Current - potential difference graphs

Objectives:

• Be able to build a circuit to vary, and measure, the voltage and current through a component.

• Know that the resistance of a bulb increases with temperature.

• Be able to describe the relationship between voltage and current for a bulb and a resistor.

Task 1 - Starter

Remind the person next to you:

          • The units of current

          • The units of resistance

          • The units of voltage

          • The definition of an Ampere

          • How voltage, current and resistance are connected

Task 2

Your teacher will demonstrate how to build a circuit with a variable resistor to measure the voltage across a bulb and the current flowing through it.  Watch carefully - you will need to do this in a minute!

Task 3

Follow the instructions on this powerpoint presentation to measure the voltage and current for both a 3.5Vbulb and a 10Ω resistor.  Record your results neatly in a table.

Task 4

1.) Plot a graph of current against voltage for both components.

• plot both sets of data on the same graph

• use a sheet of A4 graph paper in a landscape orientation

• make sure current is on the y-axis and voltage is on the x-axis

2.) Which component has a higher current when the voltage is 0.5V?  Which component has the higher resistance?  Explain your answer!

3.) Which component has a higher current when the voltage is 2.5V? Which component has the higher resistance now? Once again, explain your answer!

3.) The trend line for the resistor is straight, showing that the resistance does not change.  What happens to the resistance of the bulb as the voltage is increased? How does your graph show this?

4.) At what voltage do the resistance of the bulb and the resistor become equal? Explain how you were able to tell.

Teacher note: the finished graph should look like this.

Task 5

You will notice that the resistance of the bulb increased as the current got higher.  Listen carefully as your teacher explains why this happened, by telling you a story...

How is this analogous to the current flowing through a bulb?  Share your ideas with the person next to you.  Your teacher may ask some of you to share your ideas with the class.

Now stick in this sheet, which has a copy of the diagram above.  Show the path of the man in each of the scenarios described.  Remember to show the trees (atoms!) shaking during the earthquake for the second diagram.

Homework

You can use Ohm’s Law (V = I × R) to calculate the resistance of the bulb at various points on the graph. Use this principle to answer these questions:

1.) What is the resistance of the bulb when:

          a) The voltage is 2.5V

          b) The voltage is 5V

          c) The current is 0.15A

2.) A bulb has a resistance of 12.5Ω. What current will flow through the bulb when a voltage of 20V is applied?

3.) Another bulb is rated as having a resistance 12Ω when the current is 0.6A. What voltage is required to produce this current?

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