PHYA1 Extras

How to investigate how resistance thermistor varies with temperature

• Thermistor connected with voltmeter in parallel, ammeter in series
• The candidate gives details of how the thermistor is heated in a beaker of 
• water or a water bath and a thermometer is used to measure the 
• temperature at small regular intervals. 
• The candidate states that the resistance is found at various temperatures 
• either directly with an ohmmeter or by dividing voltage by current. 
• The candidate may mention that the water must be stirred to ensure that the 
• thermistor is at the temperature measured by the thermometer. 
• The candidate may give some indication of the range of temperatures to be 
• used. 
• The candidate may refer to repetition of whole experiment. 
• The candidate may plot a graph of resistance against temperature. 
• The candidate may use a digital thermometer.

• An excellent candidate will have a working circuit diagram with correct description of measurements including range of results and processing. 
• Uses a range of results and finds a mean value
• Uses a graphical method, eg I-V characteristics. They also mention precision eg use of vernier callipers. 

• length with a ruler 
• thickness/diameter with vernier callipers/micrometer 
• measure voltage 
• measure current 
• calculate resistance 
• use of graph, eg I-V or resistance against length 
• use of diameter to calculate cross-sectional area 
•  mention of precision, eg vernier callipers or full scale readings for V and I 
• flat metal electrodes at each end to improve connection 

The explanation expected in a competent answer should include a coherent account of the significance of discrete energy levels and how the bombardment of atoms by electrons can lead to excitation and the subsequent emission of photons of a characteristic frequency. 

• electrons bombard atoms of vapour and give energy to electrons in atom 
• electrons move to a higher energy level 
• electrons are excited 
• excited electrons move down to lower energy levels losing energy by emitting photons 
• photons have energy hf 
• photons of characteristic frequencies emitted from atoms of a particular element 
• this is because atoms have discrete energy levels which are associated with particular energy values

An alternating current supply provides an output voltage of 12 V rms at a frequency of 50 Hz. Describe how you would use an oscilloscope to check the accuracy of the rms output voltage and the frequency of the supply.

The candidate states that the power supply is connected to the input of the oscilloscope. The time base is switched off and the y gain adjusted until a complete vertical line is seen on the screen. The length of the line is measured and this is converted to peak to peak voltage using the calibration. The peak voltage is divided by root two to get the rms voltage and this is compared with the stated value. The time base is now switched on and adjusted until a minimum of one cycle is seen on the screen. The length of one cycle is measured and this is converted to time using the time base setting. Frequency is the reciprocal of this time.