why does a bimetallic strip bend when heated

All these effects can be explained by the expansion
of solids when they are heated. When a material gets hot it expands - this is
because the molecules in it are moving about more vigorously and so need more
room. As a solid is heated the molecules vibrate more violently and the solid expands in
all directions. We will just look at the increase in length for simplicity. The hotter it gets and
the longer it was to start with the more the solid expands. Different materials expand by
different amounts for the same rise in temperature. Solids do not expand very much and so we have to find a way of
magnifying this expansion. You can do this in the laboratory with a bar of iron or steel and a
slide projector, the bar need only be about 50 cm long. The bar is fixed in a retort stand and
the end of the bar is placed so that it sticks into the beam of the projector where the slide
carrier would have been. The shadow of the bar is focused onto a wall on the other side of
the laboratory.

The bar is now heated strongly in a Bunsen flame and you will see
the shadow grow. If you measure the width of the bar and the shadow you can work out the
magnification of the projector and then the actual expansion of the bar. Other example experiments: This piece of apparatus shows that it is very difficult to stop
metals from expanding when you heat them and shrinking when you cool them. A strong steel bar is fixed in the frame of the apparatus by a large nut at one end a cast iron peg at the other. When the bar is heated the peg breaks because of the huge force in the bar. It is also possible to make the peg break when the bar contracts on cooling by tightening the nut when the bar expands. This is a strip of two different metals welded together, one side is brass (high expansion) the other is iron (low expansion). When the strip is heated it bends with the brass always on the outside of the curve.

Mechanical mechanisms are sensitive to temperature changes which lead to errors in time keeping. A bimetallic strip is used to compensate for this in some mechanisms. The most common method is to use a bimetallic construction for the circular rim of the. As the spring controlling the balance becomes weaker with increasing temperature, so the balance becomes smaller in diameter to keep the period of oscillation (and hence timekeeping) constant. In the regulation of heating and cooling, that operate over a wide range of temperatures are used. In these, one end of the bimetallic strip is mechanically fixed and attached to an electrical power source, while the other (moving) end carries an electrical contact. In adjustable thermostats another contact is positioned with a regulating knob or lever. The position so set controls the regulated temperature, called the set point.

Some thermostats use a connected to both electrical leads. The angle of the entire mechanism is adjustable to control the set point of the thermostat. Depending upon the application, a higher temperature may open a contact (as in a control) or it may close a contact (as in a or ). The electrical contacts may control the power directly (as in a household iron) or indirectly, switching electrical power through a or the supply of or through an electrically operated valve. In some natural gas heaters the power may be provided with a that is heated by a pilot light (a small, continuously burning, flame). In devices without pilot lights for ignition (as in most modern gas clothes dryers and some natural gas heaters and decorative fireplaces) the power for the contacts is provided by reduced household electrical power that operates a relay controlling an electronic ignitor, either a resistance heater or an electrically powered generating device.

A direct indicating dial (such as a patio thermometer or a meat thermometer) uses a bimetallic strip wrapped into a coil. One end of the coil is fixed to the housing of the device and the other drives an indicating needle. A bimetallic strip is also used in a. consists of a tri-metallic helix. Simple toys have been built which demonstrate how the principle can be used to drive a. [ Bimetal strips are used in miniature to protect circuits from excess current. A coil of wire is used to heat a bimetal strip, which bends and operates a linkage that unlatches a spring-operated contact. This interrupts the circuit and can be reset when the bimetal strip has cooled down. Bimetal strips are also used in time-delay relays, lamp flashers, and. In some devices the current running directly through the bimetal strip is sufficient to heat it and operate contacts directly.

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