Current-voltage characteristics

Welcome to Current-Voltage Characteristics

In this chapter, we explore the "personalities" of different electrical components. Just as people react differently to pressure, electrical components react differently when you change the potential difference (voltage) across them. By looking at Current-Voltage (I-V) characteristics, we can predict how a component will behave in a real-world circuit.

Don't worry if this seems like a lot of graphs at first! Once you understand the why behind the shapes, they become much easier to remember.

1. What is an I-V Characteristic?

An I-V characteristic is simply a graph showing how the current (\(I\)) flowing through a component changes as the potential difference (\(V\)) across it is increased or decreased.

By convention, we usually plot current (\(I\)) on the vertical y-axis and potential difference (\(V\)) on the horizontal x-axis. The shape of this graph tells us whether the component's resistance is constant or if it changes under different conditions.

2. Ohm’s Law: The "Ideal" Behavior

You have likely heard of Ohm's Law. In the AQA syllabus, it is treated as a special case. It states that for some conductors, the current is directly proportional to the potential difference, provided the physical conditions (like temperature) remain constant.

Mathematically: \(V = IR\)

If a component follows Ohm's Law, its I-V graph will be a straight line passing through the origin (\(0,0\)). The steeper the gradient of an I-V graph, the lower the resistance (because \(R = \frac{V}{I}\), which is the reciprocal of the gradient).

Quick Review:
- Ohmic Conductor: Resistance is constant. Graph is a straight line.
- Non-Ohmic Conductor: Resistance changes. Graph is a curve.

3. The Big Three: Essential I-V Graphs

You need to be able to recognize, draw, and explain the I-V characteristics for three specific components:

A. The Ohmic Conductor (e.g., a Fixed Resistor)

As long as the temperature doesn't change, a standard resistor is the most predictable component. The Shape: A straight line through the origin.
The Explanation: The resistance remains constant regardless of which direction the current flows or how much voltage is applied. If you double the voltage, you double the current.

B. The Filament Lamp

Think of a traditional lightbulb. As current flows through the thin wire (filament), it gets white-hot to produce light. The Shape: An "S" shaped curve that levels off at high voltages (in both positive and negative directions).
The Explanation: As the current increases, the temperature of the filament increases. This causes the metal ions in the filament to vibrate more vigorously. These vibrations make it harder for the charge-carrying electrons to pass through, meaning the resistance increases. Because resistance is higher at higher voltages, the current doesn't increase as fast as it used to, causing the curve to flatten out.

C. The Semiconductor Diode

A diode is like a "one-way valve" for electricity. The Shape: In the reverse direction (negative V), the line is flat on the x-axis (zero current). In the forward direction, the current stays at zero until it reaches a threshold voltage (usually around \(0.6V\) to \(0.7V\)), where it then shoots up very sharply.
The Explanation: Diodes are designed to have extremely high resistance in one direction. In the forward direction, they only allow current to flow once the "barrier" (threshold voltage) is overcome. After this point, the resistance becomes very low.

4. Temperature and Resistance

The AQA syllabus requires you to understand how temperature specifically affects two types of materials: metals and NTC thermistors.

Metals (Positive Temperature Coefficient)

As we saw with the filament lamp, when a metal gets hotter, its resistance increases.
Memory Trick: Metal = More heat, More resistance.

NTC Thermistors (Negative Temperature Coefficient)

A thermistor is a "thermal resistor." The ones you study are NTC (Negative Temperature Coefficient). The Rule: As the temperature increases, the resistance decreases.
The Explanation: Thermistors are made of semiconductor material. When they get warmer, the extra energy releases more charge carriers (electrons) that were previously "stuck" to atoms. With more carriers available, it is easier for electricity to flow, so resistance drops.
I-V Shape: The curve gets steeper as voltage increases, because the current heats the thermistor, lowering its resistance and allowing even more current to flow.

Real-world use: Thermistors are used in digital thermometers and thermostats. When the room gets too cold, the resistance changes, and the circuit "knows" to turn the heating on!

5. Required Practical: Measuring I-V Characteristics

You will likely perform a practical to find these curves. To get a full range of data, you need to vary the potential difference from negative values to positive values.

Step-by-step setup:
1. Connect the component in series with an ammeter and a power supply.
2. Use a variable resistor (or a potential divider) to change the voltage across the component.
3. Connect a voltmeter in parallel across the component.
4. Record \(I\) and \(V\) at small intervals.
5. Pro-tip: To get negative values, simply swap the connections to the power supply or the component!

6. Summary and Key Takeaways

Key Terms to Remember:
- Ohmic: \(V \propto I\) (straight line).
- Non-Ohmic: Resistance changes (curved line).
- Threshold Voltage: The voltage at which a diode starts conducting (\(\approx 0.6V\)).
- NTC Thermistor: Temperature UP = Resistance DOWN.

Common Pitfall to Avoid:
Students often forget that resistance at a specific point on a curved I-V graph is NOT the gradient. To find the resistance at a certain voltage, you must use the formula \(R = \frac{V}{I}\) using the specific coordinates of that point, rather than finding the "slope" of the curve at that point.

Did you know? Superconductivity is a state where some materials have zero resistance when cooled below a "critical temperature." While it's a fascinating topic, for your I-V characteristics chapter, just remember that under normal conditions, everything has some resistance except superconductors at very low temperatures!