•Curie point, also called Curie
Temperature, temperature at which certain magnetic materials undergo a sharp
change in their magnetic properties.
•This temperature is named for the
French physicist Pierre Curie, who in 1895 discovered the laws that relate some
magnetic properties to change in temperature.
•At low temperatures, magnetic dipoles
are aligned. Above the curie point, random thermal motions nudge dipoles out of
An example of a curie pendulum which
utilizes the effects of heat on a ferromagnetic substance’s magnetization. The
motion is periodic and follows the heating/cooling process of the swinging bob.
•The heat engine uses a principle of
magnetism discovered by Curie. He studied the effects of temperature on
•Ferromagnetism covers the field of
normal magnetism that people typically associate with magnets. All normal
magnets and the material that are attracted to magnets are ferromagnetic
•Pierre Curie discovered that
ferromagnetic materials have a critical temperature at which the material loses
their ferromagnetic behavior. This is known as its Curie Point.
•Once the material reaches the Curie
Point, it will lose some of its magnetic properties until it cools away from
the heat source and regains its magnetic properties. It is then pulled into the
heat source again by the engine magnet to cycle through again.
The heat source could be a flame or even a light
depending on the material of the bob.
•A heat engine transfers energy from a
hot reservoir to a cold reservoir, converting some of it into mechanical work.
•No engine operating between two heat
reservoirs can be more efficient than a Carnot engine operating between the
•Examples of heat engines:
–Stirling Engine – Steam engine
Diagram of Apparatus
H: magnetic field intensity
Carnot Cycle PV Diagram
Our heat engine is not exactly a
Carnot cycle, however there are similarities between it and our HM diagram.
Paramagnetism vs. Ferromagnetism
are made up of magnetic domains, which contain atomic dipoles coupled together
in some direction.
these domains are aligned in random directions, and so there is no overall
presence of a magnetic field, domains parallel to the field grow while others
materials have a positive magnetic susceptibility. Ferromagnetic materials have
a strong positive susceptibility.
materials can remain magnetized after the external field is removed.
The Curie Point
Gadolinium has a Curie Temperature of
This is equivalent to around 20 degrees Celsius. This
makes for a good material in a light based Curie Pendulum.
Data from F. Keffer, Handbuch der
Physik, 18, pt. 2, New York: SpringerVerlag, 1966 and P. Heller, Rep. Progr.
Phys., 30, (pt II), 731 (1967)
Temperatures of Interest
•Nickel has a Curie Temperature of
•We expect the bob to follow an
oscillatory path that orbits the curie point with a period under 10 seconds
(from observation of examples).
We could, if the magnet crosses its
Curie point, it will permanently lose its magnetization. But from the flame
profile in the previous slide the magnet is kept at a safe enough horizontal
distance to avoid damage. The magnet stayed cool enough to touch throughout the
Example of Basic Apparatus
Period of about 7 seconds.
Period of about 5 seconds.
Stefan-Boltzmann Law - Cooling
Energy radiated by a
blackbody radiator per second per unit area is proportional to the fourth power
of the absolute temperature and is given by the Stefan‐Boltzemann Law:
• But not every radiator is ideal – in
which case the proportionality constant for emissivity is
introduced: P 4
(ideal radiator: e = 1) eT
Stefan-Boltzmann Law - Cooling
•If the hot object is radiating energy
to its cooler surroundings at temperature Ts then the total energy
•Our concern with this is the candle
soot that builds up on the pendulum bob could affect the emissivity of the
nickel and change the properties of the system over time.
Data - Magnetic Field
Measurements with a magnetic field
probe, output a voltage corresponding to two different sweet spots that
measured the Axial and Transverse potential at a given point.
Horizontal distance from edge of magnet. Positive away.
vertical distance from center of magnet. Positive down.
State 1, the position of the bob when it is at its
maximum magnetic field amplitude.
State 2, the position of the bob after it falls from
Missing Data - Temperature
•Optimally we would have spot welded a
thermocouple to the bob to measure its temperature at the different
spots. This would allow us to have a definitive number for the amount of work
we have harvested from the candle.
•The thermocouple is not without mass,
this could affect how the bob interacts with the magnet, change the period,
•The Thermocouple may be ferromagnetic
itself and have a different curie point than that of the bob, this could alter
how the magnet interacts with the bob as well.
– How can we benefit?
–To make a more efficient heat engine.
–Optimally we would have a
ferromagnetic pendulum bob with a non‐ferromagnetic, non‐conducting
–Use a Ratchet system to take energy
–The soot from the candle can effect
the cooling and heating rate of the bob.
–The candle flame is not stable
–The current pendulum rod is both
conducting and ferromagnetic.
–Due to the geometry of the bob, there
are two nodes that it can stably reside in. For the most part during operation,
it prefers the lower of the two, which is optimal.
• Mark III – Rotary Curie Engine
Examples of Rotary Type
Aluminum disk with a continuous
Aluminum disk with individual nickel
Example of Rotary Curie Engine
The period of this engine was
approximately 1 rotation every 5 seconds at an average of 16°C outside
temperature. The temperature between the disk and light fluctuated between 65°C
and 85°C and the temperature next to the light was about 90°C.