Chemical and Heating Effects of Electric Current
Chemical and Heating Effects of Electric Current
This topic sits at the boundary of physics and chemistry, and it gives 1 to 2 questions in the Part-III science paper. Electroplating and the heating effect are both asked together with their everyday applications, and those applications carry more marks than the underlying theory does.
The Heating Effect
When current passes through a conductor, the conductor gets hot. The heat produced depends on the current, the resistance and the time.
Heat produced H = I² × R × t
Note that heat depends on the square of the current, so doubling the current gives four times the heat. That relationship is worth stating precisely, because it explains why overloading a circuit is dangerous.
| Application | How the effect is used |
|---|---|
| Electric bulb | Tungsten filament glows white hot |
| Electric heater and iron | Nichrome coil heats up |
| Fuse | Thin wire melts and breaks the circuit |
| Immersion rod, geyser | Coil heats the surrounding water |
Filaments use tungsten because of its very high melting point, and heating elements use nichrome because of its high resistance and resistance to oxidation. Those two materials are asked by name.
✗ Bulb filaments are made of copper | ✓ They are made of tungsten, which has a very high melting point
The Chemical Effect
When current passes through a conducting liquid, chemical changes occur. This is why a liquid that conducts is called an electrolyte.
- Bubbles of gas may appear at the electrodes.
- Deposits of metal may form on an electrode.
- The colour of the solution may change.
- Any of these is evidence that a chemical effect has taken place.
The simple tester used in the class 8 laboratory has an LED rather than a bulb, because an LED glows even with a very weak current that would leave a bulb dark. That detail is asked as a why question.
Electroplating
Electroplating is the deposition of a layer of one metal on another using electric current. It is the most important application of the chemical effect and appears regularly.
| Object plated | Metal used | Reason |
|---|---|---|
| Iron and steel taps | Chromium | Shine and resistance to corrosion |
| Iron sheets for cans | Tin | Tin does not react with food |
| Ornaments | Silver or gold | Appearance at lower cost |
| Iron for outdoor use | Zinc (galvanisation) | Prevents rusting |
In electroplating the object to be plated is made the cathode, the negative electrode, and the plating metal is the anode. Getting this the wrong way round is the standard error.
Note the food-safety reason for tin: cans are tin-plated because tin is less reactive than iron and does not contaminate the food. That specific reason is asked rather than the general idea of coating.
उष्मीय प्रभाव H = I²Rt — धारा दोगुनी होने पर ऊष्मा चार गुनी।
TRE pointer: Electroplating is asked as an application-and-reason pair, so learn chromium for taps, tin for cans, zinc for galvanisation with the reason attached to each. The tungsten and nichrome pairing is the other repeat item. The heat depending on the square of the current is the piece of theory most likely to appear in a reasoning question, and with five options and −1/3 negative marking, the material names are cheaper marks than the formula.
60-Second Recap
- Heat produced is I²Rt, so it grows with the square of the current.
- Tungsten is used in bulb filaments for its high melting point.
- Nichrome is used in heating elements for its high resistance.
- A conducting liquid is an electrolyte; gas bubbles or deposits show a chemical effect.
- An LED tester detects weaker currents than a bulb tester.
- In electroplating the object is the cathode; chromium for taps, tin for cans, zinc for galvanising.
Frequently Asked Questions
What is electroplating and where is it used?
It is the depositing of a thin layer of one metal onto another using electric current. Taps are chromium plated for shine and corrosion resistance, food cans are tin plated because tin does not react with food, and iron is zinc plated to prevent rusting.
Why are food cans coated with tin rather than another metal?
Because tin is much less reactive than iron and does not react with the food inside, so the contents stay safe. An uncoated iron can would rust and contaminate the food. The reason, not just the fact of coating, is what questions usually ask for.
Why is tungsten used in electric bulb filaments?
Because it has an extremely high melting point and can glow white hot without melting. Heating elements in irons and heaters use nichrome instead, chosen for its high resistance and its resistance to oxidation at high temperature. Both metals are asked by name.
Why does an LED work better than a bulb in a tester?
Because an LED lights up even when the current is very weak, whereas a bulb filament needs a much larger current before it glows visibly. When testing whether a liquid conducts, the current is often tiny, so an LED tester gives a reliable result.