Page 133 - Learn To Program With Scratch

This application has no sprites,
but when the green flag is clicked, the
script shown in Figure 5-25, which
belongs to the Stage, is executed.
This script takes care of the math
for us and displays the results in the
readouts on the Stage. Note that while
the slider controls for resistors R2 and
R3 can change from 0 to 10, the mini-
mum value for R1 was intentionally set
to 1. This ensures that Rtot is always
greater than 0 and lets us avoid divid-
ing by 0 when calculating the value of Figure 5-25: Script that runs when the
the current. green flag is clicked
Most of the work for this applica-
tion went into designing the interface
(that is, the background of the Stage). After that, all we had to do was to
position the displays and sliders at the right locations on the Stage.

try it out 5-4
SeriesCircuit Open the series circuit simulator application and run it . Experiment with different
WithSwitch .sb2 values of R1, R2, R3, and V . Watch the calculated values of V1, V2, and V3 as
you drag the slider controls . What is the relationship between the voltage sum
(V1 + V2 + V3) and the battery voltage? What does this tell you about the voltage
relation in series circuits? You can make an interesting enhancement to the appli-
cation by adding an image of a switch that opens or closes the circuit, as shown
below . When the switch is open, no current will flow in the circuit . Try to imple-
ment this change using the hints given below .

Script for the
Switch sprite
Change how the main script calculates the current (I).

Same as before
Switch sprite has
two costumes
(On, Off).
Switch if Off. Set current to 0.

Visualizing a Sphere’s Volume and Surface Area
Sphere .sb2 Our third example is an interactive application for calculating the volume
and surface area of a sphere. The user changes the sphere’s diameter by
clicking some buttons on the user interface, and the application automati-
cally calculates and displays the corresponding volume and surface area.

Variables 111

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