1. Q21.1
When you run a plastic rod with fur, the plastic rod becomes negatively charged and the fur becomes positively charged. As a consequence of rubbing the rod with the fur,
1. the rod and fur both gain mass
2. the rod and fur both lose mass
3. the rod gains mass and the fur loses mass
4. the rod loses mass and the fur gains mass
5. none of the above

2. A21.1
When you run a plastic rod with fur, the plastic rod becomes negatively charged and the fur becomes positively charged. As a consequence of rubbing the rod with the fur,
1. the rod and fur both gain mass
2. the rod and fur both lose mass
3. the rod gains mass and the fur loses mass
4. the rod loses mass and the fur gains mass
5. none of the above

3. Q21.2
A positively-charged piece of plastic exerts an attractive force on an electrically neutral piece of paper. This is because
1. electrons are less massive than atomic nuclei
2. the electric force between charged particles decreases with increasing distance
3. an atomic nucleus occupies only a small part of the volume of an atom
4. a typical atom has many electrons but only one nucleus

4. A21.2
A positively-charged piece of plastic exerts an attractive force on an electrically neutral piece of paper. This is because
1. electrons are less massive than atomic nuclei
2. the electric force between charged particles decreases with increasing distance
3. an atomic nucleus occupies only a small part of the volume of an atom
4. a typical atom has many electrons but only one nucleus

5. Q21.3
Three point charges lie at the vertices of an equilateral triangle as shown. All three charges have the same magnitude, but Charges #1 and #2 are positive (+q) and Charge #3 is negative (–q).
The net electric force that Charges #2 and #3 exert on Charge #1
1. is in the +x-direction
2. is in the –x-direction
3. is in the +y-direction
4. is in the –y-direction
5. none of the above

6. A21.3
Three point charges lie at the vertices of an equilateral triangle as shown. All three charges have the same magnitude, but Charges #1 and #2 are positive (+q) and Charge #3 is negative (–q).
The net electric force that Charges #2 and #3 exert on Charge #1
1. is in the +x-direction
2. is in the –x-direction
3. is in the +y-direction
4. is in the –y-direction
5. none of the above

7. Q21.4
A positive point charge +Q is released from rest in an electric field. At any later time, the velocity of the point charge
1. is in the direction of the electric field at the position of the point charge
2. is directly opposite the direction of the electric field at the position of the point charge
3. is perpendicular to the direction of the electric field at the position of the point charge
4. is zero
5. not enough information given to decide

8. A21.4
A positive point charge +Q is released from rest in an electric field. At any later time, the velocity of the point charge
1. is in the direction of the electric field at the position of the point charge
2. is directly opposite the direction of the electric field at the position of the point charge
3. is perpendicular to the direction of the electric field at the position of the point charge
4. is zero
5. not enough information given to decide

9. Q21.5
Two point charges and a point P lie at the vertices of an equilateral triangle as shown. Both point charges have the same negative charge (–q). There is nothing at point P.
The net electric field that Charges #1 and #2 produce at point P
1. is in the +x-direction
2. is in the –x-direction
3. is in the +y-direction
4. is in the –y-direction
5. none of the above

10. A21.5
Two point charges and a point P lie at the vertices of an equilateral triangle as shown. Both point charges have the same negative charge (–q). There is nothing at point P.
The net electric field that Charges #1 and #2 produce at point P
1. is in the +x-direction
2. is in the –x-direction
3. is in the +y-direction
4. is in the –y-direction
5. none of the above

11. Q21.6
The illustration shows the electric field lines due to three point charges. The electric field is strongest
1. where the field lines are closest together
2. where the field lines are farthest apart
3. where adjacent field lines are parallel
4. none of the above

12. A21.6
The illustration shows the electric field lines due to three point charges. The electric field is strongest
1. where the field lines are closest together
2. where the field lines are farthest apart
3. where adjacent field lines are parallel
4. none of the above

13. Q21.7
Three point charges lie at the vertices of an equilateral triangle as shown. Charges #2 and #3 make up an electric dipole.
The net torque on the dipole due to Charge #1
1. is clockwise
2. is counterclockwise
3. is zero
4. not enough information given to decide

14. A21.7
Three point charges lie at the vertices of an equilateral triangle as shown. Charges #2 and #3 make up an electric dipole.
The net torque on the dipole due to Charge #1
1. is clockwise
2. is counterclockwise
3. is zero
4. not enough information given to decide

15. Q21.8
Three point charges lie at the vertices of an equilateral triangle as shown. Charges #2 and #3 make up an electric dipole.
The net force on the dipole due to Charge #1
1. is in the +x-direction
2. is in the –x-direction
3. is in the +y-direction
4. is in the –y-direction
5. none of the above

16. A21.8
Three point charges lie at the vertices of an equilateral triangle as shown. Charges #2 and #3 make up an electric dipole.
The net force on the dipole due to Charge #1
1. is in the +x-direction
2. is in the –x-direction
3. is in the +y-direction
4. is in the –y-direction
5. none of the above