How does the pull down resistor work in this circuit?

Question

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I get what the pull down resistor is there to do, but I don't really understand exactly how it works.

When there is no voltage applied through the base wire, why does any induced current then go through R2, but when a voltage is applied, it goes to the transistor? My understanding is that voltage always follows the path of least resistance, so if applying say, 5v turns the transistor on, that means that the transistor has a lower resistance than R2, so I would also assume any induced current would also flow through the resistor - so why does it then go through R2?

Answer 1

I've seen some comments here stating that the pulldown is needed to keep the transistor turned off, instead of floating, or for noise reasons. The base pulldown does in fact help keep the transistor off, but no one has answered why. This is why I chose to answer, rather than extend the comments. User alexan_e is thinking correctly about it, so I'll elaborate here.

There is a Miller capacitance between the collector and the base of all BJTs. Most designers know all too well about the MOSFET's Miller Capacitance and forget that the BJT has some as well. The BJTs Miller capacitance provides a leakage path from collector to base, injecting charge carriers into the base region, which can be amplified by the BJT's Hfe (gain). This allows noise current to flow from collector to emitter. The inclusion of the base pulldown will provide a path to ground to discharge the Miller capacitance and keep the BJT hard off and noise free.

Answer 2

When the left side of R1 gets disconnected then there is no current flow through R1 or R2 and that means that there will be no voltage drop across it.
If there is no voltage drop across R2 the base level becomes 0v and the transistor is held in an off state.

On the other hand when there is a voltage applied to the left side of R1 then R1 and R2 form a voltage divider that drives the base and with R1 being much smaller than R2 the voltage drop across R1 is fairly small.