Aim How do capacitors behave in a circuit

Aim: How do capacitors behave in a circuit? Do Now: What is the coefficient of friction for a 20 kg box being pulled across a floor with 80 N force at constant speed?

Summary: How is current, voltage and resistance all related? Symbol Name Units V Potential Difference Volts (V) PE = q. V Current Amps (A) I=q/t Resistance Ohms (Ω) R = ρL/A I R Equation

Capacitance ------+++++++ • Capacitors are charged by batteries • Electrons want to flow to the positive terminal of the battery, so they leave the bottom plate as electrons gather on the top one.

Once switch is thrown, current flows counter clockwise and electrons flow clockwise. 20 V ------+++++++ This lasts ONLY until the potential difference across the capacitor matches that across the battery, then current stops and capacitor charged!

Demo I have set up a simple circuit with a resistor and one capacitor. What do you expect to happen to my graph when I turn the power on? The graph is current vs. time

Describe uncharged and charged capacitor once the circuit is closed • An uncharged capacitor behaves like a wire, so current flows through each resistor initially. • Charged capacitor has a potential difference, but no current, so imagine nothing is there! The 200Ω branch has no current or potential. It isn’t even included in the equivalent resistance, which is now only 400Ω!

Example: Defibrilator • • Q is the charge on accumulated by each plate. V is the potential difference between them. Unit: Farads (F) Symbol:

What else can affect capacitance? • THE PLATES PROPERTIES AND LOCATION! • εo: permittivity of free space 8. 85 x 10 -12 C 2/N. m 2 • K: dielectric constant, used if some material fills the gap between plates to increase resistance. This is given depending on material.

Envelope Activity • Answer the following two questions with a partner regarding Capacitance.

Practice 1. Calculate the capacitance of a parallel plate capacitor whose plates are 20 cm x 3 cm and are separated by a 1 x 10 -3 m air gap. a. C = 5. 3 x 10 -11 F 2. What is the charge on each plate if a 12 V battery is connected across the plates? a. Q = 6. 4 x 10 -10 C

Summary 1. What decides an objects resistivity? 2. How does a capacitor behave in a circuit? 3. What are the equations for capacitance?