V Current Electricity Introductory Physics Canadian AcademyGroup Members: Kaiya Yamada
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 electricity circuit-construction-kit-dc • Define resistance and state the factors that affect resistance in a metal wire • Define potential difference (voltage) Your finished work should be • Explain the effect of potential difference uploaded to SlideShare (or and resistance on a current Google Docs if it works) and • Draw basic circuit diagrams involving batteries, lamps, switches and wires embedded into a blog post. • 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 Build this circuit. Define current electricity. • a flow of electric charge through a medium. Current electricity is the movement of electrons through materials and is therefore a transfer of charge. It can be contrasted with static conventional current electricity, which can be said to be electricity held by a material with electron flow electric charge. A Label the direction of flow of electrons and the direction of the conventional current. What is the difference between them? They are opposite in direction. The conventional current direction is from plus to minus around the circuit. It is indicated by the green arrow. Electrons flow the other way. They are negatively charged, so they are repelled by negative charge and pushed out of the negative terminal. It is indicated by the red arrow. 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. • If the cell is not included in the circuit, the circuit will not work because there is no source for electricity. Without electricity, the bulb would not light and the ammeter would indicate 0 amps.
Switches and current Build this circuit. Measure the current with the switch in the open position. • 0 amps Close the switch and measure the current. Explain your answer. • 0.9 amps • This happens because when the current is in the open position, the circuit is broken and the electricity can not A flow. However, when the switch is closed, the circuit is also closed so the electricity can flow through and light the bulb and change the reading on the ammeter to 0.9 amps. Move the ammeter to different positions in the circuit and measure the current. Does position matter? • No, it does not. Charge is not destroyed so the current strength will be the same anywhere. Are electrons ‘created’ in the cell? Electrons are not created in the cell. An imbalance in the electrons in the cell caused by chemical reactions in the battery make the electrons flow Are electrons ‘used up’ in the circuit? around the circuit back to the cell. The chemical reactions in the cell make the existing electrons move towards one side of the cell which is the anode. No, they are not. They keep flowing Because of the imbalance between one side of the battery having a heavy until they arrive back at the source. concentration of electrons and the other side lacking this, the electrons try to spread out and diffuse the concentration. However, the battery prevents The energy of the electrons is used up the electrons from going directly from the anode to the cathode. This and and changed into heat and light. The the repulsion force between each electron causes the electrons to flow around the circuit back to the cell. electrons return to the cell.
Potential Difference (voltage) Modify the circuit to increase the potential difference by including two, then three, cells. What happens to the bulb? • The bulb’s brightness increases as we add more bulbs. Too many cells cause the bulb to burst. Measure the current and potential difference. A Cells Current (A) Potential Difference (V) 1 0.9 9.00 2 1.8 18.00 3 2.7 27.00 Define potential difference (voltage). • the difference in electric potential energy per unit charge V between two points. Explain your results. • Each time I increased a battery, the current increased by Complete the circuit 0.9 amps and and the voltage increased by 9 volts. That diagram for three cells. means that each battery has 0.9 amps and 9.00 volts worth of power. Basically what we see is that when we double the current, we double the energy difference.
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/filamentresistance/ Explain in your own words how moving charges cause a bulb to glow. What energy transfers and transformations are taking place? When a bulb is connected to a battery, first, potential chemical energy is converted to electrical energy, and current flows from one contact to another through wires and a filament. As electrons move along the filament, they bump into atoms that make up the filament. There is a transfer of energy. The energy of each impact vibrates the atoms. Basically the current heats the atoms up. Here, electrical energy is transferred into heat energy. If the filament is thin or made of a certain material, it will heat up more easily because it is more resistant to the flow of electrons. The electrons in the vibrating atoms can be boosted to a higher energy level. When they return to their normal level, they release energy in the form of photons. Here, electrical energy is transferred into light energy. Explain what has happened when a cell (battery) has run out. When a battery runs out, all the chemicals that mix to cause the imbalance in the electrons run out. This means that the imbalance of electrons will not be created anymore. Therefore, no more current will flow from the battery unless it is recharged (if it is rechargeable)
Resistance (incandescent bulbs or lamps) Build this circuit. Add bulbs and record your observations. Bulbs Current (A) Observations 1 1.8 The bulbs seem to getting 2 0.9 dimmer and dimmer as I A increase the number of 3 0.6 bulbs. The number of amps 4 0.45 is decreasing with each 5 0.36 reading as well. Explain your findings in detail. The current flowing through the circuit with just one bulb is 1.8 amps. The current flowing through the circuit with two bulbs is 0.9 amps. The current flowing through the circuit with three bulbs is 0.6 amps. From these results, we can see that the number of amps is originally 1.8 amps and from there it becomes one half, one third, etc. This means that the numbers of amps flowing through this circuit is 1.8 amps divided by the number of bulbs. Remove all the bulbs to create a circuit and measure the current. Explain. The number of amps flowing through the circuit drastically increases to 12260.87 amps and the light bulbs catch on fire. Because the current electricity is not being used up anywhere as there is no load, the charge keeps building up showing the high amp count. Heat builds up in the circuit and can cause a fire.
Conclusions Use your findings in the circuits so far to write your own Laws. Law of current in a loop. “This is just a statement of energy conservation. As a current flows through resistors in a circuit, potential drops. The sum of the potential drops is the same as the supplied potential.” Law of voltage and current. (what’s the relationship?) “The relationship between the two values is direct. As voltage increases, current increases at the same rate. E(voltage)=I(current)R(resistance) “ Law of resistance and current. (what’s the relationship?) “The relationship is indirect. As the resistance is increased, the current speed decreases. As the resistance is decreased, current speed increases. The equation is resistance = potential difference/current. “ Why is it dangerous to have too little resistance in a circuit? When there is too little resistance in a circuit, the charge or current builds up without being used. This buildup causes the circuit to heat up. This can cause a short circuit, which can lead to fires.
Parallel Circuits How many different routes can current take through this circuit? • The current can take two different routes if the switches are closed. Close the lower switch only. Observe the bulb and measure the current. • The lower bulb lights up very brightly and the ammeter shows that the current is 1.80 amps. The upper bulb does not light up and the upper ammeter shows the current is 0 amps. Close the upper switch only. Observe the bulb and measure the current. • The upper light bulb shines very brightly and the upper ammeter shows the current is 1.80 amps. The lower bulb does not light up and the lower ammeter shows the current Draw a circuit diagram for is 0 amps. this set-up below: Close both switches. Observe the bulbs and measure the current at different positions. • Both light bulbs light up. They are just as bright as the bulbs in the cases above. Each ammeter shows the current is 1.8 amps. The power supply fixes the potential difference, so each component has the same potential difference across it. Observe the animation carefully. What happens to the electrons at junctions? • The group of electrons split up in half. Half of them go one way and the other half goes the other. The one half goes through the lower branch and the other half of the electrons travel through the upper branch. At the other end, the electrons come back together. Because of the idea of conservation of charge, we can understand that the total current flowing out of a junction is the same as the total current flowing in.
Law of Parallel Circuits Write your own Law, based on observations. Law of parallel circuits. “The parallel circuit has more than one path for current to flow. The same voltage is applied in each branch. If load resistance in each branch is the same, the current in each branch is the same. “ 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. The number of amps flowing through each part of the circuit is still the same: 1.8 amps. The brightness of each light bulb has not changed either.
More Parallel Circuits Close one switch at a time and record your observations. • By closing the lower switch, the bottom circuit is completed. The bottom circuit has two light bulbs so the bulbs only glow half as bright as with just one bulb. Also, the number of amps that is shown is half of the previous 1.80 amps which is 9.00 amps. • By closing the upper switch, the upper circuit is completed. The upper circuit has one light bulb so the bulbs glow twice as bright as the bulbs in the lower circuit. The upper ammeter also had an amp count twice that of what was shown on the lower ammeter: 1.80 amps. Close both switches and record your observations. • The same observations can be made as those made above. They are just happening at the same time. Draw a circuit diagram for Carefully observe the junctions. What is happening? Explain with reference to resistance and junctions. this set-up below: • Because there are more light bulbs in the lower circuit, the resistance is twice as great as the resistance in the upper circuit. Therefore, the number of amps in the lower circuit is half that of the upper circuit and the electrons are travelling a lot slower(approximately half the speed) through the lower circuit than they are through the upper circuit. This also means that for every electron that goes through the lower branch with two light bulbs, two electrons travel upwards through the upper branch. This also means that for every electron that leaves the lower branch to go back into the main circuit stream, two electrons leave the upper branch back into the main electric circuit stream back towards the cell. We also, of course, have to understand that the sum of the current leaving each junction is the same as the sum of the current entering each junction. Do you need to modify your Law of Parallel Circuits? Yes, I do.
Law of Parallel Circuits Modify your Law. Law of parallel circuits. “The parallel circuit has more than one path for current to flow. The same voltage is applied in each branch. If load resistance in each branch is the same, the current in each branch is the same. If the load resistance in each branch is different, the current in each branch will be different. Also, if one branch is broken, current will continue to flow through the other branch. “
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 electrons are moving a lot more slowly through the light bulb with the higher resistance. The number of particles interfering with the electrons’ passage is greater in the higher resistance light bulb. 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: In this parallel circuit, the lower branch had a light bulb with a resistance of 20 ohms and the upper bulb had a resistance of 10 ohms. The bulb with the lower resistance shone muchIn this series circuit, the brighter and the ammeterammeter showed that the showed that its currentcurrent was 0.6 amps. The which was 1.80 amps wasbulbs were a bit dim, but they twice that of the lower branch which was 9.00 amps.were the same brightness.
Electrical Power Define electrical power and state its unit. • The rate at which electric energy is converted to other forms of energy such as heat, light, and kinetic energy. The unit is the watt. What is the relationship between electrical power and ‘power’ as we have studied in the previous unit? • In the previous unit, we defined power as the rate at which work is done. One equation for power was Power=work/time. With electrical power, the concept is basically the same. Electrical power is the rate at which electrical energy is supplied to a circuit or consumed by a load. Equations for it would be Power=work done on charge/time or Power= energy consumed by load/time. Compare two methods of generating electrical power: one fossil-fuel based and one renewable. How do they work? What are the benefits/ disadvantages of each? Coal is a fossil fuel that is burned to boil water, which causes steam, which turns turbines. The spinning motion of the turbines causes the electromagnets to generate electricity. This electricity is stored and sent out. There is a limit on this, however, because coal is a fossil fuel and there are only so many fossil fuels in the world. It is a non-renewable energy source. Wind power is a renewable power source and it works a bit differently. Wind power uses turbines as well. The wind turns the turbines and produces electricity. Advantages/disadvantages: Advantages of coal: Coal has a very high energy density (a great deal of energy is released from a small mass of coal). It is relatively easy to transport and cheap compared to other energy sources. It can be used directly in the home to provide heating. Disadvantages of coal: Coal produces pollution and acid rain. Coal also gives off green house gases. The extraction of coal can damage the environment. Coal is non-renewable. Coal-fired power stations need a large amount of fuel. Advantages of wind energy: It is a cleaner energy source(no harmful chemical by-products), renewable, and free (of course the turbines are not free, but the wind is). Disadvantages of wind energy: It has low energy density(a large area would be needed for a great amount of energy). It is unreliable as sometimes the wind does not blow. Wind turbines can also be very noisy and expensive to produce. They need a large amount of land.
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. I did check the laws I wrote and they do concur with published information. I reread Kirchhoff’s first and second law. I learned them in grade 7, but it was a good review. I have studied circuits a lot as I am interested in electricity. I used to love doing experiments with static electricity and balloons and using it to bend water and make pictures. I like how it is used to paint cars and in photocopying. DC is direct current. It flows in only one direction. AC is alternating current. It constantly changes direction, going first one way and then the other. I don’t have time to build and draw circuits, but I did many at my old school. I liked using all the symbols for fuses, circuit breakers, meters, etc.
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