## Class 8

CLASS OUTLINE

• Electric forces from multiple charges
• Electric fields
• Electric field lines

CONCEPTESTS

CT1A positively charged object is placed close to a conducting object attached to an insulating glass pedestal (a).  After the opposite side of the conductor is grounded for a short time interval (b), the conductor becomes negatively charged (c).

Based on this information, we can conclude that within the conductor

1. both positive and negative charges move freely.
2. only negative charges move freely.
3. only positive charges move freely.
4. We can't really conclude anything.

CT2.  A pair of equal and opposite charges are placed as illustrated.

The electric field at point P is
1. along +x.
2. along -x.
3. along +y.
4. along -y.
5. in another direction.
6. The electric field at P is zero.

CT3.  A negatively charged object is placed in an electric field as shown below

The electric force on the object
1. is to the right.
2. is to the left.
3. is neither to the left nor to the right.
4. depends on whether the field is created by a positively or negatively charged object.
5. There is no force on the object at the location shown in the figure.

CT4. Consider the four field patterns shown. Assuming there are no charges in the regions shown, which of the patterns represent(s) a possible electrostatic field:

1. (a)
2. (b)
3. (b) and (d)
4. (a) and (c)
5. (b) and (c)
6. some other combination
7. None of the above.

CT5An electrically neutral dipole is placed in an external field. In which situation(s) is the net force on the dipole zero?
1. (a)
2. (c)
3. (b) and (d)
4. (a) and (c)
5. (c) and (d)
6. some other combination
7. none of the above

CT1The answer is 4.  All we know is that there is a net surplus of positive charge in one position and a net surplus of negative charge in another.  This could be caused by negative charge carriers moving toward the rod or positive charge carriers moving away.  There is no way of discerning the microscopic behavior by examining the net charge imbalance remaining.

Our knowledge of ordinary matter tells us that electrons and protons carry charge and that electrons are usually more able to move about.  However, if someone clever manufactured some material in which positive charges could move about more easily, exactly the same result would be achieved.

CT2: The answer is 1. The y components of the electric field from +q and -q will cancel since their distance to P is the same but the y component of their fields have opposite directions (one would repel a positive charge, the other would attract it).  But their x components both line up in the x direction (remember, electric fields run from + to - charges). So the net electric field is in the +x direction.  Alternatively, use your knowledge of what the electric field lines look like in this situation, remembering that they proceed from negative to positive charges.

CT3:  The answer is 2.  At the object's position, the electric field is to the right.  Therefore, the force on the negatively charged object is in the opposite direction, to the left.  It doesn't matter what charges created the field, the field itself gives sufficient information to answer the question.

CT4:  The answer is 2.  Pattern (a) can be eliminated because field lines cannot simultaneously emanate from and converge at a single point; (c) can be eliminated because there are no charges in the region, and so there are no sources of field lines; (d) can be eliminated because electrostatic field lines do not close on themselves.

CT5:  The answer is 5.  An electric dipole in a uniform electric field experiences a zero net force because the forces exerted on each end are of equal magnitude but opposite direction.  In (a) and (b) the speading of the field lines indicates a diminishing field strength to the left.  There is a torque on (d), but no net force.