18 . E —Most of these statements drive at the fundamental principle that the value of an electric potential
can be set to anything; it is only the difference in electric potential between two points that has a
physical usefulness. Usually potential is set to zero either at the ground or, for isolated point charges, a
very long distance away from the charges. But potential can, in fact, be set to zero anywhere, meaning
that the potential could easily be less than zero everywhere on a wire. (And a proton, a positive
charge, is forced from high to low potential, not the other way around.)
19 . E —This is a uniform electric field. The force on a charge in an electric field is given by F = qE .
Therefore, as long as the electric field is the same at all three points, the force on the charge is the
same as well.
20 . C —Only forces can go on free-body diagrams, and the electric field is not itself a force. The force
provided by an electric field is qE ; the weight of the electron is mg .
21 . D —Capacitance is a property of the structure of the capacitor. Changing the charge on (or the voltage
across) a capacitor does not change the capacitance. The capacitance of a parallel-plate capacitor is
given by the equation
Decreasing d , the distance between the plates, will increase the capacitance.22 . B —These resistors are in parallel with the battery; thus, they both must take the voltage of the
battery, 10 V. The total current in the circuit is 2.0 A, but that current splits between the two resistors,
leaving 1.0 A through each. This can also be determined by a direct application of Ohm’s law—
because we know both the voltage and resistance for each resistor, divide V/R to get the current.
23 . E —Use the right-hand rule for the force on a charged particle in a magnetic field: point your right
hand in the direction of the velocity, curl your fingers toward the magnetic field, and your thumb points
into the page. The charge is positive, so your thumb points in the direction of the force.
24 . B —This question uses the right-hand rule for the magnetic field produced by a current-carrying wire.
Point your thumb in the direction of the current; your fingers wrap around the wire in the direction of
the magnetic field. To the wire’s left, your fingers point out of the page.
25 . A —Only a changing magnetic flux induces a current. Flux is given by BA cos θ , where B is the
magnetic field, and A is the area of the loop of wire. Obviously, then, choices B, C, and E change the
flux and induce a current. Choice D produces a flux by changing θ , the angle at which the field
penetrates the loop of wire. In choice A, no current is induced because the field doesn’t change and
always points straight through the loop.
Interpretation: How Ready Are You?
Now that you have finished the diagnostic exam and checked your answers, it is time to try to figure out
what it all means. First, remember that getting only about 60% of the answers correct will give you a 5 on
the AP exam; about 30–40% correct is the criterion for a qualifying score of 3. You’re not supposed to get
90% correct! So relax and evaluate your performance dispassionately.
Next, see if there are any particular areas in which you struggled. For example, were there any