Page 131 - Learn To Program With Scratch

The slider for the battery voltage (V) has a range of [0, 10], and the
slider for the resistor (R) has a range of [1, 10]. When the user changes V
or R with the sliders, the application calculates the corresponding value of
the current (I) that flows in the circuit. The brightness of the bulb changes
in proportion to the value of the current passing through it: The higher the
current, the brighter the light bulb. The sizes of the V, I, and R letters in the
figure also change to indicate the relative values of these quantities.
In total, the application has five sprites
(named Volt, Current, Resistance, Equal, and
Light) and three variables (named V, I, and
R). Everything else you see in Figure 5-21
(the battery, wires, socket, and so on) is part
of the Stage’s backdrop image. The main
script that drives the application, which
belongs to the Stage, is shown Figure 5-22.
The script initializes the values of V and
R and then enters an infinite loop. On each
pass through the loop, it calculates I using Figure 5-22: Main script of
Ohm’s law application
the present values of V and R, which are set
by the user through the slider controls. It
then broadcasts a message to the other sprites
in the application to update their appearance in relation to the calculated
values. Fig ure 5-23 shows the response of the Volt, Current, Resistance, and
Light sprites (which show the letters V, I, R, and the light bulb, respectively)
when they receive the Update message.

Script for the Volt sprite Script for the Current sprite

(which shows the letter V) (which shows the letter I)

Script for the Resistance sprite Script for the Light sprite
(which shows the letter R) (which shows the light bulb)
Figure 5-23: Scripts triggered in response to the Update message

When the Update broadcast is received, the Volt, Current, and Resistance
sprites change their size (from 100 percent to 200 percent of their original
size) in relation to the current values of their respective variables. The Light
sprite executes the set ghost effect to command to change its transpar-
ency level in proportion to the value of I. This gives the light bulb a realistic
visual effect that simulates an actual bulb.

Variables 109

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