Question 1
A 12 V battery is connected across a single 4 Ω resistor. What current flows through the resistor?
- A.
0.333 A
- B.
3 A
- C.
36 A
- D.
48 A
Answer: B. 3 A
Ohm's law: I = V/R = 12/4 = 3 A. V²/R = 36 is the power in watts, not the current.
MCAT · Chemical and Physical Foundations · Physics
Electricity and circuits
Electricity and circuits is part of Physics in the MCAT Chemical and Physical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.
Question types you will see here include equivalent resistance of a network, power dissipation and bulb brightness, capacitor charge, voltage and energy and rC charging and discharging.
Common traps: adding parallel resistors like series ones; thinking an extra parallel branch raises resistance; splitting voltage in a parallel arrangement.
Sample questions
Question 2
In a test of an electrostatic dust filter, two small charged beads repel each other with a force of 0.36 N. If the charge on one bead is doubled and their separation is tripled, what is the new force?
- A.
0.040 N
- B.
0.080 N
- C.
0.24 N
- D.
0.72 N
Answer: B. 0.080 N
Coulomb's law gives F ∝ q₁q₂/r², so F = 0.36 N × 2/3² = 0.36 × 2/9 = 0.080 N (B). A forgets the doubled charge, C scales force as 1/r instead of 1/r², and D doubles the charge but ignores the larger separation.
- A.
Question 3
A polar molecule with a permanent dipole moment is placed between two parallel charged plates, where the electric field is uniform. Which describes the effect of the field on the molecule?
- A.
A net force toward the positive plate, but no torque
- B.
No net force and no torque, because the molecule's net charge is zero
- C.
A net force toward the negative plate, and a torque that turns the dipole against the field
- D.
No net force, but a torque that tends to align its dipole moment with the field
Answer: D. No net force, but a torque that tends to align its dipole moment with the field
In a uniform field the forces on the dipole's equal and opposite charge centers cancel, so there is no net force, but they act along different lines and produce a torque toward alignment with E (D). A and C predict a net force that exists only in a nonuniform field, and B ignores the torque from the separated charges.
- A.
Question 4
A charged parallel-plate capacitor in a humidity sensor is disconnected from its battery. A dielectric slab with dielectric constant 3 is then slid in to fill the gap. How do the voltage across the plates and the stored energy change?
- A.
Both triple.
- B.
The voltage is unchanged and the energy triples.
- C.
The voltage falls to one-third and the energy falls to one-ninth.
- D.
Both fall to one-third of their original values.
Answer: D. Both fall to one-third of their original values.
Disconnected, the charge Q is fixed while C triples, so V = Q/C falls to one-third and U = Q²/(2C) also falls to one-third (D). A treats the dielectric as lowering capacitance, B describes a capacitor still connected to its battery, and C uses U = ½CV² while forgetting that C itself tripled.
- A.
Question 5
A warming pad contains three identical 30 Ω heating elements: two connected in parallel, and that pair connected in series with the third, across a 45 V supply. What is the current through one of the two parallel elements?
- A.
0.50 A
- B.
1.0 A
- C.
1.5 A
- D.
3.0 A
Answer: A. 0.50 A
The parallel pair is 15 Ω, so the circuit is 45 Ω and draws 1.0 A; the pair splits this equally, giving 0.50 A in each element (A). B is the total current, which flows only through the single series element; C puts the full 45 V across one element; D puts the full 45 V across the 15 Ω pair.
- A.
Find your real gaps
A few questions show the style. The free scan shows which MCAT skills cost you the most points, then builds a plan around them.