How to evaluate, how much current flows in the circuit?

Question

Please take into advisement, that I'm quite a beginner in subjects of physics and especially about electricity. Lately I've decided, that I want to learn more on basics of electronics and I've started reading about it, but a few things still are unclear for me.

I understand, that the voltage is the force, which pushes electrons through the wire. I also understand, that current is amount of electrons pushed by the wire in time. It's also clear for me, that power source, such as battery, has defined voltage, which it can provide. What is unclear:

1. How can I determine, how much current will flow through the circuit? Suppose, that I connect a 1.5V lightbulb to a 1.5V battery - how much current will flow through this circuit?

2. Too high voltage will fry the lightbulb (or other resistor), because voltage (being an actual electrical energy, as I understand that) will cause too much heat in the resistor. But is it possible to apply too much current to a resistor?

3. Battery capacity is expressed in mAh. Say, that the battery has 1200 mAh = 1.2 Ah = 72 Amin (ampminutes) = 4320 Asec (ampseconds). Is it actually (physically) possible to drain 4320 amps from this battery in a second? If not, what is the restriction, which prevents one from doing so? (not that I want to try :))

Answer 1

1. By Ohm's law, which states V = IR, where V is the voltage accross a resistor, I the current thru it, and R the resistance. The units work out so that no additional proportionality constant is required when V is in Volts, I in Amps, and R in Ohms.

   For example, if the 1.5 V battery is connected to a 47 Ω resistor, then 32 mA will flow.

2. Of course you can apply too much current to a resistor, but note that Ohm's law tells you that's the same thing as applying too much voltage. The voltage accross a resistor and the current thru it are proportional to each other. The resistance of the resistor is the proportionality constant.
   

   Note that the power put into a resistor is the voltage times the current. Since these two are proportional to each other, the power into a resistor is proportional to both the square of the voltage and the square of the current. By knowing P = IV and applying Ohm's law, you can see that P = V²/R = I²R.

3. The Amp-hour figure is only a rough guide of battery capacity. Batteries have complex behaviors with temperature, age, current, discharge history and other parameters that a hard to nail down. In theory if a battery is rated for 1 Amp-hour, then you should be able to draw 1 A from it for 1 hour, or 2 A for 1/2 hour, or 250 mA for 4 hours, etc. In practise, there is a lot of slop, in particular with respsect to temperature and how fast you are draining the battery. Go look up a battery datasheet and see the details it goes into.