PHYS 1402 Lab – 5: Current and Voltage

Name: _____________________

 

Objectives:

· To learn to design and construct simple circuits using batteries, bulbs, wires, and switches,

· To understand the measurement of current and voltage.

 

OVERVIEW

In the following labs, you are going to discover and extend theories about electric charge and potential difference (voltage) and apply them to electric circuits. What you learn will be one of the most practical parts of the whole physics course since electric circuits form the backbone of modern technology. Without an understanding of electric circuits, we wouldn’t have lights, air conditioners, automobiles, TV sets, telephones, computers, or photocopying machines.

A battery is a device that generates an electric potential difference (voltage) from other forms of energy. The type of batteries we typically use are known as chemical batteries because they convert internal chemical energy (energy stored in chemical bonds) into electrical energy.

As a result of a potential difference, electric charge is repelled from one terminal of the battery and attracted to the other. However, no charge can flow out of a battery unless there is a conducting material connected between its terminals. If this conductor happens to be the filament in a small light bulb, the flow of charge will cause the light bulb to glow.

Electric Current

The rate of electric charge flow is called electric current. If charge Dq flows through the cross section of a conduction in time Dt, then the average current can be expressed mathematically by the relationship

 

Instantaneous current is defined as the charge per unit time passing through a part of a circuit at an instant in time. It is defined by using the limit:

The unit of current, called the Ampere (A), represents the flow of one coulomb of charge in a time of one second. Another common unit is the milliampere (mA): 1 ampere = 1000 milliampere (mA). Usually, people just refer to current as “amps” or “milliamps”.

In the next activity, you can begin exploring electric current by lighting a bulb with a battery. You will be using PHET simulation Circuit Construction Kit: DC – Virtual Lab (https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc-virtual-lab)

 

Activity 1: Arrangements that Cause a Bulb to Light

Find some arrangements in which the bulb lights and some in which it does not light. For instance, try the arrangement shown in figure below.

 Figure 1

Question 1-1: Sketch two arrangements for which the bulb lights.

Question 1-2: Sketch two arrangements for which the bulb doesn’t light. Upload your sketches.

Question 1-3: Describe as fully as possible the conditions needed for the bulb to light. Explain why the bulb fails to light in the arrangements drawn in answer to Question 1-2.

Activity 2: Other Materials Between the Battery and the Bulb

Next you will explore materials connected between a battery and a bulb allow to light. Since it seems that something flows from the battery to the bulb, we refer to materials that allow this flow as conductors and those that don’t as nonconductors or insulators .

Set up the wires, battery, and bulb so that the bulb lights, example, one of the arrangements drawn in you answer to Question 1-1. Then, stick a variety of the common objects available (paper clip, eraser, etc.) between the battery and the bulb.

Question 2-1: List some materials that allow the bulb to light.

Question 2-2: List some materials that prevent the bulb from lighting.

Question 2-3: What type of materials seem to be conductors? What types seem to be nonconductors?

 

Activity 3: Measure Current in a Circuit

Construct a circuit containing one battery, one resistor, and wire to close the circuit. The order and orientation do not matter, but it should look something like the figure below. You can show the values of the components by clicking the Values checkbox in the display at the upper-right corner of the panel. Use the default values of the battery and resistor.

A resistor connected to a battery. Figure 2

You should see the blue electrons flowing through the circuit.

Question 3-1: In what direction is the (conventional) current flowing through the circuit? Recall that current is the flow of positive charge.

a) The current flows from the negative terminal, through the wires and resistor, and into the positive terminal.

b) The current flows from the positive terminal, through the wires and resistor, and into the negative terminal.

 

To measure the current through a part of the circuit, you must break open the circuit at the point where you want to measure the current and insert the current measuring device called ammeter. That is, disconnect the circuit, put in the ammeter, and reconnect with it in place.

Set up an electrical circuit as shown in figure below. Connect an ammeter between battery and light bulb and ammeter between the light bulb and the switch.

Light bulb connected to switch, ammeters, and a battery. Figure 3

Question 3-2: Did you observe a significant difference in the currents flowing at these two locations in the circuit, or was the current the same?

Question 3-3: What is the direction of current flowing through the circuit? Is current flowing in same or opposite direction through the ammeters?

Connect the ammeter in parallel as shown in Figure 4 below. Slowly increase the power to the circuit and observe the ammeter and the resistor.

An electrical circuit with an ammeter connected in parallel to battery and light bulb. Figure 4

Question 3-4: What do you observe when you close the switch?

Summing up: When the ammeter is connected correctly, the circuit behaves as it did when no meters were connected (increasing the power increases the brightness of the bulb). When the power is increased, the ammeter shows increased current in the circuit. When the ammeter is connected incorrectly, the bulb does not light all although the ammeter shows significant current.

Question 3-5: The correct method is to connect the ammeter to the circuit in

a) Series

b) Parallel

 

Activity 4: Measure Voltage

To measure potential difference or voltage, you do not have to break the circuit. The voltage measuring device used is called voltmeter. You must connect the voltmeter leads one on each side of the circuit element. Standard practice is to touch the red lead of voltmeter to the positive terminal of the battery and the black lead to the negative terminal.

Set up an electrical circuit as shown in figure below.

 Figure 5

Question 4-1: Connect the voltmeter across the battery. What is the voltmeter reading?

Question 4-2: Connect the voltmeter across the light bulb. What is the voltmeter reading?

Question 4-3: What do you observe when you swap the leads of the voltmeter across the light bulb? Did it affect the current flow?

Summing up: When the voltmeter is connected correctly, the circuit behaves as it did when no meters were connected (increasing the power increases the brightness of the bulb). When the power is increased, the voltmeter shows increased voltage in the circuit. When the voltmeter is connected incorrectly, the bulb does not light at all although the voltmeter shows significant voltage.

Question 4-4: The correct method is to connect the voltmeter to the circuit in

a) Series

b) Parallel

 

Activity 5: Ohm’s Law

1. Connect a resistor, battery and an ammeter in series as shown in the figure below.

Connect a resistor, battery and an ammeter in series. Figure 5

2. Click on “values”.

3. Click on resistor. Slide the control till resistance value R1 = 75 ohms.

4. Increase the battery voltage until the current indicated on the ammeter is 0.1 A.

5. Record the corresponding voltmeter reading across the resistor and enter the value in the table below.

6. Repeat steps 4 and 5 for current values of 0.20 A through 0.60 A.

7. Replace the first resistor with second resistor of resistance value R2 = 33 ohms.

 

Current I (amperes A) Resistor R1 = 75 W

Voltage V (volts V)

 Resistor R2 = 33 W

Voltage V (volts V)
0 0 0
0.1 7.5  
0.2    
0.3    
0.4    
0.5    
0.6    

 

 

8. Using Excel, plot Scatter graphs of “Voltage” versus “Current” for both resistors. Plot both data sets on one set of axes. Voltage will be vertical axis; current will be the horizontal axis. Scale the graph to accommodate all your data points.

9. Place mouse on the graph and right-click. From the drop-down menu, choose “Add trendline”.

10. Format trendline will appear on the right. Default trendline will be linear.

11. Select “Display equation on chart” and “Display R-squared value on chart” to determine the slope of each best-fit line.

Question 5-1: What is the slope of each best-fit line? Identify each slope by its corresponding resistor value.

Resistor R1 = 75 W: best-fit line slope = _______________

Resistor R2 = 33 W: best-fit line slope = _______________

 

Observe the similarity between the values of the slope and the values of the corresponding resistance.

Does your slopes value match with the actual resistance values?

Upload your graph!

 

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