Presentation Transcript

1. Electricity

   J. McLeod Spring 2013
2. Charges Atoms contain particles called protons, neutrons, and electrons. Protons (+) Electrons (-) Neutrons (0)
3. Charges & Forces Electric Charge – a property that causes subatomic particles to attract(unlike charges) or repel each other(like charges) Electric Forces – the force of attraction or repulsion between electrically charged objects (depends on distance)
4. Charging Objects Rubbing two materials together can result in a transfer of electrons. Then one material is left with a positive charge and the other with an equal number of negative charge. The process of transferring charge by touching or rubbing is called charging by contact.
5. Charging Objects Conduction – the contact of a charged object to a neutral object. Friction – when two objects are rubbed together, one becoming negatively charged and the other becomes positively charged. Induction - A method used to charge an objectwithout actually touching the object to any other charged object.
6. Charging Objects Law of Conservation of Charge – charge cannot be created or destroyed only transferred. The accumulation of excess electric charge on an object is called staticelectricity.
7. Conductors A material in which electrons are able to move easily is a conductor. Metals are the best electrical conductors because they have freelymoving electrons.
8. Insulators A material in which electrons are notable to move easily is an insulator. Plastic, rubber, and glass are good insulators because they hold onto their electrons tightly.
9. Electricity Electric Current – the continuous flow of electric charge (Measured in Amps (A)) Electrons in metals are in constant motion in all directions. When an electric current flows in the wire, electrons continue their random movement, but they also drift in the direction that the currentflows.
10. Circuit A circuit is a closedpath through which electrons can flow
11. Circuits Scientist usually draw electric circuits using symbols
12. Circuits A - battery C - light bulb B - switch D - resistor
13. Circuits A closed (on) switch means that the circuit through the switch is connected, this is called a closetcircuit. A broken wire or an off switch both leave gaps in a circuit this is called an opencircuit.
14. Electricity An electric field surrounds every electric charge and exerts the force that causes other electric charges to be attracted or repelled. Any charge that is placed in an electric field will be pushed or pulled by the field.
15. Voltage Voltage causes current to flow through an electrical circuit measured in volts - V A Voltage Source (battery or generator) is required to maintain the electrical potential in a circuit.
16. Copper - low resistance Tungsten - high resistance Resistance Resistance opposition the flow of electrons electrical energy is converted to thermal energy & light measured in ohms ()
17. Resistance Resistance depends on the conductor (wire) thickness - less resistance in thicker wires temp - less resistance at low temps length - lessresistance in shorter wires
18. Circuits Series Circuit current travels in a single path one break stops the flow of current current is the same throughout circuit lights are equal brightness each device receives a fraction of the total voltage get dimmer as lights are added
19. Circuits Parallel Circuits current travels in multiple paths one break doesn’t stop flow current varies in different branches takes path of least resistance “bigger” light would be dimmer each device receives the total voltage no change when lights are added
20. Circuits Safety Features: fuse - metal melts, breaking circuit circuit breaker - bimetallic strip bends when hot, breaking circuit
21. V I R Voltage Ohm’s Law V = IR V – Volts (V) I – Current (amps, A) R – Ohms () If voltage remains constant: current & resistance are inversely proportional
22. Voltage Practice Ohm’s Law What is the voltage if the resistance is 3 ohms and the current is 3 amps? V = IR R = 3 ohms I = 3 amps V = 9 volts
23. Voltage 2. A light bulb with a resistance of 160 ohms is plugged into a 120-V outlet. What is the current flowing through the bulb? V = 120 volts R = 160 ohms I = V/R I = 0.75 A
24. Voltage 3. What voltage produces a current of 200 amps with a resistance of 20 ohms? 4. What is the resistance of a lamp operating at 100 volts and using 0.5 amps of current? 5. How much current is used when a 10 ohm heater is connected to a 120 volt source? 6. What is the voltage if the resistance is 6 ohms and the current is 5 amps? 4000 V 200 Ω 12 A 30 V
25. P I V Power Electrical Power – rate at which electrical energy is converted to another form of energy. P = IV P = power (W) I = Current (A) V = potential difference (V)
26. Power Electrical Power Practice A calculator has a 0.01-A current flowing through it. It operates with a potential difference of 9 V. How much power does it use? I = 0.01 A V = 9V P = 0.09 W
27. Power How much power does a 27 A clothes dryer use if it is connected to a 240 V electrical source? If you use 20 kilowatt-hours of electric energy to operate an appliance for 2.0 hours, how much electrical power do you use? A power tool used about 12 amperes of current and has a power rating of 1440 watts. What voltage does the tool require? A stereo receiver used a current of 2.2 amps from a 120 volt line. What is its power? 6480 W 10 kW 120 V 264 W