Question 1: Define a Light-Dependent Resistor (LDR) and explain how its resistance changes with light intensity.

Answer: An LDR is a resistor whose resistance decreases as light intensity increases. It responds to light levels by changing its electrical resistance accordingly.

Question 2: What is a thermistor and how does its resistance vary with temperature?

Answer: A thermistor is a resistor whose resistance varies with temperature, typically decreasing as temperature increases (NTC) or increasing as temperature increases (PTC).

Question 3: Describe the basic mechanism by which an LDR detects changes in light.

Answer: An LDR detects changes in light by altering its resistance; more light causes resistance to decrease, allowing more current to flow in the circuit.

Question 1: An LDR has a resistance of 10 kΩ in darkness and 1 kΩ in bright light. Calculate the percentage decrease in resistance when moving from darkness to bright light.

Answer: Percentage decrease = [(10,000 - 1,000) / 10,000] * 100 = 90%

Question 2: A thermistor's resistance drops from 50 kΩ at 20°C to 10 kΩ at 50°C. Using the formula R = R0 * e^(αT), estimate the temperature coefficient α assuming exponential change.

Answer: Using logarithms: ln(R2/R1) = α(T2 - T1). So, ln(10,000/50,000) = α(50 - 20). Calculate α = ln(0.2) / 30 ≈ -1.609 / 30 ≈ -0.0536 per °C.

Question 1: Outline a simple experiment to test how an LDR's resistance changes with different light intensities. Include the variables you would control and measure.

Answer: Set up a circuit with an LDR, a voltmeter, and a power source. Vary the light intensity by moving a light source closer or further away or using different light sources. Control variables include the voltage supply and ambient temperature. Measure the voltage across the LDR or current to determine resistance changes.

Question 2: Identify two safety precautions to consider when conducting experiments with thermistors and LDRs.

Answer: Avoid exposing components to excessive heat or direct sunlight that could damage them. Ensure electrical circuits are properly insulated to prevent short circuits or electric shocks.

Question 1: A student measures the resistance of an LDR in a room at different light levels. In darkness, resistance is 15 kΩ; in dim light, it is 5 kΩ; in bright light, it is 1 kΩ. Describe how the resistance changes with increasing light intensity and explain why.

Answer: The resistance decreases as light intensity increases. This is because the LDR's resistance is inversely proportional to light levels; more light causes more electrons to be excited, reducing resistance.

Question 1: Explain how thermistors can be used in temperature measurement devices. Include the principle of operation in your answer.

Answer: Thermistors are used in temperature measurement by exploiting their resistance change with temperature. In an NTC thermistor, resistance decreases as temperature increases. When incorporated into a circuit, the resistance change affects the current or voltage, which can be calibrated to measure temperature accurately.

Question 1: Provide an example of how an LDR is used in a household device. Explain the principle behind its operation.

Answer: LDRs are used in automatic street lighting systems to turn lights on at dusk and off at dawn. The LDR detects changes in light levels, and its decreasing resistance in bright light causes the circuit to turn off the light, while increasing resistance in darkness turns it on.

Question 2: Explain why thermistors are suitable for use in digital thermometers.

Answer: Thermistors have a rapid and predictable change in resistance with temperature, allowing for precise and quick temperature readings, making them ideal for digital thermometers.