Chapter 9 Static and Current Electricity

Electricity and Magnetism The electromagnetic force is what binds electrons to the nuclei of atoms Responsible for all of chemistry When we liberate electrons from individual atoms, we can make them do work for us So, the topic is extremely important if we are to understand the world around us

Electricity and Magnetism Many of the fundamental particles have something we call electric charge We don’t know what this is, we can only describe the results when particles have the property There are two kinds of charges which we call Positive and Negative

Electricity and Magnetism We observe that like charges repel We observe that unlike charges attract The forces are inverse square law forces, just like gravity, but very much more powerful Atoms have a positive nucleus with surrounding negatively charged electrons

Electricity and Magnetism Each electron in an atom is identical to every other electron so they all have the same mass and the same negative charge The nucleus is composed of positively charged protons and uncharged neutrons All protons are identical and the charge of the proton is exactly the same size as the charge of the electron, but it is opposite

Electricity and Magnetism Normal atoms have identical numbers of protons and electrons Atoms with a missing electron are called positive ions Atoms with an extra electron are called negative ions

Coulomb’s Law Relationship describing the force between two charged particles Charge is measured in coulombs Force in newtons Distance in meters

Coulomb’s Law

Charge Polarization Make a tiny difference in the average positions of electrons in an atom Distorts electrons like tides on the earth and pulls the nucleus to the right

Electric Field How do things interact with each other when they don’t physically touch? When we talked about gravity, we said that objects appear to experience a force attracting them to each other. Place a charged particle in space and it creates an electric field Using Coulomb’s Law, we can calculate the force on a test charge at every point in space

Electric Field

Electric Potential Lift an object into the air and we give it gravitational potential energy

Electric Potential Separate a positive charge from a negative charge and give it electric potential energy

Electric Potential Define electric potential as the electric potential energy per unit charge

Voltage Sources If we want to move charge from one place to another, we must apply a force to make it move Another way of thinking about this is to say that we must give the charges some potential energy

Voltage Sources We can give an object gravitational potential energy by lifting it into the air We have to do work on the object to lift it into the air Doing work uses energy Conservation of Energy

Voltage Sources

Voltage Sources We need an pump that pumps charges! The simplest charge pump is a battery It uses chemical reactions to separate charges and thus create electrical potential energy More convenient to talk about PE/charge or electric potential measured in volts So, a battery is a kind of electric pump

Voltage Sources Another kind of source is a generator A generator converts mechanical energy into electrical energy In any case, what we need is a device to separate charges!!!

Electric Current The movement of charge is called a current Metals have some electrons that are not needed to bond the atoms together in the solid They are pretty free to just roam about the material and are not associated with any individual metal atom

Electric Current Compare to the flow of water in a hose

Electric Current We measure the flow of water in gallons/minute We measure the flow of charges in coulombs/sec 1 coulomb/sec = 1 ampere Remember this is 6.25 x 1018 electrons moving past a point in a wire per second

Electric Current In a wire, the electrons actually move quite slowly, less than 0.01 meters/sec However, their electric field moves at the speed of light!!! So, we can send signals down a wire very quickly, because the information moves at the speed of the changing electric field which is at the speed of light

Electric Current Electron flow + -

Electric Current In the 1700’s people figured out that charges could move They had two choices Positive charge moves Negative charge moves They guessed WRONG!!!!! We know electrons move

Electric Current Hook up a battery and electrons flow from minus terminal to plus terminal Ancients defined current as positive charge flow Direction of “current” is from plus to minus

DC and AC Hook up a battery and we have direct current, i.e. current flows ALWAYS in one direction Modern generators in power plants reverse the polarity of the output terminals 60 times per second This is called alternating current It is what comes out of the wall plug

DC and AC

Electrical Resistance So, we hook up a wire between the terminals on a battery The question is, how much current flows? We have lots of free electrons able to move in the metal composing the wire So, how much charge moves past any point in the wire per second?

Electrical Resistance The amount of current that flows is determined by how much resistance there is to the flow of the charges What makes this resistance? Collisions of the electrons with each other and with the “stationary” atoms of the metal Atoms are actually vibrating in the lattice

Electrical Resistance Resistance depends on the structure of the material and the temperature The higher the temperature, the more the atoms vibrate and the more electrons make collisions with the atoms How much current flows was discovered by George Ohm, and we honor him by naming the unit of resistance an ohm

Ohm’s Law Ohm’s discovery was that current is proportional to voltage and inversely proportional to resistance

Ohm’s Law Three forms are convenient

Electric Shock We all know not to stick our finger into a light bulb socket or to drop an electrical appliance into our bath tub What causes the damage to our bodies? It is the amount of current that flows through the body that can cause problems Human skin has resistance ranging from 100 ohms to 500,000 ohms (wet to dry)

Electric Shock We can use Ohm’s Law to calculate current based on the size of the applied voltage You can feel 0.001 ampere (1 milliampere) 0.005 amperes hurts 0.010 amperes causes muscle spasms 0.015 amperes loss of muscle control 0.070 amperes disrupts heart rythyms (fatal)

Electric Shock There must be a potential difference between one part of your body and another You must become a conductor of electricity OK for birds to sit on a 5000 V transmission line as long as no part of the bird touches something else Squirrels get across the transformers

Electric Circuits A circuit is a path where a current can flow If the flow is to be continuous, the can be no gaps in the path Intoduce gaps in the form of switches, so we can control completing a circuit Most circuits have more than one device that we want to provide with electrical energy

Electric Circuits There are two ways to connect multiple devices to a voltage source One is called series The other is called parallel Each has unique properties which we now examine

Series Circuits

Series Circuits A single pathway through the circuit The current is the same everywhere in the circuit Each device provides resistance and total resistance is the sum of the devices Voltage divides among the devices Voltage drop across each device is Irdevice

Parallel Circuits

Parallel Circuits Each device connects to the voltage source Voltage is the same across each device Current from source divides into devices Total current is the sum of device currents Current in each device is just V/R Add devices, lower total resistance

Circuits Ignore resistance of wires for calculations Just as we ignored air resistance Ohm’s Law answers all questions Let’s do some sample calculations

Series Circuit Calculation 10 ohm 20 ohm 30 ohm 12 Volt

Parallel Circuit Calculation 10 ohm 20 ohm 30 ohm 12 Volt

Electric Power Moving charges do work We can heat the filament in a light bulb We can turn the rotor in a motor The rate at which work is done is power Electric Power = current x voltage Units are watts = joules/sec = amps x volts

Electric Power

Power Calculation 10 ohm 20 ohm 30 ohm 12 Volt

Power Calculation 10 ohm 20 ohm 30 ohm 12 Volt

Energy Loss in Power Lines Wires have some resistance To transport energy from Point A to Point B, we connect wires Each wire has resistance Energy Loss in one second is I2R Make I small to minimize Transformers convert AC Voltages

Fuses Limit the current that runs through wires in your house These wires have some resistance Energy loss by I2R converts to heat Hot wires can start a fire Limit the current with a fuse or circuit breaker