Working Scientifically : Graphs

Working Scientifically : Graphs

Teachers Notes This sequence of slides is designed to introduce, and explain, the idea of Graphs in practical work, as explained on pages 363-364, 393 in Physics for You, 5th edition. Note : When you start this PowerPoint if you see a message about Read-only embedded fonts then you are recommended to select Open Read-Only as this (i) gives a clearer font for those at the back of the room and (ii) ensures that the text-highlighting of key words is correct. On each slide the key points are revealed step by step, at the click of your mouse (or the press of a key such as the space-bar). Before making the next mouse-click you can ask questions of the class or make statements about what is about to be revealed. This should help students to become clearer about the ideas involved.

Naturally it pays to have quick practice-run first. To start the slide-show, press function-key F5 (or right-click->Full Screen) (to return to normal view press the key). For more (free) PowerPoint presentations, visit www.physics4u.co.uk Working Scientifically: Graphs Physics for You, 5th edition, pages 363-364, 393 Learning Objectives

You should learn : About different types of graphs, How to draw them when you are doing your practical work, How to interpret the different shapes. Drawing a graph When should and when

I drawI draw a should a bar-chart ? ? line-graph Drawing a graph Look at the table of your results: If this column has only certain fixed values,

use a bar-chart: a continuous range of values, use a line-graph: Drawing a graph What is the best way to draw a line-graph? 5 steps in drawing a graph 1. Choose simple scales. For example:

1 large square = 1 newton (1 N) or 1 large square = 2 N, or 5 N, or 10 N But never choose an awkward scale, like 1 square = 3 N or 7 N Choose a scale that will make your graph use most of the sheet of paper. 5 steps in drawing a graph 1. Choose simple scales. Put the dependent

variable on the y-axis and the independent variable on the xaxis 5 steps in drawing a graph 2. Plot the points neatly. To mark the points we usually use an X x x x Usually you

need 5 or more points for the graph. x x x Re-check each one before your next step.

5 steps in drawing a graph 3. If the points form a straight line draw the best straight line through them li n f o e x

s be t fi t x x x

x x Check that it looks the best straight line. 5 steps in drawing a graph 4. If the points form a curve draw a free-hand curve of best fit Do not join the points like a dot-todot.

5 steps in drawing a graph 5. If a point is not on the line use your apparatus to check this measurement again x x x This is called an anomalous point.

x x x You can decide to ignore anomalous points. 5 steps in drawing a graph In summary: 1. Choose good scales, with the dependent variable on the yaxis 2. Plot the points carefully 3. Draw a line of best fit using a ruler for a straight line graph,

4. or draw free-hand for a curved graph 5. Check anomalous points. Types of graphs Lets look at some examples of graphs Types of graphs 1 length length

A straight line graph: weigh t An example would be the length of a spring against the weight on it. weight

Types of graphs 2 A special case is when the straight line goes through the origin : origin In this case the two quantities are

directly proportional. If one doubles, then the other one also doubles. See page 392. Types of graphs 2 If you think your graph should go through the origin, then draw it exactly through

the origin. Types of graphs the extension of a spring against the weight on it. extension Example 1:

2 weight Types of graphs the current in a resistor against the p.d. across it. current

Example 2: 2 voltag e This illustrates Ohms Law. Types of graphs 3

A curved graph, rising : The dependent variable rises quickly at first Here are some examples: and then more slowly

Types of graphs the velocity of a falling object against the time. velocity Example 1: 3

time Eventually the object will reach its terminal velocity. Types of graphs the current in a filament lamp against the p.d. current Example 2:

3 voltag e Types of graphs 4 A curved graph, falling : The dependent

variable falls quickly at first and then more slowly Here are some examples: Types of graphs the activity of a radioactive source against the time.

activity Example 1: 4 tim e The time to fall to half is called the halflife. acceleration

Types of graphs Example 2: 4 the rate of change is shown by the gradient of the graph. tim e

This is discussed in the next PowerPoint. Learning Outcomes You should now: Know how to draw a line-graph correctly, Be able to give examples of graphs with different shapes, Be able to interpret graphs with different shapes. For more details, see:

Physics for You, 5th edition, page 364, 393 For more free PowerPoints, visit the web-site at www.physics4u.co.uk If you are connected to the web at the moment, click below to see whats available: http://www.physics4u.co.uk/

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