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

2. Summary: How is current, voltage and resistance all related?
Symbol
Name
Units
Equation V
Potential Difference
Volts (V)
PE = qV I
Current
Amps (A)
I=q/t R
Resistance
Ohms (Ω)
R = ρL/A
What is the graph of Voltage vs. Current? From Lab students know Slope = Resistance

3. 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.

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

5. 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

6. 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!

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

8. What else can affect capacitance?
THE PLATES PROPERTIES AND LOCATION!
εo: permittivity of free space
8.85 x C2/N.m2
K: dielectric constant, used if some material fills the gap between plates to increase resistance. This is given depending on material.

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

10. Practice
Calculate the capacitance of a parallel plate capacitor whose plates are 20cm x 3cm and are separated by a 1 x 10-3 m air gap.
What is the charge on each plate if a 12V battery is connected across the plates?
C = 5.3 x F
Q = 6.4 x C

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