Activity 1.5 - Electric element in a dc circuit

• Understanding that the potential difference across the conductor causes a current through it
• Exploring the relationship between the current flowing through a conductor and a potential difference across it.
• Interpreting the current – voltage diagram of a conductor
• Understanding the concept of resistance
• Investigating the behaviour of different electric elements in a direct electric circuit
• Interpreting the current – voltage diagram of different electric elements

Variable power source (up to 10V), leads, resistors of different values of resistance (e.g. 20 Ohms or higher), bulb (e.g. 6V/0,05A), other electric elements (e.g. semiconducting diode) limiting resistor to be used in a circuit with diode, computer, interface and software (e.g. CMA Coach6), current sensor, voltage sensor (if computer with interface is not available, ammeter and voltmeter can be used)

Divide the class into small groups of 2-3 and hand out Classroom Material: Electric element in a dc circuit. In this activity students carry out an experiment in order to investigate the relationship between the current flowing through a resistor and the voltage across it and to understand the concept of resistance. We expect students to be confident enough in measuring current and voltage with the help of sensors. Firstly, they are introduced the concepts of resistor and resistance – as a physical quantity defined as . During the investigation students discover that this relationship is linear and they learn to understand the current-voltage diagram and the concept of resistance. The important point of this investigation is to connect the real experiment with its graphical representation and to develop the ability to grasp the required information from the graph. Students learn to understand the physical meaning of diagram features (linear, non-linear, slope of the line) and so that to interpret the diagram correctly. In the next step they investigate the behaviour of other electric components in the direct electric circuit, such like bulbs and diodes and compare it with the behaviour of a resistor. In the activity students follow the instructions in the worksheet in a guided- discovery way. In order to enhance conceptual understanding of Ohm’s law, the activity can be followed up by the additional activity carried out in the inverse sequence. The components are hidden in a “black box” and on the basis of their current- voltage relationship measurement students reveal the black box content. This activity is carried out as a bounded inquiry when students having the problem to solve and materials available design the experiment in order to find the problem solution. The black boxes can contain a resistor, a bulb, a diode and a thermistor so that the students decide about the component that behaves differently than the other ones. Let them plan the measuring procedure without significant help. In this activity there are wide opportunities to connect this knowledge with Industry.

This can involve the application of resistors and other electric elements and electronic components, here are several examples:

• Standard applications of resistors in electric circuits in various devices. When using variable resistor, with the changing current the brightness of the lamp can be changed. In some electric model race car sets squeezing the trigger the variable resistor changes its value and hence the current through the car motor can increase so the car speeds up.
• The use of electric resistance in a wider context, e.g. in archaeology for electrical resistance survey, when metal probes are inserted into the ground to obtain a reading of the local electric resistance. Soil resistivity testing is used to find potential excavation sites. Scientists use meters to find and map out man-made areas beneath the surface. Roads and building foundations tend to be dry and compacted, producing high soil resistivity. Covered ditches and trenches have high moisture content and readily conduct electricity.
• Light-dependent resistors decrease in resistance with more light. They are widely used in light-night to switch the light on at night. Thermistors decrease in resistance when their temperature rises.
• Diodes gave basis for the wide development of electronics. Light-emitting diodes (LED) give out light when a current passes through.

LEDs hardly ever fail, and are used instead of bulbs. Teaching this topic gives a good opportunity to visit a place where electronic devices (such like TVsets, radios, computers, mobile phones, etc.) are dismantled into electronic components and separated according to their possible reuse. When dealing with electronic waste a question of their influence on the environment should be taken into account and may be discussed.

• What does the voltage applied across a circuit element cause to happen?
• Do different elements (resistor, bulb, diode) behave the same way in a dc circuit?
• How is the current flowing through an element influenced by the voltage applied across it? Does the current increase, decrease or stay constant when applying increasing voltage? What is the mathematical relationship between current and voltage (I-U relationship) for a circuit element?
• How could you distinguish between several elements knowing their I-U diagram?