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ELECTRICITY What do you already know about it? Voltage? Resistance? Current? 1 Do Smarty Demo Electric Circuit A path over which electrons travel, out through the negative terminal, through the conductor, back in through the positive terminal. 3 http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/Conductivity.htm http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/Conductivity.htm Electric Current http://www.youtube.com/watch?v=0oTmAP_EESI&feature=player_embedded Alternating Current and Direct Current If a single electron has a charge of 1.60 × 10-19 Coulombs, then 1.60 1019 Coulombs 1Electron Now flip to get the number of electrons in 1 coulomb of charge 1 Electron 19 1.60 10 Coulombs Electron 6.24 10 Coulombs 18 A collection of 6.24 × 1018 electrons has a charge of one Coulomb Electric current is the flow of electrons through a wire. Measured in Amperes (A). 1 Amp is 1 coulomb of charge per second. That means that 6.24 × 1018 electrons flow past any single point in the circuit. Current is the amount of charge flowing through a circuit in a set amount of time then the formula for current would be q I t 10 Example: A battery can deliver 14.7 coulombs of charge in 30 s. Find the current. q 14.7C I 0.49 A t 30s 11 Voltage can be thought of as, “The driving force which causes the current to flow” Current flows from high electric potential to low electric potential. Just like a ball will roll down a hill in an attempt to shed off its gravitational potential energy, an electron will flow towards a proton to shed off its electrical potential energy. 12 Electric Potential - work needed or energy acquired when a positive charge is moved from infinity to a certain point in an electric field. 13 Potential Difference – The change in the potential energy of a charge as it is moved between 2 points within an electric field. 14 The difference in this electric potential energy is what is known as voltage Current does not flow unless there’s a potential difference (voltage) present. 1 Volt is when a battery cell provides 1 Joule of electrical potential energy to each coulomb of charge as it moves through the cell. 1Joule 1Volt 1Coulomb 15 In a battery the potential difference (voltage) between the positive and negative terminals sets up an electric field. This electric field causes electrons to accelerate in the direction of the positive terminal thus causing current to flow. 16 As a charge moves through an electric field it gains either Potential Energy or Kinetic Energy If "Opposites" move closer they gain Kinetic Energy They speed up If 'Opposites' separate they gain Potential Energy 17 If "Likes" separate they gain Kinetic Energy If "Likes" move closer they gain Potential Energy 18 If an amount of Energy is supplied to charge as it moves between 2 points there is an electrical potential difference between the points. E V q V = Potential Difference (V) E = Energy (Joules) q = Charge (Coulombs) 19 Example: If a battery cell provides 3.0 J of electrical potential energy in moving 2 coulombs of charge through the cell, what is the potential difference (or voltage) of the cell? 3.0 Joules E V 1.5 Volts q 2.0 Coulombs 20 Example: It takes 6 Joules of work to move 2 coulombs of charge between 2 points in an electric field. What is the potential energy difference (voltage) between these 2 points? W E 6 Joules E W V 3 Volts q q 2 Coulombs 21 Resistance As electrons move through a material they experience collisions with other atoms. These collisions cause energy to be given off in the form of heat and can be thought of as a type of resistance to the movement of the electrons. 22 Electrical resistance is when a material resists or does not allow current to flow through the material. Electrical resistance is measured in ohms () Electrical resistance is the “friction” of the electrical world* 23 Resistance Depends on 4 Factors: 1. Temperature Increasing the temperature causes an increase in the collision of particles, which then in turn increases the resistance. 2. Length of Material Increasing the length of the material causes an increase in the number of collisions, which in turn increases the resistance . 24 3. Cross Sectional Area of Wire Increasing the space, decreases the number of collisions, which then decreases the resistance. 4. Nature of the Material Metals differ in both the number of the free electrons and the space available for electron movement. The number that indicates the resistance of materials per meter in length of 1m2 crosssectional area, at a given temperature, is called resistivity of the material. 25 Insulators have very high values of resistivity whereas good conductors have very low resistivities. Resistors are materials with a mid-range resistivity. 26 Resistance http://www.youtube.com/watch?v=WzUTOeoxREM&feature=related http://www.stmary.ws/highschool/physics/home/notes/electricity/intro/wiresResistanceCurrent.swf Ohms Law The potential difference across a load equals the product of the current through the load and the resistance of the load. V = IR R = Resistance (Ohms ) V = Potential Difference (Volts V) I = Current (Amperes A) 29 Example: A 120 Volt potential difference applied to the terminals of a light bulb produces a 2.2 A current through the bulb. Calculate the resistance of the light bulb. V 120 R 55 I 2.2 30 Electrical Power: The rate at which electric energy is converted to other forms of energy is called the power output in the circuit. E P t P = Power in J/s (Watts) E = Energy in J t = time in seconds 31 We can use unit analysis to put the equation for power in to a different form as well. Since Volts are J/C and Amps are C/s. J C J C s s Therefore: (P=VI) must also be true. 32 Example: Find the power consumed by the light bulb in the following circuit. P = VI = 1.5 0.1 = 0.15 W 33 Electricity http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/ResistanceWire.htm Homework Pg. 692 #’s 1, 2 Pg. 696 #’s 4, 6 Pg. 714 #’s 21, 23 Pg. 737 #’s 40, 41, 42 RRHS Handout 35