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I'm sure I'm missing something basic, but here we go:

If adding resistors in parallel decreases total resistance, why doesn't the voltage in the circuit change?

Does the current increase to match the decreased resistance?

Thanks!

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    \$\begingroup\$ -1 for a badly formulated question: as Houston's answer shows, without knowing the rest of the circuit, there is no way to know what will happen. \$\endgroup\$ – Wouter van Ooijen Aug 30 '15 at 20:59
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Does the current increase to match the decreased resistance?

Absolutely correct. KVL says that the voltage doesn't change, so the current must change instead in order to compensate.

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In the case of an ideal voltage source, the source will deliver as much current as is required to maintain the voltage. Adding additional resistors will draw more current. This is just a voltage source doing it's job.

schematic

simulate this circuit – Schematic created using CircuitLab

In the case of a current source however the voltage will decrease.

schematic

simulate this circuit

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    \$\begingroup\$ This assumes a perfect voltage source. A battery, for instance, will change voltage with changing current. \$\endgroup\$ – WhatRoughBeast Aug 30 '15 at 20:20

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