Power and Energy

What is power?

Power is the rate of using or supplying energy:
Power =  Energy       Power is measured in watts (W)
Energy is measured in joules (J)
Time is measured in seconds (s)
 Time
Electronics is mostly concerned with small quantities of power, so the power is often measured in milliwatts (mW), 1mW = 0.001W. For example an LED uses about 40mW and a bleeper uses about 100mW, even a lamp such as a torch bulb only uses about 1W.
The typical power used in mains electrical circuits is much larger, so this power may be measured in kilowatts (kW), 1kW = 1000W. For example a typical mains lamp uses 60W and a kettle uses about 3kW.

Calculating power using current and voltage

There are three ways of writing an equation for power, current and voltage:
Power = Current × Voltage   so   P = I × V  or 
I =  P
 V
 or 
V =  P
 I
where: P = power in watts (W)
V = voltage in volts (V)
I  = current in amps (A)
or: P = power in milliwatts (mW)
V = voltage in volts (V)
I  = current in milliamps (mA)

P I    V 
You can use the PIV triangle to help you remember the three versions of the power equations. Use it in the same way as the Ohm's Law triangle. For most electronic circuits the amp is too large, so we often measure current in milliamps (mA) and power in milliwatts (mW). 1mA = 0.001A and 1mW = 0.001W.

Calculating power using resistance and current or voltage

Using Ohm's Law V = I × R   we can convert P = I × V to:
P I²   R           P    R 
PI²R triangle         V²PR triangle
P = I² × R
or
P = V² / R
where: P = power in watts (W)
I  = current in amps (A)
R = resistance in ohms (ohm)
V = voltage in volts (V)


Wasted power and overheating

Normally electric power is useful, making a lamp light or a motor turn for example. However, electrical energy is converted to heat whenever a current flows through a resistance and this can be a problem if it makes a device or wire overheat. In electronics the effect is usually negligible, but if the resistance is low (a wire or low value resistor for example) the current can be sufficiently large to cause a problem. You can see from the equation P = I² × R that for a given resistance the power depends on the current squared, so doubling the current will give 4 times the power.
Resistors are rated by the maximum power they can have developed in them without damage, but power ratings are rarely quoted in parts lists because the standard ratings of 0.25W or 0.5W are suitable for most circuits. Further information is available on the Resistors page.
Wires and cables are rated by the maximum current they can pass without overheating. They have a very low resistance so the maximum current is relatively large.

Energy

The amount of energy used (or supplied) depends on the power and the time for which it is used:
Energy = Power × Time
A low power device operating for a long time can use more energy than a high power device operating for a short time. For example:
  • A 60W lamp switched on for 8 hours uses 60W × 8 × 3600s = 1728kJ.
  • A 3kW kettle switched on for 5 minutes uses 3000W × 5 × 60s = 900kJ.
The standard unit for energy is the joule (J), but 1J is a very small amount of energy for mains electricity so kilojoule (kJ) or megajoule (MJ) are sometimes used in scientific work. In the home we measure electrical energy in kilowatt-hours (kWh). 1kWh is the energy used by a 1kW power appliance when it is switched on for 1 hour: 1kWh = 1kW × 1 hour = 1000W × 3600s = 3.6MJ
For example:
  • A 60W lamp switched on for 8 hours uses 0.06kW × 8 = 0.48kWh.
  • A 3kW kettle switched on for 5 minutes uses 3kW × 5/60 = 0.25kWh.

Comments

  1. Thіs pаragraph giѵes clеar ideа designeԁ for the new visitors of bloggіng, thаt
    аctually how to do blogging.

    Heгe is my web-site resistance of a resistor

    ReplyDelete
  2. If some one ωіshes expert view on thе topiс of blogging then і suggest
    him/hег to go to see thiѕ weblog, Keep uρ the gоoԁ woгk.


    Heгe is my blog post ... http://school1.izmuroma.ru/mediawiki/index.php/%d0%a3%d1%87%D0%b0%d1%81%d1%82%d0%bd%d0%b8%d0%ba:kristinas

    ReplyDelete
  3. Hеllo eveгyone, it's my first visit at this web page, and post is genuinely fruitful for me, keep up posting such articles or reviews.

    Feel free to surf to my website Resistor Code Calculator

    ReplyDelete
  4. Write more, thats all I have to ѕаy. Literallу, it seems as though you relіed on
    the video to make yοur poіnt. You сlearly know what
    youre talking about, why thгow awaу your intеlligenсe on just posting νіdeοs
    to your weblοg when you could bе giѵing us something enlightening to гead?


    Feel free to surf tο my blog ... wirе wounԁ
    resiѕtor :: 194.150.248.140 :
    :

    ReplyDelete
  5. Hey there! Woulԁ you mіnd if I ѕhаrе your blоg wіth
    my faсeboοk group? Τhегe's a lot of people that I think would really appreciate your content. Please let me know. Thank you

    Also visit my blog: Wirewound Resistor

    ReplyDelete
  6. Greаt site. A lot of helpful info herе.

    I am sending it to several pals ans aԁditionallу
    shaгing in delicious. And of cоurse, thank you for уouг effort!


    Μy site varistors

    ReplyDelete

Post a Comment

Popular Posts