lab

PHYS 140

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2

Lab – 2: Electric Field

Name: _____________________

Objectives:

1. To understand the concept of electric fields.

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2. To understand the spatial distribution of the electric field for a variety of simple charge configurations.

INVESTIGATION: THE ELECTRIC FIELD

Most of the forces you have studied up until now resulted from the direct action or contact of one object on another. (The only exception was the gravitational force.) From your observations in Investigations 1 and 2, it should be obvious that charged objects can also exert forces on each other at a distance. How can that be? The action at a distance that characterizes electrical forces is in some ways inconceivable to us. How can one charge feel the presence of another and detect its motion with only empty space in between? Since all atoms and molecules are thought to contain electrical charges, physicists currently believe that all contact forces are electrical forces involving small separations. So, even though forces acting at a distance seem inconceivable to most people, physicists believe that all forces act at a distance.

To describe action at a distance, Michael Faraday introduced the notion of an electric field emanating from a collection of charges and extending out into space. More formally, the electric field due to a known collection of charges is represented by a vector at every point in space. Thus, the electric field vector, is defined as the force, , that would be experienced by a very small positive charge (called a test charge) at a point in space, divided by the magnitude of the charge qo . Thus, the electric field is in the direction of the force on the small positive test charge and has a magnitude of .

Activity 1: Electric Field Vectors from a Positive Point Charge

To investigate the vector nature of an electric field, you will use PHET simulation

Charges and Fields

(https://phet.colorado.edu/sims/html/charges-and-fields/latest/charges-and-fields_en.html)

1. Open the PHET simulation. This simulation allows you to place multiple positive and negative point-charges in any configuration and look at the resulting electric field.

2. Start the simulation. You can click and drag positive charges (red) or negative charges (blue) into the main screen. If you select Electric Field in the menu, arrows will appear, showing the direction of the electric field. Faint arrows indicate that the electric field is weaker than at locations where the arrows are brighter (this simulation does not use arrow length as a measure of field magnitude).

3. Feel free to play around with the simulation. When you are done, click the Reset button.

4. Select Electric Field and Grid in the green menu. Drag one positive charge and place it near the middle of the screen, right on top of two intersecting bold grid lines. You should see something similar to the figure below.

Question-1: What is the direction of the electric field at various points around the positive charge?

5. Now, let’s look at how the distance from the charge affects the magnitude of the electric field. Select Values on the menu, and then click and drag one of the yellow E-Field Sensors. You will see the magnitude of the electric field given in units of V/m (volts per meter, which is the same as newtons per coulomb). Place the E-Field Sensor 1 m away from the positive charge (1 m is two bold grid lines away if going in a horizontal or vertical direction) and look at the resulting field strength.

Question-2: Consider the locations to the right, left, above, and below the positive charge, all 1 m away. For these four locations, the magnitude of the electric field is________________.

a) greatest below the charge.

b) greatest to the left of the charge.

c) greatest above the charge.

d) greatest to the right of the charge.

e) the same.

Question-3: The magnitude of the electric field 1 m away from the positive charge is _______________ the magnitude of the electric field 2 m away.

a) one-half

b) equal to

c) two times

d) four times

e) one-quarter

Question-4: How does the magnitude of the electric field vary with distance from the positive charge?

Question-5: If you move the sensor around the charged particle in a circle, does the electric field value vary? Explain why or why not.

Question-6: If the field strength is E = 9 V/m a distance of 1 m from the charge, what is the field strength E a distance of 3 m from the charge?

6. Remove the positive charge by dragging it back to the box at the bottom and drag a negative charge (blue) toward the middle of the screen.

Question-7: Determine how the electric field is different from that of the positive charge. Describe the differences in the electric field due to a negative charge as compared to a positive charge?

7. Now, remove the negative charge, and drag two positive charges, placing them 1 m apart, as shown below.

Let’s look at the resulting electric field due to both charges. Recall that the electric field is a vector, so the net electric field is the vector sum of the electric fields due to each of the two charges.

Question-8: Where is the magnitude of the electric field roughly equal to zero (other than very far away from the charges)?

Question-9: Consider a point 0.5 m above the midpoint of the two charges. As you can verify by removing one of the positive charges, the electric field due to only one of the positive charges is about 18 V/m. What is the magnitude of the total electric field due to both charges at this location?

8. Make an electric dipole by replacing one of the positive charges with a negative charge, so the final configuration looks like the figure shown below.

Question-10: The electric field at the midpoint is ________________.

9. Make a small dipole by bringing the two charges very close to each other, where they are barely touching. The midpoint of the two charges should still be on one of the grid point intersections (see figure below). Measure the strength of the electric field 0.5 m directly above the midpoint as well as 1 m directly above.

Question-11: Does the strength of the electric field decrease as 1/r2?

Question-12: The electric field 1.5 cm from a very small charged object points toward the object with a magnitude of 180,000 N/C. What is the charge on the object?

Question-13: Two point charges of -20 μC and -35 μC are separated by a distance of 20.0 cm. What is the magnitude of electric field due to these charges at a point midway between them? Show your calculation.

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