Power is measure up in watts (W)Energy is measure in joules (J)Time is measured in seconds (s)

Electronics is mostly came to with small quantities that power, therefore the strength is oftenmeasured in milliwatts (mW), 1mW=0.001W. For example an LED uses around 40mWand a bleeper uses about 100mW, even a lamp such as a torch bulb only uses around 1W.

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The typical power supplied in mains electrical circuits is lot larger, therefore this power may bemeasured in kilowatts (kW), 1kW=1000W. For instance a common mains desk lamp uses60W and also a kettle uses about 3kW.

The equations

Power = current × Voltage

There are three means of creating an equation because that power, current and also voltage:

P = i × V
I =
V =


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


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

The PIV triangle


You deserve to use the PIV triangle to assist you psychic these 3 equations.Use it in the same means as the Ohm"sLawtriangle:

To calculate power, P: placed your finger over P,this leaves IV, so the equation is P=I×VTo calculate current, I: put your finger over I,this leaves P end V, so the equation is I=P/VTo calculate voltage, V: put your finger end V,this leaves P over I, therefore the equation is V=P/I

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

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

The equations

Using Ohm"s regulation V = ns × R

we can transform P = i × V to:





P = power in watts (W)I = present in amps (A)R = resistance in ohms (

)V = voltage in volts (V)

Normally electrical power is useful, making a lamp light or a motor revolve for example.However, electrical energy is convert to warmth whenever a current flows through aresistance and also this deserve to be a problem if it renders a an equipment or wire overheat. Inelectronics the impact is usually negligible, however if the resistance is low (a cable or lowvalue resistor for example) the current can it is in sufficiently large to reason a problem.

You deserve to see native the equation P=I²×R that for a givenresistance the power counts on the existing squared, so copy the present will give 4 time the power.

Resistors space rated by the maximum power they can have emerged in them without damage,but power ratings are hardly ever quoted in components lists due to the fact that the standard ratings that 0.25W or 0.5W are suitablefor most circuits. Further information is easily accessible on the resistors page.

Wires and cables are rated through the maximum existing they can pass there is no overheating.They have actually a very low resistance therefore the maximum current is fairly large. For additional information aboutcurrent rating please watch the cables page.

The amount of power used (or supplied) depends on the power and also the time for which that is used:
Energy = strength × Time
A low power machine operating because that a lengthy time can use more energy than a high strength 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 because that 5 minutes provides 3000W×5×60s = 900kJ.

The traditional unit for power is the joule (J), but 1J is a very small amount of energy for mains electricityso kilojoule (kJ) or megajoule (MJ) are sometimes used in scientific work.

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

1kWh = 1kW × 1 hour

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

You might need to transform the kWh residential unit to the scientific power unit, the joule (J):

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

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