In one line
An ideal source gives a fixed voltage (or current) no matter what load is connected, while a practical source has internal resistance, so its output changes with load.
Explained simply
A brand-new mobile battery shows about 3.7 V. When the phone starts a heavy game, the voltage dips a little. That dip happens because the battery has some resistance inside it. An ideal battery would never dip.
Technically: an ideal voltage source has zero internal resistance and keeps its terminal voltage constant for any load current. A practical voltage source is an ideal source in series with a small internal resistance \( r \). An ideal current source has infinite internal resistance and delivers constant current to any load. A practical current source is an ideal current source in parallel with a large internal resistance \( r \).
How it works
- Practical voltage source: current \( I \) flowing through \( r \) causes a drop \( Ir \) inside the source, so terminal voltage falls as load current rises.
- The smaller \( r \) is compared with the load, the closer the source behaves to ideal.
- Practical current source: part of the source current leaks through the parallel \( r \), so load current falls as load resistance rises.
- The larger \( r \) is compared with the load, the closer it behaves to ideal.
| Type | Internal resistance | Model |
|---|---|---|
| Ideal voltage source | 0 | E alone |
| Practical voltage source | small, in series | E with r in series |
| Ideal current source | infinite | I alone |
| Practical current source | large, in parallel | I with r in parallel |
Key formula
Practical voltage source: \( V = E - Ir \). Practical current source: \( I_L = I - \dfrac{V}{r} \).
\( E \) = open-circuit EMF (V), \( V \) = terminal voltage (V), \( I \) = source current (A), \( I_L \) = load current (A), \( r \) = internal resistance (\( \Omega \)).
Quick example
A 12 V battery with \( r = 0.5\,\Omega \) feeds a \( 5.5\,\Omega \) load. \( I = 12/(0.5+5.5) = 2 \) A. Terminal voltage \( V = 12 - 2 \times 0.5 = 11 \) V. An ideal 12 V source would have given 12 V across the load.
Common mistakes
- Drawing the internal resistance of a practical current source in series (it goes in parallel).
- Saying an ideal current source has zero resistance; it is infinite.
- Assuming terminal voltage equals EMF even when current flows.
How MU asks it
Typically a 5-mark short: "Distinguish between ideal and practical voltage and current sources" with diagrams. For full marks, draw all four symbols/models, sketch the V-I graph (horizontal line for ideal voltage source, drooping line for practical), give the equations above and state internal resistance in each case. A comparison table scores well.