PHY 102 1st Edition Outline of Last Lecture i Continued Examples a Example 1 b Example 2 c Example 3 d Example 4 ii Other Properties of Waves a Polarization b Diffraction c Interference d Doppler Shift Effect iii Light Properties iv Electricity a Example Discharges b Electrical Charges c Electrical Field Strength d Coulomb s Law Outline of Current Lecture I Why Gravity Dominates the Universe These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute a Electrical charge Gravitational Force II Compasses and Lodestones a Magnetic compass III Earth s Magnetic Field a Reducing magnetic field IV Fields in Physics V Fields cont d VI Fields cont d VII Magnetism from Electricity VIII Faraday s Lines of Force Current Lecture I Why Gravity Dominates the Universe a Because Fe Fg 4 17 x 1042 and Fe Fg then why does gravity dominate the universe i Electrical forces are SO much greater than gravitational forces ii No electrical force dominating the earth because the earth is neutral P e p Forces are opposite and equal and cancel out electrical effect II b The universe is neutral in terms of overall charge that is the number of electrons is balanced by the number of protons c Individual objects are most commonly neutral because they contain roughly equal numbers of positive and negative charges Analogues corresponding parts a Fe keq1q2 r2 where ke 9 x 109 Nm2 C2 i Analogously Fg Gm1m2 r2 ii However Fg is only attractive unless we are talking about the grand scale of the universe whereas Fe can be either attractive or repulsive b Fe qE Force due to electrical field III i Analogously W mg force due to gravitational field 1 W is downward 2 m is a scalar 3 g is directed downward ii This is why g is sometimes called the gravitational field strength and is expressed in units of N kg Sample Problems a What is the electrical field strength 1 meter away from a 1C positive charge Hint Use both formulas on last slide eliminating one q Want to find the electrical field E 1C 1018 p E Object 1C or 1018 p 1m E Fe Eq Fe kQq r2 Eq kQq r2 E kQ r2 9x109 nm2 c2 1C 1m 2 IV b What is the direction of the field 1 m away from the 1C charge in or out of the 1C charge Answer away c What force would a 1 4 x 10 5 C negative charge experience at 1m distance from the 1C positive charge F kQq r2 F 1 4 x 10 5 1C 1m 2 d Other notes i If an electric force is greater than 0 it is considered repulsive ii If an electric force is less than 0 it is considered attractive Compasses and Lodestones a Magnetic compass known from ancient times i Chinese probably the first Marco Polo reported 1 Chinese would take needles and stroke them on lodestones and set them in water which would point north to tell directions ii Compass needle prepared from lodestone iii Needles float on water V VI VII VIII iv Compasses have north and south ends 1 The north end of a compass is the end that points north v Like poles repel opposites attract vi Magnetic monopoles do not exist 1 If you cut a magnet in half it becomes north south not just north or just south 2 Electric monopoles do exist Earth s Magnetic Field a Earth acts as though there is a large bar magnet at its center i With its south magnetic pole around 4 from Earth s north geographic pole b On the Move and Weakening i The north end of the compass points to the south magnetic pole of the Earth ii The magnetic south pole is located north of Canada and is on the move iii Strength is currently decreasing at a rate of about 6 3 per century iv Electrical currents do not like magnetic fields interfere Fields in Physics 1 a A field is a hypothetical construct design to explain action at a distance b Two types of forces i Contact forces 1 Pushing and pulling ii Action at a glance forces 1 Gravitational force 2 Electrical force 3 Magnetic force 4 Certain nuclear force weak and strong Fields in Physics 2 a Gravitational i Arrows indicate direction motion of any matter 1 Test particle in magnetism is a gram of matter 2 Always attractive on a small scale b Electrical i Arrows indicate direction of a motion of a positive test particle p a proton ii Opposites attract likes repel c Magnetic i Arrows indicate direction of a motion of a north monopole WHICH DO NOT EXIST Fields in Physics 3 a Concentration of field lines indicate field strength i Closer the lines are the more intense the field ii The more intense the field the stronger the attractive or repulsive force IX X iii When lines point in the same direction this indicates attraction and vice versa Magnetism from Electricity a Oersted shows that direct one way flowing electricity produces a constant magnetic field i Teaching a lecture and uses a wire to show electricity ii Has a compass near the electrical current carrying wire iii When the current begins in the wire the compass needle moves 1 When you run electricity through a wire it produces a magnetic field b Ampere shows that parallel current carrying wires have an attractive force between them c Ampere argues that currents flowing within the earth are responsible for the planet s magnetic field i Dynamo theory to explain electrical magnetic fields d Moon has a small nonexistant magnetic field because it has little volume compared to the surface area so it does not have an electrical current strong enough Faraday s Lines of Force a Field lines trace out magnetic fields b Magnetic fields are directed from N to S c A long current carrying wire produces a circular consistent with the right hand rule i Thumb points in directions of current flow ii Fingers point in directions of magnetic field iii Current carrying wire is held in the fingers straight up and down d Magnetic fields of currents the solenoid i If your magnetic field is stationary there is no electric current ii If the magnetic moves in relation to a wire the electric current moves in relation to the magnet e Charged particles and moving in a magnetic field experiences a force F but only so long as v is not parallel to B f The direction of the force is given by right hand rule 2 i Current is defined to flow in the same direction as a positive test particle