What is the primary difference between an Ohmic and a non-Ohmic conductor regarding their I-V graphs?

An Ohmic conductor produces a straight-line graph through the origin, indicating constant resistance. A non-Ohmic conductor produces a curved graph, indicating that resistance changes with voltage or current.

How does the I-V characteristic of a filament lamp differ from that of a fixed resistor as voltage increases?

A fixed resistor maintains a constant gradient (constant resistance), while a filament lamp's gradient decreases as voltage increases. This is because the filament heats up, increasing the resistance and limiting the current flow.

Compare the forward and reverse bias behavior of a semiconductor diode on an I-V graph.

In forward bias, the diode conducts current easily after reaching a threshold voltage (approx. 0.6V). In reverse bias, the diode has extremely high resistance and allows virtually no current to flow.

Why is it a mistake to calculate the resistance of a non-Ohmic component by finding the gradient of the I-V curve?

Resistance is defined as the ratio $V/I$ at a specific point, not the change in $V$ over the change in $I$. The gradient of the curve represents the 'dynamic resistance,' which is different from the static resistance at that point.

What happens to the I-V data if the component is allowed to heat up significantly during the experiment?

The resistance will increase due to the temperature rise, causing the data points to deviate from the expected curve. This introduces a systematic error where the component appears more resistive than it would be at a constant temperature.

What error occurs if a voltmeter is accidentally placed in series with the component being tested?

Because a voltmeter has very high resistance, it will significantly reduce the total current in the circuit to nearly zero. The component will not receive its intended voltage, and the ammeter will show a negligible reading.

Define 'Threshold Voltage' in the context of a diode's I-V characteristic.

The threshold voltage is the minimum forward potential difference required for a diode to start conducting significant current. For silicon diodes, this is typically around $0.6$ to $0.7$ Volts.

What is an Ohmic conductor?

An Ohmic conductor is a material (like a metal wire at constant temperature) where the current is directly proportional to the potential difference across it, meaning its resistance is constant.

What does the gradient of an I-V graph represent if Current (I) is on the y-axis and Voltage (V) is on the x-axis?

The gradient represents the conductance, which is the reciprocal of the resistance ($\frac{1}{R}$). Therefore, a steeper line indicates a lower resistance.

Why does the resistance of a filament lamp increase as the current through it increases?

Increased current leads to more frequent collisions between electrons and metal ions, which increases the thermal energy of the ions. As the ions vibrate more vigorously, they further obstruct the flow of electrons, raising the resistance.

Investigating I-V Characteristics | WJEC GCSE Physics

1. Definition & Core Concepts

I-V Characteristic: This is a graphical representation showing how the current ( $I$ ) through a component changes as the potential difference ( $V$ ) across it is varied. It serves as a 'fingerprint' for identifying the electrical behavior of different materials and devices.
Ohmic Conductor: A component that follows Ohm's Law, meaning the current is directly proportional to the potential difference, provided physical conditions like temperature remain constant. This results in a straight-line graph passing through the origin.
Resistance ( $R$ ): Defined as the ratio of potential difference to current ( $R = \frac{V}{I}$ ). On an $I$ - $V$ graph (where $I$ is on the y-axis), the resistance at any point is the reciprocal of the gradient of the line connecting that point to the origin.
Non-Ohmic Conductor: Components where the resistance changes as the current or voltage changes. Their $I$ - $V$ graphs are non-linear (curves), indicating that the ratio of $V$ to $I$ is not constant.

A circuit diagram showing a power source, ammeter in series, component in parallel with a voltmeter, and a variable resistor to control current.

2. Underlying Principles

Charge Carrier Collisions: In metallic conductors, current is the flow of free electrons. As these electrons move, they collide with the positive ions of the metal lattice, creating resistance and generating thermal energy.
Temperature Effects: In a filament lamp, as current increases, the temperature of the metal filament rises. This causes the metal ions to vibrate with greater amplitude, increasing the frequency of collisions with electrons and thus increasing resistance.
Semiconductor Behavior: In devices like diodes, the material is engineered to allow charge flow only when a specific energy threshold (threshold voltage) is met, and typically only in one direction due to the internal potential barrier.

3. Methods & Techniques

Circuit Setup: Connect the component in series with an ammeter and a variable resistor (or use a potential divider). Place a voltmeter in parallel across the component to measure the voltage drop specifically across it.
Varying the Independent Variable: Use the variable resistor to change the resistance of the circuit, which alters the current and voltage. Alternatively, use a variable power supply to step through specific voltage increments.
Data Collection Strategy: Take readings of current ( $I$ ) for a range of voltages ( $V$ ). It is crucial to record both positive and negative values by reversing the connections to the power supply to see how the component behaves in both directions.
Consistency and Accuracy: To prevent the component's temperature from changing (which would alter its resistance), switch off the circuit between readings to allow it to cool down.

4. Key Distinctions

Component	Graph Shape	Resistance Behavior
Fixed Resistor	Straight line through origin	Constant resistance (Ohmic)
Filament Lamp	S-shaped curve	Resistance increases as $V$ and $I$ increase
Diode	Horizontal then sharp rise	Infinite resistance in reverse; low resistance after threshold voltage

Potential Divider vs. Rheostat: A potential divider allows the voltage to be adjusted from $0V$ up to the maximum supply voltage, whereas a simple rheostat (variable resistor in series) may not allow the voltage to reach zero depending on the component's resistance.

5. Exam Strategy & Tips

Gradient Interpretation: Always check the axes. If $I$ is on the y-axis and $V$ is on the x-axis, the gradient is $\frac{1}{R}$ . A steeper gradient means a lower resistance.
Origin Check: Most I-V graphs should pass through the origin $(0,0)$ because zero potential difference should result in zero current. If it doesn't, check for systematic errors like a zero error on the meters.
Symmetry Analysis: For resistors and filament lamps, the graph is usually symmetrical in the positive and negative quadrants. For diodes, the graph is highly asymmetrical, which is a key identifying feature.
Unit Awareness: Ensure current is in Amperes ( $A$ ) and potential difference is in Volts ( $V$ ). If current is given in $mA$ , convert it ( $1 mA = 10^{-3} A$ ) before calculating resistance in Ohms ( $\Omega$ ).

6. Common Pitfalls & Misconceptions

Resistance vs. Gradient: A common mistake is assuming the resistance is the gradient of the curve at a point. Resistance is always the absolute ratio $\frac{V}{I}$ at that specific point, not the tangent gradient $\frac{dV}{dI}$ (which is 'dynamic resistance').
Meter Placement: Students often swap the ammeter and voltmeter. Remember: Ammeters have near-zero resistance and must be in series; voltmeters have near-infinite resistance and must be in parallel.
Heating Effects: Ignoring the fact that the component might heat up during the experiment can lead to non-linear results for an otherwise Ohmic conductor.

Investigating I-V Characteristics

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Investigating I-V Characteristics

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions