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MCAT Physics Problem

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

A. 2 A

B. 8 A

C. 14 A

D. 32 A

Click for Explanation

This problem requires 3 electrical circuit concepts:

1) V = IR, which relates voltage, current, and resistance

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

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

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

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

C. 14 A, correct.

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

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