Kaplan Question of the Day on DC circuit

[MedGrl@2022]

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A simple DC circuit consists of a battery and a resistor wired together. If another resistor is placed in parallel to the original resistor, what is true about the current in the new configuration?

A. The current going through each resistor is the same.

B. The current leaving the battery is greater than the current that flows before the new resistor is inserted.

C. The current going through the original resistor decreases after the new resistor is inserted.

D. The current leaving the battery is unchanged.

Kaplan says that B is correct but couldn't both B and C correct? Perhaps C is only true in some situations?

This was kaplan's explanation: The fastest way to the correct answer is to remember that when resistors are placed in parallel, the equivalent resistance of the pair is less than either one of the original resistances. The reason why parallel setups are used in electrical circuits doesn’t have anything to do with resistance –parallel wiring is used to apply the same voltage to multiple instruments. Answer choice (B) is correct–if the total resistance goes down, then the total current must increase.

You can see that choice (A) is a trap–the voltage across each resistor is the same, not the current. The current will depend on the strength of each resistor. The current that travels through the original resistor remains unchanged when another resistor is added in parallel. So choice (C) is incorrect. Since the original resistor has the same potential across it, the current through it must be the same. Finally, choice (D) is wrong – since the equivalent resistance of the circuit changes, Ohm’s Law guarantees that the total current will change as well.

 

[slz1900]

C can't be true. The amount of current flowing through a resistor is a property of the resistor and the voltage source. Neither is changing in the question. This was confusing to me at first, but it makes sense that a resistor will always have as much current flowing through it as the voltage source allows, and that is obviously not going to be reduced by another resistor somewhere else in the circuit.

 

[MedGrl@2022]

C can't be true. The amount of current flowing through a resistor is a property of the resistor and the voltage source. Neither is changing in the question. This was confusing to me at first, but it makes sense that a resistor will always have as much current flowing through it as the voltage source allows, and that is obviously not going to be reduced by another resistor somewhere else in the circuit.

I thought that when resistors are placed in parallel the current splits between the resistors. Therefore, the current travelling through just 1 resistor versus two resistors in parallel could potentially be less or more (in other cases).

 

[slz1900]

It does split, but total current is greater since the equivalent resistance drops when you have resistors in parallel.