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## Class 10 Physics (India)

### Unit 3: Lesson 7

Electric power and heating effect of current# Electric power & energy

Let's learn how to calculate electric power delivered & consumed in any circuit. Created by Mahesh Shenoy.

## Want to join the conversation?

- 1)why is kWh a commercial unit of power?

2)does the concept of heating effect of electric current work in these LED bulbs?(2 votes)- 1) kWh is
**not**the commercial unit of*power*. It is the commercial unit of*energy*. Why? Because we use a lot of energy, and watt-second (power 'watt' * time 'second') is not sufficient (or else our power bills would run into hundreds and millions of watt-seconds!)

2) Contrary to popular belief, LEDs**do**generate heat, even though it's*much, much lesser*than other traditional bulbs. Its technology uses the maximum energy applied to it and converts it into light, leaving less heat energy.

Hope this helps. :)(7 votes)

- At3:22he says we don not pay for "voltage or current but energy we consume". But basically energy provided is what we call volts.

For eg:5v is equal to 5 joule of energy per coulomb(2 votes)- Hey there,

I had a similar question and I assume the answer is the same, quite simple really,

By energy we consume he means**total**energy,

Say 5V is the magnitude of the source,

that isn't the total amount of energy consumed by us but rather energy consumed for 1C,

I guess it's safe to say that no-one would use only 1C.

the total amount of energy used by a device =VQ has to be the same as total amount of energy produced (*by conservation of energy*).

Think about what I have replied and you may get the total answer,

Hope this helps

and if I'm wrong do let me know.(1 vote)

- At6:47,he said that P=V*I. This also means that Voltage is inversely proportional to current. But ohm's law (V=I*R) tells that voltage is directly proportional to current. Isn't this a contradiction?(2 votes)
- 1 kWh equals to?(0 votes)
- The kilowatt hour (symbolized kW⋅h as per SI) is a composite unit of energy equivalent to one kilowatt (1 kW) of power sustained for one hour. One watt is equal to 1 J/s. One kilowatt hour is 3.6 megajoules,[1][2] which is the amount of energy converted if work is done at an average rate of one thousand watts for one hour.

An electric heater consuming 1000 watts (1 kilowatt), and operating for one hour uses one kilowatt hour of energy. A television consuming 100 watts operating for 10 hours continuously uses one kilowatt hour. A 40-watt electric appliance operating continuously for 25 hours uses one kilowatt hour. In terms of human power, a healthy adult male manual laborer will perform work equal to about half a kilowatt hour over an eight-hour day.(2 votes)

- I really don't get why we pay for the
**energy we use**and not for the**voltage**we use. Could someone please elaborate on it?(1 vote) - He said that the formula for power is energy/time but he multiplied voltage with current, I don`t get it.(1 vote)

## Video transcript

if you worked at bubs you may have seen something written like this 10 watts or hundred watts what does this wattage mean and how do we calculate it that's what we learn in this video so this number 10 watts actually represents something called the electric power so power in General Electric or otherwise is a measure of how much energy how much energy is consumed consumed or delivered or delivered in unit time per time so it's units become look at these units now the unit of energy is joules the unit of time is seconds and joules per second is what we call as watts which we represent by W it's named after the scientist James Watt so over here 10 watt 10 watt means 10 10 joules per second 10 joules per second this means that this bulb consumes 10 joules of electrical energy every second and when I say consumed what I really mean is that it converts electrical energy into some other form in this example it could be heat and light but there could be other examples in speakers we convert it into sound in fans we convert into rotation or mechanical energy and so on here's a list of power ratings of typical devices that we might find in our houses for example air conditioners have pretty high power ratings they can be as high as thousand and go all the way to two thousand two thousand and five hundred watts a thousand watt means our air conditioner consumes thousand joules of electrical energy every but if you look at your mobile phones for example they consume extremely tiny amount of power less than five watts now here's a question where do these energies come from I mean it has to come from somewhere right well you may guess that in mobile phones and laptops these energies are delivered by the batteries but in bigger circuits like our in our domestic circuits then the energy is delivered by the large generators in the power stations and again what I mean by delivered here is that these things convert some other form of energy into electrical energy for example batteries convert chemical energy into electricity and you may learn more about that in chemistry generators convert rotational mechanical energy into electrical energy and we'll learn about this fun stuff in future videos and guess what for the electricity that we get at our houses we have to pay for the energy that we consume we don't pay for voltage or current but it's the energy that we pay for so it's important to be able to calculate electrical power and so that's what we will do now we will see how to calculate electric power in terms of voltage and current so let's take an example imagine we have a bulb through which two amps of current is passing and let's say the potential difference across the ends of the bulb is let's say five volts I'm looking at these two numbers let's see if we can figure out what's the power consumed by this bulb to do that we'll first recall what current and voltage means two amperes means two coulombs per second this means through any point in the circuit two coulombs of charge are passing by every second two coulombs per second and what is voltage mean volt is joules per Coulomb this means that when a Coulomb moves across the bulb from one point to another it delivers 5 joules of energy to the bulb so what we know from these two is that for every Columba passes by bulk consumes 5 joules of energy and in 1 second 2 coulombs are passing through it so using these two numbers can we calculate how much energy how many joules of energy is being consumed by the PAS by the bulb every second pause the video and see if we can try and do this yourself all right let's do this let's write that down over here I'm going to wait for one second and see what happens in one second I know two coulombs of charge will pass through but each Coulomb will deliver 5 joules of energy so 2 coulombs will deliver 2 times 5 2 times 5 10 joules of energy and therefore in one second 10 joules of energy will be delivered to the bulb and so the power would be 10 watt and if you look at the unit's also we get the same thing notice two coulombs are passing every second into 5 joules per Coulomb the Coulomb Coulomb cancels and we get 10 joules per second which we call as 10 watts now if the current was let's say 3 amperes then in 1 second three coulombs would pass by and so this would be 3 on the other hand if the world it was let's say 20 world then for every Coulomb that passes by 20 joules of energy would be delivered to the boat and so this would be 20 so can you see that in order to calculate power all we are doing is multiplying the current with voltage and so in general we can now write that power or electric power is just the product of voltage and current so if you know these two just multiply them and we get electric power also if we know the power rating of a device and we know what is the voltage across that device then using this equation we can calculate what the current would flow through that device so to summarize what we learned in any electrical circuit there is a source like batteries or generators which will deliver electric power and there are devices which consume electric power and we define in general power as how much energy is consumed or delivered per second and we measure it in watts and to calculate the electric power delivered or consumed by any device we just multiply the voltage across that device and the current to that device and I'd like to remember this by using a saying in electricity with great power comes great voltage and current