---
title: "Internal resistance and practical source"
subject: "Basic Electrical Engineering"
chapter: 1
key: "semester_1/basic_electrical_engineering:practical-source-deviation-factor"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/practical-source-deviation-factor
site: BCT Engineering
---

# Internal resistance and practical source

Basic Electrical Engineering · Chapter 1: General Electric System

- Asked in: 2070 Asar, 6 marks
- Video: [Ideal and Practical Voltage Sources](https://www.youtube.com/watch?v=qjf9L-fmNWg) — Neso Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/practical-source-deviation-factor
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_engineering/chapter-1

## Question

What is the factor responsible for the deviation of the practical sources from their ideal behavior? Explain the effect of this factor on the terminal characteristics of the voltage source.

## Short answer

Internal resistance is the factor. For a voltage source, terminal voltage V = E − I·Rs; as load current I rises, the internal IR drop increases, causing V to fall linearly from the open-circuit value E.

## Answer

The factor is the **internal resistance** of the source (*Rs* for a voltage source, *Rp* for a current source). An ideal voltage source has Rs = 0 and an ideal current source has Rp = ∞; real sources do not.

#### Effect on a practical voltage source
A practical voltage source is an ideal source *E* in series with *Rs*:

V = E − I·Rs

- At no load (I = 0), V = E.
- As the load current increases, the internal drop I·Rs increases and the terminal voltage falls linearly.
- At short circuit, V = 0 and I = E/Rs (the maximum current).
- The V-I characteristic is a straight line with slope −Rs. The larger Rs is, the steeper the fall.

```text
  V
  E |-------------------  ideal (horizontal line at E)
    |  \
    |     \               practical (slope = -Rs)
    |        \
    +-----------\------- I
               E/Rs
```

#### Voltage regulation
Regulation = (V_no load − V_full load)/V_full load. It is small when Rs is small, so a good voltage source has a very small internal resistance.

Example: E = 24 V, Rs = 2 Ω. At 3 A the terminal voltage is 24 − 6 = 18 V, not 24 V.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What causes practical sources to deviate from ideal behaviour?
   A. Their internal resistance
   B. Their size
   C. The type of load
   D. The frequency of operation
   Answer: A. Ideal sources have Rs = 0 (voltage) or Rp = infinity (current); real ones do not. (Key point: Deviation factor)

2. For a practical voltage source V = E - I Rs, how does the V-I characteristic look?
   A. Horizontal line at E
   B. Vertical line at E/Rs
   C. A straight line falling from E at no load to 0 at I = E/Rs
   D. A curve rising with current
   Answer: C. V falls linearly with slope -Rs from E (no load) to zero at short-circuit current E/Rs. (Key point: Terminal characteristic)

3. What happens to the characteristic as Rs increases?
   A. The line becomes flatter
   B. The line becomes steeper, so the voltage falls faster
   C. The no-load voltage increases
   D. Short-circuit current increases
   Answer: B. Slope is -Rs, so larger Rs means a steeper fall; short-circuit current E/Rs actually decreases. (Key point: Effect of larger Rs)

4. E = 24 V, Rs = 2 ohm. What is the terminal voltage at 3 A?
   A. 24 V
   B. 30 V
   C. 6 V
   D. 18 V
   Answer: D. V = 24 - 3 x 2 = 18 V. (Key point: Example)

## Related in Basic Electrical Engineering

- [Ideal and practical sources, conversion](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Aideal-practical-sources-conversion) — Basic Electrical Engineering, chapter 2
- [Voltage and current sources, terminal voltage](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Avoltage-current-sources-terminal-voltage) — Basic Electrical Engineering, chapter 1
- [Ideal current and voltage sources](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Aideal-practical-sources) — Basic Electrical Engineering, chapter 1, asked 3×
- [Temperature coefficient and effect on resistance](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Atemperature-coefficient-effect-on-resistance) — Basic Electrical Engineering, chapter 1
- [RL coil on sinusoidal source](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Arl-coil-200sin314t-20ohm-0-1h) — Basic Electrical Engineering, chapter 6

## Related in other subjects

- [T-N characteristics; effect of rotor resistance and voltage](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atorque-speed-rotor-resistance-voltage) — Electrical Machines, chapter 5
- [T-S characteristics, max torque, rotor resistance effect](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Ats-max-torque-rotor-resistance-variation) — Electrical Machines, chapter 5
- [Effect of rotor resistance on T-S characteristics](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Arotor-resistance-effect-ts) — Electrical Machines, chapter 5
- [Terminal voltage above emf and armature reaction](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aterminal-voltage-greater-ar-effect) — Electric Machines II, chapter 1