Review: Kirchoff’s Rules Activity 13C Achieved level: Qn. 1; Merit: Qn. 2, 3, 4, 5, 6 Excellence: Qn. 3 d, 6 b) iv. Challenge Problem on paper at the front Two further challenge problems in your workbooks

Capacitors and Capacitance DC Electricity:

Demonstration: Charged Parallel Plates Observe the electric field established between the parallel charged plates. What can we say about the charge stored in the plates? What happens to the charge on the plates if we increase the applied voltage? The pair of parallel “plates” is a capacitor.

Capacitance: The amount of charge stored on the plates of the capacitors is linearly proportional to the potential difference across the plates. The constant of proportionality is called the capacitance. Q is the charge on the plates, in coulombs (C) V is the potential difference across the plates, measured in volts (V) Capacitance is measured in farads (F) (coulombs per volt) Capacitance is a physical property of the capacitor. Capacitance is a measure of how much charge can be stored on the plates of a capacitor

Practice Problem: a) How much charge can be stored on the plates of a 100 μ F capacitor when it is connected to a 12 V battery? b) How many electrons have been moved by the battery from one plate to the other? (the charge on an electron is 1.6 ∙ C)

Capacitors A capacitor consists of a pair of conducting plates which store charge when a potential difference is applied across them. Because it breaks the circuit, current cannot flow across a capacitor. When a battery is connected to a capacitor, it pulls charge from one plate and pushes it onto the other plate. When there is a potential difference across a capacitor, an electric field will be set up between the plates. Its strength is equal to the voltage applied divided by the distance between the plates: circuit symbol

Factors Determining Capacitance How can we increase the capacitance of a parallel-plate capacitor? I.e. how can we increase the amount of charge that can be stored on the plates for a given applied voltage? Increase the plate area (it will be easier to put more charge on the plate) Reduce the distance between the plates (the negative plate will attract charge onto the positive plate more) Put an insulating material (dielectric) between the plates (these become polarised and attract more charge onto the plates)

Parallel Plate Capacitor The capacitance of a capacitor made from parallel plates of area A, separated by a distance, d, with no dielectric is given by: Where ε 0 is a constant called the permittivity of free space, and has a value of 8.84 ∙ Fm -1

Parallel Plate Capacitors

Practice Problem Complete problem 1 on page 224 in your red books.

Dielectrics A dielectric is a material placed between the plates of the capacitor to increase its capacitance. It does this by a process of polarisation – negative charges within the dielectric are attracted to the positive plate, while positive ones are attracted to the negative plate.

Equation for Capacitors with Dielectrics ε r is called the dielectric constant It is unitless (it is the ratio of two capacitances) It is a measure of how much the dielectric increases the capacitance.

Practice Problems: Complete problems in activity 14A in the Y13 Study Guide Lichtenberg figures are formed in special dielectric materials that have been “filled” with electrons.

Next Time: Combinations of capacitors Charging and discharging capacitors

Combinations of Capacitors: Capacitors in Parallel: Capacitors connected in parallel in a circuit have an equivalent capacitance equal to sum of the individual capacitances.

Combinations of Capacitors: Capacitors in Series The inverse of the equivalent capacitance of capacitors connected in series is equal to the sum of the inverses of the individual capacitances: