(Original post by blobbybill)
Plugged in numbers about 20 times now, I see that the same otucome always occurs.
However, I have a question. Since I would increase when R decreases, how come that it never happens where the voltage increases (for example, in V=IR where R only decreases a little and I increased a lot)? I can't figure out why that never happens (even in normal circuits).
Is it because you are using R in the calculation of the current (I=V/R)? I don't understand why that isn't possible, as the current always increases when resistance falls and voltage falls, so why is it not possible for V=IR that V could increase, why doesn't it ever happen where I increases a lot when R decreases only a little (and in that case, make the value of V increase)? Why does that not occur? Surely it could?
I think you are missing some important concepts:
Current is the flow (rate) at which charge (electrons) move through the circuit. Think of electrons in a conductor like water moving in a pipe as an analogy.
The supply voltage is a measure of the energy available to the circuit per quantity of electrons.
Resistors do what they say on the tin: they 'resist' the flow of electrons (current). In doing so, they act like a radiator in a central heating system by causing electrons to give up the energy supplied by the battery.
The energy given up when current flows in the resistance is also measured as a voltage - this time the voltage is developed across the resistance and is known as a potential difference. (pd)
In other words, the voltage of the supply is a source of energy, the voltage across the resistance is energy lost.
In the LDR example, the total resistance in the path of the current is Rfixed
The supply voltage Vsupply
provides the pressure to drive current around the circuit. The current is limited by the resistance in the electrons path.
Because the current flows through a single series path, the value of that current is the same at all points in the circuit.
The energy given up when the current passes through the LDR is measured in volts. And this is simply the current Itotal
In other words, VLDR
If the LDR resistance changes, then because the supply voltage is the same
, the total current in the circuit must also change. If RLDR
decreases, then Itotal
must increase since the fixed resistor does not change and the total resistance as fallen. If RLDR
increases, then Itotal
must decrease since the fixed resistor does not change and the total resistance has increased. (See eq (1))
Critically, because the total current has changed, then the energy lost across the resistances must also change:
If the LDR resistance increases, then the total current must fall (the supply voltage has not changed eq 1). And because the current is the same at all points in the circuit, the voltage developed (energy lost) across the LDR must also fall. (See eq (2))
If the LDR resistance decreases, then the total current must increase (the supply voltage has not changed eq 1). And because the current is the same at all points in the circuit, the voltage developed (energy lost) across the LDR must also increase. (See eq (2))
You must make the conceptual leap that the supply voltage is energy provided while the voltage (pd) across the resistor is energy lost.