In regards to jump-starting a car battery.

Why wouldn't dropping either the black or the red leads cause a short circuit?

Do not connect the black leads first and the red leads after. If you do that and accidentally drop the red cable onto the car's frame, a massive short-circuit will form, possibly welding the clamp to the chassis.

From wiki: http://www.wikihow.com/Jump-Start-a-Car

And then there's the part about:

There is no electrocution hazard performing a jump on most cars and light trucks. The voltages are in most of these cases, is just 12. Twelve volts hasn't electrocuted anyone, however just a small spark near a battery has caused explosions that have cause serious injury or burns. A spark caused by an accidental short circuit is large due to the amount of current or amps, not the voltage.

If 12v isn't pushing enough current to hurt, how come that same 12v in the short circuit causes such a huge spark. Isn't it still the same amount of push?

 

The black, or negative terminal of the battery, is already connected to the frame and most of the sheet metal. Shorting a piece of wire to itself will not cause electricity to flow because wires don't store electricity inside themselves.

It's the same amount of push, but humans have a lot more resistance than the frame of your car.

I'm gonna attach a few pages I wrote for ppl who are new to electronics. See if reading them helps. If it doesn't help, you can laugh at me.

 

The car frame IS the black wire. Well, actually the black wire (or - terminal) is connected to the car frame. This is done so you only have to run 1 wire to any device in the car. It can then use the frame as the return or (-) terminal.

If the red touches it, it is a short between the black and red wires (or terminals)

Many manufactures tell you NOT to connect the black jumper cable to the battery, but to connect it to the frame. This keeps sparks away from the vents of the battery that may be venting hydrogen gas.

12v has nothing to do with hurt. Its the HUGE amount of amperage that car batteries are capable of dumping out that will hurt you.

Pushing 12,000v at .001 amps is a tickle. but 12,000A at .001v is death (and actually 50mv at 120v or less can kill you)

Car batteries are a different animal. The starter motors that start the cars engine are HUGE hogs. They need to be. They have to fight all that compression to get the motor started. That requires some real torque. Torque requires amps.

[ed]
your a quick one bychon!
[/ed]

 

It's easy to be quick when you have a pre-fab answer on your desktop.

 

Do not connect the black leads first and the red leads after. If you do that and accidentally drop the red cable onto the car's frame, a massive short-circuit will form, possibly welding the clamp to the chassis.

Better advice is to get properly-made jumper cables with the handles insulated on the outside so that an accidental short can't happen.

 

There are a pretty nice pair of jumper cables I have at work that has a switch in the center of the cables. It lights an orange light if all is good and doesnt conduct until it is powered properly.

I believe it is an SCR in there that waits for both sides to connect before conducting.

Nice and easy, and no accidental reverse hookups or shorts.

 

Ok, so the car battery has only 12v of force to push amperage out. Your body has too much resistance to allow the 12v to do any harm to you. If you touched both terminals, there would still be some amperes flowing through you tho right? How many milliamps would you guess? How much greater force is in the 12v car battery as opposed to the 9v battery you stick to your tongue to check if it's still good?

 

Speaking about jump starting cars;

After both cars are running but BEFORE disconnecting the cable; Always turn on your headlights and your windshield wipers before disconnecting. The reason for this is simple, the car with the dead battery is still pulling a huge amount of current attempting to recharge its battery, the car with the good battery/alternator is sending all of this current thru the jumper cables. When the load instantly disappears (disconnecting the cable), all of the current from the alternator suddenly has no load, it takes the field in the alternator a few milliseconds to drop so all of the current in the system now becomes this huge voltage spike within the cars electrical system. I have seen this spike kill on-board computers and other electronics. The headlights and the wiper motor give this voltage spike a low resistance place to safety dump and burn up.

 

Ok, so the car battery has only 12v of force to push amperage out.

Actually, a fully charged car battery is ~12.7v-12.8v @ 25°C/77°F.

Your body has too much resistance to allow the 12v to do any harm to you. If you touched both terminals, there would still be some amperes flowing through you though, right? How many milliamps would you guess?

It depends on the skin resistance. It depends upon the individual, whether the skin is wet or dry, how much salt is present (we're mostly water; pure water is an insulator, salt in the water makes it conductive), and conductivity of the skin can also vary depending on how much voltage is applied, and whether the voltage is AC or DC.

How much greater force is in the 12v car battery as opposed to the 9v battery you stick to your tongue to check if it's still good?

V(car_battery) / V(9v_battery)
Voltage is, basically, electrical pressure (electromotive force).
http://en.wikipedia.org/wiki/Electromotive_force

So, if you have a car battery that measures 12.7v, and a 9v battery that measures 8.4v, the car battery has ~1.5 times as much EMF as the 9v battery.

The game changes as the load across the batteries decreases in resistance. A car battery has a very low internal resistance, in the mOhms range (milliohms). A 9v "pp3" battery starts out when fresh at around 3 to 5 Ohms, and increases quickly as the battery is discharged.