Introduction to Current-Voltage Characteristics

In this chapter, we are going to look at the "personality" of different electrical components. Just like people behave differently under pressure, electrical components behave differently when you change the voltage across them. We call these behaviors Current-Voltage (I-V) characteristics.

By the end of this page, you will be able to recognize and explain the specific I-V graphs for three key components: ohmic conductors, filament lamps, and semiconductor diodes. Understanding these is vital for mastering the Electricity section of your AQA AS Physics course!

1. What is an I-V Characteristic?

An I-V characteristic is simply a graph that shows how the current (\(I\)) flowing through a component changes as the potential difference (\(V\)) across it is varied.

  • The Vertical Axis (y): Usually represents Current (\(I\)), measured in Amperes (\(A\)).
  • The Horizontal Axis (x): Usually represents Potential Difference (\(V\)), measured in Volts (\(V\)).

Quick Tip: Remember from the previous chapter that Resistance (\(R\)) is defined by the equation \(R = \frac{V}{I}\). This means the shape of the graph tells us everything we need to know about a component's resistance!

2. Ohmic Conductors and Ohm's Law

An ohmic conductor is the simplest type of component. A common example is a standard resistor or a metal wire, provided the temperature stays the same.

The Graph Shape

The I-V graph for an ohmic conductor is a straight line passing through the origin (\(0,0\)).

Key Rules:

1. Direct Proportionality: The current is directly proportional to the potential difference (\(I \propto V\)). If you double the voltage, the current doubles too.
2. Constant Resistance: Because the line is straight, the resistance stays the same regardless of the voltage or current.
3. Ohm's Law: This behavior is a special case known as Ohm's Law. It states that the current through a conductor is directly proportional to the potential difference across it, provided physical conditions (like temperature) remain constant.

Quick Review: If a graph is a straight line through the origin, the component is Ohmic. If it curves, it is Non-Ohmic.

3. The Filament Lamp

A filament lamp (a traditional light bulb) is a non-ohmic conductor. It doesn't follow a straight line because its temperature changes as current flows through it.

The Graph Shape

The graph looks like an 'S' shape that levels off at high voltages (both positive and negative).

Why does it curve?

1. As the potential difference (\(V\)) increases, more current (\(I\)) flows.
2. This current causes the metal filament to heat up.
3. In metals, higher temperatures mean the atoms vibrate more, making it harder for electrons to pass through.
4. This results in an increase in resistance.
5. Because the resistance is increasing, the current doesn't increase as fast as the voltage, causing the graph to "flatten out."

Analogy: Imagine trying to run through a hallway. If the people in the hallway (atoms) are standing still, it’s easy. If they start dancing wildly (heating up), it’s much harder for you to get through!

4. The Semiconductor Diode

A diode is like a one-way valve for electricity. It is designed to let current flow in only one direction.

The Graph Shape

The graph is flat (zero current) for negative voltages and low positive voltages, then shoots up suddenly after a certain point.

Key Features:

1. Forward Bias: This is the direction in which the diode allows current to flow. However, it won't conduct immediately. It needs a "threshold voltage" (usually about \(0.6V\) to \(0.7V\)) before the resistance drops and current flows easily.
2. Reverse Bias: If you flip the battery and apply voltage in the opposite direction, the diode has a very high resistance. Almost no current flows at all.
3. Threshold Voltage: Don't worry if you see a small delay on the graph before the line goes up—that is just the diode "waking up" once it hits its required voltage.

5. Comparing the Three Characteristics

When you are in the exam, you might be asked to identify these graphs. Use this quick summary:

  • Ohmic Conductor: A perfect straight diagonal line through the origin.
  • Filament Lamp: A curve that gets shallower as voltage increases (looks like an elongated 'S').
  • Semiconductor Diode: Flat on the left, flat just past the origin, then a sharp curve upwards on the right.

Common Mistakes to Avoid

1. Assuming Gradient = Resistance: On an \(I-V\) graph, the resistance at a specific point is \(V / I\). Note that the gradient (steepness) of the graph is actually \(1/R\). If the graph gets steeper, the resistance is decreasing. If the graph gets shallower (flatter), the resistance is increasing.

2. Forgetting the Origin: All these graphs must pass through the origin (\(0,0\)). If there is zero potential difference, there must be zero current!

3. Negative Values: Students often forget that \(I-V\) graphs have four quadrants. If you reverse the connections on your power supply, you get negative voltage and negative current. Ohmic conductors and filament lamps look the same in the negative quadrant, but the diode stays flat (zero current).

Key Takeaways Summary

- Ohmic conductors: \(I \propto V\); straight line; constant resistance.
- Filament lamps: Temperature increases with current; resistance increases; graph curves/flattens.
- Diodes: Conduct in one direction only; require a threshold voltage (\(\approx 0.6V\)) to start conducting.