LABORATORY EXPERIMENTS

HOLT PHYSICS Sample Invention Lab Report continued
series with the light bulb and the doormat provides
a potential difference to the circuit. The
doormat acts as a switch in this circuit. When the
two surfaces of the doormat are not touching, as
when no weight is applied to the doormat, the
switch is open and there is no current in the circuit.
When weight is applied to the doormat and
the two foil-covered surfaces touch, the switch is
closed and there is current in the circuit according
to the potential difference and the resistance present
in the circuit. This relationship is given by the
following equation:
I = ⎯ ∆ V ⎯
R
We developed our circuit using a 9 V dc battery
and a 6.3 V, 115 mA light bulb.
When there is current in the circuit, the light bulb,
which acts as a resistor in the circuit, will light.
When the weight is removed from the doormat,
the plates of the doormat will separate, opening
the switch, and there will be no current in the circuit.
The light bulb will no longer be lighted.
8. Conclusion, Ramifications, and Scope of
Invention: The Doormat Lighting System is a
security lighting device that uses a resistor in a dc
circuit with a battery. The doormat itself operates
as a switch in this circuit, and a light bulb operates
as a resistor. When the doormat is stepped on, the
switch is closed and the light bulb lights. When the
weight is removed from the doormat, the switch is
opened and the light stops.
In this design, pieces of plastic drinking straws are
used to separate the two conducting parts of the
doormat. Other items, such as springs, may be
used in place of the drinking straws. Any material
used for this purpose must be flexible, so that it
will compress when weight is applied and will
return to its original position when the weight is
removed, and it must not conduct electricity. In
fact, another type of separator may be better,
because the drinking straws become flattened with
use and will need to be replaced often to maintain
the required distance between the two pieces.
The dimensions of all the parts of this system,
from the size of the doormat to the length of the
wires, depends upon the desired use. This system
may be used to place a doormat outside the door
of a house and light a lamp above the door, or it
may be used to light a lamp placed inside the
house or at another location. The battery and light
bulb must be selected so that the battery provides
enough potential difference to light the selected
bulb but not enough to cause a fire or short
circuit.
Another possible use of the design would be to use
a resistor other than a light bulb. For example, the
circuit could contain a buzzer or some other
device. In this way, the circuit could operate as an
alarm system or a doorbell. In addition, the switch
in the circuit could be designed for use in any
device that required a pressure-sensitive switch.
The switch could be placed in the bottom of a
mailbox and wired to a light or buzzer inside the
house; this system would notify someone inside
the house that the mail had been delivered.
Because aluminum foil conducts electricity, it
would be necessary to cover the entire switch in
insulating material before using this device.
HRW material copyrighted under notice appearing earlier in this book.
viii
HOLT PHYSICS Laboratory Experiments