Lecture 15: Static Electricity
Static Electricity: Detailed Study Notes
Introduction to Static Electricity
Introductory Scenario: A woman rubs her feet on the carpet and gives an electric shock to her identical twin. If the twin also rubs her feet, the shock will be:
Options Available:
larger
smaller
the same size
Observations about Static Electricity
Characteristics of Static Electricity:
Builds up on certain objects
Clothes in the dryer often develop static charge
Static charged objects may cling to or repel each other
Causes electrical shocks
Can make hair stand on end
Questions Regarding Static Electricity
Why do some clothes cling while others repel?
Why do clothes normally neither cling nor repel?
Why does distance reduce static effects?
Why do clingy clothes stick to uncharged walls?
Why do clingy clothes crackle as they separate?
Why do some things lose their charge quickly?
Electric Charge (Part 1)
Definition of Electric Charge:
A physical quantity that allows matter to exert electrical forces and to feel electrical forces from other charges.
Types of Electric Charge:
Positive (+) and Negative (-)
Interactions:
Charges of the Same Type: Repel (+ vs. + & - vs. -)
Charges of Different Types: Attract (+ vs. -)
Determining Charge Type:
Requires a known reference charge for comparison of attraction/repulsion.
Clothes acquire charges while in the dryer:
Clinging clothes carry opposite charges
Repelling clothes carry the same charges
Measurement of Electric Charge:
Measured in SI units called Coulombs (C)
Quantization of Charge:
Comes in whole number multiples of the fundamental unit of charge:
Fundamental unit of charge: \text{||}q\text{||} = 1.6 \times 10^{-19} Coulombs
One Coulomb represents a very large amount of charge; common scenarios use milli-, micro-, or nanoCoulombs.
Electric Charge (Part 2)
Atomic Structure of Matter:
Matter is composed of atoms; atoms consist of:
Electrons: Charge = -1
Protons: Charge = +1
Neutrons: Charge = 0
Electrical Forces:
Attraction between protons and electrons holds the atom together.
Humans harbor approximately 10^{28} units of positive and negative charge in equal amounts.
Net Charge:
Net charge is the sum of all individual charges (+ and -).
An object that is electrically neutral has equal + and - charges, resulting in zero net charge.
Most objects, including clothes, are neutrally charged and do not experience attraction or repulsion.
Charge Transfers
Mechanism of Charging:
Involves the transfer of electrons; protons remain fixed in the nucleus and do not transfer.
Electrons can move freely and their behavior results in:
Gaining electrons: results in a net negative charge.
Losing electrons: results in a net positive charge.
Contact Transfer:
Direct contact can influence electron transfer, where:
An object with a stronger affinity for electrons gains a negative charge.
An object with less affinity loses electrons and gains a positive charge.
Rubbing causes better contact and greater charge transfer, such as in clothes in a dryer.
Concept Questions on Charge Interactions
Revisiting the Introductory Question:
When the woman shocks her twin, she transfers her built-up charge. Both having rubbed their feet enhances the static buildup and results in: larger shock.
Does rubbing guarantee charge exchange?
No, there are alternative methods of acquiring charge, but one object must derive its charge from another.
Experiment and Observations
Experiment with Adhesive Tape:
Pulling strips of tape apart results in:
Initial Result: They repel due to identical charges acquired during separation.
Finger Contact Experiment:
Touching strips with fingers causes a transfer of charge, resulting in electrically neutral strips that neither attract nor repel.
Final Pull Apart Experiment:
Adhesive sides stick together, then upon separation, charge transfer occurs and they attract each other due to opposite charges.
Distance and Static Effects
Why Distance Reduces Static Effects:
Charged objects push/pull on one another with equal magnitude and opposite direction forces, even through empty space.
Coulomb’s Law of Electrostatic Forces:
F = k \frac{\text{||}q1 q2\text{||}}{r^2}
Where F is the force between charges, k is Coulomb's constant, q1 and q2 are the amounts of charge, and r is the distance between them.
Questions on Physical Behaviors
Why Do Clingy Clothes Stick to Uncharged Walls?
The wall is neutral but upon the approach of a negatively charged object (balloon), a redistribution occurs:
The wall's positive charges move closer, resulting in electric polarization.
This allows neutral objects to experience attractive forces despite having no net charge.
Electrostatic Potential Energy (EPE)
Definition:
The electrical forces between charges store electrostatic potential energy (EPE).
Work is required to either pull unlike charges apart or push like charges together, storing potential energy.
Energy Conversion:
Lowering potential energy translates into increased kinetic energy as the charges move.
Voltage Measurement:
Defined as EPE per unit charge, measured in Volts (Joules per Coulomb).
Changes in voltage result from position changes of charges and their interaction with surrounding charges.
Conductors vs. Insulators
Behavior of Conductors:
Have mobile charges (typically electrons).
Electrons can freely move, allowing charges to disperse or escape.
Behavior of Insulators:
Lack mobile charges; electrons are fixed in place and cannot flow freely.
Static Charge and Discharge:
Conductors can lose charge quickly, especially when grounded, providing a pathway to the Earth for charge dissipation.
Summary of Static Electricity
Neutral objects contain numerous charges; can transfer charge during contact.
Clothes acquire net charges through the drying process.
Oppositely charged clothes cling and can spark when separated, increasing voltage and energetic placement.
Conductors neutralize more rapidly than insulators due to their structure and charge mobility.