Required practical activity 4: use circuit diagrams to construct…

The idea

In this required practical you plot characteristics for three components: a resistor at constant temperature, a filament lamp and a diode. Build the same test circuit each time, with the component, an ammeter and a variable resistor in series and a voltmeter across the component, then adjust the variable resistor to take about ten pairs of readings. Reverse the connections to the supply to get negative values, which is essential for the diode, where the reverse readings are all 0.0 A. Plot current against potential difference for each component. A diode needs a protective resistor in series so the forward current cannot damage it, and the resistor must be given time to cool so its temperature stays constant.

What earns the marks

• A circuit with the component, ammeter and variable resistor in series and the voltmeter in parallel.
• A method of varying the potential difference across the component.
• Reversing the supply connections to obtain readings in the reverse direction.
• Plotting current against potential difference and describing the shape obtained.
• A precaution such as a protective resistor with a diode or allowing the resistor to cool.

Common mistakes and how to put them right

• Taking only a few readings, so the curve for the lamp cannot be seen.

• Leaving the current flowing continuously, which heats the resistor and destroys the constant temperature condition.

• Omitting the reverse readings, so the diode graph looks like a resistor graph.

• Connecting the diode without a series resistor and drawing too large a forward current.

Exam tips

• Describe one circuit and then say it is used for each component in turn; the examiner does not want three methods.
• Mention taking readings in both directions, since it is where the diode marks sit.
• Say how many readings and over what range, because a method without a range is incomplete.