad capacitor so that you can take it apart. Answer the following questions before you can take it apart:
  1. Based on your class lectures, draw a simple sketch of a capacitor.
    Hint: show 2 parallel plates separated by a distance d and a potential difference V
  2. Assume this particular capacitor you bought has two thin aluminum sheets of area 2.5cm width by 10cm length rolled together. What is the distance, d, between the plates? Assume that the dielectric material is paper with a dielectric constant of 3.
  3. It is good practice when working with a capacitors to check whether or not the capacitor is charged. Large capacitors, like yours, can cause electric shocks/sparks if improperly handled. Besides asking the shop attendant at RadioShark, how can you find out if your capacitor is charged?
  4. Assume the capacitor is charged to a voltage of 1.5V. What is the total electric charge, in C, in this capacitor?
  5. What is the total energy stored in this capacitor?
  6. You decide to carefully discharge your capacitor before taking it apart. How would you discharge it?
Capacitor
Image of a capacitor from RadioShark Show answer
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9. What are the SI units of the following electrical quantities?
  1. Resistance
  2. Voltage
  3. Current
Show answer
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