Sensing Components: Lamps, Diodes, Thermistors and LDRs

I-V Characteristics of Filament Lamps

A fixed resistor keeps its resistance steady, but the wire inside a traditional light bulb behaves completely differently as it gets hotter.
A filament lamp is a light-emitting component consisting of a thin metal wire (usually tungsten) inside a glass bulb.
It is a non-ohmic conductor, meaning it does NOT follow Ohm's Law because its resistance changes as the current changes.
As the current through the lamp increases, electrical energy is transferred into thermal energy, causing the temperature of the filament to rise.
When the temperature increases, the metal ions (atoms) in the filament lattice vibrate more with a larger amplitude.
These vibrating ions collide more frequently with the flowing electrons, resisting their passage and thus increasing the resistance.

The Filament Lamp Graph

The graph of current ( $I$ ) against potential difference ( $V$ ) for a filament lamp is a curved "S-shape" that passes through the origin.
At low voltages, the relationship is approximately linear, meaning resistance is relatively constant.
At high voltages (both positive and negative), the curve levels off and becomes shallower, indicating that a further increase in potential difference results in a smaller increase in current (higher resistance).
The graph is perfectly symmetrical in the positive and negative quadrants because resistance behaves identically regardless of the direction of current flow.

Worked Example: Calculating Resistance from an I-V Graph

You cannot find resistance just by looking at the steepness of the curve; you must calculate it using coordinates from the graph. Find the resistance of a filament lamp when the potential difference across it is $12\text{ V}$ and the current is $3\text{ A}$ .

Step 1: Write the formula for resistance.

$R = \frac{V}{I}$

Step 2: Substitute the known values.

$R = \frac{12\text{ V}}{3\text{ A}}$

Step 3: Calculate the final answer with units.

$R = 4\text{ }\Omega$

I-V Characteristics of Diodes

What if you needed to force electricity to travel down a one-way street?
A diode is a semiconductor component that allows current to flow in one direction only, known as the forward bias.
In the reverse direction (reverse bias), the diode has a very high resistance, which completely prevents current from flowing.
On an I-V graph, reverse bias is shown as a completely horizontal line along the x-axis ( $I = 0$ ).
In forward bias, resistance is initially very high until the potential difference reaches a specific threshold voltage (approximately $0.6\text{ V}$ to $0.7\text{ V}$ for silicon diodes).
Above this threshold voltage, resistance decreases rapidly, and current increases sharply, curving steeply upwards on the graph.
In practical circuits, a fixed resistor is often placed in series with a diode to limit the current and prevent the diode from overheating once it starts conducting.

Resistance of Thermistors and LDRs

Some components actually become better conductors the more extreme their environment gets.
A thermistor is a temperature-dependent resistor. In a standard NTC (Negative Temperature Coefficient) thermistor, resistance decreases as temperature increases.
An LDR (Light Dependent Resistor) changes its resistance depending on light intensity. Its resistance decreases as light intensity increases.
In physical terms, incident photons of light release more charge carriers (electrons) in the semiconductor material of the LDR, making it easier for current to flow.
An LDR has a very high resistance in total darkness (in the $M\Omega$ range) and a very low resistance in bright light (tens of $\Omega$ ).
Both components are non-ohmic conductors. On a graph of Resistance against Temperature/Light, the line forms a non-linear downward curve that does NOT touch the x-axis, because resistance never truly reaches zero.

Sensing Circuits and Potential Dividers

Every time your mobile phone screen automatically brightens in the sun, a smart circuit is doing the work behind the scenes.
A potential divider is a series circuit that shares the total supply potential difference ( $V_{supply}$ ) between components in proportion to their resistance.
The golden rule of a potential divider is that the component with the higher resistance takes a larger share of the total potential difference.
A sensing circuit uses a potential divider (typically containing a fixed resistor and a sensor like an LDR or thermistor) to monitor environmental changes and trigger an electrical response.
For example, in a heating system, as the room gets colder, the thermistor's resistance increases.
This causes the thermistor to take a larger share of the potential difference. If a heater is connected in parallel with the thermistor, this increased voltage will trigger the heater to turn on.
Similarly, in an automatic street light circuit, as the surroundings get darker, the LDR's resistance increases.
This causes the LDR to take a larger share of the potential difference. If a street lamp is connected in parallel with the LDR, this increased voltage will trigger the lamp to turn on.

Worked Example: Sensing Circuit Voltages (Higher Tier)

A $12\text{ V}$ battery is connected to a $200\text{ }\Omega$ fixed resistor and a thermistor in series. At $50^\circ\text{C}$ , the thermistor has a resistance of $100\text{ }\Omega$ . Calculate the potential difference across the fixed resistor.

Step 1: Write down the formula for voltage sharing.

$V_{out} = V_{in} \times \left(\frac{R_{out}}{R_{total}}\right)$

Step 2: Calculate the total resistance of the circuit.

$R_{total} = 200\text{ }\Omega + 100\text{ }\Omega = 300\text{ }\Omega$

Step 3: Substitute the values for the fixed resistor.

$V_{out} = 12\text{ V} \times \left(\frac{200\text{ }\Omega}{300\text{ }\Omega}\right)$

Step 4: Calculate the final answer.

$V_{out} = 12 \times 0.667 = 8\text{ V}$

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Exam Tips

Common Mistake
Students often state that the gradient of an I-V curve equals resistance. The gradient is NOT resistance; you must calculate resistance at a specific point using R = V/I with the actual coordinates from the graph.
2
In 'Explain' questions about sensing circuits, examiners expect a 4-step logical chain: state the environmental change, how the sensor's resistance changes, how the voltage share changes, and the final output response.
3
For the diode I-V graph, you must draw a completely flat, horizontal line along the negative x-axis (left side of the y-axis) to clearly show that current is zero in reverse bias.
4
Remember the simple mnemonics TURD (Temperature Up, Resistance Down) and LURD (Light Up, Resistance Down) to quickly recall how thermistors and LDRs behave under different conditions.

Key Terms(11)

Filament lamp: A light-emitting component consisting of a thin metal wire inside a glass bulb that transfers electrical energy into thermal energy and light.
Non-ohmic conductor: A conductor that does not follow Ohm's Law; its resistance changes as current or temperature changes.
Diode: A semiconductor component that allows current to flow in one direction only.
Forward bias: The condition where a diode allows current to flow, occurring once the potential difference exceeds the threshold voltage.
Reverse bias: The condition where a diode has a very high resistance and completely blocks current flow.
Threshold voltage: The minimum potential difference required for a diode to begin conducting significant current (usually 0.6 V to 0.7 V).
Thermistor: A temperature-dependent resistor whose resistance decreases as temperature increases.
NTC: Negative Temperature Coefficient, a property where resistance decreases as temperature increases.
LDR: A Light Dependent Resistor whose resistance decreases as light intensity increases.
Potential divider: A series circuit that divides the input voltage into smaller output voltages based on the ratio of the components' resistances.
Sensing circuit: A potential divider circuit used to monitor environmental changes (like light or temperature) and produce an electrical response.

Previous NoteOhmic and Non-Ohmic Circuit Elements Next NotePractical: Investigating I-V Characteristics

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Key Terms(11)

Filament lamp: A light-emitting component consisting of a thin metal wire inside a glass bulb that transfers electrical energy into thermal energy and light.
Non-ohmic conductor: A conductor that does not follow Ohm's Law; its resistance changes as current or temperature changes.
Diode: A semiconductor component that allows current to flow in one direction only.
Forward bias: The condition where a diode allows current to flow, occurring once the potential difference exceeds the threshold voltage.
Reverse bias: The condition where a diode has a very high resistance and completely blocks current flow.
Threshold voltage: The minimum potential difference required for a diode to begin conducting significant current (usually 0.6 V to 0.7 V).
Thermistor: A temperature-dependent resistor whose resistance decreases as temperature increases.
NTC: Negative Temperature Coefficient, a property where resistance decreases as temperature increases.
LDR: A Light Dependent Resistor whose resistance decreases as light intensity increases.
Potential divider: A series circuit that divides the input voltage into smaller output voltages based on the ratio of the components' resistances.
Sensing circuit: A potential divider circuit used to monitor environmental changes (like light or temperature) and produce an electrical response.

Sensing Components: Lamps, Diodes, Thermistors and LDRs

I-V Characteristics of Filament Lamps

A fixed resistor keeps its resistance steady, but the wire inside a traditional light bulb behaves completely differently as it gets hotter.
A filament lamp is a light-emitting component consisting of a thin metal wire (usually tungsten) inside a glass bulb.
It is a non-ohmic conductor, meaning it does NOT follow Ohm's Law because its resistance changes as the current changes.
As the current through the lamp increases, electrical energy is transferred into thermal energy, causing the temperature of the filament to rise.
When the temperature increases, the metal ions (atoms) in the filament lattice vibrate more with a larger amplitude.
These vibrating ions collide more frequently with the flowing electrons, resisting their passage and thus increasing the resistance.

The Filament Lamp Graph

The graph of current ( $I$ ) against potential difference ( $V$ ) for a filament lamp is a curved "S-shape" that passes through the origin.
At low voltages, the relationship is approximately linear, meaning resistance is relatively constant.
At high voltages (both positive and negative), the curve levels off and becomes shallower, indicating that a further increase in potential difference results in a smaller increase in current (higher resistance).
The graph is perfectly symmetrical in the positive and negative quadrants because resistance behaves identically regardless of the direction of current flow.

Worked Example: Calculating Resistance from an I-V Graph

Step 1: Write the formula for resistance.

$R = \frac{V}{I}$

Step 2: Substitute the known values.

$R = \frac{12\text{ V}}{3\text{ A}}$

Step 3: Calculate the final answer with units.

$R = 4\text{ }\Omega$

I-V Characteristics of Diodes

What if you needed to force electricity to travel down a one-way street?
A diode is a semiconductor component that allows current to flow in one direction only, known as the forward bias.
In the reverse direction (reverse bias), the diode has a very high resistance, which completely prevents current from flowing.
On an I-V graph, reverse bias is shown as a completely horizontal line along the x-axis ( $I = 0$ ).
In forward bias, resistance is initially very high until the potential difference reaches a specific threshold voltage (approximately $0.6\text{ V}$ to $0.7\text{ V}$ for silicon diodes).
Above this threshold voltage, resistance decreases rapidly, and current increases sharply, curving steeply upwards on the graph.
In practical circuits, a fixed resistor is often placed in series with a diode to limit the current and prevent the diode from overheating once it starts conducting.

Resistance of Thermistors and LDRs

Some components actually become better conductors the more extreme their environment gets.
A thermistor is a temperature-dependent resistor. In a standard NTC (Negative Temperature Coefficient) thermistor, resistance decreases as temperature increases.
An LDR (Light Dependent Resistor) changes its resistance depending on light intensity. Its resistance decreases as light intensity increases.
In physical terms, incident photons of light release more charge carriers (electrons) in the semiconductor material of the LDR, making it easier for current to flow.
An LDR has a very high resistance in total darkness (in the $M\Omega$ range) and a very low resistance in bright light (tens of $\Omega$ ).
Both components are non-ohmic conductors. On a graph of Resistance against Temperature/Light, the line forms a non-linear downward curve that does NOT touch the x-axis, because resistance never truly reaches zero.

Sensing Circuits and Potential Dividers

Every time your mobile phone screen automatically brightens in the sun, a smart circuit is doing the work behind the scenes.
A potential divider is a series circuit that shares the total supply potential difference ( $V_{supply}$ ) between components in proportion to their resistance.
The golden rule of a potential divider is that the component with the higher resistance takes a larger share of the total potential difference.
A sensing circuit uses a potential divider (typically containing a fixed resistor and a sensor like an LDR or thermistor) to monitor environmental changes and trigger an electrical response.
For example, in a heating system, as the room gets colder, the thermistor's resistance increases.
This causes the thermistor to take a larger share of the potential difference. If a heater is connected in parallel with the thermistor, this increased voltage will trigger the heater to turn on.
Similarly, in an automatic street light circuit, as the surroundings get darker, the LDR's resistance increases.
This causes the LDR to take a larger share of the potential difference. If a street lamp is connected in parallel with the LDR, this increased voltage will trigger the lamp to turn on.

Worked Example: Sensing Circuit Voltages (Higher Tier)

Step 1: Write down the formula for voltage sharing.

$V_{out} = V_{in} \times \left(\frac{R_{out}}{R_{total}}\right)$

Step 2: Calculate the total resistance of the circuit.

$R_{total} = 200\text{ }\Omega + 100\text{ }\Omega = 300\text{ }\Omega$

Step 3: Substitute the values for the fixed resistor.

$V_{out} = 12\text{ V} \times \left(\frac{200\text{ }\Omega}{300\text{ }\Omega}\right)$

Step 4: Calculate the final answer.

$V_{out} = 12 \times 0.667 = 8\text{ V}$

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often state that the gradient of an I-V curve equals resistance. The gradient is NOT resistance; you must calculate resistance at a specific point using R = V/I with the actual coordinates from the graph.
2
In 'Explain' questions about sensing circuits, examiners expect a 4-step logical chain: state the environmental change, how the sensor's resistance changes, how the voltage share changes, and the final output response.
3
For the diode I-V graph, you must draw a completely flat, horizontal line along the negative x-axis (left side of the y-axis) to clearly show that current is zero in reverse bias.
4
Remember the simple mnemonics TURD (Temperature Up, Resistance Down) and LURD (Light Up, Resistance Down) to quickly recall how thermistors and LDRs behave under different conditions.

Key Terms(11)

Filament lamp: A light-emitting component consisting of a thin metal wire inside a glass bulb that transfers electrical energy into thermal energy and light.
Non-ohmic conductor: A conductor that does not follow Ohm's Law; its resistance changes as current or temperature changes.
Diode: A semiconductor component that allows current to flow in one direction only.
Forward bias: The condition where a diode allows current to flow, occurring once the potential difference exceeds the threshold voltage.
Reverse bias: The condition where a diode has a very high resistance and completely blocks current flow.
Threshold voltage: The minimum potential difference required for a diode to begin conducting significant current (usually 0.6 V to 0.7 V).
Thermistor: A temperature-dependent resistor whose resistance decreases as temperature increases.
NTC: Negative Temperature Coefficient, a property where resistance decreases as temperature increases.
LDR: A Light Dependent Resistor whose resistance decreases as light intensity increases.
Potential divider: A series circuit that divides the input voltage into smaller output voltages based on the ratio of the components' resistances.
Sensing circuit: A potential divider circuit used to monitor environmental changes (like light or temperature) and produce an electrical response.

Previous NoteOhmic and Non-Ohmic Circuit Elements Next NotePractical: Investigating I-V Characteristics

Back to Resistors

Put your knowledge into practice — try past paper questions for Physics

Key Terms(11)

Filament lamp: A light-emitting component consisting of a thin metal wire inside a glass bulb that transfers electrical energy into thermal energy and light.
Non-ohmic conductor: A conductor that does not follow Ohm's Law; its resistance changes as current or temperature changes.
Diode: A semiconductor component that allows current to flow in one direction only.
Forward bias: The condition where a diode allows current to flow, occurring once the potential difference exceeds the threshold voltage.
Reverse bias: The condition where a diode has a very high resistance and completely blocks current flow.
Threshold voltage: The minimum potential difference required for a diode to begin conducting significant current (usually 0.6 V to 0.7 V).
Thermistor: A temperature-dependent resistor whose resistance decreases as temperature increases.
NTC: Negative Temperature Coefficient, a property where resistance decreases as temperature increases.
LDR: A Light Dependent Resistor whose resistance decreases as light intensity increases.
Potential divider: A series circuit that divides the input voltage into smaller output voltages based on the ratio of the components' resistances.
Sensing circuit: A potential divider circuit used to monitor environmental changes (like light or temperature) and produce an electrical response.