1. GCSE Computer Science
SAMPLE PRESENTATIONS
(c) 2018

2. Video Compatibility (c) 2018 Please note:
YouTube videos sometimes cause Powerpoint to Crash
If this happens:
File, Options
Advanced
Scroll down to Display
Disable hardware graphics acceleration
Disable Slide Show hardware graphics acceleration
(c) 2018

3. Sample Presentations (c) 2018
These are SAMPLE slides only for evaluation purposes
The slides use topics from more than one exam board
(c) 2018

4. Adding binary numbers 0 + 0 = 0 1 + 0 = 1 0 + 1 = 1 1 + 1 = 1 0
= 1 1
Remember these two additions
(c) 2018

5. Add these binary numbers
_____________
1 + 0 = 1
= 1
= 10
= 1
= 11
1 + 1 = 10
= 10
0 + 1 = 1
1 is added to the answer
0 is added to the answer 1 1 1 1 1
1 is carried to the next column 1 1 1 1
(c) 2018

6. Denary multiplication
What is 5 x 10?
How did you do this?
What about 5 x 100?
What is the rule for multiplying by multiples of 10?
Remember – denary is BASE 10
5 x 10 = 5 x 101
5 x 100 = 5 x 102
5 x 1000 = 5 x 103
5 x = 5 x 104
1 zero
2 zeros
3 zeros
4 zeros
Show students that they add the number of zeros when multiplying by multiples of 10
Move on to show them that the number of zeros equates to the power of 10 and it is the power of 10 that says how many zeros should be added and how many places the denary number would shift
(c) 2018

7. Binary multiplication
What is 12 x 210?
How did you do this?
What about 12 x 410?
What about 12 x 810?
What is the rule for multiplying by multiples of 210?
Remember – binary is BASE 2
1 x 2 = 1 x 21
1 x 4 = 1 x 22
1 x 8 = 1 x 23
1 x 16 = 1 x 24
1 zero
2 zeros
3 zeros
4 zeros
Students should explore what happens when they multiply binary numbers by multiples of denary 2
Show how the relationship between the number of places moved and the zeros added is equivalent to the power of 2 being multiplied by
(c) 2018

8. Logical shift
Used to multiply or divide 8-bit unsigned binary numbers by powers of 2
Eg, multiply by 8
(in denary 5 * 8 = 40)
Shift the binary number 3 places (23) to the left
Add 3 zeros
(just like multiplying by one thousand (103 ) 27 26 25 24 23 22 21 20
128 64 32 16 8 4 2 1 1
Use this to show how binary digits are shifted to the left when multiplied, and how zeros are added to the gaps left on the right hand side (least significant bits)
Show how 8 is 2 to the power of 3 and that’s how it’s decided to move 3 places
(c) 2018

9. Arithmetic shift left (c) 2018 Used on signed binary numbers
Same process as logical shift left BUT the MSB is left in place
The number above (-22) will be multiplied by 4
Leave MSB in place
Shift 2 places to the left
Add zeros
Check: = = 88
sign 26 25 24 23 22 21 20
-128 64 32 16 8 4 2 1 1 1 1
Only needed for EdExcel
MSB = Most Significant Bit
(c) 2018

10. Understanding overflow
The year in a date is represented by 2 denary digits
What happened when the year changed from 1999 to 2000?
= ?
Discuss whether the answer should be 100 or 00.
If the answer is 100, what happens to the 1 in 100? Is it lost, or can 3 digits be stored? Or does it cause an error because the computer can’t cope with 3 digits?
(c) 2018

11. Convert denary to hexadecimal
188 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 32 48 64 80 96
112
128
144
160
176
192
208
224
240
256
Convert the denary value 188 to hexadecimal.
188 ÷ 16
= 11
remainder 12
11 in Hex is B
12 in Hex is C
Answer = B C
Now calculate the remainder
There are 11 lots of 16 in 176 and so there are also 11 lots of 16 in 188
There are 11 lots of 16
Find the highest number below 188 in the 16 times table
There are 12 lots of 1
188 – 176 = 12
Show how 188 fits between 176 and 192 and so there are 11 lots of 16 in 176
Show how we need to calculate the remainder to know how many 1s needed to be added to the set of 16s
(c) 2018

12. OR gate 2 inputs 1 output EITHER input can be on for output to be on
Show how the on switches (green) relate to 1 and off switches (red) equate to 0
Show how the output depends on the circuit
(c) 2018

13. OR gate Truth table
Q = A OR B
Q = A v B
INPUT
OUTPUT A B Q 1
(c) 2018

14. Creating a logic diagram (1)
Create a logic diagram for the Boolean expression:
Q = NOT ((A AND B) OR (B OR C))
Start with the inner brackets
You will need one gate for AND
and another gate for OR
B is the same input for both gates
AND OR
Show how to deal with inner brackets first
Emphasise that B is used in both so only needs to be written as an input once (but it doesn’t matter if it’s written down twice)
(c) 2018

15. Creating a logic diagram (2)
Now look at the next level of brackets:
Q = NOT ((A AND B) OR (B OR C))
Use an OR gate
The outputs of the previous gates can be used as inputs for the new gate
AND OR OR
(c) 2018

16. Creating a logic diagram (3)
Now deal with what is outside the brackets:
Q = NOT ((A AND B) OR (B OR C))
A NOT gate can be added to the output of the circuit so far
AND OR
NOT OR
(c) 2018

17. Star Topology – poor diagramc
How many marks do you think this diagram would get?
What improvements could be made?
C = Client / Computer
S = Switch / Server
Direction of Flow
Discuss the importance of including a legend and the direction of flow on diagrams
(c) 2018

18. Selection A question will have at least 2 options (c) 2018
Answer = YES
Answer = NO
Selection
A question will have at least 2 options
Usually YES or NO
But can be other answers
Think carefully about what happens after each option
Use the animation to show what happens when the answer to the question is Yes and what happens when the answer is No
(c) 2018

19. Trace tables – example (question 18b)
1 val = 43
2 arr = [3, 5, 13, 43, 655, 872]
3 left = 1
4 right = arr.length
5 while left != right
6 mid =(left + right) div 2
7 if val <= arr[mid] then
8 right = mid
9 else
10 Left = mid + 1
11 endif
12 endwhile  43 1 6  1 4 3 13  1 2 3 4 5 5
655 2 1 2 3 4 4 43 1 2 3 3 2 3 1 1 1 2 3
This is one method of dealing with trace tables.
The ticks are useful to show which lines have been dealt with when adding values to the trace table
When it comes to the iteration, the number for each iteration help to keep track. Similarly, showing the number of each iteration for each line in the trace table helps to see if any mistakes have been made.
This is good practise for an exam. 1 2 3 4
(c) 2018

20. Bubble sort (c) 2018 8 students stand in any order
We’re going to sort into height order
Look at first student
Are they taller than next student?
Yes: swap (bubble moves up the list)
No: stay in place
Repeat for other students from second to last
What happens to the tallest person during the first pass?
Repeat from start until there are no swaps
What happens to the second tallest person during the second pass?
(c) 2018

21. (c) 2018

22. Show the steps in a bubble sort
Show the list after each PASS
Pass 1: T R H B F T R H B F
Show how the first pass works:
T and R swap because R comes before T
T and H swap because H comes before T
T and B swap because B comes before T
T and F swap because F comes before T then T slots into place at the end of the list – highest value has ‘bubbled’ to the top
(c) 2018

23. Show the steps in a bubble sort
Show the list after each PASS
Pass 2: T R H B F R H B F T
Show how the second pass works:
T is already in place
R and H swap because H comes before R
R and B swap because B comes before R
R and F swap because F comes before R then R slots into place – the second highest value has ‘bubbled’ towards the end of the list
(c) 2018

24. Show the steps in a bubble sort
Show the list after each PASS
Pass 3: T R H B F H B F R T
Show the 3rd pass:
H and B swap because B comes before H
H and F swap because F comes before H – H slots into place and has ‘bubbled’ towards the top of the list
4th pass:
Nothing happens because B comes before F and so no action required.
(c) 2018

25. Video Source: www.tiny.cc/InsertionSort
Play to 1:49
(c) 2018

26. Looking for programming errors
The algorithm below should calculate the smallest score from a list of scores.
min_score = score[0]
for counter = 1 to score.length
if score[counter] > min_score then
minscore = score[counter]
endfor
next counter
Identify the syntax errors.
Identify the logic errors.
Find syntax error(s) first
eg spelling = syntax
Find logic errors next
eg <, >, <=, >=
- 1
See if students can find the errors.
Encourage them to find syntax errors first as they are often easier to find
Then move onto the logic errors.
(c) 2018

27. Spot the mistake – character set
Letters, numbers and symbols
Characters on a keyboard
Characters used by people
Characters used by a computer
Number of characters available
Characters used for coding
Symbols a computer understands
Set of symbols used by the computer
The character é is part of a computer's character set. Describe [define] what is meant by a character set. [1]  
This slide is designed to encourage students to be specific in an exam and ensure they use the correct terminology
Discuss why each is right or wrong 

28. Sound terminology (c) 2018 OCR uses some non-standard terminology:
Define each of these terms.
OCR’s Term
Computing Term
Definition
Sample Size
Bit depth
Bit Rate
Bit rate
Sampling frequency
Sample rate
There is a similar slide for AQA
(c) 2018

29. Sound terminology - definitions
OCR’s non-standard terminology you need to learn:
OCR’s Term
Computing Term
Definition
Sample Size
Bit depth
Number of bits used for each sample
Bit Rate
Bit rate
Number of bits used per second
Sampling frequency
Sample rate
Number of samples per second
(c) 2018

30. Sound file size
A music file has a sample size of 8 bits. The sampling frequency is 10 kHz. The sound lasts for 40 seconds and has 2 channels. Calculate the file size in bytes.
file size (bytes) = sampling frequency x sample size x channels x seconds ÷ 8
file size (bytes) = 10 x 1000 x 8 x 2 x 40 ÷ 8
remove [ x 8 ÷ 8 ]
multiply 10 x 1000 = 10,000
file size (bytes) = 10,000 x 2 x 40
file size (bytes) = 10,000 x 80 = 800,000 bytes or 800 kB
(c) 2018

31. Calculating sound file size
Calculate the size in bits of the following sound files:
A 10 second telephone call recorded with a sampling rate of 8,000 Hz and a sample resolution (bit depth) of 8.
A 10 second music file that has a sampling rate of 10 kHz and a sample resolution (bit depth)of 16
A 3 minute 20 second voice recording that has a sampling rate of 10,000 Hz and a sample resolution (bit depth)of 8.
Extension: calculate each of the above in bytes and then calculate in kB or MB
(c) 2018

32. define: abstraction
hiding
unnecessary
detail
(c) 2018

33. What’s wrong?
Identify a suitable storage device for transferring homework files to and from school.
Answer:
a USB
a USB stick
Hard disk drive
Flash memory stick
Discuss what is wrong with each of these answers:
USB is just a connection method
USB stick could be a WiFi dongle or Bluetooth dongle or wireless keyboard dongle
Hard disk drive would need to be external hard disk drive in an exam 
(c) 2018

34. Spot the mistake – giving reasons
It’s secure
It’s more secure
It’s more secure than HTTP
Sensitive medical data will be encrypted
So he knows he is visiting the correct site
To improve the page ranking in a search engine
So hackers can’t see his password
Charlie uses HTTPS to book an appointment with his doctor. Give two reasons why he uses HTTPS instead of HTTP. [2]   

35. Even parity 1 1 (c) 2018 The total number of on (1) bits must be even1
The total number of on (1) bits must be even
There are currently four on bits (even number)
An 8th bit is added as zero
to make the total number of on bits (4) even
It is then received as
there are an odd number (3) of on bits
even parity check has spotted an error 1
(c) 2018

36. Watch
Watch this supermarket conveyor belt analogy of run length encoding on YouTube. (
Read out the words and press pause as required
(c) 2018

37. Run length encoding – supermarket analogy
Source:
(c) 2018

38. Choosing your impact themes
ultural C
egal L
thical E
nvironmental
rivacy P
takeholders S
(c) 2018

39. Structuring your essay
Positive impact
Negative impact
Technology
Environment
Stakeholder
Ethics
Conclusion
Take each impact area in turn
Explain positive impacts and/or
Explain negative impacts
Repeat for each impact area
Conclude
(c) 2018