Chapter 16 Electric Forces and Fields

Section 1

Electricity

Static Electricity- a buildup of electrons- Ex: sliding your feet across the

carpet

Current Electricity- flow of electrons- Ex: car battery

Electric Charge

Electric Charge- electrical property of matter that creates electric and magnetic forces and interactions

There are two kinds of electric charge- Positive Charge- Negative Charge

Like charges repelUnlike charges attract

Electric Charge cont’d

Electric charge is conserved

Example:When a balloon is rubbed against your hair, electrons are transferred to the balloon. Your hair becomes positive and the balloon gains a negative charge

Electric Charge

Atoms consist of protons, electrons, and neutrons

Unit of electric charge: Coulomb, C

Mass of proton: +1.6 x 10^-19Mass of electron: -1.6 x 10^-19

Transfer of Electric Charge

Charging by friction: - one material gains electrons and

becomes negatively charged while the other loses electrons and becomes positively charged

Ex: sliding your feet across the carpet

Transfer of Electric Charge

Charge by contact:

When a negatively charged object touches a neutral object. Electrons flow from the rod to the doorknob. The doorknob now has a negative charge.

Transfer of Electric Charge

Induced charge:

A negatively charged rod is brought near a neutral doorknob. The charges on the doorknob will redistribute themselves.

Transfer of Electric Charge

Electrical conductors- a material in which charges can move freely

- Ex: copper, aluminum

Electrical insulators- a material in which charges cannot move freely

- Ex: glass, rubber, silk, plastic

Transfer of Electric Charge

Semiconductors- have electrical properties between insulators and conductors

Superconductors- have zero electrical resistance; can conduct electricity indefinitely without heating

Chapter 16 ElectricitySection 2

Electric Force

Electric force- the force of attraction or repulsion between objects due to charge

Depends on charge and distance

Electric Force

Electric Field- a region in space around a charged object that causes a stationary charged object to experience an electric force

Current

Electrical Potential Energy- the ability to move an electric charge from one point to another

The electrical potential energy between two negative charges decreases as the distance between them increases.

Current

Potential Difference- the work that must be done against electric forces to move a unit charge from one point to the other

SI unit: volt, V

a.k.a voltage

Current

Current- the rate that electric charges move through a conductor

SI unit: ampere, A (amp)

Electrical Resistance

Resistance- the opposition posed by a material or a device to the flow of current

Caused by internal friction, which slows the movement of charges through a conducting material.

Electrical Resistance

SI unit of resistance: ohm, Ω

Equation:resistance = voltage R = V

current I

Example

A nine volt battery supplies power to a cordless curling iron with a resistance of 18 ohms. How much current is flowing through the curling iron?

Example

A 110 volt wall outlet supplies power to a strobe light with a resistance of 2200 ohms. How much current is flowing through the strobe light?

Example

A CD player with a resistance of 40 ohms has a current of 0.1 amps flowing through it. Sketch the circuit diagram and calculate how many volts supply the CD player?

Chapter 16 ElectricitySection 3

Circuits

Electric Circuit- a set of electrical components connected such that they provide one or more complete paths for the movement of charges.

An electric circuit is a path through which charges can be conducted

Circuits

Closed Circuit- this is the conducting path produced when (for example) a light bulb is connected across a battery’s terminals

Open Circuit- results when there is no complete path, this means there is no charge flow and no current

Circuits

Switch- used to open and close a circuit- Ex: Light switch in your home

Schematic Diagram- a diagram that depicts the construction of an electrical circuit or apparatus

Circuits

Series- the components of a circuit that form a single path for current

Parallel- a circuit in which all of the components are connected to each other side by side

Circuits

Electrical Energy- the energy that is associated with charged particles because of their positions

Electric Power is the rate at which electrical energy is used in a circuit

Circuits

Electric Power Equation:

Power = current x voltageor

P = IV

SI unit: watt, W

Circuits

Example:

When a hair dryer is plugged into a 120 V outlet, it has a 9.1 A current in it. What is the hair dryer’s power rating?

Circuits

Fuse- an electrical device that contains a metal strip that melts when current in the circuit becomes too great

Fuses “blow out” when the current in the circuit reaches a certain level

Circuits

Circuit Breaker- a switch that opens a circuit automatically when the current exceeds a certain value

When the current exceeds a certain level the circuit breaker acts as a switch and opens the circuit. These can be reset unlike fuses.

Chapter 16 Electricity Capacitance

Capacitance

Ability of a conductor to store energy

Energy is electricity

Capacitance Formula

Capacitance = charge on each plate potential difference

SI Unit: farad, F

Capacitance

Depends on:1. Size of capacitor2. Shape of capacitor

Capacitance

Also depends on material between capacitor’s plates

Called a dielectric: insulating material (air, glass, rubber, or waxed paper)

Capacitance

A dielectric material between plates increases the capacitance

When plates of capacitor are connected, they will discharge the stored energy

Resistors in Series

Carry the same current

Total current depends on amount of resistors in the circuit

Resistors in Series

To find total current you must find the equivalent resistance

Then use it to find the current

Equivalent Resistance

Sum of the resistor’s in a circuit

Resistors in Parallel

Have the same voltage (potential difference) across them

Sum of currents in parallel resistors = total current

Resistors in Parallel

Calculated by formula on page 653 in textbook

AC Current

Current changes direction

Electrons flow first one way, then in the opposite direction

Direct Current

Current always flows in one direction

Example: Batteries

Voltage in the United States

Transferred at 120 volts

Must be stepped up or stepped down by transformer