AP Physics Physics: Electricity & Magnetism 5 — Questions and Answers
Question 1: A transformer has 100 primary turns and 500 secondary turns. If the primary voltage is 120 V, what is the secondary voltage?
- 24 V
- 600 V (Correct answer)
- 120 V
- 12 V
Correct answer: 600 V
Using the transformer equation V_s/V_p = N_s/N_p: V_s = 120 × (500/100) = 600 V.
Question 2: The work done by the electric force to move a positive charge Q from point A to point B equals:
- Q(V_B − V_A)
- Q(V_A − V_B) (Correct answer)
- Q·V_A·V_B
- Q/(V_A − V_B)
Correct answer: Q(V_A − V_B)
Work done by the electric force is W = Q(V_A − V_B); the charge loses potential energy moving from high to low potential.
Question 3: Which statement about electric field lines is INCORRECT?
- They begin on positive charges and end on negative charges
- They can cross each other at points of equal potential (Correct answer)
- Their density indicates the field strength
- They are perpendicular to equipotential surfaces
Correct answer: They can cross each other at points of equal potential
Electric field lines never cross because the field has a unique direction at every point; crossing would imply two directions simultaneously.
Question 4: In an RL circuit connected to a DC source, after a long time the current reaches:
- Zero
- ε/R (Correct answer)
- ε/L
- Infinity
Correct answer: ε/R
After many time constants, the inductor acts as a wire (zero resistance), so the steady-state current is simply I = ε/R.
Question 5: A uniform electric field of 500 V/m exists between two parallel plates separated by 0.02 m. The potential difference between the plates is:
- 25,000 V
- 10 V (Correct answer)
- 0.04 V
- 250 V
Correct answer: 10 V
For a uniform field, V = Ed = 500 × 0.02 = 10 V.
Question 6: Kirchhoff's Voltage Law (KVL) is a consequence of:
- Conservation of charge
- Conservation of energy (Correct answer)
- Newton's third law
- Faraday's Law
Correct answer: Conservation of energy
KVL states that the sum of voltage drops around any closed loop is zero, which follows from the conservation of energy for the electric potential.
Question 7: A conducting sphere of radius R has total charge Q. The electric potential at its surface is:
- kQ/R²
- kQ/R (Correct answer)
- kQ²/R
- Zero
Correct answer: kQ/R
The potential at the surface of a conducting sphere is V = kQ/R, the same as if all charge were at the center.
AP Physics Physics: Electricity & Magnetism 5
A transformer has 100 primary turns and 500 secondary turns.
If the primary voltage is 120 V, what is the secondary voltage?