Overview
This lecture covers the principles, uses, and calculations of potential divider circuits, including potentiometers, galvanometers, and variable resistance components such as LDRs and thermistors.
Potential Divider Circuits
- Potential dividers use resistors in series to split input voltage (Vin) into a chosen output voltage (Vout).
- Purposes: provide variable voltage, select specific voltages, or split voltage among components.
- Vout is calculated as: Vout = (R2 / (R1 + R2)) × Vin, where R2 is the resistor across which Vout is measured.
- The larger the resistance in a resistor, the greater its share of the total voltage (V = IR).
Worked Example: Potential Divider Calculation
- To find Vin: Rearranged equation is Vin = ((R1 + R2) / R2) × Vout.
- Apply correct resistor values to solve for unknowns.
Potentiometers
- A potentiometer is a variable resistor set up as a potential divider, providing continuously adjustable output voltage.
- Circuit symbol: resistor with a sliding arrow.
- Moving the slider alters the proportion of resistance, changing the output voltage.
- Maximum output voltage is when slider is at the end of one extreme, minimum at the other.
Worked Example: Potentiometer Output Calculation
- With three resistors and a sliding contact, output voltage depends on which resistors are included between output and ground.
- Vout = ((R2 + R3) / (R1 + R2 + R3)) × Vin for maximum; Vout = (R3 / (R1 + R2 + R3)) × Vin for minimum.
Galvanometer
- A galvanometer is a sensitive ammeter for detecting small currents and measuring EMF in potentiometer circuits.
- Circuit symbol: arrow inside a circle; pointer shows current direction.
- Null deflection (pointer upright) means zero current and equal potentials on both sides.
- Used for comparing unknown and known EMF values.
Variable Resistance Components
- LDRs (Light Dependent Resistors) and thermistors affect potential divider output depending on light or temperature.
- Higher light or temperature decreases LDR/thermistor resistance, lowering their potential difference and increasing the fixed resistor's.
- Applications include automatic lighting and temperature control circuits.
Worked Example: Sensory Component in Divider
- If thermistor temperature decreases, its resistance and voltage increase; the fixed resistor's voltage decreases, total remains constant (Kirchhoff’s second law).
Key Terms & Definitions
- Potential Divider — Circuit splitting input voltage into a specified output using series resistors.
- Potentiometer — Variable resistor configured as a divider for adjustable voltage.
- Galvanometer — Sensitive instrument detecting small currents; shows balance in potentiometers.
- LDR (Light Dependent Resistor) — A resistor whose resistance decreases as light intensity increases.
- Thermistor — A resistor whose resistance decreases with increasing temperature (NTC type).
Action Items / Next Steps
- Practice calculating Vout in potential divider circuits using different resistor configurations.
- Review circuit symbols for potentiometer, galvanometer, LDR, and thermistor.
- Prepare for questions involving sensory components in divider circuits.