Download Electric Charge and Force

Lab 1: Electric Charge, Force & Vector Field
Objectives: to produce objects that are deficient in electrons, and that have excess electrons;
to treat “excess or deficit electrons” as having negative or positive charge quantity;
to see that charges move easily in a conductor but not readily in an insulator;
to show that the total electric charge is constant, in separation and sharing scenarios;
to verify that the electric Force on a charged object due to a second charged object
(1)
is repulsive for same-type charges, (2) is attractive for opposite-sign charges,
(3)
decreases with distance, plausibly as the square of the distance;
to see that electric charge tends to spread over the surface Area of a conductor;
to start to consider the Electric Field vector as mediator for electric Forces.
Apparatus:
rod-in-ring electroscope, small conducting balls on insulating thread, variety of
materials for triboelectric surfaces, conducting balls on insulating rods.
Background: All electrical phenomena result from electric charges and the motion of those charges.
Matter contains mostly protons (p+ , with positive electric charge), neutrons (n , with
total charge zero they are electrically neutral) and electrons (e– , with negative charge).
An atom has Z protons (its atomic number ignores neutrons); if it has the same number
of electrons as protons, the atom’s total charge is zero. We deduce that electric charge
adds in a normal manner (keep track of ± signs!). This also means that the charge in an
electron is the exact negative of the charge in a proton; the absolute value of this
“elementary charge unit” is denoted e ; it takes 6.25•1018 e to make 1 Coulomb.
The p and n are very dense, holding nearly all the atom’s mass in a region less than
about 4•10–15 meter across (the atom’s nucleus). In contrast, the electrons form a cloud
out to about 2•10–10 m from the nucleus, staying nearby because they are electrically
attracted to the protons. Like-sign charges repel; opposite-sign charges attract.
Electric Forces behave in a way that is analogous to gravity: the Force on an object
(say, an outer electron) is the object’s property (electric charge, or gravitational mass)


multiplied by its environment’s vector field (Electric field E , or gravitational field g )




electric : on q F  qE [C ]  [ CN ]
gravitational : on m F  mg [kg]  [ kgN ] .
The inward electric Force applied to an outer electron, unlike gravity, occurs because
the electron’s negative charge (–e) is immersed in an outward Electric field (not –g).
This Electric field is caused by other (“source”) charges Q , and how close they are.
The other charge is positive; an Electric field must point away from a positive Qsource .
Coulomb’s Law has the strength decrease as the square of the distance from the source:

kQ
GM

electric : Eby Q  2 (away)
gravitational : g by M   2 (away)
r
r
Where k = 8.99E9 Nm2/C2 is Coulomb’s universal constant (but depends on units!).
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If a positive charge is displaced from a negative charge (along the dipole axis), the pair
has become a dipole. Although its total charge is zero, the Electric Field near a dipole
is not zero (see Lab #2) so it can apply electric Force to nearby charges. Conversely,
unless the Electric Field at a polar molecule is uniform (the same at different locations)
there will be a non-zero Force applied to the molecule.
An atom’s outermost electron is attracted to the Z protons in its nucleus, but is repelled
by the (Z-1) other electrons – the net Force on it, caused by the average Electric Field,
is not very strong. If an outer electron is pulled away by some passing molecule, the
extra electron makes the molecule negatively charged, while the atom it came from has
become a positive-charge ion. Some surfaces (PVC, PET, polyester, rubber) are prone
to collect electrons (–), but others (hair, glass, silk, nylon) tend to lose electrons (+).
{but be aware that tiny amounts of water can overwhelm these tendencies.}
In many conducting materials (like most metals) each positive atom core is stuck to its
neighbors, but its outer electron moves from atom to atom. Electrons have little inertia,
so they accelerate quickly into an Electric field – leaving positive cores behind them,
they can move ¼ meter through metal in a nano-second, to the other metal surface – so
the net Force on a free electron, inside a conductor, stays essentially zero.
If spread over a larger surface Area, the Electric field piercing that surface is weaker:


Eoutward  Apierced  4 kQinside (Gauss’ Law is just Coulomb’s Law re-arranged)
Activity 1. A. Qualitative Forces with Scotch Tape Alone
Both lab partners start with a fresh piece of tape 50 – 100 mm long; fold one end over
as a non-sticky handle, and stick the rest of it to the table. Label tape “A1”.and “A2”.
Quickly peel the tape off the table; do not let the tapes curl back and touch your hand.
Bring the two tape pieces close to the other (non-glued sides!) – do not let them touch.
Do the free ends attract or repel, when you bring them near each other?
Repeat enough times to be confident that results are consistent, rather than random.
You and your lab partner should have no trouble coming to consensus on this …
Did the effect seem to be stronger, or weaker, with large distance between the tapes?
Should the charge excess on tape A1 be the same sign, or different, than on tape A2?
Explain what would cause them to be the same, or different:
When you and your lab partner reach consensus, summarize on page 6.
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Activity 1. B. Qualitative Forces with Scotch Tape –Bottom and Top
Both lab partners start with a fresh piece of tape 50 – 100 mm long , with handle.
Stick each tape to the table, and label them “B1” and “B2” (for bottom).
Both lab partners get a slightly shorter piece of tape, with handle. Apply carefully on
top of each B tape – make sure it does not touch the table – label “T1” and “T2”.
First pull each pair of tapes off the table, then separate T from B, into two hands each.
Now do four tests (without stopping to write your results between tests …):
place B1 and B2 tapes near each other – how do the tapes react?
place T1 and T2 tapes near each other – how do the tapes react?
place T1 and B1 tapes near each other – how do the tapes react?
predict the reaction of T2 when near B2 … check your prediction.
Do T1 and T2 have the same sign charge as each other, or different sign of charge?
What evidence have you observed for this?
What reason would you cite as the cause for this?
Do T1 and B1 have the same sign charge as each other, or different sign of charge?
What evidence have you observed for this?
What reason would you cite as the cause for this?
Does A have the same sign of charge as B , or same as T , or different than either?
Explain your reasoning for this expectation:
What evidence would support or refute this?
Obtain evidence. When lab partners reach consensus, summarize on page 6.
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Activity 2. A. Charges on Insulators and Conductors
Show where excess charge is, by drawing +’s and –’s , to explain results below.
The small pith balls (5mm diameter, 0.03gram) a coated with a conductive paint; you
want to keep their threads as “clean” as possible so that they remain non-conductive –
so try to not touch the threads except at the ends (water condensation is a conductor).
1) Rub the hard rubber rod with fur; then touch the rubbed surface to a pith ball.
Rod now { attracts | repels } ball. The rod’s charge is { + | – | 0 }
The ball’s charge is { + | – | 0 }
what caused the ball to have charge:
your evidence for it having charge:
Which do you think has more excess charge? { ball | rod | same }.
why do you think so?
Near the rod, the Electric field points { toward | away from } the rod. Draw it.
Force on ball is { along | against } the Electric field there.
does this agree with equation 1? show how:
How far from the rod does it still have a noticeable Force?
The Electric Field is stronger { near to | far from } the rod.
2) Remove excess charge from the pith ball by touching it.
Investigate the behavior of an un-charged pith ball
when near charged rods (but not close enough to spark).
Is an uncharged ball ever repelled by the rod?
How close must the ball be, to start getting a noticeable Force?
draw + and – signs on the sketch to explain the ball’s behavior
also draw arrows for the Electric Field due to the rod.
Can you explain the net Force on the ball, just by using equation 1?
Explain the net Force on the ball, by also using equation 2
(reach consensus with your lab partner about all important features):
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Activity 2. B. Measuring Charge Quantity with an Electroscope
1) Rub the rubber rod with fur; then touch rod to electroscope ball.
To what angle does the electroscope needle deflect?
Draw the charges q on the electroscope needle?
Label the charges Q that push on the needle.
How did charges get onto the electroscope needle?
Bring the rubber rod near the electroscope ball (do not spark)
needle deflection { increases | decreases } . Explain how:
Charge the glass rod with polyester, bring near electroscope ball.
needle deflection { increases | decreases } . Explain how:
2) Draw the charge sign on needle, ball, rod, and finger during each step:
a. Bring a charged rubber rod near the electroscope ball (no spark);
b. lab partner’s finger touches ball briefly, then remove finger;
c. charged rod is removed; d. bring charged glass rod near (no spark)
3) Ground two metallic balls (on insulating stands) set to touch one another. Charge a
rod; bring rod near one of the balls (no spark!) to chase electrons onto the other ball;
separate the balls before removing charged rod. ground electroscope;
a. hold small ball 0.15m above electroscope ball; record needle angle _______ .
b. hold large ball 0.15m above electroscope ball; record needle angle _______ .
c. touch small ball to electroscope ball (avoid ring!); record needle angle ______ .
d. ground electroscope; touch large ball to electroscope; record needle angle ______.
which ball had more charge on it? { large | same | small }
which ball had denser surface charge? { large | same | small }
Which ball had stronger Electric field? { large | same | small }
Activity 3. A. Coulomb’s Law, Force as a function of Distance
Obtain 2 small balls on insulating thread – please handle these only by their hooks.
Hang them from a ruler, with hooks a few mm apart.
mass m_____________ thread Length L ___________
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Charge them both from highly charged metallic ball.
record data in the table below, for a few different hook spacings.
hook spacing h
[mm]
ball spacing b
[mm]
sin  =
(b – h)/2L
horizontal F =
m g tan 
Qaverage,theory =
F b2 / kC
Activity 3. B. Coulomb’s Law, Force as a function of Charge
Touch one ball briefly, to ground; when released, it will be attracted to the charged ball
{why?}. After they touch, each should have half as much charge as in the table above.
Measure the ball spacing for the closest hook spacing, and compute their charges.
Lab 1 Summary Sheet
Name : ______________________
Lab Partner : ____________
Tape Alone Summary (charges and Forces) :
Top and Bottom Tape Summary (reasons and evidence) :
Explain why the attraction in 2A2 was weaker than the repulsion seen in 2A1:
Charges on Electroscope : The needle deflection angle changed as a charged rod was brought near
the electroscope ball, even though the total charge on the electroscope was constant.
What charge is it that actually determines the electroscope needle angle?
Charge on Metallic Balls : The large ball has about twice the Area of the smaller ball ... Explain
why the small ball had just as much charge (absolute value) as the large ball had.
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If the small ball had the same charge as the large one, but half the Area, then the
Electric field through that Area should have been twice as intense ... Explain why the
needle deflection was essentially the same for the two different balls.
Coulomb’s Law: Does your data show the Electric Force weaken with distance between charges?
Does your data show the Electric Force decreasing as 1/r2 ?
Use your average value for charge Q to calculate the Electric field at the ball surface.
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Questions:
Fuel delivery trucks can become electrically charged as they travel, and might spark
when the nozzle nears the tank. How can this happen, and how can it be prevented?
What electrostatic experiment can you do to tell whether two charges have the same
magnitude?
Explain how a charged comb (which has been run through your hair) can attract small
bits of paper near it.
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