Plan for Today (AP Physics I)

• Lecture/Notes on Capacitors

Capacitor Review

• What happens if we attach a capacitor to a 6 V battery?

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

Picture of Capacitor and Electrons

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

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)

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

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

• How could we put more charge/more electrons on the plate?– Put more energy into the system– Use a bigger battery

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

Graph

• For a given set of plates

• What do we do with this?– Slope of line!– Slope = q/V = C /V

– So Capacitance C = q/V– Units: Coulombs/Volt = 1 farad

• 1 farad is a big number• Normally, C is in , n, and pf

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

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

Question

• A 5 pf capacitor is hooked up to a 10 V battery. What is the charge on the plates?

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

Question

• If we hook the same capacitor up to a 25 V battery, what is q?

Question

• If we hook the same capacitor up to a 25 V battery, what is q?

• q = C * V = 5 * 10^-12 * 25

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?

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?

Why bringing the plates together works

• Imagine 2 electrons repelling each other• What could cause a reduction in repulsion– Bring a positive charge close

Equation for Charge on a Capacitor

• C = ?– Area – proportional to C– Distance – inversely proportional

Equation

• But we want to convert the “proportional to” to an equality

• Need proportionality constant

Equation

Example Problem

• What is the size of a 1 f capacitor if d = .5 mm?

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

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?

• Could we add something that could help between the plates?

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

New equation

Chart of k’s

• P. 575 in book

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?

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?

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?

Example

• A 10 pf capacitor with plates 1mm apart with water in between

• What is the size of the plates?

Example

• A 10 pf capacitor with plates 1mm apart with water in between

• What is the size of the plates?

Symbols for Circuits

• Capacitor

• Battery

Circuit

• Circuit – complete path for electrons to follow– Flow from +

Assignment

• Do worksheet in small groups