How does the resistance of an NTC thermistor change as temperature increases?

The resistance decreases as temperature increases. This happens because the thermal energy liberates more charge carriers within the semi-conductor, allowing current to flow more easily.

What is the primary difference between the circuit symbols for an LDR and an LED?

An LDR has two arrows pointing towards the rectangle, indicating it is light-dependent. An LED has two arrows pointing away from the triangle/line symbol, indicating it emits light.

Why should the power supply be kept at a low voltage when investigating thermistors?

High voltages cause higher currents, which lead to 'self-heating' of the component. This internal heating would lower the thermistor's resistance independently of the external temperature being measured, creating an inaccurate result.

In an LDR experiment, why is it necessary to use a dark room?

A dark room ensures that the only light intensity affecting the LDR is the controlled source (the lamp). Background light would act as a systematic error, making the resistance lower than it should be for the specific lamp setting.

How do you calculate the resistance of a component using a voltmeter and an ammeter?

Resistance is calculated using the formula $R = V / I$. The voltmeter must be connected in parallel to the component to measure potential difference, and the ammeter in series to measure current.

What does 'NTC' stand for in the context of thermistors?

NTC stands for Negative Temperature Coefficient. it means that the relationship between temperature and resistance is negative (inverse); as one goes up, the other goes down.

Compare the resistance of an LDR in a bright room versus a dark room.

In a bright room, the LDR has low resistance because high light intensity liberates many charge carriers. In a dark room, the resistance is very high because fewer charge carriers are available to conduct electricity.

Why is a 'waiting period' required before taking readings in these experiments?

The components need time to react to the change in the environment (thermal lag or photo-sensitivity delay). Waiting ensures the component has reached a stable state so the recorded $V$ and $I$ values accurately reflect the new conditions.

What is the difference between an Ohmic conductor and a thermistor?

An Ohmic conductor has a constant resistance regardless of external conditions (at constant temperature), resulting in a straight-line I-V graph. A thermistor's resistance changes with temperature, making it non-ohmic and resulting in a curved graph.

What error occurs if the voltmeter is accidentally connected in series with the LDR?

A voltmeter has very high resistance; if connected in series, it will significantly reduce the total current in the circuit to near zero. This prevents the LDR from functioning correctly and results in an unusable ammeter reading.

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Investigating Resistance in Thermistors & LDRs

Summary

Thermistors and Light-Dependent Resistors (LDRs) are non-ohmic components whose electrical resistance is determined by environmental factors. This investigation focuses on the inverse relationship between temperature/light intensity and resistance, utilizing standard circuit measurements to derive and analyze these characteristics.

1. Definition & Core Concepts

Thermistors (NTC): A Negative Temperature Coefficient (NTC) thermistor is a sensor whose resistance decreases as the surrounding temperature increases. They are primarily used in temperature-sensing circuits like thermostats and digital thermometers.
Light-Dependent Resistors (LDRs): An LDR is a component whose resistance is sensitive to light intensity. As the light falling on the LDR increases, its resistance decreases, making it ideal for automatic lighting systems.
Non-Ohmic Behavior: Unlike standard resistors, these components do not follow Ohm's Law linearly ( $V \propto I$ ) because their resistance is not constant; it is a function of external physical stimuli.

2. Underlying Principles

Graph showing the non-linear inverse relationship between resistance and environmental stimulus (temperature or light intensity) for thermistors and LDRs.

3. Methods & Techniques

Circuit Assembly: Connect the component (LDR or Thermistor) in series with an ammeter and a power supply. Connect a voltmeter in parallel across the component to measure the potential difference specifically across it.
Varying the Independent Variable: For an LDR, use a lamp with a dimmer switch or vary the distance between the lamp and the LDR. For a thermistor, use a heater with a dimmer switch or place the thermistor in a water bath that is gradually heated.
Data Collection: Record the current ( $I$ ) and voltage ( $V$ ) at regular intervals of the stimulus (e.g., every 5 degrees Celsius or every 10 cm of distance). Ensure the environment is controlled, such as performing LDR tests in a darkened room to avoid background light interference.
Equilibrium Time: Always wait a few seconds after changing the stimulus before taking readings. This allows the component to reach thermal or photo-equilibrium with its surroundings.

4. Key Distinctions

5. Exam Strategy & Tips