A Current Electricity Introductory Physics Canadian AcademyGroup Members: Christina and YJ
Is this plausible? Could you really power a house with static electricity? http://www.youtube.com/watch?v=scUosAkxlvo
Current Electricity Construct and explain. Work in pairs or by yourself for these tasks. With each question: • Build it in the PhET simulation • Build it in the lab if possible • Draw the circuit diagram and answer the questions on the slide. You should be able to:http://phet.colorado.edu/en/simulation/ • Define current electricitycircuit-construction-kit-dc • Define resistance and state the factors that affect resistance in a metal wire • Define potential difference (voltage) • Explain the effect of potential difference and resistance on a current • Draw basic circuit diagrams involving batteries, lamps, switches and wires • Define electrical power including the relationship to voltage and current
Some basic circuit symbolsYou can use these to build the circuits on the next slides. wire cell battery junction + - + - cathode anode electron flow bulb/ lamp resistor conventional current What do these two components measure? switch V A voltmeter ammeter
A simple series circuit e− Build this circuit. Define current electricity. • Flow of electrons Label the direction of flow of electrons and the A direction of the conventional current. What is the difference between them? Use the non-contact ammeter to measure the current in the circuit. • 0.90 Amps What happens if the cell is not included in the circuit? Explain. • There is no current electricity.
Switches and current Build this circuit. Measure the current with the switch in the open position. • 0.90 Amps Close the switch and measure the current. Explain A your answer. • 0.00 Amps. There is no current electricity because the switch was off. Move the ammeter to different positions in the circuit and measure the current. Does position matter? Are electrons ‘used up’ in the circuit? • Position does not matter. • No, electrons are circulating inside Are electrons ‘created’ in the cell? the circuit. • No, electrons are not created in the cell. Instead, transfers negative electrons.
Potential Difference (voltage) Modify the circuit to increase the potential difference by including two, then three, cells. What happens to the bulb? A • The light becomes lighter/stronger. What happens to the current measurements? 1 cell: 0.90 Amps 2 cells: 1.80 Amps 3 cells: 2.70 Amps Explain your answers. • The more cells, the more negative electrons transferred. As a result, the current flow is rapid. Define potential difference (voltage). • The voltage difference between any two points, which makes the current flow, in a circuit
Resistance (incandescent bulbs or lamps) Go to the following applet and see resistance at a molecular level and how a light bulb works. http://micro.magnet.fsu.edu/electromag/java/filamentr esistance/ Explain in your own words how moving charges cause a bulb to glow. The greater the current, the greater the collisions between the electrons and the molecules, therefore, more transfer of energy, which converts into heat and light energy.
Resistance (incandescent bulbs or lamps) Build this circuit. Add bulbs and record your observations. Bulbs (A) Observations 1 1.80 Very bright A 2 0.90 Bright 3 0.60 Normal 4 0.45 Almost dark 5 0.36 Very dark Explain your findings in detail. •As the number of light bulbs increased, the brightness of the bulbs decreased. In other words, the Amperes decreased. Remove all the bulbs to create a short circuit and measure the current if you can. Explain. •As the circuit becomes shorter, Amperes increases. As a result, the circuit catches on fire.
Conclusions Use your findings in the circuits so far to write your own Laws. Law of current in a loop. “ Electrons flow from negative to positive when powered by a battery and also when it is fully connected.“ Law of voltage and current. (what’s the relationship?) “ The brightness of the light bulb becomes brighter as there are more batteries. Also, the current becomes faster. “ Law of resistance and current. (what’s the relationship?) “The voltage of a circuit decreases as the number of light bulbs increase. Also, the current becomes slower when there are more bulbs. “ Why is it dangerous to have too little resistance in a circuit? THE BATTERIES CAUGHT ON FIRE!
Parallel Circuits How many different routes can current take through this circuit? • Two routes? Close the lower switch only. Observe the bulb and measure the current. • 1.80 Amps except for the second row. Close the upper switch only. Observe the bulb and measure the current. • 1.80 Amps except for the first row. Draw a circuit diagram for this set-up below: Close both switches. Observe the bulbs and measure the current at different positions. • No current (0 Amps) Observe the animation carefully. What happens to the electrons at junctions? • The electrons move to towards the positive side of the batteries. The electrons transferred from the batteries move separate into two routes, towards the upper and lower bulbs.
Law of Parallel Circuits Write your own Law, based on observations. Law of parallel circuits. “ The total Amps is the sum of both parallel circuits. “ Now test your Law using a third bulb in parallel. Draw the circuit diagram below and write your observations of the bulbs and of current. When you add another light bulb in parallel with the others, the total Amps increase because there are three routes.
More Parallel Circuits Close one switch at a time and record your observations. • The current is flowing through the middle section when the lower switch is closed (0.90 Amps). When the other switch is closed, it is a normal current, 1.80 Amps. Close both switches and record your observations. Draw a circuit diagram for • No current this set-up below: Carefully observe the junctions. What is happening? Explain with reference to resistance and junctions. • The middle section has slower current (0.90 Amps) because there are two resistance while there is a normal current (1.80 Amps) in the upper section. Also, the lower section has 2.70 Amps. Do you need to modify your Law of Parallel Circuits? Yes
Law of Parallel Circuits Modify your Law. Law of parallel circuits. “ The sum voltage is equal to zero. Electrons flow most through the path of least resistance. “
More Resistance Use two cells and two bulbs in a circuit. Use CTRL-click to adjust the resistance of the bulbs (one is 20 ohms (Ω), the other is 10Ω). What is the difference between these two bulbs on a molecular level? • The one with the greater resistance is dimmer. Wire up the bulbs in two different circuits: series and parallel. Draw the circuits below. Under each circuit, record and explain your observations. Series: Parallel: When the bulbs are in series, the bulbs When the bulbs are parallel, the battery share the same current (more resistance). provides double current for parallel circuits. Therefore, the bulbs get dim. Therefore, the bulbs that are parallel become brighter because they receive double the current.
Electrical Power Define electrical power and state its unit. • The rate at which energy is converted in an electrical circuit. What is the relationship between electrical power and ‘power’ as we have studied in the previous unit? • Electrical power is the power done in an electrical circuit. • Work – flow of electrons Benefits Disadvantages Wind Wheel -Never run out -Hard to generate the same quantity of electricity -No pollution as fossil fuels -Can only generate electricity on windy days Coal -Can generate large quantities of electricity -Will run out eventually -Causes global warming A coal-fired power station burns coal to heat up water, which is converted into steam. The steam turns the turbine, which turns a generator, that generates electricity. When a wind blows, it turns the wind turbine, which produces electricity.
Extension If you finish with extra time: • Check the Laws you have written against published information. Do they concur? • Find out more about circuits and their components. • Find out about the difference between AC and DC. • Build your own circuits and draw the circuit diagrams below.
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