ELECTRIC CURRENT AND CIRCUITS TOPIC 1 – ELECTRICAL ENERGY AND VOLTAGE TOPIC 2 – CAPACITANCE TOPIC 3 – CURRENT AND RESISTANCE TOPIC 4 – ELECTRIC POWER TOPIC 5 – EQUIVALENT RESISTANCE AND CIRCUITS

TOPIC 2 – CAPACITANCE Learning Goal: You will understand how capacitors store charge and can be used as a source of electrical energy. Success Criteria: You will know you have met the learning goal when you can truthfully say: 1.I can relate capacitance to charge and voltage. 2.I can determine the capacitance of a parallel-plate capacitor. Image(s) from Bing Images

Success Criteria 1: I can relate the capacitance of a parallel-plate capacitor to charge and voltage. Batteries store electrical energy to be used gradually over time. Another device that is commonly used to store electrical energy is a capacitor. Capacitors, however, release their energy all at once.

Success Criteria 1: I can relate capacitance to charge and voltage. A capacitor is made of two conducting surfaces separated by an insulating surface (or by empty space). When a voltage is supplied from a battery or an electrical outlet, the two surfaces become oppositely charged. When the surfaces come in contact with one another, the charge immediately flows between them. Most electronic equipment uses capacitors in some form or another.

Success Criteria 1: I can relate capacitance to charge and voltage. The capacitance of a capacitor is how much charge it can store: C = Q/ Δ V The unit for capacitance is the farad (F), named after Michael Faraday, an important early physicist in the field of electricity. Most commonly used capacitors are in the pico- to microfarad range. A 1 F capacitor will thus hold a charge of 1 C on each plate when a 1 V potential is applied to it. A larger capacitor will hold more charge for a given voltage. To find the U electric stored on a capacitor, which tells you how much work discharging it can do: U electric = ½Q Δ V

Success Criteria 1: I can relate capacitance to charge and voltage. Think of a capacitor kind of like a spring. Charging the capacitor is analogous to stretching a spring (adding energy). Discharging the capacitor is like releasing the spring, allowing all of the built-up energy to be released at once.

Success Criteria 1: I can relate capacitance to charge and voltage. Task (6 points): Answer the questions. a)Go to the “Applications of Capacitors” Wikipedia page and write down five uses of capacitors. b)How would the capacitance of a capacitor change if it were charged with a 4.5 V battery instead of a 1.5 V battery? c)How much charge can a 320 nF capacitor hold on each plate when connected to a 9.0 V battery? d)If a capacitor holds 0.05 C of charge on each plate with a 220 V potential, how much energy is stored on the capacitor? e)Suppose 5.28 mJ of energy are stored on a capacitor with a potential difference of 12 V. What is the charge on each plate? f)Do you think it would generally be more dangerous to touch both plates of a charged capacitor or to touch both terminals of a battery? Why?

Success Criteria 2: I can determine the capacitance of a parallel-plate capacitor. Different equations describe the capacitances of difference objects of different shapes. Here, we’ll look at how the dimensions of a parallel-plate capacitor determine its capacitance: C = ε 0 A/d The Greek letter epsilon ( ε ) refers to a concept called permittivity, which is the ability of something to store electrical energy in a field. ε 0 is the permittivity of a vacuum, which is a constant: ε 0 = 8.85x C 2 /(N x m 2 ) A = the area of one plate on the capacitor. d = the distance between the plates.

Success Criteria 2: I can determine the capacitance of a parallel-plate capacitor. Task (5 points): Answer the questions. a)You make a capacitor out of two metal plates, each of which is 15 cm x 25 cm. If you place the plates 6.0 cm apart, what is the capacitance of your capacitor? b)If each plate on your capacitor from part a) were supplied with 26.5 pC of charge, what would the voltage be between the plates? c)How much potential energy would there be stored in the capacitor from part a)? d)What are two ways you could increase the capacitance of a parallel-plate capacitor? e)How would doubling the distance between the plates affect the capacitance?

Success Criteria 2: I can determine the capacitance of a parallel-plate capacitor. Task (4 points): Answer the questions. You set up a capacitor and take the following measurements: Length of each plateWidth of each plateDistance between platesBattery voltage 262 mm342 mm18.0 mm3.30 V a)What is the capacitance in picofarads? b)What is the charge on each plate in picocoulombs? c)What is the potential energy stored in the capacitor in picojoules? d)If you wanted the capacitor to be able to store 1.00 nJ of energy, how close together would you have to bring the plates?

Task (3 points): Write at least 6 things you learned in this topic (1/2 point each). If you do this in your notebook, please do it in list form, rather than paragraph form.