ohms law class 12 | ohmic and non ohmic devices | limitations of ohms law | 12th class physics

Summary

This video explains the concept of Ohmic and Non-Ohmic devices in electrical circuits. It delves into the relationship between voltage and current, highlighting how Ohmic devices follow a linear I-V graph, while Non-Ohmic devices, like bulbs, thermistors, and diodes, exhibit non-linear behaviors. The video explores how temperature changes affect resistance in thermistors and how the current flow in diodes is restricted until a certain voltage threshold is reached. Through practical examples, it clarifies the differences between these devices and their impact on current-voltage relationships in electrical systems.

Takeaways

• 😀 Ohm's law states that the relationship between voltage and current is linear in Ohmic devices, meaning the current increases proportionally with voltage.
• 😀 Non-Ohmic devices, such as bulbs and thermistors, do not follow a linear current-voltage relationship due to changing resistance with temperature.
• 😀 A device follows Ohm's law if its current increases consistently with a uniform increase in voltage, forming a straight-line graph.
• 😀 When a device does not obey Ohm’s law, its graph will deviate from a straight line and become curved.
• 😀 A bulb heats up over time, increasing its resistance, which in turn decreases the current flow despite a constant voltage increase.
• 😀 Positive Temperature Coefficient (PTC) thermistors have resistance that increases with temperature.
• 😀 Negative Temperature Coefficient (NTC) thermistors have resistance that decreases as temperature increases, allowing more current to flow.
• 😀 In a thermistor with a negative temperature coefficient, an increase in temperature causes a decrease in resistance, enhancing the flow of current.
• 😀 Diodes exhibit non-Ohmic behavior, as no current flows through them until the voltage reaches the threshold (around 0.7V for silicon diodes).
• 😀 Once the threshold voltage for a diode is surpassed, the current increases significantly, but the relationship between voltage and current is not linear, creating a curve in the graph.

Q & A

What is the behavior of ohmic devices when voltage is increased?

Ohmic devices follow Ohm's Law, which means that as voltage increases, the current increases uniformly. This results in a linear relationship, represented by a straight line on the voltage-current graph.

How do non-ohmic devices differ in their behavior from ohmic devices?

Non-ohmic devices, like a bulb or thermistor, do not follow Ohm's Law. As voltage increases, the current does not increase linearly. For example, in bulbs, as they heat up, the resistance increases, which causes the current to rise less with each additional increase in voltage.

Why does the voltage-current graph for a bulb not form a straight line?

In a bulb, as the filament heats up, its resistance increases. This rise in resistance reduces the current flow despite an increase in voltage, which causes the voltage-current graph to curve rather than form a straight line.

What is the reason for the increased resistance in a bulb as it heats up?

As the bulb filament heats up, the atoms in the material vibrate more, which impedes the flow of electrons. This leads to an increase in resistance, which in turn limits the increase in current.

What is a thermistor and how does it behave with temperature changes?

A thermistor is a type of resistor whose resistance changes with temperature. In a thermistor with a negative temperature coefficient (NTC), as the temperature increases, the resistance decreases, allowing more current to flow. Conversely, in a positive temperature coefficient thermistor, the resistance increases with temperature.

How does the behavior of current change in a thermistor as its temperature rises?

In a thermistor with a negative temperature coefficient, as its temperature increases, the number of free charge carriers (electrons) increases. This leads to a reduction in resistance and an increase in current flow.

What happens to the current when a diode is connected to a circuit?

When a diode is connected to a circuit, current does not flow until the voltage exceeds a certain threshold (typically around 0.7V for silicon diodes). Below this voltage, the current remains very small or zero. After surpassing the threshold, the current increases significantly.

Why does current only flow in a diode after reaching the threshold voltage?

A diode has a built-in voltage threshold (around 0.7V for silicon diodes). Until the applied voltage reaches this threshold, the diode behaves as an insulator, and no significant current flows. Once the voltage exceeds the threshold, the diode allows current to flow more freely.

What is the main reason for the non-linearity in the current-voltage graph of a diode?

The non-linearity in a diode's current-voltage graph is due to the diode's behavior, where current only begins to flow after the applied voltage exceeds the threshold (around 0.7V for silicon diodes). Before this voltage, the current is almost negligible, but once it surpasses the threshold, the current increases sharply.

What is the effect of increasing temperature on the resistance of a thermistor with a negative temperature coefficient?

For a thermistor with a negative temperature coefficient, increasing temperature decreases its resistance. This reduction in resistance allows more current to flow through the thermistor, as the number of free charge carriers increases with the temperature.