Models for teaching electric circuits Physics Teachers Conference 18 February 2011 Christina Hart.

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Models for teaching electric circuits Physics Teachers Conference 18 February 2011 Christina Hart

Making a globe light Connect the battery and globe to make the globe light; use connecting wires if you need to or want to. Find connections that: do make the globe light? do not make the globe light? Draw diagrams of one connection : where the globe does light, where it does not. Explain why the globe does light with some connections but not with others.

Making connections Why does the globe have to be connected in a particular way for it to light? How are connections made from the globe holder? How the connections made in a household light globe and holder? Illustrate your answers with diagrams. filament With a hand lens look at the globe Identify the filament.

Electric charge and electrical energy Understanding how electric circuits work depends on understanding the difference between: electrical energy electric charge These two terms are confounded in everyday language: we talk about ‘ electricity We cannot experience electrical energy directly: only electrically charged particles can have electrical energy.

Electric charge and electrical energy Electric charge and electrical energy are different. A charged particle (electron, proton, ion) may carry electrical energy. But a charged particle does not have electrical energy just because it is charged. Separating electrons from the atoms they ‘ belong ’ to requires energy. The energy is transferred to the separated charges: they then have electrical energy.

Photo: Pym Prescott - Canberra January 27 2006: flickr.com/photos/ppym1/92003809/ Reproduced under Creative Commons Licence: http://creativecommons.org/licenses/by-nc/2.0/deed.en Attribution-Noncommercial 2.0 Generic lightning aplet

Placing the switch in a circuit Set up a simple circuit. Predict Predict: Where might you place a switch to turn the globe on and off. Mark on the diagram Write down you reason for placing it at that point. Observe Observe: Connect the switch into to the circuit at the point you marked. Does changing the position of the switch make any difference to the effect of turning it on or off? Note: the different positions you try anything that surprises you

+ - Direction of electron flow Direction of conventional current I Electron flow and conventional current In most circuits the current is due to the movement of (negatively charged) electrons Electrons move from the negative terminal of the energy source to the positive terminal. Conventional current is the equivalent flow of positive charges, from the positive terminal of the energy source to the negative terminal.

Model 1 The role of the battery Conventional current Why a complete circuit is needed A break at any point in the circuit will stop the flow of charges Applet that illustrates the flow of electrons: http://www.mste.uiuc.edu/users/Murphy/Hol eFlow/default.html http://www.mste.uiuc.edu/users/Murphy/Hol eFlow/default.html

Circuits and diagrams Experiment with connecting 2 globes, battery and switch in different arrangements. For each arrangement: draw the circuit diagram; ask your teacher to check your circuit and diagram; carefully note the relative brightness of the globes and which globes are affected by the switch; make a note of any questions you have, or puzzling observations you notice. Other similar activities in Brainbox kit

Electricity questions How come, the longer the circuit or the more things you put in it, the light comes on instantly? Speed of electricity – how long/big could a circuit be? Why, when the circuit has more stuff in it, does the light fade on? Does the number of wires matter? Can you make circuits that ring bells/buzzers rather than a light? Why is the globe furthest from the battery brighter? Shouldn ’ t the one nearer the battery be brighter?

Electricity questions How does a battery work? What is the role of the positive and negative in batteries? Why does a battery have a positive and a negative side, what role to positive and negative play? Why doesn ’ t the battery operate when two batteries are placed together at the flat ends? We know positive and negative have to alternate, but unsure what the exact reason is. What happens when you recharge a battery? Why are there no more power black outs any more?

Electricity questions How does fruit conduct electricity? Does any other food conduct electricity? Is that why when you eat fruit/food you get energy?

Electricity questions Why does this stay on until we press the switch, then it goes off?

Model 2 Current and resistance Why the globe gets hot Resistance at one point in the circuit causes a decrease in current right round the circuit

Conductors and insulators

About practical work At CONASTA in 1982) 1 discussed a lesson which involved a class of 12 year old children dealing with a simple battery-bulb circuit (Osborne, 1982). The circuit was drawn on the board (Figure 1) and the pupils were asked to construct it. They were then required to use the circuit to identify which common materials appeared to "conduct an electric current". Osborne, R. (1984). “ Children's science meets scientists' science. ” Lab Talk 28(1): 2-7.

About practical work Figure 2 showed the circuit that was finally constructed by two of the children, despite considerable teacher help. What did the students find out when they connected different materials in their circuit? Osborne, R. (1984). "Children's science meets scientists' science." Lab Talk 28(1): 2-7.

About practical work Figure 2 showed the circuit that was finally constructed by two of the children, despite considerable teacher help. Osborne, R. (1984). "Children's science meets scientists' science." Lab Talk 28(1): 2-7. They were very happy with their circuit because in their view it was important that the light glowed every time the switch was closed. They found all the things they tested were conductors.

About practical work However, they seemed to have no problem in ignoring their conclusions when the teacher's end-of-lesson conclusions were written up on the board: most metals unless coated with some non-conducting material will conduct electricity, e.g. gold, aluminium, copper. most plastic based products, e.g. foam, cellotape, plastic bags, will not conduct electricity. products from nature, e.g. leaves, wood, fruit pips, will generally conduct electricity. These statements were happily copied by the pupils into their books. The pupils did not appear to be concerned that the teacher's statement bore no relationship to their views and experiences; indeed they did not even seem to be aware that this should be so. Osborne, R. (1984). "Children's science meets scientists' science." Lab Talk 28(1): 2-7.

About practical work did What lesson did these students learn?

Investigating a hand-held torch Dismantle a battery operated hand held torch. Identify the closed path followed by the electrons when the torch is operating. How does the switch operate? Make a diagrammatic drawing of the main parts of the torch. Show the closed circuit with a solid coloured line. Draw the corresponding circuit diagram. Label the main components of the torch on the circuit diagram: A, B, C etc. Mark the corresponding components on the diagrammatic drawing of the torch with the same letters.

Two globe circuits

Model 3 Resistance and current in two globe circuits

Model 4 Energy in a simple circuit Electrical charge vs electrical energy Brightness of the globe Other analogies

Model 5 Energy in two globe circuits Energy distribution Current distribution Brightness of the globes Energy in circuits with two batteries

Model 5 (cont) globes with different resistance

Electricity questions How come, the longer the circuit or the more things you put in it, the light comes on instantly? Speed of electricity – how long/big could a circuit be? Why, when the circuit has more stuff in it, does the light fade on? Does the number of wires matter? Can you make circuits that ring bells/buzzers rather than a light? Why is the globe furthest from the battery brighter? Shouldn ’ t the one nearer the battery be brighter?

Electricity questions How does a battery work? What is the role of the positive and negative in batteries? Why does a battery have a positive and a negative side, what role to positive and negative play? Why doesn ’ t the battery operate when two batteries are placed together at the flat ends? We know positive and negative have to alternate, but unsure what the exact reason is. What happens when you recharge a battery? Why are there no more power black outs any more?

Electricity questions How does fruit conduct electricity? Does any other food conduct electricity? Is that why when you eat fruit/food you get energy?

The difficult bits! Why does this stay on until we press the switch, then it goes off?

The difficult bits! How do electrons ‘ know ’ what is in the circuit?

Model 6 Alternative models for the energy carried by electrical charges dominoes bicycle chain rope circuit

The difficult bits! How do electrons ‘ know ’ what is in the circuit alternative models: bicycle chain rope circuit Alternating current Power supply, earth return and electrical safety

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