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Stevens High School Physics Laboratory Manual

Students: This will be an in-class report. I expect that you will fill in the blank sections with neat work, and I will check it on the next class day. You will be called to the board to present your solutions.

Purpose: The purpose of this lab is to give you some experience with constructing circuits and measuring current (amperage) and electric potential difference (voltage) before working with actual electrical components in follow-up labs. Concept: Electricity, as the term is commonly used, refers to the phenomena associated with the motion of charged particles. Moving charged particles produce an electric current, I [A], and they lose energy as they move through elements of an electric circuit. The degree to which an element of a circuit (a wire, battery, bulb, etc) impedes the flow of current is referred to as the resistance, R []. In electric circuits, energy is measured in volts, which is actually energy per charge [1 V = 1 J/C]. Ohm’s Law gives the relationship between these quantities:

I= ΔVR

(1 )

The basic laws of conservation of charge and energy still apply to electric circuits, and they form very useful tools for analyzing them. For instance, in a series circuit like that shown below, the current through each resistor is the same (no electrons are lost!).

Figure 1: Series arrangement of resistors

In a series circuit, electrons are forced to go through one resistor after another, giving an effective resistance that is the sum of the individual resistances:

Req=R1+R2+…+Rn

or

Req=∑i=1

n

Ri

(2 )

In contrast, in a parallel circuit like that shown below, multiple paths for electrons are available, so the current in each branch is not necessarily the same, but it adds up to the total. In parallel, the potential difference is the same across every branch. This is necessary so that any complete path through an electric circuit gives a net

11

Simulated Circuits


potential difference of zero (energy is conserved). The electrons flow through all paths simultaneously (with the magnitude of each current given by equation (1) above), and the resistance is added as reciprocals:

Figure 2:Parallel arrangement of resistors

1R eq

= 1R1

+ 1R2

+…+ 1Rn

or

Req=(∑i=1n

Ri)−1

(3 )

You will be investigating the properties of both of these circuits as well as a combination circuit in this lab.

Materials: Computer with connection to the internet Paper and writing implement Calculator

Procedure: 1. Navigate to the Phet simulation: Circuit Construction Kit (DC only). You’ll find it here:

https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

Series:2. Now, build a series circuit with two light bulbs by dragging circuit elements onto the layout area. The

schematic for what I would like you to build is below along with a screen shot of the simulation in the data section.

3. You can vary the values of each element of your circuit by right-clicking on it. Set the following parameters:

a. Bulb 1: resistance = 10 .b. Bulb 2: resistance = 20 .c. Battery: voltage = up to you! I recommend at least 20 V so that you can see the light bulb

brightnesses. 4. Record your measurements in the data section of this report below. To measure voltage, you will need

to drag a voltmeter onto your work space and place the leads across each bulb, one at a time. Don’t worry about any negative signs – they are merely telling you that the red lead of the voltmeter is at a lower electric potential than the black lead.

https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc


Parallel:5. Now, build a parallel circuit with two light bulbs by dragging circuit elements onto the layout area. The

schematic for what I would like you to build is below in the data section.6. You can vary the values of each element of your circuit by right-clicking on it. Set the following

parameters:a. Bulb 1: resistance = 10 .b. Bulb 2: resistance = 20 .c. Battery: voltage = up to you! I recommend at least 20 V so that you can see the light bulb

brightnesses. 7. Record your measurements in the data section of this report below.

Combination:8. Now, build the circuit with 4 bulbs as shown in the schematic below. You should set the resistances as I

have indicated, but may pick whatever voltage you would like for the battery, then complete the measurements in the data section below.

Data: Record your measurements and observations in the spaces provided on the following pages.


Series

Figure 3a: Schematic of series circuit for the laboratory.

Data:Potential Difference

(V, volts)Current (I, amps)

Resistance (R, ohms)

Bulb 1Bulb 2

Battery/Total n/a

Observations:1. Are electrons ‘used up’ as they flow through the circuit? How can you tell?

2. How do the individual potential differences of the two bulbs compare to the battery voltage you selected?

3. Which bulb appears brighter? Explain why you would expect it to be brighter.

Figure 3b: Screen shot from the simulation.


Parallel

Figure 4: Schematic of parallel circuit for lab.




Bulb 1Bulb 2

Battery/Total n/a

Observations:1. How do the currents through each individual bulb compare to the total current?

2. Which conservation law does your answer to question one verify?

3. How do the potential differences across each bulb compare to the battery voltage?

4. Which bulb appears brighter? Explain why you would expect it to be brighter.


Combination

Figure 5: Schematic of a combination circuit for the laboratory.




Bulb 1Bulb 2Bulb 3Bulb 4

Battery/Total n/a

Observations:1. Explain what you notice about the currents and the potential differences.

Data Analysis: 1. Summarize your results as follows.

a. Resistors (or bulbs) in a series circuit have the same _____________.

b. The brightest bulb in a series circuit is the one with the (circle one): largest/smallest resistance.

c. Resistors (or bulbs) in a parallel circuit have the same ________________.

d. The brightest bulb in a parallel circuit is the one with the (circle one): largest/smallest resistance.

public.rcas.org · web viewin electric circuits, energy is measured in volts, which is actually...

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