Power= Energy Time
Power is measure in watt (W)Energy is measure in joules (J)Time is measure up in seconds (s)

Electronics is mostly came to with tiny quantities that power, for this reason the power is oftenmeasured in milliwatts (mW), 1mW=0.001W. For instance an LED uses about 40mWand a bleeper uses around 100mW, also a lamp such together a torch bulb only uses around 1W.

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The common power used in mains electric circuits is much larger, therefore this power might bemeasured in kilowatts (kW), 1kW=1000W. For example a usual mains desk lamp uses60W and also a kettle uses around 3kW.

### The equations

 Power = existing × Voltage

There room three ways of creating an equation for power, current and also voltage:

 P = ns × V
I =
 P V
V =
 P I

where:

P = power in watt (W)V = voltage in volts (V)I = present in amps (A)

or:

P = power in milliwatts (mW)V = voltage in volts (V)I = present in milliamps (mA)

### The PIV triangle

 P I V

You deserve to use the PIV triangle to help you psychic these 3 equations.Use that in the same method as the Ohm"sLawtriangle:

To calculation power, P: placed your finger over P,this pipeline IV, for this reason the equation is P=I×VTo calculation current, I: placed your finger over I,this pipeline P end V, therefore the equation is I=P/VTo calculation voltage, V: placed your finger over V,this pipeline P end I, so the equation is V=P/I

The amp is quite big for electronic devices so we frequently measure existing in milliamps (mA) and power in milliwatts (mW).

1mA = 0.001A and 1mW = 0.001W.

### The equations

Using Ohm"s law V = ns × R

we can convert P = ns × V to:

 P=I²×R

and

P=
 V² R

where:

P = power in watts (W)I = current in amps (A)R = resistance in ohms ( )V = voltage in volts (V)

Normally electric power is useful, making a lamp irradiate or a motor revolve for example.However, electrical energy is converted to warmth whenever a existing flows v aresistance and also this can be a trouble if it renders a maker or cable overheat. Inelectronics the effect is commonly negligible, yet if the resistance is low (a wire or lowvalue resistor for example) the existing can be sufficiently big to cause a problem.

You can see native the equation P=I²×R the for a givenresistance the power depends on the present squared, so copy the existing will provide 4 times the power.

Resistors room rated by the maximum power they have the right to have emerged in them there is no damage,but power ratings are rarely quoted in parts lists due to the fact that the typical ratings the 0.25W or 0.5W room suitablefor most circuits. More information is obtainable on the resistors page.

Wires and cables space rated by the maximum existing they have the right to pass there is no overheating.They have a really low resistance so the maximum existing is reasonably large. For additional information aboutcurrent rating please see the cables page.

The quantity of energy used (or supplied) relies on the power and also the time for which that is used:
 Energy = power × Time
A low power an equipment operating for a long time have the right to use more energy than a high power deviceoperating for a brief time.

For example:A 60W lamp switched on for 8 hrs uses 60W×8×3600s = 1728kJ.A 3kW kettle switched on for 5 minutes supplies 3000W×5×60s = 900kJ.

The typical unit for energy is the joule (J), but 1J is a very tiny amount of energy for mains electricityso kilojoule (kJ) or megajoule (MJ) are occasionally used in clinical work.

In the house we measure electric energy in kilowatt-hours (kWh), frequently just referred to as a "unit"of electricity when the context is clear. 1kWh is the energy used by a 1kW strength appliance once it is switched on because that 1 hour:

 1kWh = 1kW × 1 hour

For example:A 60W lamp switched on for 8 hours uses 0.06kW×8 =0.48kWh.A 3kW kettle switched on because that 5 minutes offers 3kW×5/60 =0.25kWh.

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You might need to transform the kWh domestic unit come the scientific energy unit, the joule (J):

1kWh = 1kW × 1 hour =1000W×3600s =3.6MJ