(455) Thermistors, LDR, potentiometers [IB Physics SL/HL]

Resistors and Ohm's Law

• Not all resistors obey Ohm's law.

  • Ohmic Resistors: Follow Ohm's law.

  • Non-Ohmic Resistors: Exhibit variable resistance.

Types of Variable Resistors

• Variable Resistor Symbol:

  • Common symbol includes a square and an arrow indicating change.

Light-Dependent Resistor (LDR)

• Definition: Resistance varies with light intensity or wavelength.

• Symbol: Similar to a variable resistor, but indicates light input (two arrows).

• Applications: Automatic dimming in devices like phone screens.

Thermistors

• Definition: Resistance changes with temperature; typically made of metal.

• Symbol: Similar to variable resistor with an added stick in the graphic.

• Applications: Used in engines or household thermostats.

Potentiometers

• Definition: Resistance changes based on the contact point along a coil.

• Symbol: Arrow indicates the variable contact point.

• Applications: Adjustment of resistance in circuits based on connection point.

Circuit Applications of Variable Resistors

• Basic Circuit Description: Includes a sensor (LDR, thermistor) connected to a voltmeter to monitor voltage changes.

  • Voltage and Current Calculation: Using Ohm’s Law, I = V/R.

  • For resistors in series: Total resistance R = R1 + R2.

Example Calculation: Automatic Light Switch with LDR

• Given: LDR activates the light switch when V across it is 5V; Battery EMF = 12V; LDR Resistance = 200 kΩ.

• Equation: V_out = (R2 / (R1 + R2)) * V_in.

• Solve for R1:

  • Rearranging gives R1 = (R2 * V_in / V_out) - R2.

  • Calculation leads to R1 = 280 kΩ to activate the switch when the LDR is at 200 kΩ.