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A –Level Physics: Capacitor Basics Objectives: FLASHBACK FLASHBACK: Explain with a labelled circuit diagram, how the internal resistance of a battery can be determined Starter Activities 1) Draw the circuit symbols for the following: • Diode • Thermistor • Variable Resistor • Capacitor 2) Calculate the resultant resistance of three 20 ohm resistors in series and then in parallel. Then calculate the resultant resistance of one 30 ohm resistor in series with two 40 ohm resistors in parallel with one another. Capacitors “A capacitor is a device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator” In a situation whereby there are two plates with an air gap in between, electrical charge is initially unable to pass. However, gradually the charge builds up on the negative terminal when the capacitor is attached to a power source. As electrons still flow away from the other terminal (and to the power source), the other becomes ‘positive’ It’s a little like the negative terminal is full of negative charge carrying electrons whereas the other terminal is lacking! -+ Capacitors The attraction between these opposite charges causes an electric field between the plates which gradually increases until the p.difference across them = the power supply! The amount of charge a capacitor can store (per volt across it) is known as the CAPACITANCE (C) and is measured in FARADS (F). 𝐶ℎ𝑎𝑟𝑔𝑒 𝑆𝑡𝑜𝑟𝑒𝑑 (𝐶) 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒 (𝐹) = 𝑃. 𝑑 𝑎𝑐𝑟𝑜𝑠𝑠 𝑡ℎ𝑒 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑜𝑟 (𝑉) 𝑄 𝐶= 𝑉 PRACTICE: what is the capacitance of a capacitor that can store 18mC when 6V is across it. How much charge would be stored if the voltage was increased to 20V? Energy Stored in a Capacitor A charged capacitor becomes a store of electrical potential energy and this energy can subsequently be transformed into other forms. Usually E=QV, but that equation assumes a constant voltage and charge. If we plot a voltage-charge graph for a capacitor, naturally both begin at zero. They are then directly proportional meaning that the energy stored (area under the graph) is now E= ½ QV Alternative Equations Knowing that Q=CV and E= ½ QV, you can work out some alternative equations. What is the energy stored on a fully charged 100microfarad capacitor with 3mC of charge? Find the equation linking energy stored to capacitance and voltage 𝑬=? 𝑬 = ½ CV2 Find the equation linking energy stored to charge and capacitance 𝑬=? 𝑸𝟐 𝑬=½ 𝑪 Independent Study Research and write half a page (OF YOUR OWN WRITING…I will be checking…) on the use of capacitors in ‘car courtesy light systems’