Cathy's Question: Motor on/off with LDR and Resistor?

[CathyLou]

I've no idea how to start the following question so any help or guidance would be really appreciated.

A potential divider circuit is set up with a LDR and a fixed resistor, R. The LDR has a resistance of 200 Ω when it is light and 2500 Ω when it is dark. A motor is set so that it comes on if 4.0 V is applied across it. A 6.0 V battery provides the emf (assume it has no internal resistance).

When it is dark, does the motor come on if R is (a) 3000 Ω (b) 4000 Ω (c) 5000 Ω?

When it is light, does the motor come on if R is (a) 300 Ω (b) 400 Ω (c) 500 Ω?

If the motor were moved so that it is across the LDR instead, what would R have to be so that the motor came on in the light?

Thank you.

Cathy

 

[denverdoc]

need to post at least some thought about this: maybe you don't have a diagram, and that is the issue: I read it as such. The diffference between
v1 and v3 is just the applied EMF of 6Volts.

V1-----R------v2------Rx------v3. where Rx=variable depending on whether dark or light. So for the potential difference between v1 and v2, to be >=4v,
what must R be. Does this help at all?

 

[m2003]

, it sounds like you are working on a project involving a potential divider circuit and a LDR (light-dependent resistor). This type of circuit is commonly used for controlling devices based on the amount of light present in the environment. In this case, the device in question is a motor that turns on at a certain voltage.

To answer your first question, we need to understand how the potential divider circuit works. When the LDR is exposed to light, its resistance decreases, causing the voltage at the motor to increase. If the voltage at the motor reaches 4.0 V, the motor will turn on. So, in order for the motor to come on when it is dark, the total resistance in the circuit must be low enough to allow for a voltage of 4.0 V at the motor.

For (a) 3000 Ω, the total resistance in the circuit would be 3200 Ω (200 Ω for the LDR and 3000 Ω for the fixed resistor). This is lower than the resistance in the dark state (2500 Ω for the LDR and 3000 Ω for the fixed resistor), so the motor would come on.

For (b) 4000 Ω, the total resistance in the circuit would be 4500 Ω (2500 Ω for the LDR and 4000 Ω for the fixed resistor). This is higher than the resistance in the dark state, so the motor would not come on.

For (c) 5000 Ω, the total resistance in the circuit would be 5500 Ω (2500 Ω for the LDR and 5000 Ω for the fixed resistor). Again, this is higher than the resistance in the dark state, so the motor would not come on.

In the light state, the opposite is true. The LDR has a lower resistance, so the total resistance in the circuit must be higher in order for the motor to turn on.

For (a) 300 Ω, the total resistance would be 2300 Ω (200 Ω for the LDR and 300 Ω for the fixed resistor). This is lower than the resistance in the light state, so the motor would not come on.

For (b) 400 Ω, the total resistance would be 2400 Ω (200 Ω for the LDR and 400 Ω for the fixed resistor). This is equal