Power (watts) = Intensity of Current (amps) * Electric Pressure (volts)

What some people suggest may help is to use the diagram below and simply cover the value you are looking for with a finger and the remaining is the required calculation.

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]]>Yes.

The other important measure in a dc circuit is power measured in watts.

Power (watts) = * Intensity of Current (amps) * Electric Pressure (volts)

P = I * E

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I think it was my dad who taught me to remember the formulas like this:

Women Are Virgins Watts = Amps * Volts

Virgins Are Rare Volts = Amps * Resistance

As simple as that! And I never forgot it, even after 50 more years!

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Electrical Pressure (voltage) = Intensity of Current (amps) x Resistance (ohms)

E = I x R

where voltage, ampere and ohms are the units of measure like inches are to Distance.

So the reciprocal of this (if I wanted to solve for I) would be I=E/R correct?

]]>Drop a very big stone from 1m high = high intensity/big Amps

Drop a small stone from 100m high = high voltage/big Volts

Drop a very big stone from 100m high = don't stand under it ! ?

]]>To sum it up with regards to your post.

Volts are a measure of Electric Force (pressure) between two points, the same as air pressure in your car tyre is a measure of the pressure difference inside the tyre compared with air pressure outside the tyre. An electric current, measured in amperes (amps), is the amount of electricity flowing past a give point at any time like the rate at which air exits a car tyre when you remove the valve. You can vary the rate at which the air exits the tyre by placing a finger over the outlet thus increasing the Resistance.

The mAh capacity rating **refers to the storage capacity** available for a particular battery.

How much electricity the battery **actually** delivers at any time depends on the resistance of the electric load in the circuit. An electric load can be a motor, an Arduino board, a light bulb, a heater and so on.

Current (amps) = Pressure(volts) / Resistance(ohms)

]]>**How much electricity actually flows out at any given time depends on the voltage AND the resistance in the circuit.**

Electrical Pressure (voltage) = Intensity of Current (amps) x Resistance (ohms)

E = I x R

where voltage, ampere and ohms are the units of measure like inches are to Distance.

Battery by the way refers to a collection of electric cells. A 1.5v battery has one cell. A 9v battery will have six cells. It is the size of a cell and the chemicals they use which determines the amount of electricity they have stored. If you pull a little 9v battery apart you will find it is made up of six tiny 1.5v cells connected in series.

The voltage of a battery is the pressure the battery will try and maintain between it's + and - terminals. Batteries also have an internal resistance which limits the current (gallons per minute) they can deliver.

As you can see here the Arduino is being powered by 9volts made up of six 1.5 cells connected in series.

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]]>I haven't come across any suitable videos pertaining to your question. However, the two links below will give you a starting point. Basically the current capacity (mAh) of a battery or cell will be dependent on the size of its electrodes. The larger the electrodes making contact with the electrolyte of the cell, the larger the surface area for the electron producing chemical reaction, and therefore the number of instantaneous electrons available to flow.

In the case of your AA vs 9V comparison, the single AA Cell has a much larger electrode surface area than the 9V battery. If you were to disassemble a 9V battery(Not recommended, It's messy and there are plenty of sharp pieces to cut yourself on) you would find six smaller 1.5 volt cells inside. It is the small electrode surface area of the smaller parts of the 9V battery that reduces the current capacity. From the first link below you find that the AA has a current capacity of 1000 mAh and the 9V battery has a current capacity of 500 mAh. This shows the AA cell has electrodes approximately twice the size of the individual electrodes in the 9V battery.

There is another capacity that is important to consider when selecting a battery, That is the Power Capacity. If you take the rated Current Capacity of a cell/battery and multiply it by the output voltage, you will have an approximation of the power capacity in milliwatt hours (mWh). In this case the AA cell has a power capacity of 1,500 mWh while the 9V has a power capacity of 4,500 mWh. If you only needed 1.5 volts (at 10mA say) for your circuit, the 9V battery would last longer.

Note my calculated numbers are approximate because the output voltage of the cell/battery will drop while it is being used. Also there are losses doe to efficiencies in a regulator (required to drop 9V to 1.5V) that would impact on total available power capacity of the 9V battery.

Basic cell/battery data for comparison purposes: http://www.techlib.com/reference/batteries.html

More detailed cell/battery data: https://en.wikipedia.org/wiki/List_of_battery_sizes

Hope this helps.

]]>I think I am getting it. Volts are the potential energy while current is the amount of energy flowing through the circuit. Can you recommend any videos that talk about how devices use the current and deplete the battery and why batteries have different ratings like a 1.5v AA battery has more mAh than a 9v battery.

]]>Imagine a battery is a tank full of water and a water pump. The pump can supply a certain amount of pressure (voltage). How much water flows out in a given time (current) depends on the resistance in the pipe and pump itself. The analogy isn't exactly right but sort of gave me a handle on visualising how electricity works providing you also begin to understand the differences. So how long your battery lasts depends on how large the battery is (how full the tank) and the rate at which the water (electricity) flows out. The amount of electricity (water) used by the Arduino depends on what it is doing and how often it does it just as how long your water tank stays full depends on how much water you take out of it over any given time. However you are jumping ahead of yourself here, first learn the basics then later you will be able to understand the explanations. For example I have a little book "Programming Arduino Next Steps" that has a chapter on power consumption of Arduino Boards and when and how to reduce it.

With your 3.3v arduino and 4.6v servo and a 9v power source you can reduce the voltage (pressure) to each part where required. How this is done depends on the requirements and can be a simple voltage divider to a much more complex circuit with feedback.

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