ELECTRIC CURRENT Chapter 13.2 Voltage, Current and Resistance
The flow of electricity depends upon… Voltage Current Resistance http://cache.eb.com/eb/image?id=100742&rendTypeId=4 Current (electron flow) voltage resistance - +
Voltage Force that causes charges to move (the push or the electric field) Voltage exists if there is a potential difference in charges (negatively charged vs. positively charged) SI unit is Volt (V)
Does the size of the cell matter? D cellsAA cells 1.5 volts
Electric Current The flow of charged particles. Charges move from a position of higher to lower electric PE (or higher voltage to lower voltage). SI unit is ampere (A or amps)
Resistance Resistance restricts the flow of electric charges. The cause: internal friction as moving charged particles collide. Unit for resistance is ohm
Factors influencing the resistance of a wire length temperature cross-sectional area conductivity of the material
Increasing Resistance Which is more resistant? Long or short wire? long Thin or thick wire? thin Hot or cold wire? hot Less conductive or more conductive wire? Less conductive
Circuits Series circuit only one path for electric current to flow bulbs dim if more bulbs are added in series other bulbs go out if one bulb burns out Parallel circuit two or more paths for electric current to flow bulbs maintain same brightness if more bulbs are added in parallel other bulbs stay lit if one bulb goes out
Series circuit Parallel circuit http://iss.cet.edu/electricity/pages/a17.xml http://iss.cet.edu/electricity/pages/a17.xml
Match statements with the correct circuit Series circuit vs. Parallel circuit 1. other bulbs go out if one bulb burns out 2. bulbs maintain same brightness if more bulbs are added 3. two or more paths for electric current to flow 4. bulbs dim if more bulbs are added 5. other bulbs stay lit if one bulb goes out 6. only one path for electric current to flow 1 2 4 3 6 5
Series or Parallel? 1 2 3 4 7 5 6 parallel series parallel
Hewitt and electric circuits Series circuits __________ resistance and decrease in _________ and __________ as more light bulbs are added. Adding more light bulbs to parallel circuits causes the resistance of the circuit to __________, the voltage across each bulb to ___________ the ________, and the current flowing through the entire circuit to ____________.
Draw these circuits using schematic symbols in your spiral notebook. Series, 1.5 V dry cell, three bulbs, one closed switch Parallel, 1.5 V dry cell, three bulbs in parallel, one open switch controlling only the second bulb.
What’s wrong with this picture? http://www.fire-extinguisher101.com/article0705-electrical-fire-safety.html In a parallel circuit, each device will receive the amount of electric current needed to operate. This can lead to wires overheating, which can cause fires.
Fuses and Circuit Breakers For overloaded current flow, the path will be broken fuse burns out breaker springs open. Each parallel circuit in your home has a fuse or circuit breaker rated at a certain amount of amperes (15A, 20A, 30A, and so on) As more electrical devices are added to the parallel circuit, the resistance of the circuit decreases. This means that more electric current can flow.
Circuit Breaker and Fuse Box http://jimspearsfusebox.tripod.com/help/oldfusebox.jpg http://www.smarthome.com/7103.html
Exit Question: Draw these two circuits using schematic symbols. Series, three 1.5 V dry cells, two bulbs, one open switch Parallel, one 6 V battery, three bulbs, one closed switch controlling one bulb, another open switch controlling all three at once.
Series or parallel? http://www.pz.harvard.edu/ucp/curriculum/circuits/s5_lessonplan.htm http://mrskingsbioweb.com/IntegratedScience.htm http://www.fortunecity.com/greenfield/eagles/180/current.html http://www.inclusive.net/exemplars/exemplar_b_4_1.shtml http://www.school-for-champions.com/science/dc_circuits.htm
Circuits: Basic Questions Series circuits How many paths can an electron follow from one end of the terminal to the other? Do the bulbs get brighter, dimmer or stay the same if more bulbs are added in series? What happens to the other bulbs if one bulb goes out? Parallel circuits How many paths can an electron follow from one end of the terminal to the other? series Do the bulbs get brighter, dimmer or stay the same if more bulbs are added in series What happens to the other bulbs if one bulb goes out? parallel?
Ohm’s Law Resistance = Voltage/Current R = V / I R is resistance, unit is ohm V is voltage, unit is volt (V) I is current, unit is ampere (A)
Using Ohm’s Law Use to calculate electrical resistance, amperage or voltage. Ohm’s Law predicts the degree of electric shock one can experience Do practice problem #1 on page 443.
Adding load in parallel causes a decrease in resistance (more current is available, voltage stays the same) Adding load causes an increase in resistance and decreases voltage across each device.
How batteries produce voltage A chemical reaction facilitated by an electrolyte causes electrons to move internally from one terminal of the battery (one type of metal) to the other (a different type of metal). This difference in charges produces voltage. Positive end (cathode) loses electrons Negative end (anode) gains electrons The reaction starts only after a conducting wire connects both terminals. Unconnected batteries still react, but very slowly. This means they have a shelf life. The speed of the reaction determines how many electrons will be transferred. Batteries wear out when the reaction slows or is impeded and cannot keep up with the demand of the electric device (power!)
What’s A Coulomb? A coulomb is a measure of charge consisting of a large group of electrons (6.24 x 10 18 electrons) One ampere contains one coulomb of charge passing by each second. 1 volt is one joule of energy per coulomb A 100 W bulb in a 120 V circuit needs A 100 W bulb needs 100 joules of energy per second. (power = work or energy/time)
Electric Shock 0.001 amps is equal to 1 milliamp! 5 milliamps 70 milliamps
Electric Shock For you to receive an electric shock… There must be a voltage difference between one part of your body and another. The current will pass along the path of least electrical resistance. Average wet skin… 1000 ohms Dry skin… 100,000 ohms Range 100 to 500,000 ohms Wet skin reduces your resistance to the flow of current. DANGER!
How to Prevent Electric Shock Increase the resistance to the flow of charged particles (electric current). Cut off the pathway for electric current. Re-route the pathway for electric current (lightning rods and grounding wires)
Ground Fault Current Interrupter The GFCI senses changes in current and will trip if the leakage current reaches 0.005 A (5 milliamps).
Lightning Rods Lightning rods leak off charges to prevent build up of charges. Lightning rod systems act as a conductor, directing the electrical energy away from the house to the ground.