Transcript Lecture 2: Power, Energy, and Ohm`s Law

Ohm’s Law, Energy, Power,
Series and Parallel Circuits
Circuits 1
Fall 2005
Harding University
Jonathan White
Outline
► Ohm’s
Law
 Georg Ohm
 Relationship between resistance, current, and
voltage
► Energy
► Power
 In general
 In electrical circuits
► Series
and parallel circuits
Georg Simon Ohm
(1787-1854 )
German physicist who experimentally
determined that the if the voltage
across a resistor is increased, the
current through the resistor will
increase.
► Ideas were published in 1827, but
they were rejected by his peers. He
lived in poverty for several years
before taking a teaching position.
► Called the Mozart of electricity.
►
Ohm’s Law
►
V=I*R
 For a constant resistance, if the current increases, the voltage
increases at the same rate
►
I=V/R
 For a constant resistance, if the voltage increases, the current will
increase at the exact same rate
►
R=V/I
 For a constant resistance, if the voltage increases, the current must
increase at the exact same rate.
►
Using the water pipe example, for a given size of valve (R),
if you increase the pressure at which you push the water
through the pipe (V), the amount of water that flows
through the pipe will increase at the same rate(I).
Energy
Energy is the fundamental capacity to do work.
► A charge can have potential energy (voltage) because of
its place in space.
► In a power station, gas, coal, or nuclear energy is
transformed into electrical energy – much like a battery.
► Note that energy can neither be created or destroyed, only
transformed.
► Energy is measured in joules and symbolized by the letter
w. (lowercase w).
► Energy is the amount of power consumed over time, which
can be written as w=∫p dt from t0 to t
►
Power
► Power
is the rate at which energy is used.
► p = dW/dt, where W is energy and t is time
 Since i=dq/dt and v = dw/dq, p can be rewritten as
(dw/dq) * (dq/dt)
 p=v*I
► Power
in electrical circuits is measured in watts
and symbolized by the letter W. Note: energy
uses a lowercase letter and power uses an
uppercase.
 For example, a 60 watt light bulb uses 60 joules of
energy in 1 second.
Power in Circuits
► Consider
a device with a voltage across it and a
current through it.
 The voltage is a measure of the potential energy that a
unit charge dissipates when it passes through a device.
If the device is a resistor, then the energy lost will
appear as heat.
 The current is the number of charges that flow through
a device in 1 second.
► If
each couloumb dissipates V joules, and I
couloumbs flows in one second, then the rate of
energy dissipation is the product, VI.
 That’s what power is, the rate at which energy is
consumed.
Power Formulas
►P
= VI
 Note: Power can be negative. If the device is a
battery, it is losing power so that the loads can do work.
Can the power ever be negative for a resistor? By
convention, if the current enters through the positive
terminal of an element, the power is positive. If the
current leaves an element (enters through the negative
terminal), it is producing power, and the power will be
negative.
► Using
Ohm’s Law, you can state the above formula
in a number of ways:
 P = I2 * R
 P = V2 / R
Energy Conversion in a resistor
► The
power consumed by a resistor can be written
as w=∫v*i dt from t0 to t, since p=v*i.
► As you’ve seen, when there is a current through a
resistance, electrical energy is converted to heat
energy.
► This heat is caused by the collisions of the free
electrons within the atomic structure of the
resistive material.
► When a collision occurs, heat is given off and the
electron gives up some of its acquired energy as it
moves through the material.
Review
► Ohm’s
Law:
 V= I * R; I = V/R; R = V*I
► Energy
Definition: Capacity to do work.
Measured in joules.
Symbol is w
Energy can neither be created or destroyed,
only transformed.
 A battery stores energy




Review
► Power:





Definition: The rate at which energy is used.
Measured in watts.
Symbol is W.
P=W/t
In electrical circuits,
►P
= V * I, P = I2 * R, P = V2 / R
 Power can be negative, if something is supplying power.
► Voltage
drops: Charge loses energy as it travels
through circuits.