Capacitors Goal of the class Distinguish between electrical

Capacitors

Goal of the class • Distinguish between electrical potential energy, electric potential, and potential difference. • Solve problems involving electrical energy and potential difference. • Question of the Day: What is the value of the electric field inside a conductor? • Previous answer: No previous question

Question • The battery shown has a potential difference of 6. 0 volts. It has just been connected to two metal plates separated by an air gap. There is no electrical connection between the two plates and air is a very poor conductor.

Capacitors ‐ + • The two metal plates are electrically neutral before the switch is closed. What will happen when the switch is closed if the left plate is connected to the negative terminal of the battery? • Electrons will flow toward lower PE. • From the battery to the left plate • From the right plate to the battery

Parallel Plate Capacitors • Electrons build up on the left plate, giving it a net negative charge. The right plate has a net positive charge. – Capacitors can store charge or electrical PE.

Capacitance • Capacitance measures the ability to store charge • SI unit: coulombs/volt (C/V) or farads (F) • In what way(s) is a capacitor like a battery? • In what way(s) is it different?

Capacitance • How would capacitance change if the metal plates had more surface area? – Capacitance would increase. • How would it change if they were closer together? – Capacitance would increase.

Capacitance • is a constant that is determined by the material between the plates ( 0 refers to a vacuum). • Combining the two equations for C yields:

Dielectrics • The space between the plates is filled with a dielectric. – Rubber, waxed paper, air • The dielectric increases the capacitance. – The induced charge on the dielectric allows more charge to build up on the plates.

Capacitor Applications • Connecting the two plates of a charged capacitor will discharge it. – Flash attachments on cameras use a charged capacitor to produce a rapid flow of charge. • Some computer keyboards use capacitors under the keys to sense the pressure. – Pushing down on the key changes the capacitance, and circuits sense the change.

Energy and Capacitors • As the charge builds, it requires more and more work to add electrons to the plate due to the electrical repulsion. – The average work or PE stored in the capacitor is (1/2)Q V. – Derive equivalent equations for PEelectric by substituting: Q = C V and V = Q/C

Practice Problem • A 225 F is capacitor connected to a 6. 00 V battery and charged. a) How much charge is stored on the capacitor? b) How much electrical potential energy is stored on the capacitor? • Answers: 1. 35 x 10‐ 3 C , 4. 05 x 10‐ 3 J

Question • If a light bulb replaced the two metal plates and the battery was connected, electrons would flow out of the negative and into the positive terminal. Will this also occur with the two metal plates? ‐ If not, why not? ‐ If so, is the flow similar or different from that with the light bulb? Explain.

Homework • Please complete questions on page 616 • Q 10, 13, 17‐ 19