Investigating Non-Ohmic Resistors Learning Outcome Set up the circuits in order to calculate or directly measure the resistance of variable resistors. To be able to plot a graph of the results.

Current-Potential Graph for a filament bulb Use your apparatus from last lesson. This time you are going to use your rheostat as a variable resistor to vary the current through a filament bulb. But you will measure the voltage across the bulb this time as the current varies.

Control the pd/current using the variable resistor to give you even values easy to plot on a graph. Current (A) P.D 1 (V)P.D 2 (V)P.D 3 (V)P.D Average (V)

Now replace an LED for the bulb and repeat the investigation. Record and plot your results. You will need a multimeter (milliamps) Turn the diode, now what do you notice. Next replace the LED with a Thermistor, but repeat the investigation at two temperatures of water. You will need a multimeter (milliamps) Record and plot your results. Next replace the Thermistor with an LDR, but repeat the investigation at two different light intensities (Cover with different thicknesses of tracing paper.) You will need a multimeter (milliamps) Record and plot your results.

Multimeter as Ammeter Set to milliamps. Remember all readings will by

Extension activities To extend more able pupils, the following investigations have been included and link with AS level investigations. Set multimeter as Ohmmeter.

Physics Practical project The electronic thermometer An ordinary thermometer with a liquid inside a thin tube works because the liquid EXPANDS when it is heated. The length of the thread of liquid is used to tell the temperature.

The electronic thermometer The thermometer has to be CALIBRATED first. What does that mean? When it is made, the tube with liquid in has no numbers on it so what do the manufacturers do? Which temperatures do we know that have well-known values?

The electronic thermometer The freezing point of pure water is called 0 0 C so they put the blank tube into melting ice and mark where the liquid falls to. The boiling point of pure water is called C so they place the blank tube at the surface of boiling water and mark where the liquid rises to. They then divide the space in between into 100 equal gaps.

The electronic thermometer The length between the two points at 100 and 0 is divided into 100 equal intervals and these are marked on the glass. You can then measure any temperature between the two.

The electronic thermometer A glass tube can be easily broken and mercury could leak out so there is a need for other kinds of thermometer, especially in automatic electronic devices. e.g. A washing machine should only start working when the water is at the correct temperature. How will the electronic controls know that the water is hot enough?

The electronic thermometer A greenhouse should automatically open its windows if the atmosphere inside becomes too warm. How will the electronic controls know if the air is too warm? Answer: by using an electronic thermometer. But what is one of those? What will change with temperature?

The electronic thermometer There is a COMPONENT whose RESISTANCE changes with temperature. It is called a THERMISTOR – the ‘therm’ part suggests heat. A multimeter can be used to measure its resistance which will be measured over a range of temperatures.

The electronic thermometer To investigate the thermistor, you need a circuit with a multimeter connected in series with a thermistor. The multimeter is set to measure RESISTANCE. The dial in the middle is turned 8 clicks anti-clockwise until it points to the 20kΩ setting. (see picture below) Then connect the mounted thermistor to the bottom right two connecting sockets on the multimeter. The reading should be between 4.0 and 6.0kΩ depending on the temperature of the room. (kΩ means kilo-ohms)

The electronic thermometer To CALIBRATE the thermistor, you must expose it to a range of temperatures that you can measure using a mercury thermometer. The thermistor is surrounded by HOT water in a small beaker. The resistance will show a much LOWER value than at room temperature. A thermometer is placed in the water. As the water cools, the resistance rises and results should be recorded at regular intervals of temperature.

Temperature/ 0 CResistance/ kΩ Temperature/ 0 C Resistance/kΩ Record results as the water cools. Plot the points on graph paper. Join the points with a smooth CURVE.

To test your calibrated electronic thermometer, move your thermistor and meter to another beaker of water which your teacher will have. Allow it to read the resistance. Use your graph to give the temperature. Tell the teacher your answer!

Variable Resistors Copy the graph of current – potential for a resistor (figure 3 p. 155) and label it. Define what an ohmic conductor is, how does resistance vary with the direction of current? Answer Qs 1 and 2 p. 155 in full in your book. More Current-Potential Graphs p Draw how the current and potential difference varies through a filament bulb (Fig 1). Use the information to describe why the graph is this shape. Use R = V/I and the graph to answer question a. Draw how the current and potential difference varies through a diode (Fig 2). Use the information to describe why the graph is this shape. Answer question b Draw how the current and potential difference varies through a Thermistor and LDR (Figs 4 & 5). Use the information to describe why the graph is this shape. Answer Questions c & d Complete the summary questions on page 157 in full.