- 2068 Jestha (old course) · 8 marks
Dielectric heating vs induction heating
Short answer
Dielectric heating uses a high-frequency electric field (10–50 MHz) to heat insulating materials via molecular friction. Unlike induction heating which heats conductors by eddy currents and hysteresis, dielectric heating provides uniform volumetric heating. Applications include drying wood, gluing plywood, welding plastics, and food processing.
What is dielectric heating? How is it different from induction heating? State the applications of dielectric heating.
Dielectric heating is the heating of an electrically non-conducting (insulating) material by placing it between two metal plates (electrodes) connected to a high-frequency AC voltage (about 10–50 MHz, a few kV). The alternating field makes the molecules of the dielectric flip and rub repeatedly (dielectric hysteresis loss), so heat is generated uniformly throughout the material itself.
HF oscillator (10–50 MHz)
| |
===+=== electrode |
[ material (charge) ] ← heat produced inside
===+=== electrode |
|________________|
Heat produced: P = 2π f C V² tan δ ≈ 2π f C V² cos φ, so heat rises with frequency f and voltage V. The material behaves like a lossy capacitor (loss angle δ).
Difference from induction heating
| Point | Dielectric heating | Induction heating |
|---|---|---|
| Material heated | Insulators (wood, plastic, rubber, food) | Conductors (metals) |
| Heating principle | Dielectric (molecular) loss | Eddy current and hysteresis loss |
| Field used | Electric field between plates | Alternating magnetic field of a coil |
| Frequency | High: 10–50 MHz (RF) | Low to medium: 50 Hz – 500 kHz |
| Heat distribution | Uniform through the whole volume | Mainly near the surface (skin effect) |
| Device | Capacitor plates (electrodes) | Induction coil (inductor) |
| Typical use | Drying, gluing, plastic welding | Melting, hardening, brazing metals |
Applications of dielectric heating
- Drying of timber, paper, textiles and tobacco.
- Gluing of plywood and laminated wood (heat cures glue uniformly inside).
- Welding of PVC and plastic sheets (raincoats, bags).
- Pre-heating of plastic moulding powder and vulcanising rubber.
- Food processing: baking, dehydration, sterilisation; the microwave oven works on the same principle at 2.45 GHz.
- Medical diathermy (heating body tissue).
✓ Answer checked by Claude Opus 5.5 (high effort)
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