Introduction to I-V Characteristics

In the world of electronics, every component has a "personality." Some components are very predictable, while others change their behavior as they get hotter or as the electricity flows in a different direction. In Physics, we call this personality the Current-potential difference (I-V) characteristic.

By plotting a graph of current \( I \) on the y-axis and potential difference \( V \) on the x-axis, we can see exactly how a component behaves. This is a fundamental skill for Unit 2: Electric Circuits. Don't worry if it seems like a lot of graphs at first—once you understand why the lines curve, the patterns become much easier to remember!

Note: If you need a refresher on what current and potential difference are, check out the chapter on "Charge, current, potential difference and resistance."

1. Ohmic Conductors

An Ohmic conductor (like a standard resistor at a constant temperature) is the simplest component to understand. It follows Ohm's Law, which states that the current is directly proportional to the potential difference, provided the temperature remains constant.

The Graph

  • Shape: A straight line passing through the origin \( (0,0) \).
  • Meaning: Because the line is straight, the resistance \( R \) is constant.
  • Calculation: Since \( V = IR \), the gradient of an \( I-V \) graph is equal to \( \frac{1}{R} \). This means a steeper line represents a lower resistance.

Quick Tip: If the graph is a straight line through the origin, the component is "Ohmic." If it curves, it is "non-Ohmic."

2. The Filament Bulb

A filament bulb is a non-Ohmic conductor. As more current flows through the thin wire (filament), the wire gets hot and glows. This heat changes the way the atoms in the wire behave.

The Graph

  • Shape: An "S" shape that starts straight at the origin but levels off (becomes less steep) at higher voltages.
  • Why it curves: As the potential difference \( V \) increases, the current \( I \) increases. This causes the temperature of the filament to rise.
  • The Physics: As the metal filament gets hotter, the lattice ions (the atoms in the metal) vibrate more vigorously. These vibrations make it much harder for the free electrons to pass through, which increases the resistance.

Key Takeaway: For a filament bulb, as \( V \) increases, temperature increases, which causes resistance \( R \) to increase.

3. The Semiconductor Diode

Think of a diode as a one-way valve for electricity. It only allows current to flow in one direction.

The Graph

  • Forward Bias (Positive V): The diode conducts. Usually, nothing happens until a "threshold voltage" (often around \( 0.6 \text{ V} \)) is reached. After this point, the current rises very sharply, meaning resistance drops significantly.
  • Reverse Bias (Negative V): If you flip the battery, the diode has a very high resistance. The graph stays flat on the x-axis, meaning virtually zero current flows.

Did you know? Diodes are used to protect sensitive electronics from being damaged if the batteries are put in the wrong way round!

4. NTC Thermistors

A thermistor is a "thermal resistor." The most common type you need to know for the Pearson Edexcel syllabus is the NTC (Negative Temperature Coefficient) thermistor.

The Graph

  • Shape: The curve gets steeper as the voltage increases.
  • The Physics: This behavior is the opposite of a filament bulb. In a semiconductor like a thermistor, heating it up provides enough energy to release more conduction electrons (charge carriers).
  • Result: Even though the lattice vibrations increase, the massive increase in the number of available charge carriers \( n \) outweighs this. Therefore, as temperature increases, resistance decreases.

Memory Trick: NTC stands for Negative... Temperature... Conducts more! (As Temp goes up, Resistance goes down).

Summary of Factors Affecting Resistance

The syllabus requires you to explain why resistance changes in different materials. Here is a quick reference guide:

Metals (like the Filament Bulb):
Temperature Up \( \implies \) Lattice vibrations increase \( \implies \) Resistance Increases.

NTC Thermistors:
Temperature Up \( \implies \) More conduction electrons released \( \implies \) Resistance Decreases.

LDRs (Light Dependent Resistors):
Illumination (Light) Up \( \implies \) More conduction electrons released \( \implies \) Resistance Decreases.

Common Exam Mistakes to Avoid

  • Mistaking the Gradient: Remember that on an \( I-V \) graph, the resistance is not the gradient. Resistance at a point is the value of \( \frac{V}{I} \) at that specific point.
  • Forgetting the Origin: All \( I-V \) characteristic graphs for single components should pass through \( (0,0) \). If there is no potential difference, there can be no current!
  • Vague Explanations: When explaining the filament bulb, always mention lattice vibrations. When explaining thermistors, always mention the increase in number of conduction electrons (or charge carriers).

Quick Review Questions

1. What happens to the resistance of an NTC thermistor as it gets colder?
(Answer: The resistance increases because fewer charge carriers are released.)

2. Why does the \( I-V \) graph for a filament bulb level off at high voltages?
(Answer: Increased temperature causes increased lattice vibrations, which increases resistance and limits the current.)

3. What is the definition of an Ohmic conductor?
(Answer: A conductor where current is directly proportional to potential difference, provided temperature is constant.)