Calculating Power dissipated in a given circuit

[zmint]

I am trying to grasp few concepts and formulas, for better understanding of electronics as a hobby. So I am here with two queries.
A) Help regarding calculating the power dissipated in a given circuit. (P=I2R=V2/R=IV.) I, well understood the formula and used it with success.

The wiki answer states : Power dissipated in a given circuit is power that is converted to heat and then conducted or radiated away from the device.

I came across two types of calculations :
One is from the LED wizard and other is from one of our respected members.

1)

The LED wizard has calculated the Power two times(once for resistors and second for LED’s), and then total the two results to get the final power dissipated.


2)

Power of the circuit is calculated only once and the voltage drop of the Diod 1N2007 has not be taken into consideration. My inquisitive electronic mind was anxious that, Like the LED wizard, why was power dissipated not calculated two times (once in LED and other for Resistor) to get the total power.

The question may seem stupid, But, as it goes, "If you don't ask, you remain stupid".

Let me make it very clear that, I don't ever mean to say that the calculation are wrong in any way. Only I want to get the clear cut picture in layman language

a) What is power ?
b) How do we calculate power dissipated in a given circuit and why ?
c) What are the effects of power on the circuit ?
d) When finding the expected life of battery in relation to a given circuit, do we consider the current drawn by the circuit or the total power consumption of the circuit. (or I am trying to hold the same ear from another direction)
e) Is power different in AC and DC.
f) Is it the same thing, that is quoted on common Bulb and CFL ?


B) My second query is related to my experiment with white LEDS :
In this experiment with the LED array :
Power source : 6v SLA Battery, LED : 3V, 20mA
Connected :
1) 10 LED, 5 strings(2 LEDS in series) connected in parallel, with 1 ohm resistor with every string resulted Amp reading of 60 mA
2) 10 LED , connected in parallel (it is not a good practice I know), with a single 15 ohm resulted in Amp reading of 175 mA
3) 15 LED, Connected in parallel, with a single 10 ohm, resulted Amp reading of 210 mA
These reading are not what I expected theoretically. Where am I getting it wrong? May be the LEDs are poorly (wrongly) rated, may be it to do something with maximum discharging rate of a battery or what?

Thanks
Zmint (Trying to be more stable with electronics)

 

[audioguru]

The LED wizard calculates the power in the resistors so you can select a resistor power rating that does not fry them.
It calculates the power in the LEDs so when they are crowded together then you can provide enough venting to cool them.

Battery running time is determined by current but you can calculate the current if you know the power dissipated by the load.

A common incandescent bulb is a simple resistor whose resistance increases about 10 times when it is hot.
A CFL is a complicated circuit that has a high phase-shift of its current which appears higher than it actually is.

Your "6V" battery is not 6.0V. It is 6.3V when fully charged and sitting for a couple of hours with no load. Its voltage drops when loaded and as its charge runs down. Its voltage is about 7.2V fully charged and fresh from the charger.
Your "3V" LEDs are maybe 2.8V to 3.6V.
Your "10 ohms" resistor is 9.5 ohms to 10.5 ohms.
I don't know the accuracy of your meter.

 

[JimB]

In addition to what AG has said, in some circumstances is possible that putting a meter into a circuit to measure the current will add significant extra resistance and reduce the current flowing, thus the indicated current is lower than the actual current without the meter,

a) What is power ?
JimB: The rate of "doing work" or using energy*

b) How do we calculate power dissipated in a given circuit and why?
JimB: Power is calculated using the formulae P = I²R, or P = V²/R or P = VI. It is necessary to know the power so that suitably rated components can be used.

c) What are the effects of power on the circuit ?
JimB: Power makes the componets get hot.

d) When finding the expected life of battery in relation to a given circuit, do we consider the current drawn by the circuit or the total power consumption of the circuit. (or I am trying to hold the same ear from another direction)
JimB: Not sure about holding ears?, but the life of a battery is dependant on the current (the rate at which energy is taken from the battery) and the total quantity of energy taken from the battery.

e) Is power different in AC and DC.
JimB: I am going to say no. BUT, there are complications, there are several values which can be given to an alternating voltage or current. When power is being calculated we use the RMS (Root Mean Square) value. Also, in reactive circuits we must take the power factor into account, power factor is a measure of the phase difference between voltage and current.

f) Is it the same thing, that is quoted on common Bulb and CFL ?
JimB: I am not sure what you are asking here, but incandescent lamps (common bulbs) and CFLs are power they take form the supply.

*It is important to understand the difference between POWER and ENERGY.
Consider if you have a can full of petrol, it will have a certain amount of energy (just how much I dont know just sitting here).
If we use that petrol in a Formula1 racing car, the engine will produce a large amount of power (1000BHP, 746kW) for a short time, if we use it in a small motor cycle it will produce a small amount of power (10BHP, 7.46kW) for a long time, but the energy used (one can of petrol) will be the same.

JimB