ELECTRIC CHARGES & CURRENT By Mr. Frank Todd Notes provided by Mr. Joe Martonik A. Electric Charges & Static Electricity • • • • • 1. Types of Electric Charge & Interaction Protons= + charge Electrons= - charge Like charges repel (+ +, - -) Unlike charges attract ( + - ) • 2. Electric Fields • The area around electric charges that has the force of the charge exerted on it. • When a charge is placed in an electric field, it is pulled or pushed. • The field is the strongest near the charged particle. • 3. Static Charge: Results from a build up or redistribution of charge on an object. • Charged Objects: – Some materials hold their electrons well (insulators). Some do not and the electrons move freely from atom to atom (conductors). – A neutral object can become charged by gaining or losing electrons. – A build up of charges is called static electricity. – (The charges build up but do NOT FLOW) Give up electrons easily Triboelelectric series Gain electrons easily •Human hands (usually too moist, though) Very positive •Rabbit Fur •Glass •Human hair •Nylon •Wool •Fur •Lead •Silk •Aluminum •Paper •Cotton •Steel Neutral •Wood •Amber •Hard rubber •Nickel, Copper •Brass, Silver •Gold, Platinum •Polyester •Styrene (Styrofoam) •Saran Wrap •Polyurethane •Polyethylene (like Scotch Tape) •Polypropylene •Vinyl (PVC) •Silicon •Teflon Very negative • Transferring Charge: Charges can be transferred by the following ways. – Friction—by rubbing materials against one another • Transferring Charge: Charges can be transferred by the following ways. – Conduction—by direct contact between two objects • Transferring Charge: Charges can be transferred by the following ways. – Induction—movement of electrons to one side of an object. (Caused by an electric field of another object) • Static Cling: Clothes stick together because each piece of clothing contains opposite charges. What caused the clothes to be charged? • Rubbing between clothes as they tumble about in the dryer. • 4. Static Discharge: – When there is a loss of static electricity as electric charges move off an object, it is called static discharge. – Humidity: Water molecules in the air can pick up electrons which may be in the process of being transferred. Charge then cannot build up on an object. • 4. Static Discharge: – Sparks & Lightning: – A spark results when electrons jump from one object to another. – Lightning is a giant spark created when water droplets become charged by swirling wind during a storm. Electrons collect in the lower part of the clouds. To restore a neutral condition, electrons jump from the clouds to the ground. • 5. Detecting Charge: – An electroscope is a device used to detect electric charge. – The device cannot tell if it is storing + or – charges. – Can only detect the presence of a charge. B. Circuit Measurements • 1. Electrical Potential: • It is the potential energy per unit of electric charge. • Electrons flow from places of higher potential to places of lower potential. – This called Potential Difference. • 2. Voltage & Sources: • Voltage is measured by a voltmeter. • Potential difference provides the force that pushes charge through a circuit. • This force is called the Volt. • Electrons will flow in a circuit as long as there is a potential difference or voltage between 2 parts of a circuit. • As voltage increases the flow of electrons increases. • Batteries and Generators serve as voltage sources. • 3. Resistance: The opposition to flow of electric charges in a circuit. • The greater the resistance in a circuit, the less current or flow of electrons for a given voltage. • Influences on Resistance: – A wires length and thickness – Thick + Short wires = low resistance – Temperature—Low temperature= lower resistance • Ohms= unit for measuring Resistance • 4. Current: Is the number of electrons passing a point in a given amount of time. • Current always flows through a path of least resistance. • Current will increase/decrease based on the battery construction or source of electrons. • Amps=unit of measuring current • Measured by an Ammeter. Common tools and their Amperage bug killer 1-2 fan = 1-3 hedge trimmer 23 weed trimmer 2-4 electric drill 3-6 saber saw 4-8 sander 4-8 band saw 5-12 lawn mower 6-12 grinder 7-10 chain saw 7-12 drill press 7-14 belt sander 7-15 shop vac 8-14 lawn edger 9-10 table saw 12-15 air compressor 9-15 snow blower 1215 circular saw 1215 1/4 HP motor = 6 amps 3/4 HP motor = 14 amps 1 HP motor = 16 amps router 8-13 1/2 HP motor = 10 amps • • • • • • 5. Ohm’s Law Resistance = Voltage/Current Ohms = Volts/Amps R= V/I V= RxI I = V/R V : : R x I • Sample Problem: • A 0.40 amp current is produced when a 12 Volt battery is connected to a headlight. How much resistance is produced by the headlight? • R=V/I • R= 12 V/0.40Amps • R= 30 Ohms C. Series and Parallel Circuits • 1. Series Circuits • A circuit where all the parts are connected “one after the other” creating only one path for the flow of electrons. • If one bulb in a series circuit burns out or is removed the circuit is broken and all the bulbs go out. • As more bulbs are added, they get dimmer b/c the resistance increases. • Ammeters should be wired in series. C. Series and Parallel Circuits • 2. Parallel Circuits • Different parts of the circuit are on different branches. This type of circuit provides different paths for the electric current. (Each bulb has it’s own path.) • If one bulb is removed or burns out there are other paths for the current to take so the other bulbs stay lit. • Parallel circuits glow brighter because more current flows to the bulbs. • As more branches are added to a parallel circuit the resistance decreases. • Voltmeters should be wired in parallel. Label each type of circuit below. Bulb Series Parallel • 3. Household Circuits: • Must be wired in Parallel • Electricity is fed into a home by Thick and Heavy wires called lines. These have low resistance. • Parallel branches extend from the lines to the wall sockets and Appliances. • Switches are placed to control branches of circuits one at a time. • Voltage in house circuits is 120 Volts. D. Electrical Safety • 1. Becoming Part of a Circuit • Short Circuit—A connection that allows a current to take an unintended path. • If you touch an exposed wire in the house, 120 Volts of current will pass into your body. This is called a SHOCK. • 2. Grounding: • Additional wire in circuit to protect a person from shock. • Plugs have a 3rd prong. This connects the metal shell of the appliance to the ground wire of the building. • Lightning rod-metal rod mounted to the roof of a building. If lightning strikes the rod the energy flows down the rod to a ground wire and then into the Earth. • 3. Fuses and Circuit Breakers: • When a wire carries more current than it is designed to carry it will get HOT. The insulation will then burn. • Fuse-A device with a thin strip of wire (metal) that will melt if too much current flows. This is part of the circuit. (When the fuse melts or “blows” the circuit is broken.) • Circuit Breaker- device which uses an electromagnet to turn off a circuit when it is overloaded. • 4. Electric Shocks: • The human body depends on tiny electrical pulses to control many processes (ex. Heart beat) An electrical shock may disrupt these processes. • The severity of the shock depends on the current. A current of 0.2amps will burn and travel across the body and could stop the heart. • Current of an electric shock is related to voltage and resistance. • Your body has a low resistance (ions in fluids) • When wet your bodies resistance is hundreds of times lower.