Physics 208 – Introductory Physics II

Nazareth College

Department of Chemistry & Biochemistry

Robert F Szalapski, PhD – Adjunct Lecturer

www.CallMeDrRob.com

Spring 2012

Exam 4

Electric Charge, Fields & Forces

## § Problem 1

A droplet of oil with a mass of and unknown charge is suspended in an electric field with a strength of downward.

### § (a)

Create a free-body diagram showing the forces acting upon the oil drop. You may create your diagram on the figure shown or next to it. (10 points)

### § (b)

What is the value of the charge ? (10 points)

Note that the charge must be negative because the electric field and the force are in opposite directions for the oil drop to be suspended.

## § Problem 2

An unknown charge is placed from a charge. In order to determine the value of a third charge with a value is placed at the midpoint between the other two, and it is observed to experience a force of towards the charge. Compute . (30 points)

There is definitely more than one way to proceed. I choose to consider the two forces acting upon the charge taking into account that we know their sum, the net force or towards the charge. First compute the force from that charge which will be attractive and hence appear as shown on the diagram.

Notice that this is smaller than the net force also shown on the diagram, and hence the force of the unknown charge will need to be repulsive pushing the smallest charge towards the charge, hence it has been drawn as shown. In other words,

Hence

Because the force is repulsive

## § Problem 3

Three charges, , are placed at three vertices of a square with side lengths of placed and oriented as in the figure. Use the coordinate system provided.

### § (a)

What are the components and of the electric field due to evaluated at the fourth vertex at the bottom of the figure? Draw and label a vector representing on the figure.

(10 points)

Notice that, as drawn, the component is opposite to the indicated angle and to the right while the component is adjacent to the angle and downward.

### § (b)

What are the components and of the electric field due to evaluated at the fourth vertex at the bottom of the figure? Draw and label a vector representing on the figure.

(10 points)

The magnitudes are identical to the previous step, but the directions of the components must be reconsidered.

### § (c)

What are the components and of the electric field due to evaluated at the fourth vertex at the bottom of the figure? Draw and label a vector representing on the figure.

(10 points)

Notice the increased separation between the opposite corners of the square as indicated on the diagram. Also notice that the direction is straight down, hence purely component.

### § (d)

What are the components and of the resultant electric field due to all three charges evaluated at the fourth vertex at the bottom of the figure? Draw a vector representing on the figure. What is its magnitude, , and its direction? Draw and label a vector representing on the figure.

(10 points)

downward, or in the direction. Could also state CW from the axis.

### § (e)

Suppose an electron is placed at the fourth vertex. What force, , magnitude and direction, would the electron experience?

(5 points)

The charge of the electron is

Because the charge is negative, the force will be opposite to the field, upward, or in the direction with a magnitude

### § (f)

What acceleration, , magnitude and direction, would the electron experience?

(5 points)

upward, aligned with the force and opposite to the electric field.