Electricity and Magnetism Chapter 8 Electricity and Magnetism

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

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

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

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

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=9.0𝑥1 0 9 𝑁 𝑚 2 𝐶 2 Section 8.1

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

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

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

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

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

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

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

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 (1787-1854) – 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

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

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

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

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 110-120 V Section 8.2

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

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

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

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

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

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

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

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

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

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

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

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

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

Sec 8.4 Magnetism Section 8.4

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

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

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

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

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

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

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

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

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