Difference between current and power

Question

I am a bit confused about the difference between current and power. I am hoping someone can explain the difference to me maybe using an analogy or something.

I do understand that current at a point is a measure for how/much charge (in coulomb) is passing through the point (per second). I think I understand Ohm's law which says that over some component with fixed resistance R and voltage drop V there will be a current of \$V \over R\$.

I understand that power is much energy in joules is "deposited" in the component per second.

Some examples of where my confusion comes from: Usually when one buys appliances they have a wattage rating. So, my microwave oven uses (up to) 800 W. So it seems that wattage is the important thing to understand in many cases.

A power supply is (from what I understand) usually given with a (max) voltage and a (max) current. Why is the wattage not given here?

If I have an LED that can handle maximum 3V and I also have a 9V battery, I would need a resistor. But to find the size of the needed resistor I need to know how much current the LED uses. I have been told that one should find this in the datasheet of the LED, but that LEDs rarely come with a datasheet when you buy them. I have been told that it is common to assume that the LED uses about 20 mA. So I can understand how I can calculate the needed resistor. Where does the wattage come into to this situation? Why isn't the wattage listed?. Also, if one has a complex circuit that can handle only 5V, but one only has a 9v power source, then how would one know the current?

Maybe my confusion is that it seems that a certain current should always give a certain wattage.

Answer 1

Basically, think of current as a flow of water in a pipe, current is the amount of water being moved, more current means more 'water' flowing.

Voltage is the force or pressure pushing the 'water' along, more voltage means the 'water' is pushed harder and consequently moves faster through the pipe (that's why cranking up the voltage usually results in more current as well).

Now power is the amount of effort required to do all this, think about how much effort it takes to suck water up a straw... now try with a 50mm drainage pipe, you'll be lucky to move anything at all, it takes more effort to move more water even if the pressure is the same (head height is one way of measuring water pressure).

And for completeness, resistance would be the size of the pipe, a tiny pipe means you need to push harder to move the same amount of water at the same rate (1m^3 a second with a 1m pipe is easy, the water'll flow at an easy going meter and a half a second, try achieving that with a garden hose and you'll find you need to break the sound barrier). Trying to achieve the same flow rate with a higher resistance means you need more pressure and as a result you need to expend more effort to do so. Volts (pressure) = Amps (flow) * Resistance and Power (effort) = Volts*Amps (or flow * change in pressure, which interestingly still gives you power in watts)

Answer 2

Answering all your questions properly will take some effort. Let us consider the case where we have a voltage supply with a fixed output voltage. This is the most common case for off-the-shelf products.

AC appliances mostly use a fixed voltage (depending on country). Since the voltage is fixed and known, I can calculate power if I know current, or if I know current, I can calculate power using P = I * V. This is probably why you consider them to be kind of redundant or closely related.

Now let's consider a different case. Suppose I have a 3.7V battery. I want to use it to power a 5V circuit that uses 100mA. In order to do that, I have to raise the voltage to 5V (using a boost converter). Now, power must be considered separately from current. The power required by the circuit is 5 * 0.1 = 500 mW. Due to conservation of energy, I will need at least 500mW from the battery. In reality, I will probably need around 600 mW, due to less than perfect conversion efficiency of the boost converter. So that is around 3.7V / 0.6W = 162mA.

Power supplies may have various things specified, depending on what they are used for. Lab power supplies usually specify max current and max voltage. Adaptors for laptops probably specify maximum input power consumption, output voltage (fixed) and maximum output current.

When driving LED's, you usually start with the current you wish to run through the LED. The voltage does not vary too much with current. But when current and voltage are both known, the power can be calculated trivially (P = V * I). But in fact, white LED's intended for illumination are often rated by power. If you are buying LED's and don't have a model number or data sheet, you should consider getting your LED's from another source. It is true that 20ma is a common max current for LED indicators. But depending on the use, sometimes you can use much lower current (like 1 or 2 mA) especially for red LED's. LED's for illumination can use much higher currents than 20 mA.

A final comment. Sometimes your power supply voltage is higher than the required input voltage of your circuit. You can use a linear regulator to reduce the voltage. In this case, the current will be the same for both circuits. The linear regulator simply converts the extra power to heat. But you can also use a buck converter. A buck converter will convert the higher voltage to a lower voltage somewhat more efficiently. Typical values are 80% to 90% efficiency. This means that the buck converter will produce less heat than a linear regulator.

I skipped over some details, because I don't think you are ready for them. Perhaps some people will comment on this.

Answer 3

Power is the product of voltage and current and is the work done in moving a quantity of charge through a resistance.

For example, If a 1 volt difference exists across a 1 ohm resistor, then the charge which will flow through that resistance will be one coulomb per second,(one ampere) and the power dissipated by the resistor will be be 1 watt.

Answer 4

re: "...the difference between current and power."
"current" is measured in (A)mps & "power" is measured in (W)atts.
Power (in Watts) = V * A
Therefore, "the difference" is that current is a factor of power.
Note also, electric "current" is the flow of energy (via electric charge) over time (e.g.: an Ampère = 1 Coulomb/Second) = the current that one volt can send through one ohm of resistance in 1 Second.

re: "So it seems that wattage is the important thing to understand in many cases."
Appliances are rated in Watts (which is power) so that you can compare appliances to determine which will cost you the least to operate (e.g. refrigerators). Power ratings can also be used to determine which device may operate more vigorously than another device (e.g. blenders)--once again for potential buyer comparison purposes.

re: "A power supply is (from what I understand) usually given with a (max) voltage and a (max) current. Why is the wattage not given here?"
Many power supplies are advertised by the max wattage (e.g. a computer switching power supply). In other situations, the manufacturer &/or the seller may not include wattage information because they have no requirement to do so (e.g. some exported foreign products) and if they did provide it, then their product/s may not be as attractive to potential customers as other available products--therefore it is left out by the seller with the hope that some unsuspecting customer (hopefully with low initial power needs) will buy their product/s.

re: LED Wattage "...Why isn't the wattage listed?"
When you buy LEDs from a reputable source, datasheets will be available for their products. Those datasheets will indicate current consumption at various voltages levels. With that information, you can calculate the power (Watts) that will be consumed. When you buy LEDs online or from bulk bins at many electronics parts stores, they often do not provide datasheets for such parts. Sometimes you can get the manufacture & part numbers from such sellers & then look up the datasheets online--but that isn't always possible. Some times, technical information is not provided because the seller doesn't have such information (they may have bought their inventory at a bankruptcy auction, for example) or they are too lazy to make such information available for their potential customers.

Answer 5

Wattage only matters for appliances because that's how other appliances are specified; no one cares if a blender uses 10A because that's not how blenders are marketed.

In microelectronics power only matters when talking about a) efficiency, or b) heat generation. Outside of those concerns we really only care about voltage and current independent of power.