Electricity and Magnetism Chapter 2: Electricity Section 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: Electricity Section 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 wires 3. 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: Electricity Section 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

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