1. Electricity and Magnetism
Chapter 8
Electricity and Magnetism

2. Sec 8.1Electric Charge, Electric Force, and Electric Field
Electric charge is a fundamental quantity – we don’t really know what it is
But we can describe it, use it, control it
Electricity runs motors, lights, heaters, A/C, stereos, TV’s, computers, etc.
Electric Forces – at the microscopic level they hold atoms and molecules together
Electric Forces hold matter together
Gravitational Forces hold the universe together
Magnetism is also closely associated with electricity
Intro

3. Sec 8.1Electric Charge, Electric Force, and Electric Field
Experimental evidence leads us to conclude that there are two types charges
Positive (+)
Negative (-)
All matter is composed of atoms, which in turn are composed of subatomic particles
Electrons (-)
Protons (+)
Neutron (neutral)
The unit of charge is the Coulomb (C)
The charge on the electron is 1.6x10-19C
Section 8.1

4. Sec 8.1Electric Charge, Electric Force, and Electric Field
Two negative charges repel
Two positive charges repel
One negative and one positive attract
Repulse
Repulse
Attract
Section 8.1

5. Sec 8.1Electric Charge, Electric Force, and Electric Field
Section 8.1

6. Sec 8.1Electric Charge, Electric Force, and Electric Field
An electric force exists between any two charged particles – either attractive or repulsive
Like charges repel (positive to positive, or negative to negative)
Unlike charges attract (positive to negative)
Coulomb’s Law relates the magnitude of the force between two electric charges and:
Their separation
Their charge
𝐹=𝑘 𝑞 1 𝑞 2 𝑟 2 , k is coulomb’s constant, 𝑘=9.0𝑥 𝑁 𝑚 2 𝐶 2
Section 8.1

7. Sec 8.1Electric Charge, Electric Force, and Electric Field
Coulomb’s apparatus

8. Sec 8.1Electric Charge, Electric Force, and Electric Field
Comparison between Coulomb’s Law and Newton’s Law of Universal Gravitation
Equations look similar
F = kq1q2 / r2 & F = Gm1m2 / r2
Both depend on r2
Coulomb’s law can describe either an attractive or repulsive force – gravity is always positive
Electrical charges are much stronger than gravitational forces
Section 8.1

9. Sec 8.1Electric Charge, Electric Force, and Electric Field
The Electric Field Action-at-a-distance concept replaced by the Electric Field which surrounds the charge and represents the physical effect in nearby space.
Section 8.1

10. Sec 8.2 Current, Voltage, and Electrical Power
I = the rate of flow of electric charge = charge/time = q/t
I = electric current (amperes)
q = electric charge flowing past a point (coulombs)
t = time for the charge to pass point (seconds)
1 ampere (A) = flow of 1 Coulomb per second
Rearrange equation above:
q = It or 1 coulomb = 1 ampere x 1 second
Therefore, 1 coulomb is the amount of charge that flows past a given point in 1 second when the current is 1 ampere
Section 8.2

11. Sec 8.2 Current, Voltage, and Electrical Power
Electrical conductor – materials in which an electric charge flows readily (most metals, due to the outer, loosely bound electrons)
Electrical insulator – materials that do not conduct electricity very well due to very tight electron bonding (wood, plastic, glass)
Semiconductor – not good as a conductor or insulator (graphite)
Section 8.2

12. Sec 8.2 Current, Voltage, and Electrical Power
When work is done to separate positive and negative charges, we have electric potential energy
Section 8.2

13. Sec 8.2 Current, Voltage, and Electrical Power
Instead of measuring electric potential energy, we measure the potential difference, or voltage
Voltage – the amount of work it would take to move a charge between two points, divided by the value of the charge
Voltage = work / charge = V = W/q
Measured in volts (V) = 1 joule/Coulomb
When we have electric potential energy, this may be used to set up an electrical current
Section 8.2

14. Sec 8.2 Current, Voltage, and Electrical Power
Whenever there is an electrical current, there is resistance (R) within the conducting material
R is due to atomic/subatomic collisions
Georg Ohm ( ) – formulated a simple relationship between voltage, current, and resistance
Ohm’s Law  V = IR
V = voltage in volts, I = current in amperes, and R = resistance in ohms
1 ohm = 1 volt/1 ampere (R=V/I)
Section 8.2

15. Sec 8.2 Current, Voltage, and Electrical Power
In a simple electric circuit:
Electrons flow from negative terminal to positive terminal (provided by the chemical energy of the battery) -- negative to positive
Open switch – not a complete circuit and no flow of current (electrons)
Closed switch – a complete circuit and flow of current (electrons) exists
Closed Circuit Required – to have a sustained electrical current
Section 8.2

16. Sec 8.2 Current, Voltage, and Electrical Power
The light bulb offers resistance. The kinetic energy of the electric energy is converted to heat and radiant energy.
Section 8.2

17. Sec 8.2 Current, Voltage, and Electrical Power
The water wheel analogy
Section 8.2

18. Sec 8.3 Simple Electric Circuits and Electrical Safety
Forms of Electric Current
Direct Current (DC) – the electron flow is always in one direction, from (-) to (+)
Used in batteries and automobiles
Alternating Current (AC) – constantly changing the voltage from positive to negative and back
Used in homes.
60 Hz (cycles/sec) and Voltage of V
Section 8.2

19. Sec 8.3 Simple Electric Circuits and Electrical Safety
A series circuit
If one bulb were to burn out the circuit would be broken, and all the lights would go out
Same current all the way
Section 8.3

20. Sec 8.3 Simple Electric Circuits and Electrical Safety
A parallel circuit: Current Divides, Voltage is the same
Section 8.3

21. Sec 8.3 Simple Electric Circuits and Electrical Safety
Household circuits are wired in parallel independent branches any particular circuit element can operate when others in the same circuit do not.
Section 8.3

22. Sec 8.3 Simple Electric Circuits and Electrical Safety
A dangerous shock can occur if an internal ’hot’ wire comes in contact with the metal casing of a tool.
This danger can be minimized by grounding the case with a dedicated wire through the third wire on the plug.
Section 8.3

23. Sec 8.4 Magnetism Closely associated with electricity is magnetism.
Electric fields come from stationary electric charges
Magnetic fields come from moving electric charges
A bar magnet has two regions of magnetic strength, called the poles.
One pole is designated “north,” one “south.”
Section 8.4

24. Sec 8.4 Magnetism
Magnetic field - a set of imaginary lines that indicates the direction in which a small compass needle would point if it were placed near a magnet
These lines are indications of the magnetic force field.
Magnetic fields are vector quantities.
Section 8.4

25. Sec 8.4 Magnetism
The N pole of a magnet is where magnetic fields emanate (or come out of…).
The S pole of a magnet is where magnetic fields terminate (or go into…).
Magnets also have repulsive forces, specific to their poles, called …
Law of Poles – Like poles repel and unlike poles attract
N-S attract
S-S & N-N repel
Section 8.4

26. All magnets have two poles – they are dipoles
Sec 8.4 Magnetism
All magnets have two poles – they are dipoles
Section 8.4

27. Sec 8.4 Magnetism
The arrows indicate the direction in which the north pole of a compass would point.
Section 8.4

28. Sec 8.4 Magnetism
The source of magnetism is moving and spinning electrons.
First discovered in rocks in the Greek province of Magnesia
Magnetic rock: magnetite
Section 8.4

29. Sec 8.4 Magnetism
Hans Oersted, a Danish physicist, first discovered that a compass needle was deflected by a current-carrying wire.
Current open  deflection of compass needle
Current closed  no deflection of compass needle
A current-carrying wire produces a magnetic field: stronger current  stronger field
Electromagnet – can be switched on & off

30. Sec 8.4 Magnetism
A simple electromagnet consists of an insulated coil of wire wrapped around a piece of iron.
Stronger current  stronger magnet
Electromagnets are made of a type of iron that is quickly magnetized and unmagnetized – termed “soft.”
Section 8.4

31. Sec 8.4 Magnetism
Similar to the pattern from a giant bar magnet being present within the earth (but one is not present!)
Section 8.4

32. Sec 8.4 Magnetism
Section 8.4

33. Sec 8.5 Electromagnetism
Electromagnetism – the interaction of electrical and magnetic effects
Two basic principles:
Moving electric charges (current) give rise to magnetic fields (basis for an electromagnet).
A magnetic field will deflect a moving electric charge (basis for electric motors and generators).
Section 8.5

34. Sec 8.5 Electromagnetism
When a moving charge (a current) enters a magnetic field, the moving charge will be deflected by the magnetic field
The magnetic force (Fmag) is perpendicular to the plane formed by the velocity vector (v) of the moving charge and the magnetic field (B)
Section 8.5

35. Sec 8.5 Electromagnetism
Electrons entering a magnetic field experience a force Fmag that deflects them “out of the page”
Section 8.5

36. Sec 8.5 Electromagnetism
Electric motor – a device that converts electrical energy into mechanical work
When a loop of coil is carrying a current within a magnetic field, the coil experiences a torque and rotates
Section 8.5

37. Sec 8.5 Electromagnetism
The split-ring commutator reverses the loop current every half-cycle, enabling the loop to rotate continuously
The inertia of the spinning loop carries it through the positions where unstable conditions exist
Section 8.5

38. Sec 8.5 Electromagnetism
Generator – a device that converts mechanical energy into electrical energy
Generators operate on the principle of electromagnetic induction
Electromagnetic induction was discovered by the English scientist, Michael Faraday in 1831
When a magnet is moved toward a loop or coil of wire, a current is induced in the wire
The same effect is obtained if the magnetic field is stationary and the loop is rotated within it
Section 8.5

39. Sec 8.5 Electromagnetism
When the loop is rotated within the magnetic field a current is induced in the loop
The current varies in direction every half-cycle and is termed alternating current (ac)
Section 8.5

40. Sec 8.5 Electromagnetism
Since the secondary coil has more windings, the induced ac voltage is greater than the input voltage.
The factor of voltage step-up depends on the ratio of the windings on the two coils.
Section 8.5

41. Sec 8.5 Electromagnetism
Voltage is dramatically stepped-up at the generating plant to minimize joule heat loss during long-distance transmission. The voltage must then be stepped-down for household use.
Section 8.5