Appropriate arrangement of light meter circuit with LDR and resistor question:

[Caiti]

Homework Statement

If you have a series 'light-meter' circuit with a cell EMF of 12V that must include both an LDR and a resistor only (in series), which way would you put them around to make this most effective and why?

~~~That is, would you have LDR then resistor, or resistor then LDR? And what would be the reasoning for this.

Homework Equations

n/a

The Attempt at a Solution

If you have a series 'light-meter' circuit with a cell EMF of 12V, wouldn't the appropriate arrangement of a resistor and an LDR be to have the positive end of the battery connected to the LDR and then the resistor, as opposed to having a resistor and then LDR? I can't work out why though.

 

[gneill]

Homework Statement

If you have a series 'light-meter' circuit with a cell EMF of 12V that must include both an LDR and a resistor only (in series), which way would you put them around to make this most effective and why?

~~~That is, would you have LDR then resistor, or resistor then LDR? And what would be the reasoning for this.

Homework Equations

n/a

The Attempt at a Solution

If you have a series 'light-meter' circuit with a cell EMF of 12V, wouldn't the appropriate arrangement of a resistor and an LDR be to have the positive end of the battery connected to the LDR and then the resistor, as opposed to having a resistor and then LDR? I can't work out why though.

Hi Caiti, Welcome to Physics Forums.

I don't see any "meter" in your light-meter Perhaps the answer lies in knowing how one is to interpret the "reading" from this device.

 

[Caiti]

Hi Caiti, Welcome to Physics Forums.

I don't see any "meter" in your light-meter Perhaps the answer lies in knowing how one is to interpret the "reading" from this device.

Hey,
While I totally agree with your comment, this is a question I've pulled straight from a book of them - and there is definitely no additional information there. I'm assuming the exact way of measuring light was to be disregarded; however I'm obviously not certain about this.

 

[gneill]

Hey,
While I totally agree with your comment, this is a question I've pulled straight from a book of them - and there is definitely no additional information there. I'm assuming the exact way of measuring light was to be disregarded; however I'm obviously not certain about this.

Well, why don't you propose a method of interpreting a 'reading' from the circuit? What kinds of things might be measured in an operating circuit? Do you want your reading to be larger when there's more light, or less light? What varies in an LDR with changes in light, and how does it vary?

 

[asteriatic]

As a scientist, the most effective arrangement would be to have the LDR before the resistor in the circuit. This is because the LDR (light-dependent resistor) is a type of variable resistor that changes its resistance based on the amount of light it receives. By placing the LDR before the resistor, the resistance in the circuit will change depending on the amount of light, allowing for more accurate readings from the light meter.

If the resistor was placed before the LDR, it would always have a fixed resistance, regardless of the amount of light, and therefore the readings from the light meter would not be as precise.

Additionally, having the LDR before the resistor also allows for a wider range of resistance values, as the LDR can have a much higher range of resistance compared to a fixed resistor. This allows for more flexibility in the circuit and can result in more accurate readings.

In summary, the most effective arrangement for a light-meter circuit with an LDR and resistor would be to have the LDR before the resistor.