Handbook of Practical Physics - Tripod



Handbook of Practical Physics

The objective of the GCE ‘O’ level Practical Physics (5054/3) are to test candidates on:

a. The ability to follow instructions.

b. The ability to carry out techniques, use apparatus, handle measuring devices and materials effectively and safely.

c. The ability to interpret, evaluate and report upon observations and experimental data. (e.g. plot and use a graph to interpolate, extrapolate or find gradient of a straight line.)

d. The ability to identify problems, plan and carry out investigations, including the selection of techniques, apparatus, measuring devices and materials.

e. The ability to evaluate methods and suggest possible improvements.

Instructions to students

A. Each student should possess the following:

a. a long transparent ruler.

b. A flexicurve.

c. A silent calculator.

d. A geometrical set.

e. Sharp pencils and an eraser.

Display of data

Each student is advised to:

a. Present data concisely and precisely by tabulating them neatly, showing all observations which have been taken.

b. Heading tables using the given symbols and given units. If units are not suggested, use the most convenient symbols.

c. All physical quantities must have a unit.

d. Take at least six sets of readings by repeating the procedure, unless otherwise stated.

e. Plan your readings so that they are uniformly spread out within the given limits. In any case you must the maximum range.

Accuracy of reading

Great care must be taken to given the correct numbers of decimal places for reading taken from instruments.

|Instruments |Readings recorded to the nearest |

|Metre rule |0.001m, 0.1cm, 1mm |

|Vernier Calipers |0.01cm, 0.1 mm |

|Micrometer screw guage |0.001cm, 0.01mm |

|Measuring cylinder |0.5cm3 if scales divisions are 1cm3 apart |

|Protractor |0.5( |

|Stopwatch (Analogue) |0.1s |

|(Digital) |0.01s |

|Dial - O - gram |0.01g |

|Electronic balance |0.1g |

|Ammeter |0.01A |

|Voltmeter |0.01V or 0.05V |

|Thermometer |0.5(C |

Note:

a. All measured quantities using the same instruments should have the same number of decimal places.

b. As a guide, all reading taken from instruments should be accurate up the smallest division on the instrument.

c. Generally all calculated values should not have more than 3 significant figures.

d. In experiments involving oscillations, at least 20 oscillations should be taken, unless specified otherwise, and the timing should be repeated. An average timing should then be taken from these two sets of readings and the average reading should be kept to 1 decimal place when using the digital stopwatch. This is to correct for the reaction time.

Graph plotting

The best way to obtain the average of a set of readings is to plot a graph. Question 4 in paper 3 requires the plotting of a graph.

When plotting a graph, always remember to:

a. Label the axis with the correct quantities and their units (if any). The axes should be drawn with a sharp pencil.

b. Choose a convenient scale so that the graph will occupy more than half the area of the space available. If a scale is suggested, follow it. If not, the scale used should be an even scale (that is scales based on multiples of two or five) and should read to at least 3 significant figures on the graph paper.

c. Each point should be clearly and neatly marked with a small cross using a sharp pencil. The size of a cross should be smaller that the size of one square grid of the graph paper. All points should be clearly and accurately plotted on the grid of the graph paper.

d. Draw the graph as a single, fine straight pencil line or smooth curve, through the best mean position defined by the points. (Note: if a point is not beyond one square grid of the graph paper from the graph, it is still considered to be on the graph.)

Note: Average mark awarded to a graph is about 5. The 5 marks are allocated as follows:

i. A - Fully labelled axes and title of graph

ii. S - Appropriate scales used and graph occupies more than half of the area of the space provided. No odd scales.

iii. P- all points accurately plotted.

iv. C1 – the best smooth line or curve should be drawn through at least 80% of the points plotted.

v. C2 – right type of line or curve is obtained.

B. Calculation of gradient (straight line graph only)

a. A large triangle (more than half the graph plotted) must be chosen and clearly indicated on the graph to work out the gradient. Co-ordinates of the gradient must be indicated.

b. The method of calculating the gradient must be shown clearly.

c. If possible, give the unit to the final answer of the gradient.

Evidence on the graph

Evident on how a reading is obtained from a graph must be shown:

E.g. 1: the triangle for the calculation of gradient with the co-ordinates indicated.

E.g. 2: indicate all values read from the graph on the vertical and horizontal axis.

Some common precautions in Physics experiments

Mechanics

1. Pendulum

a. Angle of oscillation should be small, not more than 10(.

b. The first few oscillations should be ignored so that only steady oscillations are timed.

c. Reject readings when oscillations become elliptical.

d. The length of the pendulum must be measured from the point of suspension to the centre of the bob.

e. Disturbance due to wind can be avoided by switching off the fans and closing the windows.

2. Hydrometer

a. The test-tube (or straw, boiling tube, etc) often drifts towards the side of the beaker with the result that the level of the liquid in the beaker is difficult to read. The drift could be prevented by constructing a paper lid to the beaker with a hole in the centre to hold the test-tube.

3. General

a. Parallax error:

i. when using a measuring cylinder parallax error can be avoided by viewing the reading at the same level as the liquid.

ii. When reading meters such as voltmeter, ammeter etc., parallax errors can be avoided by viewing the reading vertically above the pointer.

b. The meter rule should be set vertical with the help of a plumbline.

Light

a. The two pins used to locate the path of light ray must be placed as far as possible (preferably more than 5cm) to minimise errors..

b. The pins must be vertically upright.

Heat

a. the electrical heater must not be switched on until the heating coil is immersed in the liquid, otherwise it could be burnt out.

b. The calorimeter should be covered with the lid to reduce heat loss from its contents to the surroundings (which are normally at a lower temperature) by conduction and convection.

c. The calorimeter should be placed in a shiny jacket lined with good insulating material (e.g. cotton wool) to minimise heat loss by radiation and conduction.

d. The water or liquid in the container (e.g. a calorimeter) must be stirred continuously during the experiment so that the temperature is uniform throughout.

Electricity

a. It is very easy to be misled by a false point of balance due to poor contact between the jockey and the bridge wire. So when a suppose point of balance is found, the jockey should be moved slightly to the right and left of the point to see whether the pointer of the galvanometer deflect in opposite directions. In locating the null point using a centred-zero galvanometer. Jockeys should be held vertically to obtain more precise readings. (Do not press too hard on the wire, otherwise kinks will form and resistance of wire will change.)

b. When not taking any reading, the switch should be left open to avoid heating the circuit elements like resistors.

c. Make sure there are no kinks on resistance wires used.

d. Make sure all connections are properly secured.

e. Make sure all ammeters and voltmeters are properly connected. Positive terminals of the meters should be connected the positive terminals of the battery and the negative terminals to the negative terminals of the battery.

Magneticism

a. If magnetic field is involved, any magnetised material or material that can be magnetised should be placed far away so that the original magnetic field would not be disturbed.

Notes:

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