3) Sketch E-Field Lines for the Following Situations

Electrostatics 1: Charge, Leader: German Parada electric force, E field Course: Phys 222 Supplemental Instruction Instructor: Dr. Soeren Prell Iowa State University Date: 01/27/2014

Warm-up Exercises: 1) Two charges (4nC and -3nC) are placed 5cm apart. A third charge is placed directly in line with them 3cm away from the negative charge. If the charge of this third charge is 2nC, where would it accelerate when placed? Where should this charge be placed so that it will stay stationary when released?

2) Two metallic spheres in insulating supports are initially uncharged and in contact with each other. A negatively charged glass bar is brought near one of them, the spheres are separated and the bar is removed. Sketch this process and describe the final state of the spheres. How would this be different if the spheres were made out of plastic?

3) Sketch E-field lines for the following situations:

A) All objects are conductors B) Middle bar is insulator

Problems:

1) Two small charged metal spheres are connected to each other via an elastic rubber band (“spring” constant of 39 N/m, non-conductive material). The original length of the rubber band was 10 cm and when the spheres were connected, it elongated to 15cm. Find the sign and magnitude of the charges considering that one charge has twice the charge of the other one.

2) Four charges have been placed in the corners of a square with sides of 25cm. The two charges on the top vertices are 3uC, while the one on the bottom right corner is -2uC and the one on the bottom left vertex is 1uC. a. What is the net force experienced by the 1uC charge? b. What is the E-field at that position? Describe the two ways of calculating this value (only need to do one of them)

Supplemental Instruction 1060 Hixson-Lied Student Success Center v 294-6624 v www.si.iastate.edu c. Without changing the magnitude of two of the other three charges so that the net force on the 1 uC charge is zero? Hint: Start by sketching the force vectors

3) Basic Cathode Ray Tube Older versions of CRTs deflected the beam of electrons to different parts of the screen by adjusting the Electric field between a pair of metallic plates, as seen in the figure below,

Assume the electrons enter the electric field region at a horizontal speed of 9.5x106 m/s, and that the field between the 10cm long plates is uniform. Calculate the E-field needed to deflect the electrons in order to hit a spot 20cm above the center of the screen (the screen is 60cm away from the left edge of the plates)

4) Derive the E-field for a charge located at a distance r of a thin and long (but not infinitely long) wire of length 2L and linear charge density λ

5) In the derivation for the E-field from a ring of charge (radius R and linear charge density λ), what would change if we are asked to calculate the field at a point offset R/2 from the middle of the ring at a distance of d? Set up the integrals required but do not solve them.

6) Calculate the E-field at the middle of a semicircle of wire with radius a and charge Q uniformly distributed along its lenght.

Conceptual Questions: 1) Two metallic balls attract each other when brought in close proximity. What could happen if they touch?

2) You have a positively-charged rod. Describe how you would create a positive charge on an insulated metallic object. What about a negative charge?

3) A proton and an electron are released (at different times) in an electric field. Compare the initial force, acceleration and direction of motion of the two particles.