● An electric field exists around any charged object. The field produces forces on other charged objects. ● The electric field is the force per unit charge. Creating and Measuring Electric Fields Section 21.1 Section 21.1

● The direction of the electric field is the direction of the force on a tiny, positive test charge. ● Electric field lines provide a picture of the electric field. They are directed away from positive charges and toward negative charges. They never cross, and their density is related to the strength of the field. Creating and Measuring Electric Fields Section 21.1 Section 21.1

● Electric potential difference is the change in potential energy per unit charge in an electric field. Applications of Electric Fields Section 21.2 Section 21.2

● Electric potential differences are measured in volts. ● The electric field between two parallel plates is uniform between the plates, except near the edges. Applications of Electric Fields Section 21.2 Section 21.2

● In a uniform field, the potential difference is related to the field strength by the following: Applications of Electric Fields Section 21.2 Section 21.2 ● Robert Millikan’s experiments showed that electric charge is quantized. ● Robert Millikan also showed that the negative charge carried by an electron is 1.60× C.

● Charges will move in a conductor until the electric potential is the same everywhere on the conductor. ● Grounding makes the potential difference between an object and Earth equal to zero. ● Grounding can prevent sparks resulting from a neutral object making contact with objects that have built-up charge on them. Applications of Electric Fields Section 21.2 Section 21.2

● Electric fields are strongest near sharply pointed conductors. ● Capacitance is the ratio of the charge on an object to its electric potential difference. Applications of Electric Fields Section 21.2 Section 21.2

● Capacitance is independent of the charge on an object and the electric potential difference across it. ● Capacitors are used to store charge. Applications of Electric Fields Section 21.2 Section 21.2

1.The electric field of a charge is measured twice at the same location, first by using a positive test charge of 4.0×10 –6 C (test charge A) and then by using a positive test charge of 2.0×10 –6 C (test charge B). What will be the change in the electric field of the charge in the two measurements? Chapter Assessment Questions

A.The electric field of the charge when measured using test charge A will be double of that when measured using test charge B. B.The electric field of the charge when measured using test charge B will be double of that when measured using test charge A. C.The electric field of the charge will remain the same in both cases. D.The electric field of the charge when measured using test charge A will be four times of that when measured using test charge B. Chapter Assessment Questions

Reason: According to Coulomb’s law, the force is directly proportional to the strength of the test charge, q. That is, if the charge is doubled, so is the force. Therefore, the ratio of force to charge is a constant. If you divide the force, F, by the test charge, q, you obtain a vector quantity F/q. This quantity does not depend on the test charge, but only on the force, F, and the location of the point. Hence, the electric field of a charge, that is, E = F/q in the above case, will remain the same in both measurements. Chapter Assessment Questions

2.The electric field strength at a point 0.30 m from a point charge is 4.0×10 5 N/C. What will be the electric field strength at a point that is 0.60 m from that point charge? A.4.0×10 5 N/C B.1.0×10 5 N/C C.2.0×10 5 N/C D.8.0×10 5 N/C Chapter Assessment Questions

Reason: There is an inverse square relationship between the electric field strength and the distance from the point charge. Electric field strength is written as: Chapter Assessment Questions

Hence, if the distance is doubled, the electric field strength will decrease by four times. Reason:

3.On which factors does the change in potential difference depend? Chapter Assessment Questions A.on the magnitude of the test charge and the field of the charge B.on the magnitude of the test charge and the displacement C.only on the field of the charge D.on the field and the displacement

Reason: The change in potential difference is the work done in moving the test charge. Since work depends on the field and the displacement, the change in potential difference depends only on the field and the displacement and not on the magnitude of the test charge. Chapter Assessment Questions

4.On which factors does the direction of force on a charge depend? Chapter Assessment Questions A. only on the direction of the field B.only on the direction of the field and sign of the charge C.only on the strength of the field D.only on the direction and strength of the field

Chapter Assessment Questions Reason: Recall that, F = Eq. That is, the direction of force depends on the direction of the field and the sign of the charge.

5.Suppose you move a test charge in a circle around a negative charge. What will be the change in the electric potential? A.There will be a positive change in the electric potential. B.There will be a negative change in the electric potential. C.The change in electric potential will be zero. D.The change in electric potential will be infinite. Chapter Assessment Questions

Reason: When we move a test charge in a circle around a negative charge, the force that the electric field exerts on the test charge is always perpendicular to the direction in which we moved it, so we do no work. Therefore, the electric potential difference is zero. Chapter Assessment Questions