Eduqas Physics for A Level Year 2: Study and Rev Guide

■ use graphs with error bars to nd the uncertainty in the gradient and intercept ■ combine the uncertainties in values to estimate the uncertainties in a calculated value. The techniques for these skills are covered in the AS Study and Revision Guide, Sections 3.5 and 3.6. P.1.3 Conclusions and evaluation One of the rst tasks following graph plotting is to consider whether it is consistent with the given relationship. This involves judging whether the points lie on a good straight line and possibly whether it passes through the origin. If you have plotted error bars, these involve deciding whether it is possible to draw a straight line through the error bars and whether the max/ min graphs straddle the origin, respectively. In addition, you will usually determine values of constants in the relationship by taking measurements from the graph – the gradient and intercept. Example A graph of v 2 against x is found to be a straight line of gradient 1.50 ± 0.05ms −2 and intercept 18.2 ± 0.8m 2 s −2 . (See Pointer ) Calculate the acceleration, a , and initial velocity, u . Answer The relationship is v 2 = u 2 + 2 ax So the gradient is 2 a and the intercept is u 2 . \ Acceleration = 1.50±0.04 2 = 0.75 ± 0.02 m s −2 Fractional uncertainty in u 2 = 0.8 18.2 = 0.044 \ Fractional uncertainty in u = 1 2 × 0.044 = 0.022 \ u = 18.2 (1±0.022)ms −1 = 4.27 ± 0.09 m s −1 P.1.4 Risk assessment Risk assessments form part of the NEA. They could also come up in the component tests. There are two kinds of risk assessment to consider. (a) Maintaining the safety of the experimenter This is generally performed under three headings: ■ Identi cation of a hazard , e.g. burn hazard, slip hazard, cutting hazard. ■ Identi cation of a risk , which is the speci c aspect of the activity which brings the hazard into play, e.g. spilling hot water onto yourself when pouring water into a test tube. Pointer Many physicists consider graphs to be purely numerical so the gradients and intercepts have no units. They would rephrase the Example: A graph of ( v / m s –1 ) 2 against ( x /m) is found to have a gradient of 1.50 ± 0.05 and intercept 18.2 ± 0.8 . Pointer A risk assessment is never just a restatement of normal laboratory behaviour rules, such as keeping your bag under the bench. It must be specific to the procedure. e A graph of ln y against ln x is a straight line with gradient 2.5 and intercept 1.8 on the ln y axis. Determine the relationship between x and y . (See Section M.3) QUICKFIRE QUICKFIRE QUICKFIRE f A graph of ln y against x is a straight line with gradient −0.15 and intercept 10 on the ln y axis. Determine the relationship between x and y . QUICKFIRE QUICKFIRE 192 A Level Physics: Study and Revision Guide