Electricity CHAPTER 6 Structure of Atoms Atoms are made of 3 particles Protons-Positively charges Neutrons-No Charge Found in Nucleus Electrons-Negatively Charged Found in Nucleus Found outside of Nucleus Most of the time, Protons=Electrons Electrically Neutral Charged Atoms When an atom gains or loses and electron, it becomes charged Called an ion Atoms cannot lose a proton and remain the same element 1 proton=Hydrogen 3 protons=Lithium Etc. See Periodic table The number of protons is called the atomic number Charges can be confusing… When an atom GAINS an electron, it becomes negatively charged Electrons are negatively charged If it loses an electron, it becomes positively charged Protons are what determines what element an atom is. More protons than electrons=more positive than negative Charge is measured in Coulombs (C) Static Electricity Accumulation of excess charge on an object Gained excess electrons Shoes on carpet, hair on balloon, cloths in dryer Because the object is negatively charged, it is attracted to objects with a positive charge Opposite charges attract Law of Conservation of Charge Yep, another one… Charge can be transferred, but it cannot be created or destroyed Typically, it is electrons transferred, not protons Charges Exert Forces Unlike charges attract, like charges repel The force depends on the distance and charge Called Coulomb’s Law Charges also create a field Electric Field-area around charged particles that create a force Indicated by arrows Towards negative Away from Positive Strength of Electric Force Electric force is stronger than gravitational force Why don’t we feel it? Most objects are electrically neutral Conductors and Insulators Conductor-allows electrons to easily pass Best conductors are metal Insulator-does not allow electrons to easily pass through Plastics are great insulators Charging Charging by contact Charged by touching or rubbing Charging by Induction Charged by nearby charged object Does not need to touch object Lightning Storm clouds are arranged with negative charge on bottom Becomes more negative than objects on Earth’s surface Static discharges from cloud to surface http://www.regentsprep.org/Regents/physics/phys03/alightnin/default.htm Grounding Connecting an object to Earth with a conductor Reroutes lightning and electricity Safety measure Electric Current 6.2 Current and Voltage Difference Electric Current-The net movement of electric charge in a single direction. Measured in Amperes (A) If there is no current, there is no NET movement One Ampere is equal to 1 coulomb per second Movement forward=movement backward=movement side-to-side When an electric current is applied, there is still backward and forward movement, but the electrons drift in the direction of the current. Net movement Before Current is Applied After Current is Applied Current and Voltage Difference Related to the force that causes electric charges to flow. From area of high voltage to low voltage Measured in volts (V) Electric Circuits A closed path that electric current follows Battery, wire, lightbulb Current and electron flow For historical reasons, we think of current as flow of positive charges Positive charges do not flow Due to this, current is in the opposite direction of electron flow Electrons flow from lower to higher voltage Current is from higher to lower voltage Batteries 2 Main Types: Dry-cell Wet-cell Dry-Cell Batteries Carbon Rod (positive) Zinc Container (negative) Moist paste (electrolyte) Allows electrons to flow Batteries in flashlights Metals can be different Lithium, carbon, and lithium solution Cell phone battery Wet-cell Battery 2 connected plates made of different metals in an electrolyte Unlike the paste in dry-cells, the wet-cell has a liquid Has many cells connected together. Most common type is lead-acid Lead and Lead dioxide in sulfuric acid solution Electrical Outlets The 2 holes of an outlet have a voltage difference In the U.S. usually 120 V Resistance Tendency of object to resist flow of electrons Measured in ohms (Ω) Takes some of the flowing electrons from the current This symbol is the capital form of the Greek letter “Omega” This is why batteries charging get warm The resistor is taking some of the electrons, and converts the chemical energy to thermal energy Ohm’s Law Current(A)=Voltage difference(V)/resistance(Ω) 𝐼= Current is measured in Amps (A) 𝑉 𝑅 Where I is current Example Problem The voltage difference in a graphing calculator is 6V, and the resistance is 1,200 Ω. What is the current through the batteries of the graphing calculator? Knowns: V=6V, R=1200 Ω Unknown: I=? Equation: 𝐼 = 𝑅 Solve: 𝐼 = 𝑅 = 1200Ω =0.005 A 𝑉 𝑉 6𝑉 Equations Solve for V Solve for R Guided Practice An object has a resistance of 60 Ω as a current of 2 A goes through it. What is the voltage difference across the object? You Try It! The voltage in a device is 50 V, and the resistance is 5 Ω. What is the current in the device? Types of Current Alternating Current (AC) Type found in household electrical outlets Changes direction Direct Current (DC) Found in battery-powered devices Does not change direction Converters change types (ACDC) Circuits 6.3 Series Circuits Electric circuit with only one branch. Used in things such as flashlights and lamps Amount of current is the same in all parts If a part is disconnected, the circuit is broken Light doesn’t light up Open circuits If part of the circuit is removed, the circuit is open Current does not flow, and object does not work Parallel Circuits Circuits that have 2 or more branches Current can go through any number of the branches You can turn lights off in one room, and not kill power for whole house Circuit breakers protect against overheating wires Used in homes for this reason Flips open if the wire gets too hot, stopping the circuit Fuses do a similar thing Fuses contain a metal that melts if too much current passes through Electrical Power and Energy Electrical Power=Rate at which electrical energy is converted to another form P=IV Electrical Power (W)=current(A) x voltage difference (V) Measured in kilowatts (kW) 1kW=1000W Example Problem The current in a cloths dryer is 15 A when it is plugged into a 240 V outlet. What is the power of the cloths dryer? Answer in kW. K: I=15A V=240V U: P=? E: P=IV P=IV=(15A)(240V) =3600W =3.6kW Guided Practice If a blender is plugged into a 120 V outlet that supplies 2 A of current, what amount of power is used by the blender? You Try It! If a clock expends 3 W of power from a 1.5 V battery, what amount of current is supplying the clock? Calculating Electrical Energy E=Pt Electrical energy (kWh)=electrical power (kW) x time (h) To figure out cost for electricity, Power Companies, such as Xcel, multiply kWh by cost per kWh. Currently the average in the U.S. is $0.10 per kWh. Mine was $0.07 per kWh last month. Example Problem A microwave with a power rating of 1200 W is used for 15 minutes. How much electrical energy did the power company provide for the microwave? K: P=1200W t=15 minutes U: E=? Need to convert! P=1200W (1kW/1000W)=1.2kW T=15 m (1hr/60min)=0.25h E= (1.2kW)(.25h)=0.3kWh Guided Practice A TV plugged into a standard U.S, outlet has a current of 10 A through it. If a football game took 3 hours, how much did the game cost to watch assuming the electric company charges $0.10 per kWh? You Try it! A current of 15 A runs through an appliance with resistance of 3Ω. The appliance runs 24 hours a day. If the cost per kWh is $0.10, what is the daily cost to run the appliance?