1. Chapter 9 Static and Current Electricity

2. 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

3. 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

4. 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

5. 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

6. 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

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

8. Coulomb’s Law

9. 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

10. 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

11. Electric Field

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

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

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

15. 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

16. 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

17. Voltage Sources

18. 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

19. 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!!!

20. 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

21. Electric Current
Compare to the flow of water in a hose

22. 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

23. 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

24. Electric Current
Electron flow + -

25. 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

26. 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

27. 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

28. DC and AC

29. 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?

30. 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

31. 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

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

33. Ohm’s Law
Three forms are convenient

34. 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)

35. Electric Shock
We can use Ohm’s Law to calculate current based on the size of the applied voltage
You can feel 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)

36. 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

37. 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

38. 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

39. Series Circuits

40. 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

41. Parallel Circuits

42. 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

43. 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

44. Series Circuit Calculation
10 ohm
20 ohm
30 ohm
12 Volt

45. Parallel Circuit Calculation
10 ohm
20 ohm
30 ohm
12 Volt

46. 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

47. Electric Power

48. Power Calculation
10 ohm
20 ohm
30 ohm
12 Volt

49. Power Calculation
10 ohm
20 ohm
30 ohm
12 Volt

50. 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

51. 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