# Free MCAT practice question

• by
• Nov 25, 2013
• MCAT Blog, MCAT Long Form, MCAT Physics, MCAT Prep

Item 16

The voltage drop across Rd is 8V. What is the current across Re?

a) 2A

b) 8A

c) 14A

d) 32A

Explanation:

This problem requires 3 electrical circuit concepts:

1) V=IR relating voltage, current, and resistance

2) The voltage drop across resistors in parallel is equal in each resistor

3) Kirchoff’s current law states that the sum of currents entering a node equals the sum of currents leaving a node

The voltage drop is equivalent for each resistor between red nodes A and B. The voltage drop across Rd is given as 8V, and therefore the voltage drop across resistors Rb and Rc is also 8V. Using V=IR for each resistor, the current in resistor Rb=4A, Rc=8A, and Rd=2A. Using Kirchoff’s current law at node B: 4A + 8A + 2A = 14A. The current leaving node B equals the current across Re because there are no other nodes in between.

a) 2A, incorrect, this implies that the current across Rd=Re.

b) 8A, incorrect, this implies that the current across Rd equals the voltage and the current across Re.

c) 14A, correct.

d) 32A, incorrect, this answer incorrectly assumes a voltage of 8V across Re and then uses a false relationship VR=I.