Electricity Chapter 11 Test Review

Static Electricity

The imbalance of charges on the surface of an object (is static).

Electroscope

An instrument used to detect the presence of electric charges in an object. A pith ball and a metal leaf.

Pith Ball Electroscope

An object is brought towards the neutral pith ball hanging on the string. If the object is charged, the ball will be attracted and move towards it.

Metal Leaf Electroscope

When a negatively charged object is brought near the electroscope, electrons are transferred to the leaves (by temporary induction and charge separation) which causes them to repel and spread a part until the object is removed. Is more precise and can detect the specific charge better than a pith ball.

The Law of Electric Charges

Like charges repel, opposite charges attract.

What is the law for a neutral object?

Neutral objects are always attracted to charged objects through induced charge separation.

Electric Force

A charged object can either exert an attractive or repulsive force. Its strength depends on the proximity and the amount of charges in an object.

Induced Charge Separation

When a charged object is brought near a neutral object, the electrons shift their positions. If the object is positive, electrons will move towards it, if negative, electrons will move away from it. Protons never move.

Methods of charging objects

Friction, conduction, induction

Friction

When two NEUTRAL objects rub or touch, electrons are transferred. One object will gain more electrons, one will lose more (depending on the Electrostatic Series) making them opposite charges (will attract).

The Electrostatic Series

Lists materials in order of increasing tendency to gain electrons (become more negative)

Conduction

When two objects with different charges/ amounts of charges come in contact and electrons transfer from the more negative object to the less negative object.

Conduction Type 1

With a charged object and a neutral object: electrons move until they have equal charges?

Conduction between two charged objects

Electrons always move from the object with a greater negative charge to a smaller one. The charge of the objects are not neutralized, but they both contain the same type and amount.

How can excess electric charges be removed?

Through grounding

Grounding

Removing excess charge by transferring electrons between the object and the ground (earth) since it is a large neutral object, until the object is neutral. Humans can ground things (we are fair conductors and stand on the ground).

Grounding Laws

When a positively charged object is grounded, electrons are pulled from the ground. When a negatively charged object is grounded, electrons travel into the ground until the object is neutral.

Conductor

Materials that allow the movement of electrons easily. Either good or fair. Often metals.

Insulator

Materials that prevent the movement of electrons. Often non-metals.

Good Conductors Examples

Silver, copper, gold, aluminum, iron.

Fair Conductors Examples

graphite, human body, saltwater, nichrome.

Good Insulators Examples

Oil, fur, plastic, glass, wood, rubber, paper, pure water.

Applications of Conductors/Insulators

Insulators wrap copper wires. Saltwater has ions which makes it a conductor. Any charge from a conductor will immediately pass through you and into your hand.

Induction

The process of charging a neutral object, either temporarily or permanently by bringing a charged object close to it (not touching). There is no transfer of electrons and no contact.

Temporary Induction

When a charged object is brought near a neutral object, it induces a charge separation, making one side charged. The two objects are attracted to each other. Once the charged object is removed, the neutral object goes back to normal.

Permanent Induction

The electrons from the induced charge separation are pushed out of the object through grounding (if the charged object is negative) or the electrons are pulled into the object (if the charged object is positive(, hence permanently changing its charge.

What is electric discharge?

The rapid transfer of electric charge when two charged objects come in contact/proximity with each other.

How is the intensity of electric discharge determined?

The greater the charge imbalance, the larger and more noticeable the discharge will be. Can be seen as sparks.

How does lighting work?

The excess negative charge at the bottom of the cloud induces a charge separation on Earth's surface (repels electrons away). The now temporarily positive surface of the earth (temporary induction) causes an imbalance.

Examples of Applications of Static Electricity

Electrostatic paint sprayers and electrostatic lifting apparatus.

Electrostatic Paint Sprayers

Use the properties of static charges to efficiently paint the objects. The pain is given a charge as it leaves the nozzle of the sprayer, and the object to ne painted is given the opposite charge.

Electrostatic Lifting Apparatus

Can be used to make a copy of a footprint left behind at a crime scene. A special film or foil is placed over the footprint. The film is then electrostatically charged. The dust/dirt particles from the footprint are attracted to the backside of the film. The dust particles "jump" off the floor onto the black film, revealing the details of the footprint.

Lighting

The electric discharge from a charge imbalance between clouds or clouds and the ground.