Physics 106, Summer 2012

Reading: Chapter 18 Sec 5, 10-11

Did you complete the reading assignment?

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Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links? Yes No

KEY CONCEPTS:

1) Capacitors in parallel can be reduced to one equivalent circuit capacitance with this equation.

2) Capacitors in series can be reduced to one equivalent circuit capacitance with this equation.

3) Adding a resistor to a circuit with a battery and a capacitor affects the amount of time it takes to charge or discharge a capacitor.

4) Adding a resistor to a circuit with a battery and a capacitor also affects the amount of current flowing just after a switch is opened or closed.

5) An ammeter is a measurement for measuring current. It has very little internal resistance.

6) A voltmeter is a measurement for measuring potential. It has almost infinite resistance.

Exercise 1

Go to: http://buphy.bu.edu/~duffy/semester2/c11_RC.html

To charge a capacitor, you would close a switch to complete a circuit containing a battery, a capacitor and a resistor. Manipulate the simulation for charging the capacitor. What happens to the current in a circuit in the first few seconds of the charging process?

Current increases exponentially Current decreases exponentially Current stays the same, but is non-zero

The current is zero throughout the charging process.

What happens to the voltage in a circuit in the first few seconds of the charging process?

Voltage increases exponentially Voltage decreases exponentially Voltage stays the same, but is non-zero

The Voltage is zero throughout the charging process.

To discharge a capacitor, you would close a switch in an RC circuit that contains an already charged capacitor. What happens to the current in an RC circuit in the first few seconds of discharging a capacitor?

The current is zero throughout the discharging process.

What happens to the voltage during the first few seconds of the discharging process?

The Voltage is zero throughout the discharging process.

Read over the equations and explanations under the simulation. Remember that ‘d’ in these equations is the same as Δ or ‘change in’ from our equations.

Tau ( is called the ‘time constant’. and has units of seconds. If is large will the charging process be faster or slower?

Faster

Slower

If is large will the discharging process be faster or slower? Faster

Exercise 2

Go to: http://phet.colorado.edu/simulations/sims.php?sim=Circuit_Construction_Kit_ACDC

Try building an RC circuit. Can you charge it and discharge it?

Create a circuit with two resistors in series with a battery. Change the value of one of the resistors to 25 ohms. Select a voltmeter and non-contact ammeter from the tools box on the right hand side of the screen. It should look like this:

Measure the current with the non-contact ammeter on each wire. How do the currents in the wires relate to each other? (you may want to change resistor values for a better understanding)

Increases after each resistor. Decreases after each resistor. Increases as the resistance of the resistor increases. Decreases as the resistance of the resistor increases. Stays the same.

Measure the voltage drop with the voltmeter across each resistor. How do the voltage drops in the different resistors relate to each other? (you may want to change resistor values for a better understanding)

How do the combined voltage drops across each resistor compare to the voltage drop across the battery?

More Less The same

Exercise 3

Clear the circuit and create a new circuit with two resistors in parallel with a battery. Change the value of one of the resistors to 25 ohms. Have the non-contact ammeter and voltmeter ready to take measurements again. It should look like the following:

Measure the current with the non-contact ammeter on each branch and right next to the battery. How do the currents in the branches relate to each other? (you may want to change resistor values for a better understanding)

How do the combined currents in the branches compare to the current next to the battery?

Measure the voltage drop with the voltmeter across each resistor and across the battery. How do the voltage drops across the resistors in different branches relate to each other? (you may want to change resistor values for a better understanding)

Exercise 4

Clear the circuit. Rebuild the circuits from exercise 1 and exercise 2 but with light bulbs instead of resistors. Let the resistance of each light bulb be the same for simplicity.

Which circuit has brighter light bulbs? Why?

Parallel because both light bulbs experience the same voltage drop (greater V = greater power = brighter)

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