# Short Circuit vs. Complete Circuit

I am beginning my study of electrical engineering and I apologize in advance if this question has been asked or if this question does not make any sense. I was wondering why short circuits are "bad" and complete circuits are "good".

From what I understand, when you have a battery, and short the two terminals together, you introduce a large current into the battery causing it to overheat and possibly combust.

simulate this circuit – Schematic created using CircuitLab

However, when you add some form of resistance to the circuit, you create a complete circuit and the battery source is not in jeopardy of overheating.

simulate this circuit

My question is, is there not current flowing into the battery in a complete circuit? How come the battery is unable to handle the current under its own voltage? And if some form of resistance on the circuit is required in order to prevent a short circuit scenario, how much of a resistance do I need, in order to prevent the battery from overheating? Thanks in advance. Cheers.

• Incidentally, when you use the schematic button on the editor toolbar you can save the editable CircuitLab schematic directly into your question. No need to screen-grab and upload. No background grid! – Transistor Nov 12 '17 at 0:06

My question is, is there not current flowing into the battery in a complete circuit?

Yes, but it is limited by the load resistor. The resistor is so-called because it resists the flow of electricity.

How come the battery is unable to handle the current under its own voltage?

The battery will have some internal resistance - in fact, we usually model them as an ideal voltage source with an internal series resistance - but the series resistance value in a good / new / healthy battery is usually very low. That means that in the short-circuit condition the current can be much higher than is safe for the battery. Think what would happen if you short out your car battery positive to the chassis with a spanner!

And if some form of resistance on the circuit is required in order to prevent a short circuit scenario, how much of a resistance do I need, in order to prevent the battery from overheating?

• Read the battery's specification from the datasheet and work out what the safe current is.
• From Ohm's Law we can calculate the minimum resistance as $$R = \frac {V_{batt}}{I_{max}}$$

e.g., A 12 V car battery can supply 40 A for several minutes.

$$R = \frac {V_{batt}}{I_{max}} = \frac {12}{40} = 0.3 \ \Omega$$

Incidentally, the power being delivered in the above example is given by $P = VI = 12 \cdot 40 = 480 \ \mathrm W$.

Figure 1. (a) The apparent LED key-fob lamp. (b) The more correct model showing the cell's internal resistance. Source: LEDnique.

On a related note, the internal resistance of a button cell is used in key-fob LED lamps to limit the current to a safe value for the LED.

• @MrCheezits: Thanks for accepting my answer but I really recommend that you un-accept until the Earth has spun around at least once so that everyone on the planet has a chance to answer. That way you will encourage others and, possibly, get further insights. On the other hand, if you are 100% satisfied then that's fine too. – Transistor Nov 11 '17 at 23:17
• Just un-accepted it. That is a good idea. I definitely would like more insights into this problem. Thank you for the LED explanation. I was messing around with LEDs and in a lot of literature they were mentioning the importance of putting resistors in series with your LEDs, and I was always wondering why we were introducing another electrical component into the circuit. Makes sense. – Adam Bak Nov 11 '17 at 23:19
• Another article I wrote on LED IV curves may be of assistance. – Transistor Nov 11 '17 at 23:22

Good answers here already, but nobody seems to have mentioned that a short-circuit IS a complete-circuit that happens to have zero resistance. That is, short-circuit is a subset of complete-circuit. They are not mutually exclusive as you imply.

simulate this circuit – Schematic created using CircuitLab

As for why is one is dangerous and the other is not...

If you go down a really steep hill in your car with no brakes that is bad right?

Same with circuits. If there is no resistance to impede the flow of current, bad things can happen.

There is current through battery in a closed circuit. The question is how much.

Resistance in a circuit causes heat dissipation and lowers current. Wire in itself has very low resistance. When you have a short circuit, like in your first picture, the only part offering substantial resistance is the battery. This causes massive release of heat in the battery causing it, in many cases, to blow up.

The amount of resistance you need depends on the power source. Many resistors or other components have a rated maximum voltage it can handle. Typically you won't need to worry about the value in Ohms of the resistor to prevent a blow up. Compared to wire, even a 5 Ohm resistor causes a lot more heat dissipation than 1000m of wire:

http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/wirega.html

Hope that helps.

In general, a short circuit is an unwanted connection between two points in a circuit.

As others have said, a short circuit between the terminals of a battery, or between the power and ground leads of a circuit, or between other points in a circuit where large currents can flow can lead to damage - blown fuses, burnt circuit traces or components, etc.

However, you can have short circuits that do not cause large currents or obvious damage. A short circuit between two signal lines (perhaps two inputs to a microcontroller) can cause unwanted effects on the circuit operation without causing the circuit to draw excessive current.

Occasionally, we use the term "short" to mean "make a temporary connection for test purposes" - in this case, the short circuit is intended and useful.

Many DVMs have a "continuity test". Beginners may think that "continuity" indicates a short circuit/zero ohm connection, but with this meter function it merely means a low resistance connection - often anything under ~50 Ohms will cause the "continuity" buzzer to sound.