← IGCSE Computer Science Paper 1

Capacitive touch screen

A touch screen with a charged glass layer. A finger conducts electricity and changes the charge at the touch point, so its position is found. Supports multi-touch.

Part of: IGCSE Computer Science 0478 — 3.2 Input and Output Devices — Module 12

Understanding Capacitive touch screen in IGCSE Computer Science Paper 1

What it does: The code shows the capacitive sequence: the finger touches, the charge at that point changes, and the position is worked out. The screen senses the change in charge caused by the conductive finger. Heat has nothing to do with it. Capacitive screens support multi-touch, so they can detect several fingers at once, for example for a pinch gesture. Detection depends on the conductivity of a finger changing the charge, so plastic styluses and ordinary gloves do not work.

Common mistakes: A capacitive touch screen uses a charged glass layer, not a plastic film. The line should be layer <- 'charged glass'. A plastic stylus does not conduct electricity, so it cannot change the charge on a capacitive screen. works should be FALSE.

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Learning
Duo
What does a capacitive touch screen detect when you touch it?
The pressure of two plastic layers being pushed togetherA change in the electrical charge of the glass layer at the touch point
Matches
Capacitive screens work with any object, such as a plastic stylus
↓
Capacitive screens need a conductive object such as a finger. Plastic does not conduct electricity, so it cannot change the charge.
Applying
Meanings
Term
capacitive touch screen
Meaning
A touch screen with a charged glass layer. A finger conducts electricity and changes the charge at the touch point, so its position is found. Supports multi-touch.
Practice
IF finger touches glass THEN
Testing
Gaps
IF finger = TRUE chargeChanges ← TRUE
Check Answer
Spots
1layer <- 'plastic film'
2finger <- TRUE

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Quick Questions

1. What does a capacitive touch screen detect when you touch it?
A. The pressure of two plastic layers being pushed together
B. A change in the electrical charge of the glass layer at the touch point ✓
A capacitive screen has a charged glass layer. Your finger conducts electricity and changes the charge where you touch. Pressing layers together is how a resistive screen works.
2. Why does a bare finger work on a capacitive touch screen?
A. A finger conducts electricity and disturbs the screen's electric field ✓
B. A finger is warm enough to melt the top layer of the screen
The screen senses the change in charge caused by the conductive finger. Heat has nothing to do with it.
3. A student taps a capacitive screen with an ordinary woolly glove. What usually happens?
A. Nothing, because the glove does not conduct electricity ✓
B. It works perfectly, because the screen senses pressure
Ordinary gloves are insulators, so they do not change the screen's charge. Capacitive screens do not work on pressure.
4. Which tool would NOT normally work on a capacitive touch screen?
A. A bare finger
B. An ordinary plastic stylus ✓
A plastic stylus does not conduct electricity, so it cannot change the charge at the touch point. A finger can.
5. Which gesture can a capacitive touch screen support?
A. Pinching two fingers to zoom (multi-touch) ✓
B. Only one touch point at a time
Capacitive screens support multi-touch, so they can detect several fingers at once, for example for a pinch gesture.

Match the Pairs

capacitive touch screen → A touch screen with a charged glass layer. A finger conducts electricity and changes the charge at the touch point, so its position is found.
multi-touch → The screen can detect two or more fingers at the same time, which allows gestures such as pinching to zoom in or out.
A finger conducts electricity, so it disturbs the screen's electric field at the touch point and the position is found → finger touch -> charge changes -> position found
Ordinary gloves and a plastic stylus do not conduct electricity, so a capacitive screen does not respond to them → glove or plastic stylus -> no charge change -> no touch
Capacitive screens work with any object, such as a plastic stylus → Capacitive screens need a conductive object such as a finger. Plastic does not conduct electricity, so it cannot change the charge.
A capacitive screen finds the touch point when two layers are pressed together → This describes a resistive screen. A capacitive screen finds the touch point from the change in charge caused by a conductive finger.

Key Term

capacitive touch screen
A touch screen with a charged glass layer. A finger conducts electricity and changes the charge at the touch point, so its position is found. Supports multi-touch.

Fill in the Blanks

1. IF finger = TRUE THEN chargeChanges ← TRUE
Hint: Type the keyword that introduces the action to run when the condition is true.
2. IF glove = "ordinary" THEN touch ← FALSE
Hint: Type the Boolean value showing that an ordinary glove does not conduct, so no touch is detected.
3. IF touchCount > 1 THEN multiTouch ← TRUE
Hint: Type the comparison operator that means more than one finger is touching the screen.
4. change ← chargeBefore - chargeAfter
Hint: Type the variable that holds the charge measured after the finger touches the glass.
5. OUTPUT "Pinch to zoom uses multi-touch"
Hint: Type the pseudocode keyword that displays a message to the user.

Spot the Error

Question 1
A layer <- 'plastic film'
B finger <- TRUE
C IF finger THEN
D PRINT 'Charge changes'
E ENDIF
Line A — A capacitive touch screen uses a charged glass layer, not a plastic film. The line should be layer <- 'charged glass'.
Question 2
A object <- 'plastic stylus'
B works <- TRUE
C PRINT works
Line B — A plastic stylus does not conduct electricity, so it cannot change the charge on a capacitive screen. works should be FALSE.
Question 3
A glove <- 'ordinary'
B IF glove = 'ordinary' THEN
C PRINT 'Touch detected'
D ENDIF
Line C — Ordinary gloves are insulators, so they do not change the charge at the touch point. The touch is not detected.
Question 4
A device <- 'smartphone'
B screen <- 'resistive'
C PRINT device, screen
Line B — Smartphones and tablets use capacitive touch screens. The line should be screen <- 'capacitive'.
Question 5
A finger_touch <- TRUE
B IF finger_touch = TRUE
C PRINT 'Find touch position'
D ENDIF
Line B — The IF statement is missing THEN. It should be IF finger_touch = TRUE THEN.

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