Electrical Power
When voltage is applied to
a conductor, current will flow and the amount of current flow
and voltage represent a quantity of power.
Such electric power can be
used for heating purposes, for operating a motor or in other
applications of electric energy. Since we cannot get power
for nothing, a battery or other power source must be used
to generate the energy needed. The electric power symbol (P)
is measured by the amount of voltage multiplied by the quantity
of current flow.
P = E x I
The unit of power is the
Watt, named after the Scottish inventor James Watt(17361819).
One Watt of power is equal to one ampere of current flow produced
by one volt of electric pressure.
Because the watt unit relates
to electric power, the symbol VA (VoltAmp) has been used
as well as W (Watt).
When power is calculated
in terms of time, the unit of energy is the Joule. This is
also known as the Wattsecond and represents one watt of power
for one second. In the measurement of ordinary electric power
consumed in homes, the KilowattHour (kWh) is utilized and
this refers to 1000 Watts for 1 hour. In many electronic applications,
however, only fractional power units are used, and the term
milliwatt (mW) is then utilized for convience, to express
onethousandth of a watt.
Thus, 0.0005W = 0.5 mW.
The formula P = E x I solves
for the amount of energy consumed in terms of unit watts.
Therfore if 20 volts are present across a resistance and 2
amps of current flow, the amount of energy consumed equals
40 watts.
If E voltage is unknown but
I (amps) and R (resistance) are known, the following formula
is used:
P = I² x R
Thus if 2 amps of current
are flowing and the resistance is 10 ohms the amount of power
is P = 2 x 2 x 10 = 40 watts.
Power can also be found by
dividing the voltage squared by the resistance: P = E²/R.
Because power is related
to Ohm's law, the amount of power used can be used in formulas
for finding unknown values of current and voltage.
I = P/E
R = P/I²
E = ^(P x R)
I = ^(P/R)
When dealing with large amounts
of electrical power, it may be required that you be able to
determine the cost of the power consumed.
You will be dealing with
units of kilowatts and kilowatthours (kWh), which means the
number of kilowatts used per hour. Thus, 25 kWh is equivalent
to 25 kilowatts used for 1 hour. To find the expense of an
electric usage bill, the formula is:
Expense = kW x hours x rate
per kWh
Voltage
V or E = I x R
As this formula shows, an
unknown voltage can be found by multiplying the current by
the resistance value. Thus, if the current flow through a
3 ohm resistance is 20 amps, the voltage across the resistance
is 60 volts. If the value of the resistance is desired, the
known values of current and voltage are used for solving the
problem by use of the formula:
R = E/I or V/I
As an example, assume that
the voltage across a resistance is 50 volts and the value
of the current flow is 10 amps. Dividing the E value by the
I value results in a resistance of 5 ohms.
Amps
I=E/R or V/R
This formula indicates that
the amount of current flowing in a circuit is equal to the
quotient obtained when the voltage value is divided by the
resistance value. Thus, if there are 10 volts across a 5 ohm
resistor, the current flow through the resistor is 2 amps.
An unknown value of either
voltage or resistance can also be found by rearrangement of
the formula. Voltage, for example, can be obtained if thevalues
of current and resistance are known.
Because power is related
to Ohm's law, the amount of power used can be used in formulas
for finding unknown values of current and voltage.
I = P/E
R = P/I²
E = ^ (P x R)
I = ^ (P/R)
Ohms
If the value of the resistance
is desired, the known values of current and voltage are used
for solving the problem by use of the formula:
R = E/I or V/I
As an example, assume that
the voltage across a resistance is 50 volts and the value
of the current flow is 10 amps. Dividing the E value by the
I value indicates a resistance of 5 ohms.
Because power is related
to Ohm's law, the amount of power used can be used in formulas
for finding unknown values of current and voltage.
I = P/E
R = P/I²
E = ^ (P x R)
I = ^ (P/R)
