Turn over 13 13 The diagram shows two parallel metal plates with a potential difference (p.d.) of 100 V across them. Three equipotential lines are shown. 0V +100V metal plate metal plate (a) Draw lines to represent the electric field in the shaded area. (4) 14 (b) A parallel plate capacitor consists of a thin layer of insulator of thickness d between two plates of conducting material of area A. conducting material d A insulator The capacitor has a capacitance 0.1 μF and is charged to a p.d. of 100 V by connecting it to an electrical supply. The capacitor is then disconnected from the supply and the p.d. between the two plates slowly decreases. This is because the insulator is not perfect and a small charge can flow through it. The graph shows how the p.d. varies with time. 25 30 20 15 10 5 0 60 70 80 90 100 50 40 30 20 10 0 Time / hours p.d. / V Turn over 15 The insulator is a type of plastic and should have a resistivity greater than 1014 Ω m. Deduce whether the plastic used in this capacitor has a resistivity greater than this value. A = 5.6 × 10−3 m2 d = 0.6 × 10−6 m (5) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (Total for Question 13 = 9 marks)