Why do different metals create voltage when heated?

In one line

The rule

When two different metals are joined and their junctions are at different temperatures, a voltage (potential difference) is produced between them. This is called the thermoelectric effect or Seebeck effect.

Formula: V = S × ΔT

• V = voltage produced (in volts, V)
• S = Seebeck coefficient (how strongly a metal responds to heat, in V/°C)
• ΔT = temperature difference between the two junctions (in °C)

Let's understand it simply

Imagine you have two different metal wires, say copper and iron. You twist their ends together at two points, making two junctions. Now, heat one junction (for example, with a candle) and keep the other cool (maybe with ice).

Electrons (tiny charged particles) in metals move more when heated. But in each metal, electrons react to heat in their own way. In copper, they might move faster; in iron, a bit slower. Because of this difference, electrons start to pile up more in one place than the other. This creates a push for the electrons to move—a voltage.

It's like having two roads (metals) with different speed limits. If you send cars (electrons) down both roads, but one road lets them go faster, more cars will reach the end quickly on that road. This imbalance creates a pressure (voltage) between the ends.

How it works

1. Join two different metals: Make a closed loop using two metals (like copper and iron).
2. Create two junctions: Twist or weld the metals together at two places.
3. Heat one junction: Apply heat to one junction, leave the other cool.
4. Electrons move differently: In each metal, electrons gain energy from heat, but not equally. One metal lets electrons move more easily.
5. Charge builds up: Because of the difference, electrons pile up at one end, creating a voltage between the two junctions.
6. Voltage can be measured: If you connect a voltmeter, you can see this voltage. The bigger the temperature difference, the bigger the voltage.

The story behind it

This effect was discovered by Thomas Johann Seebeck in 1821, a German physicist. He found that when he joined two different metals and heated one end, a compass needle nearby moved. This showed that a current was flowing, creating a magnetic field. Seebeck did not set out to make a thermometer—he was just curious about electricity and heat. His discovery led to the invention of the thermocouple, a device still used today to measure temperature.

Where you see it in real life

• Thermocouples in kitchens: Gas geysers and stoves use thermocouples to check if the flame is on. If the flame goes out, the voltage drops, and the gas supply is cut off for safety.
• Temperature sensors in ACs and fridges: Thermocouples help control cooling by measuring temperature changes.
• Automobile engines: Thermocouples monitor engine temperature to prevent overheating.
• Power plants: Some power plants use thermoelectric generators to convert waste heat into electricity.
• Rocket and missile testing: Thermocouples measure the temperature of exhaust gases.
• Steel factories: Used to check the temperature of molten metal, where normal thermometers would melt.

Why it matters

• Safe appliances: Thermocouples prevent gas leaks in kitchens and industries.
• Accurate temperature measurement: Used in science labs, factories, and hospitals.
• Energy harvesting: Converts waste heat (from machines, cars) into useful electricity.
• Foundation for electronics: Understanding this effect helps in making sensors and new energy devices.
• Exams and jobs: Questions on thermoelectric effect come in board exams, JEE, NEET, and are important for engineering and technician jobs.

Worked example

Let's say you have a copper-iron thermocouple. The Seebeck coefficient (S) is about 15 microvolts per °C (0.000015 V/°C).

The hot junction is at 100°C, the cold at 0°C.

Given: S = 0.000015 V/°C, ΔT = 100°C - 0°C = 100°C
Step 1: Write the rule        V = S × ΔT
Step 2: Put in the numbers    V = 0.000015 × 100
Step 3: Answer                V = 0.0015 V (or 1.5 millivolts)

Common mistakes

• Thinking all metals give the same voltage: Each metal pair gives a different voltage because their electrons react to heat differently.
• Forgetting both junctions matter: You need two junctions at different temperatures. If both are at the same temperature, no voltage is produced.
• Mixing up current and voltage: The effect creates a voltage, not a current—current flows only if there is a closed circuit.
• Assuming more heat always means more voltage: Only the temperature difference between the two junctions matters, not just the amount of heat.

Remember it

"Two different metals + heat difference = voltage at the junctions."