“Which of the following is a unilateral element?” — this one-mark question appears again and again in BCS (EEE), PGCB, BPDB, BREB, DPDC, NESCO and university circuit-theory exams. The concept is simple once you look at an element’s V–I characteristic, and it matters because important network theorems (reciprocity, Thevenin, Norton, superposition) are stated for linear bilateral networks. This post explains the difference with an animation, examples and MCQs.
- Bilateral element: behaves the same for current in either direction; its V–I curve is symmetric about the origin: I(−V) = −I(V). Examples: R, L, C, transmission line.
- Unilateral element: behaves differently for the two directions. Examples: diode, transistor, SCR, rectifier.
- Bilateral/unilateral is a different classification from linear/non-linear: a varistor is non-linear but bilateral.
- Reciprocity theorem applies only to linear, bilateral networks.
1. Definitions
Bilateral element
An element whose voltage–current relationship is the same for current flowing in either direction. If you reverse the applied voltage, the current simply reverses with the same magnitude. Mathematically, the V–I characteristic is an odd function: I(−V) = −I(V), so the curve is symmetric about the origin (lies in the 1st and 3rd quadrants).
Examples: resistor, inductor, capacitor, transformer winding, transmission line, a filament lamp, a metal-oxide varistor (MOV).
Unilateral element
An element whose behaviour depends on the direction of current — it conducts easily in one direction and poorly (or not at all) in the other. Its V–I characteristic is not symmetric about the origin.
Examples: p-n junction diode, Zener diode, LED, BJT, FET, SCR, rectifiers, vacuum tubes.
2. Seeing the Difference (Animation)
The same alternating voltage is applied to a resistor and to a diode. For the resistor, the operating point moves along a straight line through the origin — +1 V gives +1 A and −1 V gives −1 A. For the diode, positive voltage drives a large forward current, while negative voltage gives almost none.
3. Bilateral vs Unilateral – Comparison Table
| Basis | Bilateral element | Unilateral element |
|---|---|---|
| Behaviour with direction of current | Same in both directions | Different in each direction |
| V–I characteristic | Symmetric about origin (1st & 3rd quadrants) | Asymmetric |
| Interchanging terminals | No change in circuit behaviour | Circuit behaviour changes |
| Typical nature | Mostly passive | Mostly semiconductor / electronic devices |
| Examples | R, L, C, transmission line, MOV | Diode, transistor, SCR, rectifier |
| Reciprocity theorem | Applicable (if also linear) | Not applicable |
4. Other Ways to Classify Circuit Elements
| Classification | Meaning | Examples |
|---|---|---|
| Active / Passive | Active elements can deliver net energy (or provide power gain); passive elements only absorb or store energy | Active: sources, transistors, op-amps. Passive: R, L, C |
| Linear / Non-linear | Linear elements obey superposition and homogeneity (straight-line V–I) | Linear: R, L, C. Non-linear: diode, thermistor, iron-core coil |
| Bilateral / Unilateral | Same / different behaviour for both current directions | Bilateral: R, L, C. Unilateral: diode |
| Lumped / Distributed | Lumped: element concentrated at a point; distributed: spread over length (relevant when size ≈ wavelength) | Lumped: discrete resistor. Distributed: transmission line |
| Time-invariant / Time-variant | Parameters do not / do change with time | Fixed resistor vs. motor-driven rheostat |
5. Why It Matters: Network Theorems
- Reciprocity theorem — valid only for linear, bilateral networks (with a single source). Swapping the positions of source and ammeter does not change the reading.
- Thevenin’s and Norton’s theorems — usually stated for linear bilateral networks.
- Superposition theorem — requires linearity; it fails for non-linear elements such as diodes.
- Maximum power transfer theorem — applies to linear bilateral networks.
- Kirchhoff’s laws — valid for all lumped networks, linear or not, bilateral or not.
6. Examples Explained
- Resistor: V = IR in either direction → bilateral and linear.
- Capacitor / inductor: i = C dv/dt and v = L di/dt hold for both polarities → bilateral (but an electrolytic capacitor is polarised and must not be reverse-biased — a practical exception).
- Diode: forward current I = IS(eV/ηVT − 1); reverse current ≈ −IS (nA–µA) → unilateral, non-linear.
- Transistor: collector and emitter cannot be interchanged without changing behaviour → unilateral.
- Transmission line: power can flow either way with the same parameters → bilateral.
7. Practice MCQs (click to see answer)
1. Which of the following is a unilateral element? (a) Resistor (b) Inductor (c) Capacitor (d) Diode
2. A bilateral element has a V–I characteristic that is: (a) Symmetric about origin (b) Only in 1st quadrant (c) Exponential (d) Undefined
3. Reciprocity theorem is applicable to: (a) Linear bilateral networks (b) Non-linear networks (c) Unilateral networks (d) All networks
4. A transmission line is an example of a: (a) Unilateral element (b) Bilateral, distributed element (c) Active element (d) Non-linear element
5. A metal-oxide varistor is: (a) Linear bilateral (b) Non-linear bilateral (c) Linear unilateral (d) Non-linear unilateral
6. Which law is valid for both linear and non-linear networks? (a) Superposition (b) Reciprocity (c) Kirchhoff’s laws (d) Thevenin
7. An SCR is a: (a) Bilateral device (b) Unilateral device (c) Passive device (d) Linear device
8. Which of these is a bilateral semiconductor switch? (a) SCR (b) Diode (c) TRIAC (d) BJT
8. বাংলায় দ্রুত রিভিশন
- বাইল্যাটারাল উপাদান: উভয় দিকে কারেন্ট প্রবাহে একই আচরণ করে; V–I লেখচিত্র মূলবিন্দুর সাপেক্ষে প্রতিসম। যেমন: রেজিস্টর, ইন্ডাক্টর, ক্যাপাসিটর, ট্রান্সমিশন লাইন।
- ইউনিল্যাটারাল উপাদান: দিক অনুযায়ী আচরণ বদলায়। যেমন: ডায়োড, ট্রানজিস্টর, SCR, রেক্টিফায়ার।
- লিনিয়ার/নন-লিনিয়ার এবং বাইল্যাটারাল/ইউনিল্যাটারাল আলাদা শ্রেণিবিভাগ — ভ্যারিস্টর নন-লিনিয়ার কিন্তু বাইল্যাটারাল।
- রেসিপ্রোসিটি উপপাদ্য শুধু লিনিয়ার বাইল্যাটারাল নেটওয়ার্কে প্রযোজ্য; কির্শফের সূত্র সব নেটওয়ার্কে প্রযোজ্য।
- TRIAC ও DIAC দুই দিকেই পরিবহন করে (বাইল্যাটারাল সুইচ); SCR একমুখী।
Related Posts
- Electric Current and Its Types
- Semiconductor Diode: Working and Advantages
- What Is Negative Resistance?
- FET vs BJT: Key Differences
References: Hayt, Kemmerly & Durbin, Engineering Circuit Analysis; A. Sudhakar & S. Palli, Circuits and Networks. Images and animations: original work by Totalecer.