Section 1: Electric Charge and Force

Section 2: Current

Section 3: Circuits

Key Terms

Electric Charge

Electrical Conductor

Electrical Insulator

Electric Force

Electric Field

Electric Charge

An electric charge is an electrical property of

matter that creates electric and magnetic

forces and interactions

Like charges repel, and opposite charges attract

The two charges are positive and negative

Positive + Negative = Attraction

Positive + Positive = Repulsion

An object’s electric charge depends on the

imbalance of its protons and electrons

All matter is made up of protons, neutrons, and

electrons. When there is an imbalance in the

number of protons and electrons the object

becomes charged.

The SI unit of electric charge is coulomb

Proton = + 1.6x10-19 C

Electron = - 1.6x10-19 C

Conductors allow charges to flow; insulators do

not

Conductor a material in which charges can

move freely and that can carry an electric

current (Copper)

Insulator a material that does not transfer

current easily (Glass)

Objects can be charged by the transfer of

electrons

Protons and neutrons are relatively fixed in the

nucleus of an atom. The outermost electrons

can be easily transferred from on atom to

another.

The direction of transfer depends on the

material

One material gains electrons becoming

negatively charged and the other material

loses electrons become positively charged

The transfer of electrons by rubbing two

objects together is called charging by friction

Objects can also be charged without friction.

This is called charging by contact.

Charges move within uncharged objects

A negatively charged rod brought near a metal

doorknob induces a positive charge on the

side of the doorknob closest to the rod and a

negative charge on the side farthest from the

rod.

A charged object can attract and insulator by

inducing a slight change in position of the

electrons. This is called polarization

Electric Force

An electric force is the force of attraction or

repulsion between objects due to charge

Electric forces are responsible for events we

cannot see. They hold atoms together as well

as proteins which our bodies need

Electric force depends on charge and distance

The electric force between two objects is

proportional to the product of the charges on

the objects.

The electric force is also inversely proportional

to the square of the distance between two

objects

Electric force acts through a field

Electric field is a region in space around a

charged object that causes a stationary

charged object to experience an electric

field.

Electric field lines can be used to show an

electric field.

Key Terms

Electrical Potential Energy

Potential Difference

Cell

Current

Resistance

Voltage and Current

Electrical potential energy is the ability to

move an electric charge from one point to

another

Electrical potential energy between two

negative charges decrease as the distance

between them increases

Potential difference is measured in volts

Potential difference between any two points, the work that must be done against electric forces to move a unit charge from one point to the other

The SI unit for potential difference is Volts (V)

1 Volt = 1Joule per Coulomb (1 J/C)

Potential difference is some times called voltage

There is a voltage across the terminals of a

battery

The voltage across two terminals of a battery

can range from 1.5 V (small battery) to 12 V

(car battery)

Cell is a device that is a source of electric

current because of a potential difference, or

voltage, between the terminals

Table 1 on pg 539 gives examples of different

types of cells.

Electrochemical cells contain an electrolyte, a

solution that conducts electricity, and two

electrodes, each a different conducting

material. Can be either a wet cell or dry cell

Wet cell – liquid electrolyte

Dry Cell – Paste-like electrolyte

A voltage sets charges in motion

When charges are accelerated by an electric

field to move to a position of lower potential

energy, an electric current is produced.

A current is the rate that electric charges

move through a conductor.

SI unit for current is Ampere (amp)

Ampere = 1 C of charge moving past a point in

1 second

Batteries are direct current sources because

the charge always moves from one terminal

to the other in the same direction.

Conventional current is defined as movement

of positive charge

A negative charge moving in one direction has

the same effect as a positive charge moving

in the opposite direction.

Electrical Resistance

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

Resistance can be calculated from current and voltage

A conductor’s resistance indicates how much the motion of charges within it is resisted because of collisions

It is found by dividing the voltage across the

conductors by the current.

The SI unit is ohm Ω, which is equal to volts

per ampere.

Resistance Equation

resistance = voltage/current or R =V/I

Resistor is a special type of conductor used to

control current.

Equations

R = V/I = Ω

V = IR = volts

I = V/R = amp

Conductors have low resistance

A good conductor allows electrons to flow

easily under the influence of an electric

field. Metals (copper)

Some materials become superconductors below

a certain temperature

Certain metals and compounds have zero

resistance when their temperature falls

below a certain temperature called the

critical temperature.

Superconductors have been used in electrical

devices such as filters, powerful magnets,

and Maglev high-speed express trains.

Semiconductors are intermediate to

conductors and insulators

Materials that semiconductors that are in their

pure state, are insulators. As impurities are

added the semiconductor is loses some of its

insulating ability. Silicon and germanium are

examples of semiconductors .

Insulators have high resistance

Insulating materials are used to prevent

electric current from leaking.

Example: Plastic coating on wires

Sometimes it is important to provide a

pathway for current to leave a charged

object. This is done by using a ground.

Key Terms

Electric Circuit

Schematic Diagram

Series

Parallel

Electrical Energy

Fuse

Circuit Breaker

What are Circuits?

An electric circuit is a path through which

charges can be conducted

Closed Circuit - electrons are able to flow a

complete path

Open Circuit – Electrons are unable to flow a

incomplete path

Switches interrupt the flow of charges in a

circuit

Schematic diagrams are used to represent

circuits

Schematic diagram is a graphical

representation of a circuit that uses lines to

represent wires and different symbols to

represent components

Table 2 pg 548 has more examples of

Schematic Diagram Symbols

Series and Parallel Circuits

Series circuits have a single path for current

Figure 17A pg 549

If one element along the path in a series circuit is removed, the circuit will not work

The voltage across each device in a series circuit can be different

Parallel circuits have multiple paths for current

The voltage across each device is the same in a parallel circuit

When devices are connected in parallel,

charges have more than one path to follow.

The circuit can be complete even if one light

bulb burns out.

The current will take the path with least

resistance.

Electric Power and Electrical Energy

Electrical energy is 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

Electric power is the product of the total

current (I) in the voltage (V) across a circuit

Electric Power Equation

power = current x voltage

P = IV

Power is measured in watts

If you combine the electric power equation

with the equation V = IR, the power lost, or

dissipated, by a resistor can be calculated

P = I2 R = V2/R

Electric companies measure energy consumed

in kilowatt-hours

1kW x h = 3.6 x 106 J

Fuses and Circuit Breakers

If to much objects are plugged into an outlet,

the overall resistance will be lower. This

means that the electrical wires carry more

than a safe level of current

When this happens the circuit is said to be

overloaded which can lead to a fire.

When two wires touch, it creates an

alternative pathway for the electrical

current. This is a short circuit.

Grounding appliances can reduce the risk

electrical shocks from short circuits

Fuses melt to prevent circuit overloads

A fuse is an electrical device that contains a metal strip that melts when current in the circuit becomes too great.

A circuit overload will cause a fuse to blow out

Circuit breakers open circuits with high current

A circuit breaker is a switch that opens a circuit automatically when the circuit exceeds a certain value.

Circuit breakers uses a magnet or bimetallic

strip that respond to current overload by

opening the circuit

Unlike fuses circuit breakers can be reset by

turning the switch back on.