Electricity and Magnetism

Chapter 2: ElectricitySection 1: Electric Charge and Static

Electricity

Vocabulary

Electric force Electric field Static electricity Conservation of charge Conduction Induction Static discharge

Electric Charge

Remember: The charged parts of atoms are

electrons and protons When two protons come near each

other, they repel But if an electron and a proton

come near each other, they attract.

Why do protons repel protons but attract electrons? Because they have different types

of electric charge Electric charge is a property of

electrons and protons Protons and electrons have opposite

electric charge Protons have + electric charge Electrons have – electric charge

The two types of electric charges interact in specific ways

This rule of repulsion/attraction is the same as the rule for magnetic poles

There is one important difference between electric charges and magnetic poles Magnetic poles cannot exist alone We can have single electric

charges though. Meaning a negative charge can exist

without a positive charge.

Electric Force

Remember, a force is any push or pull.

In electricity, an electric force is the attraction or repulsion between electric charges.

Electric Field Just as magnetic poles exert their

forces over a distance, so do electric charges.

Also, a magnetic field extends around a magnet, so does electric charges.

An electric field is an area of electrical force around a charged object.

When one charged object is placed in the electrical field of another charged object, it is either pushed or pulled. Pushed if the objects have the same

charge, pulled if the objects have opposite charges.

Electric Field Around a Single Charge An electric field is invisible, just

like a magnetic field Just like magnetic fields, we

represent electrical fields with arrows to show the direction of the electric force

Strength of the electric field is related to the distance from the charged object. The greater the distance, the

weaker the electric field is. Like magnetic fields, the closer the

lines are to one another, the greater strength of the field.

Electric Field Around Multiple Charges When there are two or

more charges, the shape of the electric field of each charge is altered. The electric field of

each charge will combine by repelling or attracting.

Static Electricity Most objects have no overall charge

Meaning they are neutral Meaning they have an equal number of

protons and electrons. There would be no overall electric force on

the atom Some objects can become charged

But protons are bound tightly to the atom, and can’t leave

But electrons can sometimes leave their atom

With some materials, the electrons are held loosely by their atoms So these electrons can move to other

atoms.

Uncharged objects can become charged by gaining or losing electrons. If an object loses electrons, it is left with

more protons than electrons. This means it will have a positive charge

If an object gains electrons, it now has more electrons than protons This means it will have a negative charge

The build-up of charges on an object is called static electricity. In static electricity, charges build up on an

object, but do they do not flow continuously.

Transferring Charge An object becomes charged only

when electrons are transferred from one location to another.

Charges are never created or destroyed. This is known as the conservation

of charge. There are three methods by

which charges can be transferred to build up static electricity Charging by friction Charging by conduction Charging by induction

Charging by Friction

When two uncharged objects rub together, some electrons from one object can move onto the other object. The object that gains electrons

becomes negative Charging by friction is the

transfer of electrons by rubbing. Clothing sticking together after

coming out of the dryer Rubbing a balloon against

something

Charging by Conduction

When a charged object touches another object, electrons can be transferred between the objects. Note: Objects must TOUCH!

Electrons will transfer from the object that has a more negative charge to the one with more positive charge

So charging by conduction is the transfer of electrons from a charged object to another object by direct contact.

Charging by Induction

Objects to not have to be touching for electrons to be transferred

Charging by induction is the movement of electrons from one part of an object that is caused by the electric field of another object.

The electric field around a charged object attracts or repels the electric charges in a second object.

Detecting Charge

Electric charge is invisible, but can be detected by an instrument called an electroscope.

Static Discharge Charges that build up as static

electricity on an object don’t stay there forever.

Electrons tend to move, returning the object to neutral conditions.

Think of what happens when a positively charged object touches a negatively charged object Electrons transfer from the negatively

charged object to the positively charged object until both have the same charge

The loss of static electricity as electric charges transfer from one object to another is called static discharge.

Very often, static discharge produces a spark. As electrons move, they heat the air around

the path they travel until it glows The glowing air is the spark we see

Lightning is a dramatic example of static discharge Basically, just a large spark

Within clouds, water droplets can become electrically charged

Lightning hits Earth when negative charges at bottom of clouds causes Earth to become positively charged What type of charging would this be? Induction!

Electrons will then jump between the cloud and Earth’s surface, creating a giant spark (lightning).

Van de Graff Generator

Electricity and Magnetism

Chapter 2: ElectricitySection 2: Electric

Current

Vocabulary

Electric current Electric circuit Conductor Insulator Voltage Voltage source Resistance

Flow of Electric Charges Remember:

Static electric charges don’t flow continuously

However, when electric charges are made to flow through a wire (or other material) we can get an electric current.

Electric current is the continuous flow of electrical charges through a material.

The amount of charge that flows through the wire in a unit of time is the rate of electric current.

Electric Current

The unit for the rate of current is the Ampere. Often shortened to Amp or just A.

This is like any other rate So this is how much charge moves

per second.

Current in a Circuit

Electric current doesn’t automatically exist in a material.

Current requires a specific path to follow.

To produce electric current, charges must flow continuously from one place to another. Current requires an electric circuit

An electric circuit is a complete, unbroken path through which electric charges can flow.

Conductors and Insulators

Charges low through a circuit made of metal wires. But charge will not flow through a

circuit made of plastic. Electric charges will not flow easily

through every material. A conductor is a material that

transfers electrical charges easily While an insulator is a material

that does not transfer electric charges easily.

Conductors In a conductor, the atoms contain

electrons that are loosely bound. These electrons are able to

move throughout the conductor. As these electrons flow through

the conductor, they form an electric current.

When you turn on a light switch, are the charges coming from the power station to the lightbulb? No The charges are already in every

part of the circuit. The switch just makes them start to flow!

Insulators

Electrons in an insulator are tightly bound to their atoms, so they don’t move easily

Rubber, glass, sand, plastic, and wood are all good insulators.

We generally use insulators to stop the flow of charge This is why most wires come

wrapped in a rubber tube.

Charges Need to Flow

Charges in an electrical circuit move because of differences in electrical potential energy Just like a roller coaster will fall

because of a difference in gravitational potential energy at the top of the track and the bottom of the track.

A battery (or other power source) in a circuit provides the potential energy difference for the circuit.

This energy difference is related to the charges inside the battery.

Voltage The difference in electrical

potential energy between two places in a circuit is called voltage. It is measured in the unit of the volt

(V). Voltage is what causes current in an

electrical circuit. To maintain voltage, a circuit

needs a source of energy. A voltage source is a device that

creates a potential difference in an electrical circuit Which provides the energy needed

to maintain voltage Batteries and generators are

examples of voltage sources.

Resistance

Current depends on resistance Resistance is the measure of how

difficult it is for charges to flow through a material

In other words, conductors have low resistance

Insulators have high resistance The greater the resistance, the

less current there is for a given charge.

The unit of resistance is the ohm (Ω)

Factors effecting resistance There are four factors that

determine the resistance in an object

1. The material from which the wire is made

2. Length Long wires have more resistance

than short wires3. Diameter of wire

Thin wires have more resistance than thick wires

4. Temperature of the wire Hotter wires have more resistance

than colder wires

Electricity and Magnetism

Chapter 2: ElectricitySection 4: Electric

Circuits

Vocabulary

Ohm’s law Series circuit Parallel circuit

Ohm’s Law To understand circuits, we need to

understand the relationship between current, voltage and resistance.

In the 1800’s, Georg Ohm performed experiments to show how these three factors are related.

Ohm found that if he kept all of the factors that affect resistance constant, the resistance of most conductors does not depend upon the voltage across them So resistance does not depend upon

voltage Voltage only affects the current He hypothesized that conductors (and

other materials) have a constant resistance regardless of applied voltage

Calculating with Ohm’s Law The relationship between resistance,

voltage and current is summed up in Ohm’s Law. States that the resistance is equal to the

voltage divided by the current

CurrentVoltage Resistance

ResistanceCurrent Voltage Resistance

Voltage Current

What this tells us Direct relationship between

voltage and current So if we double voltage, we double

current If we have 5x the voltage, we get 5x

the current If I ½ the voltage, I get ½ the

current Inverse relationship between

resistance and current If I double resistance, I ½x the

current If I have 5x the resistance I get 1/5

the current If I have 1/3 the resistance, I get 3x

the current

Features of a Circuit

All electrical circuits have the same basic features

1. Circuits have devices that are run by electrical energy

Radio, computer, light bulb, refrigerator are all devices that transform electrical energy into some other form of energy

Devices like light bulbs and fans act like resistors in a circuit

2. A circuit has a source of electrical energy

In other words, a voltage source is needed

3. Electric circuits are connected by conducting wires

The conducting wires complete the path of the current.

Allows charges to flow throughout the circuit

Circuit Diagrams

Series Circuits

If all of the parts of an electric circuit are connected one after another, the circuit is called a series circuit. So in a series circuit, there is only

ONE path for current to take Easy to build

Just hook up device after device Disadvantages

If one part of the circuit is broken, the whole thing is

Resistors As resistors are added to a series

circuit, the resistance increases

Parallel Circuits In a parallel circuit, the different parts

of the circuit are on separate branches. There are several paths for the charge to

take Several Paths

If a light bulb burns out in a parallel circuit… Only one path (branch) is broken, but

charge can still go through other branches Resistors

If you add resistors to a parallel circuit, total resistance actually goes down

Too many appliances in an outlet

Electric Power (Actually section 5)

All electrical appliances transform electrical energy into some other form.

Remember, the rate at which energy is transformed is known as power.

Electrical power is dependent upon voltage and current. Power = Voltage x Current