- You now have T and Ty on your FBD. Clearly there is a horizontal component of T, Tx. Add that to your figure. Draw it with its tail starting at the dot. Again, be careful with your scaling.

We now have W, T, Ty, and Tx in our FBD. Are all these forces acting on the pith ball? Not exactly. We have some redundancy. T is the vector sum of Tx and Ty. So T has actually been replaced by its components.

- Draw a “≈” through the T vector to indicate that we can ignore it now.

- Look at our remaining three vectors. Does our FBD indicate that the ball is in equilibrium?

- What type of force, missing from our FBD, is keeping the ball in equilibrium?

Strong nuclear, weak nuclear, friction, gravity (its weight), electrostatic, tension, air resistance (Circle one)

- Add that force vector to your FBD and label it Fe. Draw it with its tail starting at the dot. Again, be careful with your scale.

- Draw the matching FBD for the left ball.

Your sole task is to determine the charge on either pith ball and compare it to the value given in the info box. Here are some guidelines and suggestions.

· Charge up the pith balls to start. Use a large charge for best results.

· The forces involved are gravity (weight), W, the string tension, T, and the Coulomb force, Fe. Forces that are not horizontal or vertical will need to be resolved into correctly-labeled components.

· Record any data you take in the data table provided. You may add to the table.

· Explain your method in words, referring to labeled figures. (You supply the figures.) (Item 1 below.)

For clarity’s sake, refer to the right-hand ball in your discussion.

· Clearly show your calculations using the proper variable terminology. (Item 2 below.) Define any variables that you create. E.g., Fe: electrostatic force between pith balls

Wait as long as possible before replacing your variables with numbers. (See Ex.)

Ex. a = 2.5 m/s2, Vo = 12 m/s, Vf = 22 m/s, t =? | ||||

Wrong | Right | |||

2.5𝑚/𝑠2 = 22𝑚⁄𝑠−12𝑚⁄𝑠, etc. | 𝑎 = 𝑣𝑓−𝑣𝑜, | 𝑡 = | 𝑣𝑓−𝑣𝑜, | 𝑡 = 22𝑚⁄𝑠−12𝑚⁄𝑠 = 4.0 s |

𝑡 | 𝑡 | 𝑎 | 2.5𝑚/𝑠2 |

**Table 1 Charge on a Pith Ball (you may not actually need all of these values)**

k = 9.0×109 N m2/C2

mass of a pith ball (from lab info box), m = kg charge # =

accepted value for charge on one pith ball (info box), Qa = C experimental value for charge on one pith ball, Qe = C separation between centers of pith balls, r = m

length of the pendulum, L = _ m

deflection angle of a pith ball from the vertical, θ = °

- Explain your method in words, referring to labeled figures. (You supply the figures.)

- Clearly show your calculations using the proper variable terminology. Include the calculation of percentage error for your value of the charge on a pith ball.

- What do you feel was the major source of error? Why?

- Is the assumption that the charge distribution is spherical most inaccurate for small or large r’s? Explain.

**WEEK 2**

**Purpose**

· To develop an understanding of the relationship between equipotential lines and electric field lines.

· To investigate equipotential lines produced by three different configurations of charge.

· To discover the cause of electric shielding.

**Equipment**

Electric Field Mapping Apparatus PENCIL

Note: Your teacher will assign you a power supply voltage, Vps, which you’ll use beginning in part I of the procedure.

We’ll use the Electric Field Mapping Apparatus in this lab activity. You can get quick access to help by rolling your mouse over most objects on the screen. Complete instructions are provided in Apparatus Exploration.