First Midterm Exam Tomorrow 7 9 pm in this room. Chapters 21 23 - - PowerPoint PPT Presentation

First Midterm Exam

• Tomorrow 7 – 9 pm in this room.
• Chapters 21 – 23 and labs 1 – 3.
• Bring a pen/pencil and a calculator (but you

can only use it for arithmetic; no formulas, integration, etc.)

• Exam will include a page of key equations.
• No homework this week.

Capacitors and Batteries

• A battery (or voltage source) maintains the

potential difference between its two terminals by means of a chemical reaction or other mechanism.

• Analyzing multiple capacitors in a circuit:

– The potential is constant throughout a conductor in equilibrium. – Charge is conserved.

Figure 24.9

Capacitors in Parallel

Figure 24.11b (modified)

Capacitors in Series

Effective Capacitance

• Parallel:
• Series:

1 2 eff

C C C   

1 2

1 1 1

eff

C C C   

A circuit consists of a capacitor, a battery, and a switch, all connected in series. Initially, the switch is open and the capacitor is uncharged. The switch is then closed and the capacitor charges. While the capacitor is charging, how does the net charge within the battery change?

1) It increases. 2) It decreases. 3) It stays the same

Several different capacitors are hooked across a DC battery in parallel. The voltage across each capacitor is 1) directly proportional to its capacitance. 2) inversely proportional to its capacitance. 3) independent of its capacitance.

If C1 < C2 < C3 < C4 for the combination of capacitors shown, the equivalent capacitance is 1)less than C1. 2)more than C4. 3)between C1 and C4.

Conservation

• Conservation of Charge – total charge in a

closed system is constant.

• Electric Field is Conservative –

– Independent of path – Depends only on the end points (a and b). – If endpoints are the same (b = a), – Kirchoff’s Loop (or Voltage) Rule.

b a

V   E dl   

V    

 E dl

  



Steady state

• A system (e.g. circuit) is in the steady state

when the current in every part of the circuit is constant.

– In many practical circuits, the steady state is achieved in a short time.

• In the steady state, the charge (or current)

flowing into any point in the circuit has to equal the charge (or current) flowing out.

– Kirchhoff’s Node (or Current) Rule.