Q1) Tungsten is a metal commonly used for the filaments in light bulbs due to its high melting point.
(a)  The current in a filament is given by the equation I = nqvA where v is the drift velocity. Drift velocity is defined as
the mean velocity of the free electrons in a current carrying conductor.
(i)  Explain why drift velocity is defined as a mean velocity.
(2)
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(ii)  The diagram shows a free electron in a tungsten filament. A potential difference is applied across the ends of the filament.

Draw on the diagram to show a possible path of the electron as it moves through the tungsten filament.
(2)

(b)  In a 12 V light bulb, the tungsten filament has a length of 1.3 cm and cross-sectional area of 7.9 × 10−11 m2. When the bulb is first switched on, at room temperature, the current is 1.2 A.
(i)  Calculate the resistivity of tungsten at room temperature.
(3)
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Resistivity = ........................................................... 

*(ii)  A short time after being switched on, the temperature of the filament has increased. At this temperature the resistivity of tungsten has also increased.
With reference to the lattice of the metal, explain why the resistivity has increased.
(4)
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(iii)  Suggest why a tungsten filament is more likely to break when first switched on and not after it has been switched on for some time.
(1)
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  Q2)  Wire wound resistors are made by coiling a length of wire around a ceramic core and then coating the arrangement in a protective sealant as shown.

(a)  Show that the resistance of a 1.40 m length of nichrome wire of diameter 0.0500 mm is about 1100 Ω.
resistivity of nichrome = 1.50 × 10−6 Ω m
(3)
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Q1.