Presentation on theme: "BALDWIN1 PHYSICS Mr. BALDWIN ELECTRICITYMonday, May 11, 2015 AIM: What cause charges to flow? DO NOW: Answer the following questions. 1.What is the name."— Presentation transcript:
BALDWIN1 PHYSICS Mr. BALDWIN ELECTRICITYMonday, May 11, 2015 AIM: What cause charges to flow? DO NOW: Answer the following questions. 1.What is the name given to a charged atom or molecule (one that gains or loses electrons)? 2.When an atom loses an electron, what happens to its net charge? 3.What is the name given to such atom that loses electrons? 4.When an atom gains an electron, what happens to its net charge? 5.What is the name given to such atom that gains electrons?
CHARGED PARTICLES 1.ION: When an atom or molecule gains or loses an electron, it becomes charged. 2.ANION: negatively charged ion When an atom gains electrons, its net charge is negative 3.CATION: positively charged ion When an atom loses electrons, its net charge is positive
The Electric Battery Volta discovered that electricity could be created if dissimilar metals (electrodes) were connected by a conductive solution called an electrolyte. simple electric cell This is a simple electric cell. A battery transforms chemical energy into electrical energy.
The Electric Battery Electrode that attracts anions is called an ANODE (+) Electrode that attracts cations is called a CATHODE (–)
Electric Current Electric current (I) is the rate of flow of positive charges through a conductor: qt where q is the charge and t is the time Unit of electric current: the ampere, A. 1 A = 1 C/s.
TEST YOURSELF Calculate the current where 10 coulombs of charge pass a point in 5 seconds. Calculate the current in a lightning bolt that delivers a charge of 35 coulombs to the ground in 1ms.
BALDWIN 7 PHYSICS Mr. BALDWIN ELECTRICITYMonday, May 11, 2015 AIM: How do we construct a simple circuit? DO NOW: A current of 5.0 A passes through the bulb in 20 µs. What is the quantity of charge flowing through the bulb? How long does it take a current of 5.0 mA to deliver 15 C of charge? HOME WORK – Prepare for Test on Friday http://phet.colorado.edu/en/simulation/circuit- construction-kit-dc http://phet.colorado.edu/en/simulation/circuit- construction-kit-dc
Simple Circuit A complete circuit is one where current can flow all the way around. Note that the schematic drawing doesn’t look much like the physical circuit!
Electric Current In order for current to flow, there must be a path from one battery terminal, through the circuit, to the other battery terminal AND a potential difference across the terminal Which one of these circuits will work? (c) THIS ONE
Electric Current By convention, current is defined as flowing from + to –. (High potential to low potential) Electrons actually flow in the opposite direction, but not all currents consist of electrons.
Did You Understand? 1.A 60 W bulb draws a current of 500mA. How much charge passes through the bulb in 20 s? 2.What is the average current produced by a lightning strike if a charge of 50 C is transferred in 2.0 ms?
Summarize the following terms Ion Anion Cation Anode Cathode Current Circuit
BALDWIN 14 PHYSICS Mr. BALDWIN ELECTRICITYMonday, May 11, 2015 AIM: How are the current, voltage and resistance related in a simple circuit? DO NOW: A current of 5.0 A passes through the bulb in 20 µs. If the charge of an electron is How many electrons flow through the bulb? HOME WORK – Text pg 546 # 29 -33.
Ohm’s Law: Resistance and Resistors Experimentally, it is found that the current in a wire is proportional to the potential difference between its ends:
Ohm’s Law resistance. The ratio of voltage to current in a circuit is called the resistance. Resistance is the hindrance to the flow of charge.
Ohm’s Law: Resistance and Resistors In many conductors, the resistance is independent of the voltage; this relationship is called Ohm’s law. nonohmic (b) Materials that do not follow Ohm’s law are called nonohmic. Unit of resistance: the ohm, Ω. 1 Ω = 1 V/A.
Resistors Standard resistors are manufactured for use in electric circuits; they are color-coded to indicate their value and precision.
Example1: You are given a resistor whose stripes are colored from left to right as brown, black, orange, gold. Find the resistance value. Step One: The gold stripe is on the right so go to Step Two. Step Two: The first stripe is brown which has a value of 1. The second stripe is black which has a value of 0. Therefore the first two digits of the resistance value are 10. Step Three: The third stripe is orange which means x 1,000. Step Four: The value of the resistance is found as 10 x 1000 = 10,000 ohms (10 kilohms = 10 kohms). The gold stripe means the actual value of the resistor mar vary by 5% meaning the actual value will be somewhere between 9,500 ohms and 10,500 ohms. (Since 5% of 10,000 = 0.05 x 10,000 = 500)
Example 2: You are given a resistor whose stripes are colored from left to right as orange, orange, brown, silver. Find the resistance value. Step One: The silver stripe is on the right so go to Step Two. Step Two: The first stripe is orange which has a value of 3. The second stripe is orange which has a value of 3. Therefore the first two digits of the resistance value are 33. Step Three: The third stripe is brown which means x 10. Step Four: The value of the resistance is found as 33 x 10 = 330 ohms. The silver stripe means the actual value of the resistor mar vary by 10% meaning the actual value will be between 297 ohms and 363 ohms. (Since 10% of 330 = 0.10 x 330 = 33)
Example3: You are given a resistor whose stripes are colored from left to right as blue, gray, red, gold. Find the resistance value. Step One: The gold stripe is on the right so go to Step Two. Step Two: The first stripe is blue which has a value of 6. The second stripe is gray which has a value of 8. Therefore the first two digits of the resistance value are 68. Step Three: The third stripe is red which means x 100. Step Four: The value of the resistance is found as 68 x 100 = 6800 ohms (6.8 kilohms = 6.8 kohms). The gold stripe means the actual value of the resistor mar vary by 5% meaning the actual value will be somewhere between 6,460 ohms and 7,140 ohms. (Since 5% of 6,800 = 0.05 x 6,800 = 340)
Some clarifications: Batteries maintain a (nearly) constant potential difference; the current varies. Resistance is a property of a material or device. Current is not a vector but it does have a direction. Current and charge do not get used up. Whatever charge goes in one end of a circuit comes out the other end.
BALDWIN 24 PHYSICS Mr. BALDWIN ELECTRICITY 5/11/2015 AIM: How do we determine the resistance of a conducting material? DO NOW: Go over exam. HOME WORK – Resitivity hand out
Electric Hazards 10 to 100 mA Even very small currents: 10 to 100 mA can be dangerous, disrupting the nervous system. Larger currents may also cause burns. SO…How do we control the amount of current?
Resistivity The resistance of a wire is directly proportional to its length and inversely proportional to its cross- sectional area: Where ρ, the resistivity, L is the length & A is the cross-sectional area of the material.
Resistivity & Temperature For any given material, the resistivity increases with temperature:
Practice L = 1609 m, A = πR 2 ρ= 1.7 x 10 -8 Ωm. First find the cross-sectional area: A = πr 2 = (3.14) [ (0.002117 m) / 2)] 2 = 3.519 x 10 -6 m 2 Now substitute into the above equation to determine the resistance. R = (1.7 x 10 -8 ohm m) (1609 m) / (3.519 x 10 -6 m 2 ) R = 7.8 (7.7709 ohm) Determine the resistance of a 1-mile length of 12- gauge copper wire. 1 mile = 1609 m; diameter of 12 gauge copper wire = 0.2117 cm
1.5 V30.50 Amp Battery Voltage (V) Total Resistance (Ω) Current (Amps) 3.0 V31 Amp 4.5 V31.5 Amp
BALDWIN 33 PHYSICS Mr. BALDWIN ELECTRICITY Monday, May 11, 2015 AIM: How do we measure the power in our homes? DO NOW: (Turn in) Determine the resistance of a 1-cm length of 12-gauge gold wire whose ρ= 2.44 x 10 -8 Ω·m and has a diameter d= 0.2117 cm HOME WORK – Handout – Power: #s 1-5
Electric Power Power, as you may recall, is the energy transformed by a device per unit time.(P=W/t) In electricity,
Electric Power The unit of power is the Watt, W. For Ohmic devices (V=IR), we can make the substitutions:
Electric Power What you pay for on your electric bill is not power, but energy – the power consumption multiplied by the time. We have been measuring energy in joules, but the electric company measures it in kilowatt-hours, kWh.
TRY IT The average amount of electrical energy used in 30 days is 270kW·h. The cost price for electricity is 19.2240¢/kW·h. How much will the bill be in a year?
Power in Household Circuits Fuses are one-use items – if they blow, the fuse is destroyed and must be replaced.
Alternating Current Current from a battery flows steadily in one direction (direct current, DC). Current from a power plant varies sinusoidally (alternating current, AC).
Superconductivity In general, resistivity decreases as temperature decreases. Some materials, however, have resistivity that falls abruptly to zero at a very low temperature, called the critical temperature, T C.
Superconductivity Experiments have shown that currents, once started, can flow through these materials for years without decreasing even without a potential difference. Critical temperatures are low; for many years no material was found to be superconducting above 23 K. More recently, novel materials have been found to be superconducting below 90 K, and work on higher temperature superconductors is continuing.
Electrical Conduction in the Human Nervous System The human nervous system depends on the flow of electric charge. The basic elements of the nervous system are cells called neurons. Neurons have a main cell body, small attachments called dendrites, and a long tail called the axon.
Electrical Conduction in the Human Nervous System Signals are received by the dendrites, propagated along the axon, and transmitted through a connection called a synapse.