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Variables

• The variable you change is called the independent variable
• The variable you measure is called the dependent variable
• All other variables that affect the dependent variable should be kept the same (controlled variables) to ensure you have a fair test
• E.G
• Conduct and experiment to investigate if the surface color of a beaker affects how quickly it loses heat after being filled with hot water.
• Independent variable = surface color of the water
• Dependent variable = temperature of water (after a fixed time)
• Control variable = initial temperature of water, volume of water, beaker material

Repeats and Precision

• Usually we have to take 6 experiments to eliminate the chance of any enormities
• Taking repeats allow us to check the reliability of the experiments.
• Always exclude the enormity
• Precision is to how many decimal places you have read an instrument
• Precise reading are taken with instruments that have small divisions
• Use the significant figures for the least precise measurement given as the number you give in your final answer.
• E.g
• Andrew's time in seconds : 0.44 0.46 0.44 0.48 0.43
• state the precision of Andrew's readings
• A=0.01

Accuracy

• Accuracy tells us how close our measurements are to the true values. We can compare our measurements with the true values found in a data table or on the internet.
• Ways to improve accuracy depend on the experiment but may include
• 1. Using more sensitive equipment
• 2. Avoiding parallax by placing object closer to scale/taking reading at eye level
• 3. Check zero
• 4. In a measuring cylinder, read bottom of meniscus
• 5. When measuring a very small thickness of a piece of paper, measure the thickness of a large number and divide by the number
• 6. Use large values so the percentage uncertainty is lower

Graphs

• Normally we will plot the independent variable on the horizontal axis of the graph and the dependent variable on the vertical axis of the graph
• Make sure you graph fills up at least half of the space you are given to draw a graph
• Only ever draw a line of best fit which is a straight line or smooth curve; never join up the points if you only have plotted an anomalous result make sure you ignore it when plotting a best fit line
• When plotting points, use a sharp pencil and make a neat cross. You will lose marks if you plot any points incorrectly

Interpreting Graphs

• You need to be able to describe in words the relationships show by the graphs. How does the dependent variable depend on the independent variable?
• Comment on whether the line is straight or whether it goes through the origin. If it does both we say there is a directly proportional relationship.
• If the graph is a curve comment on how the rate is changing as well as whether the dependent variable increases or decreases with the independent variable
• You may need to explain the Physics causing the relationship e.g why does the current through a light bulb filament increase with the voltage but at a decreasing rate ?

Planning an Experiment

• You need to be able to write an experimental plan. The most common experiments assessed on the IGCSE exams are:
• Measuring the speed of sound
• Measuring the density of a regular solid
• Hooke's law for a spring/metal wire/rubber band
• Measuring speed using a stopwatch/ticker timer and trundle wheel
• Investigating current/resistance of a wire as length/temperature of a wire changed
• Measuring refractive index/critical angle of a glass block
• Measuring the half life of a radioactive source

General Tips and Equations

• Say what you are measuring (dependent variable)
• Say what you are changing (independent variable)
• Describe how you will make the measurements and the apparatus used to take each measurement
• Describe what variables you are going to keep constant (control variables)
• State what measurements you are going to take i.e values; how many and the range
• State Whether you will repeat the measurements and why this is useful to do
• Speed = distance divided by time
• Weight = mass times gravitational field strength
• Work done = force times distance
• charge = current times time