What is the primary difference between the $I-V$ graphs of an ohmic conductor and a filament lamp?

An ohmic conductor produces a straight line through the origin, indicating constant resistance. A filament lamp produces an 'S' shaped curve where the gradient decreases at higher voltages because the resistance increases as the filament heats up.

How do the $I-V$ characteristics of a thermistor differ from those of a filament lamp as current increases?

In a filament lamp, resistance increases with current (gradient becomes shallower). In a thermistor, resistance decreases as it gets warmer due to increased current (gradient becomes steeper), allowing more charge carriers to flow.

What is the difference between 'forward bias' and 'reverse bias' on a diode's $I-V$ graph?

Forward bias refers to the positive voltage region where the diode conducts current after reaching a threshold. Reverse bias refers to the negative voltage region where the diode has near-infinite resistance and allows virtually no current to flow.

What is a common error when calculating resistance from a non-linear $I-V$ graph?

Students often try to use the gradient of the tangent to find resistance. However, resistance at a point is always the absolute ratio of $\frac{V}{I}$ at that specific coordinate, not the local slope of the curve.

Why is it incorrect to say a component obeys Ohm's Law if its $I-V$ graph is a straight line that does not pass through the origin?

Ohm's Law requires direct proportionality ($I \propto V$), which mathematically necessitates a y-intercept of zero. A non-zero intercept implies a constant offset or a systematic error, failing the proportionality test.

What mistake is made when interpreting a 'flattening' $I-V$ curve as decreasing resistance?

A flattening curve (bending toward the x-axis/Voltage axis) indicates that for larger increases in voltage, you get smaller increases in current. This means the resistance is actually increasing, as the gradient ($\frac{1}{R}$) is getting smaller.

Define an 'Ohmic Conductor' in terms of its $I-V$ characteristic.

An ohmic conductor is a component that maintains a constant resistance regardless of the applied voltage, resulting in an $I-V$ graph that is a straight line passing through the origin.

What is the 'threshold voltage' of a semiconductor diode?

The threshold voltage is the minimum forward potential difference (usually around $0.6$V) required for the diode to overcome its internal barrier and begin conducting significant electric current.

How is resistance ($R$) represented on an $I-V$ graph?

Resistance is the reciprocal of the ratio of the coordinates at any point ($R = \frac{V}{I}$). On the graph, it is inversely related to the gradient of the line connecting the origin to that point.

Why does the resistance of a metal wire increase when the current flowing through it increases?

As current increases, the drift velocity of electrons increases, leading to more frequent collisions with metal ions. these collisions transfer energy to the ions, causing them to vibrate more, which further obstructs the flow of electrons.

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Current-Potential Difference Graphs

Summary

Current-Potential Difference ( $I-V$ ) graphs are fundamental tools in electronics used to characterize the electrical behavior of components. By plotting current ( $I$ ) against potential difference ( $V$ ), we can determine whether a component is ohmic or non-ohmic and analyze how its resistance changes under different operating conditions.

1. Definition & Core Concepts

An $I-V$ characteristic graph displays the relationship between the current flowing through a component and the potential difference applied across it. The independent variable, potential difference ( $V$ ), is typically plotted on the x-axis, while the dependent variable, current ( $I$ ), is plotted on the y-axis.
Components are classified as Ohmic if they produce a straight-line graph passing through the origin, indicating that current is directly proportional to voltage. Non-ohmic components produce curved graphs, meaning their resistance changes as the current or voltage varies.
The physical state of the component, particularly its temperature, often dictates the shape of the graph. For many materials, as current increases, internal heating occurs, which subsequently alters the material's resistance.

I-V characteristic curves for an Ohmic resistor (straight line), a filament lamp (curving downwards), and a diode (flat then sharp increase).

2. Underlying Principles

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions