Joule's Law for Engineering Students
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ScribdTranslationsJoule's law
The Joule effect is the phenomenon by which if an electric current flows through a
conductor, part of the kinetic energy of the electrons is transformed into heat due to the
collisions they suffer with the atoms of the conducting material through which they
circulate, raising its temperature. The name is in honor of its discoverer, the British
physicist James Prescott Joule.
Causes of the phenomenon
Solids generally have a crystalline structure, with the atoms or molecules occupying the
vertices of the unit cells, and sometimes also the center of the cell or its faces. When the
crystal is subjected to a potential difference, the electrons are driven by the electric field
through the solid and must pass through the intricate network of atoms that form it. On
their way, the electrons collide with these atoms, losing part of their kinetic energy, which
is released in the form of heat.
This effect was defined as follows: "The amount of heat energy produced by an electric
current depends directly on the square of the intensity of the current, the time that it
circulates through the conductor and the resistance that the conductor opposes to the
passage of the current." Mathematically it is expressed as
Microscopically the Joule effect is calculated through the volume integral of the electric
field times the current density:
The resistance is the component that transforms electrical energy into heat energy (for
example an electric stove, an electric stove, an iron, etc.).
Using Joule's law we can determine the amount of heat that a resistor is capable of
delivering, this amount of heat will depend on the intensity of the current that flows
through it, the value of the electrical resistance and the amount of time it is connected,
then we can state Joule's law saying that the amount of heat released by a resistor is
directly proportional to the square of the intensity of the current to the value of the
resistance and the time.
Applications
This effect is the basis for the operation of various household appliances such as ovens,
toasters and electric heaters, and some devices used industrially such as welding
machines, etc., in which the useful effect sought is, precisely, the heat given off by the
conductor when the current passes through it.
However, in most applications it is an undesirable effect and the reason why electrical and
electronic devices need a fan to reduce the heat generated and prevent excessive heating
of different devices such as integrated circuits. And even incandescent lamps that produce
more heat energy than light.
Issues
1. A current of 2A flows through a 50Ω resistor. How much power is dissipated as heat?
Data
R=50Ω substitution
I=2A P=(2A )2(50Ω) P=200watts
2. A 2kw water heater is going to be connected to a 240v line, whose circuit breaker operates
when the current in the circuit is 10A. Will the breaker open when the heater is turned on?
Data
P=2kw=2000w p=IV replacement
V=240v I=P/VI=2000w/240V I=8.33
I=? No because it is less than 10A
3. A 12v battery is charged with a current of 20A for 1 hour a) What power is required to charge
the battery at this rate? b) What amount of energy is supplied during the process?
Data a) P=IV a) P=(20A)(12v) a) P=240N
V=12v b)T=PT b) T=(240w)(3600s) b) T=864000J
I=20th
T=1hour=3600s
4. What resistor should be used to generate 10k of heat per minute when connected to a 120v
source?
Data P=T/t P=100000J/60s P=166.66w
R=?
T=10kJ=1000Jb R=V2/PR=(120v)2/166.66w R=86.4Ω
T=1 min= 60s
V=120v
ENSENADA INSTITUTE OF TECHNOLOGY
ELECTRICITY AND MAGNETISM
EXPOSURE REPORT JOULE'S LAW
ING. Rafael Murillo Rizo
TEAM #6