7/30/2019 Electric Circuits Analogy

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Much research has taken place into students understanding ofsimple electric circuits. This research indicates that in manycases students use mental models of current flow which act asa hindrance to correct understanding of the electric circuits.

Some common student problems are,

(1) Lack of differentiation between electric current, energy,power and voltage.

(2) Most children initially approach their studies with asource-consumer view of electricity. They believe that thecurrent leaves the battery through a single wire towhatever device is in the circuit where it is consumed.

(3) Consumption of current. Students often believe that thecurrent returning to the battery is less than the currentleaving it. As the current passes through components in acircuit, each one will receive less than the one before. Thisis a very persistent model and one that is familiar to allPhysics teachers. The idea of consumption of current is anattractive idea for many students as conservation ofcurrent appears to be at variance to the fact that the batterymust eventually become empty.

(4) Use of sequential reasoning. The student analyses thecircuit in terms of before and after each component.Increasing the resistance of a variable resistor beforea bulb will decrease the current and consequently thebrightness of the bulb. Some students believe thatadjusting a variable resistor situated after a bulb will haveno effect on its brightness.

The research indicates that it is not easy to modify studentsideas. An analogy which is based on energy rather thancurrent may prove successful in overcoming studentsmisconceptions.

An Analogy for Current FlowThe following analogy for current flow has the advantage thatit differentiates clearly between energy and current. It offersthe student an explanation of why a battery must eventuallyget flat while at the same time demonstrating the concept ofconservation of current. In this analogy a domestic heatingsystem is compared with a basic electric circuit.

A domestic heating circuit consisting of a boiler, pump,

radiator and connecting pipes, is first described. The basicoperation of the system is explained to the students, i.e. theboiler heats the water and the pump pushes it through thesystem. The first task is to convince the students that the rateof flow of water is the same at every point in the circuit. Thiscan be done by introducing the idea of meters into thediagram which measure the flow rate in millilitres per second(ml/s) after some discussion students will have very littledifficulty in accepting that if the flow rate at point A is say 10ml/s then the flow rate at B will have to be the same (seeFigure 1.) It can be pointed out that if the flow rate is less at B,this can only mean a leak in the system and a should there bea greater flow rate at B then where can the extra water have

come from? By using similar arguments, the students can beconvinced that the flow rate is the same at all points in thecircuit.

AN ENERGY BASED ANALOGY FOR ELECTRIC CIRCUITSby Kevin P. McCarthy, Scoil Phobail Sliabh Luachra, Rathmore, Co. Kerry

Figure 1. Basic domestic heating circuit.

Figure 2. Basic electric circuit.

Students will intuitively know that something is lost by the water asit moves through the radiator. This is where the idea of energy beingcarried around the circuit can be introduced. The water before theradiator is carrying more energy/heat than the water leaving. Theenergy lost by the water is taken in by the radiator and transferredto the room. Similarities can be drawn with the idea that the currentcarries electrical energy in an electric circuit. When the currentpasses through a bulb (or other component) some of this energy isconverted into other forms such as heat and light. This will meanthat the current returning to the battery is carrying less electricalenergy than the current leaving. This will fit into the studentsknowledge that the battery will eventually get flat while alsopreserving the idea of conservation of current.

The main ideas are summarised in the table below.

WATER CIRCUIT ELECTRIC CIRCUIT

Boiler gives heat energy towater and pump pushes itaround circuit.

Water flows in only onedirection around circuit.

Rate of flow same at all points.

Energy (heat) lost by wateris gained by radiator.

The analogy can be developed further by introducing the idea thatthe electric current consists of a flow of electrons moving around thecircuit. These carry the energy and distribute it to the load.

Battery gives electrical energyto current and pushes itaround circuit.

Current flows in one directiononly around circuit.

Size of current same at allpoints.

Electrical energy is convertedto heat & light by bulb.