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Plan for Today (AP Physics I) Lecture/Notes on Capacitors
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Capacitor Review What happens if we attach a capacitor to a 6 V battery?
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Capacitors and Electrons What happens with the electrons? – An electron on the plate is attracted to the positive end of the battery – It moves off the plate and towards the positive end of the battery – As one electron enters the battery at the positive end, it pushes out 1 electron at the negative end
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Picture of Capacitor and Electrons
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Capacitors and Electrons One electron enters the positive end of the battery and 1 electron is pushed out of the negative end again and again and again Finally, the negative side gets so full, it can’t push any more electrons out to the plate – There isn’t enough energy in the battery since electrons repel each other, it takes energy to push them all together
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How much energy is on each electron? Each electron has 6V of energy with a 6V battery V = PE/q so PE = V * q = 6* (1.6 * 10^-19)
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Energy on Electrons Continued = 6eV Rather than this, we use the term electron volts 6 electron volts – means one electron is excited through 6 volts
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Capacitors What can we say about the charge on the plates of a capacitor? – It is the same on both plates – Pull 1 electron off, put one on the other side
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How could we put more charge/more electrons on the plate? – Put more energy into the system – Use a bigger battery
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What happens as we change the size of the battery? Double the size of the battery – Doubles q Triple the size of the battery – Triples q
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Graph For a given set of plates
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What do we do with this? – Slope of line! – Slope = q/V = C /V – So Capacitance C = q/V – Units: Coulombs/Volt = 1 farad
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1 farad is a big number Normally, C is in, n, and pf
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Uses of a capacitor When we need a big q (charge) for something – In a car battery – the battery is big enough to run, but not to start – need a big charge to start – Flash in a camera – Paddles in cardiac shock
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Capacitors vs. Batteries Way to think of it – Capacitor is like an empty pitcher – can fill it up – Battery is like a faucet – just keeps coming
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Question A 5 pf capacitor is hooked up to a 10 V battery. What is the charge on the plates?
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Question A 5 pf capacitor is hooked up to a 10 V battery. What is the charge on the plates? – C = q/v – Q = C * V = 5 * 10^-12 * 10
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Question If we hook the same capacitor up to a 25 V battery, what is q?
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Question If we hook the same capacitor up to a 25 V battery, what is q? q = C * V = 5 * 10^-12 * 25
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Question 2 plates are in a vacuum How can I change the amount of charge stored on them? – What if it’s for a given battery?
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Ideas Make the plates bigger – C is proportional to the area on the plates Bring the plates closer together – C is inversely proportional to distance between them – Why is this true?
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Why bringing the plates together works Imagine 2 electrons repelling each other What could cause a reduction in repulsion – Bring a positive charge close
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Equation for Charge on a Capacitor C = ? – Area – proportional to C – Distance – inversely proportional
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Equation But we want to convert the “proportional to” to an equality Need proportionality constant
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Equation
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Example Problem What is the size of a 1 f capacitor if d =.5 mm?
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Example Problem What is the size of a 1 f capacitor if d =.5 mm? – d =.5 mm – C = 1 f – E = 8.85 * 10^-12 – A = C*d/E
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But… When we do this problem, 2 sheets of aluminum to make this capacitor could cover ½ the school district This is not possible So what can we do?
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Could we add something that could help between the plates?
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What to add in between Must be an insulator Could be polar – why? Polar molecules line up Makes the effective spacing between plates much smaller
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New equation
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Chart of k’s P. 575 in book
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Example Plates with an area of 50 cm^2 are placed.5 mm apart with paper between them. They are attached to a 10 V battery. – What is the C? – What is the charge on the plates?
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Example Plates with an area of 50 cm^2 are placed.5 mm apart with paper between them. They are attached to a 10 V battery. – What is the C? – What is the charge on the plates?
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Example Plates with an area of 50 cm^2 are placed 5 mm apart with paper between them. They are attached to a 10 V battery. – What is the C? – What is the charge on the plates?
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Example A 10 pf capacitor with plates 1mm apart with water in between What is the size of the plates?
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Example A 10 pf capacitor with plates 1mm apart with water in between What is the size of the plates?
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Symbols for Circuits Capacitor Battery
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Circuit Circuit – complete path for electrons to follow – Flow from +
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Assignment Do worksheet in small groups
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