PHYS 1402 General 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 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) What type of energy
does the Genecon handle have?
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) Did you have to
exert a greater torque on the handle to turn it at the same rate now?
Q4) If you turn the
handle at the same rate, does its kinetic energy change?
Q5) If you had to exert
a greater torque, then did you have to do more work on the Genecon?
Clearly,
when you connect the leads from the Genecon together,
the amount of work you have to do increases, suggesting that the energy you are
adding to the system is being dissipated, but as what?
Disconnect
the alligator clips and connect them on opposite sides of a single strand of
steel wool. 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.
Q6) Describe what you observed.
Q7) 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.
Q8) Describe what you observe.
Q9) How is the light
bulb lighting similar to the strand of steel wool melting? How is it different?
Q10) 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.
Q11) 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
Q12) Describe what you see.
Q13) From where did the
light bulb obtain the energy to light?
Q14) Did you do work on
the battery so that it could provide the energy or was the energy already
stored in the battery (by some previous work)?
Q15) 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. If a charge, q, experiences a difference in
potential energy, ΔU, between two points, then the Potential Difference, ΔV, between those two points is defined
as ΔV = ΔU/q. The SI unit for potential difference is the
volt for which the symbol is V.
Q16) If a battery is
rated at 1.5 V, then across 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.
Q17) Record your reading. Dont forget units.
Q18) 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 to 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.
Q19) Which terminal of
the battery is at higher potential. Explain
using your answer to Q17 and Q18.
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.
Q20) Record the result.
Q21) If measuring a DC
potential difference and you obtain 0 V, what is a good thing to check on the
meter?
Part
IV Batteries in Series and Parallel
Place
a D cell into the second battery holder.
Connect the + terminal of on cell to the terminal of the second. Notice that the cells are connected one after
the other. This type of connection is
known as a series connection. When multiple cells are connected together,
the combination is known as a battery.
Q22) Which terminals will
be the + and terminals of the battery?
Explain.
Connect
the battery across a light bulb.
Q23) How does the brightness of the light bulb now
compare to the brightness when it when it was connected to a single D cell? You may want to connect the light bulb across
a single D cell again for reference.
Q24) What does this
suggest about the potential difference across a series combination of cells
compared to an individual cell.
Q25) Make sure that the DMM is on the DC setting
and use it to measure the potential difference across the battery. Record the result.
Q26) How does the
measured potential difference of the battery compare to the measured potential
difference of the individual cell?
Get
together with a nearby group and measure the potential difference for three
cells connected in series and then four cells connected in series.
Q27) Record your results. (Groups can separate
now.)
Q28) When cells are
connected in series, the potential difference of the combination is the
__________ of the individual potential differences.
Now
connect the cells so that the + terminals are connected to each other and
terminals are connected to each other.
This combination of cells is referred to as a parallel combination.
Connect
the parallel combination across a light bulb.
Q29) How does the brightness of the light bulb now
compare to the brightness when it when it was connected to a single D
cell? You may want to connect the light
bulb across a single D cell again for reference.
Q30) What does this
suggest about the potential difference of a parallel combination compared to an
individual cell?
Q31) Which are the + and
terminals of a parallel combination of cells?
Q32) Use the DMM to measure the potential
difference across the parallel combination of cells. Record the result.
Q33) When identical cells
are connected in parallel, the potential difference of the combination is the
___________ of the individual cells.
Note
only identical cells should ever be connected in parallel.
Q34) If we want a light
bulb to be brighter we want to connect cells in __________.
Q35) Most graphing calculators have four 1.5 V
cells connected in series. What
potential difference is the calculator using?
We
will discuss what applications there might be for connecting batteries in
parallel at a later date.