PHYS 1407 – Conceptual Physics
II
Introduction
to Electric Potential
Leader:
_____________________________ Recorder:
___________________________
Skeptic:
_____________________________ Encourager: _________________________
2 x D
cells Steel
wool
Socket
with 3 V light bulb Digital
Multimeter
4 x
Alligator clip wire
Safety
If the D cells used in the
following procedure ever got noticeably hot, immediately disconnect any wires.
Introduction
In this activity we will investigate the
idea of electric potential and its relation to electrical energy. We will also use a DMM to measure electric
potential and observe the behavior of batteries when connected in series and
parallel.
Procedure
Part
1 – Energy Transformations
Make
sure that the cord is plugged into the Genecon, and that the alligator clips
are not connected. Turn the handle of
the Genecon at about 1 turn per second. Note:
do not turn the handle of the Genecon too hard as this can strip the
gears inside.
Q1) When you are turning the Genecon handle, what
type of energy does it?
Q2) From where did it obtain the energy?
Connect
the alligator clips on the cord together and turn the handle of the Genecon at
about 1 turn per second.
Q3) Do you now have to exert more force to turn
the handle at the same rate as before?
Q4) If you turn the handle at the same rate, does
its kinetic energy change?
Q5) If you had to exert a greater force, then did
you have to do more work to turn the Genecon’s handle?
Clearly,
when you connect the leads from the Genecon together, the amount of work you
have to do increases. However, if you
are turning the handle at the same rate, it has the same kinetic energy.
Q6) Where do you think the extra energy is going?
Disconnect
the alligator clips and connect them on opposite sides of a small piece of
steel wool. Use the clips to gently pull
on the steel wool so that only a single strand connects the clips. Give the
Genecon a couple of quick turns and observe what happens. If nothing happens, make sure that the
connection to the steel wool is good. If
nothing happens after several tries, contact your instructor.
Q7) Describe what you observed.
Q8) What was the source of the thermal energy
that melted the strand of steel wool?
(I.e. where was the energy input?)
Connect
the Genecon leads to opposite sides of the provided light bulb socket. Turn the handle at about 1 turn per second
and observe what happens. Note if the
handle turns very easily and nothing happens check your connections. If still nothing happens contact your
instructor.
Q9) Describe what you observe.
Q10) How is the light bulb lighting similar to the
strand of steel wool melting? How is it
different?
Q11) What was the source of the thermal energy
that causes the light bulb to glow?
(I.e. where was the energy input?)
As a historical note,
You should have observed that you were
doing work on the handle and that the work you did was being dissipated as heat
by the light bulb. The Genecon is an
energy conversion device. It converts
the mechanical energy of turning the handle into electrical energy. The electrical energy is transmitted by
charge flowing in the wire.
Q12) What does the light bulb do to the electrical
energy?
Part
II – Potential Difference
Place
one of the D cells into a battery holder on the
Q13) Describe what you see.
Q14) From where did the light bulb obtain the
energy to light?
Q15) Did you do work on the battery so that it
could provide the energy or was the energy already stored in the battery?
Q16) Another term for stored energy is __________
energy.
It
turns out to be convenient in electricity that instead of talking about
potential energy, we refer to potential energy per charge. The Potential
Difference between those two points is defined as the potential energy per charge
between those two points. The SI unit for potential difference is the volt for which the symbol is V.
Q17) If a battery is rated at 1.5 V, then between
its terminals it provides 1.5 V of __________ ___________, or 1.5 J of
________________ per coulomb of __________.
Part
III – Measuring Potential Difference
In
this part of the lab we will explore measuring potential difference and
familiarize ourselves with the use of a digital multimeter, DMM. A DMM is probably the most basic tool for
making electrical measurements.
Plug
the black lead into the socket marked COM and the red lead into the socket
labeled V Ω. Turn on the DMM so
that it will read DC volts.
Simultaneously place the red lead on the + terminal of a D cell and the
black lead on the – terminal of the same D cell.
Q18) Record your reading. Don’t forget units.
Q19) Reverse the side of the battery to which the
leads are connected. How does the
reading on the DMM change?
A DMM
when set up correctly is designed so that the lead connected to the V Ω
socket is at a higher potential than the lead connected to the COM socket when
the measured potential is positive.
Q20) Which terminal of the battery is at higher
potential. Explain using your answers to
Q18 and Q19.
Some
DMMs can measure AC as well as DC potential.
A common error in using DMMs with this capability is to have the DMM on
the wrong setting when trying to measure a potential difference. Switch the DMM to the AC Volts setting and
measure the potential difference across the battery.
Q21) Record the result.
Q22) If measuring a DC potential difference and you
obtain 0 V, what is a good thing to check on the meter?