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Question 19M.2.HL.TZ1.8b.i

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Date May 2019 Marks available [Maximum mark: 3] Reference code 19M.2.HL.TZ1.8b.i
Level HL Paper 2 Time zone TZ1
Command term Calculate Question number b.i Adapted from N/A
b.i.
[Maximum mark: 3]
19M.2.HL.TZ1.8b.i

A student makes a parallel-plate capacitor of capacitance 68 nF from aluminium foil and plastic film by inserting one sheet of plastic film between two sheets of aluminium foil.

The aluminium foil and the plastic film are 450 mm wide.

The plastic film has a thickness of 55 μm and a permittivity of 2.5 × 10−11 C2 N–1 m–2.

The student uses a switch to charge and discharge the capacitor using the circuit shown. The ammeter is ideal.

The emf of the battery is 12 V.

(b.i)

The resistor R in the circuit has a resistance of 1.2 kΩ. Calculate the time taken for the charge on the capacitor to fall to 50 % of its fully charged value.

[3]

Markscheme

0.5 = e − t R C = e − t 1200 × 6.8 × 10 − 8

t = «−» 1200 × 6.8 × 10−8 × ln0.5 ✔

5.7 × 10−5 «s» ✔

OR

use of t 1 2 = RC × ln2 ✔

1200 × 6.8 × 10−8 × 0.693 ✔

5.7 × 10−5 «s» ✔

Examiners report

This question was also challenging for many candidates, with not an insignificant number leaving it blank. The candidates who did attempt it generally set up a correct equation, but ran into some simple calculation and power of ten errors. Some candidates attempted to solve the equation using basic circuit equations, which did not receive any marks.

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