PHYS 1407 – Conceptual Physics II

Introduction to Circuits

 

Leader: ___________________                      Recorder: __________________________

Skeptic: ___________________                     Encourager: ________________________

 

Materials

1 x strand hook up wire

D cell

1 x low voltage light bulb

Pasco Circuit kit

Digital Multimeter

1 x alligator clip cable

 

Introduction

      In this lab, we will investigate simple electrical circuits.

 

Part 1  Current

      You are given a battery, a single piece of wire and a low voltage light bulb.  Find a way to make the light bulb light.

 

Q1)  Sketch the arrangement that made the light bulb light.

 

 

Q2)  Find three other geometric arrangements of bulb, battery, and wire that will make the light bulb light and sketch each of them in the space below.

 

 

 

 

 

Q3)  When the light bulb is lit, a current is flowing through it.  What condition must be met so that a current flows?

 

 

 

Part 2 Potential Difference

Note:  In the following activities please disconnect the circuit when not taking data or making observations to preserve battery life.

     

      In part 1 you saw that in order for a current to flow, a conducting path must be provided from the + to – terminal on the battery.  The path is completed within the battery and hence is known as a circuit.  What factors determine how the amount of current that flows in a circuit?

 

            We can connect batteries in series (i.e. + to -, + to – and so on).  First connect one battery across a single light bulb and observe the brightness of the bulb.  Then connect two batteries in series across the light bulb and observe the brightness of the light bulb.

 

Q4)  How did the brightness of the bulb change as you connected more batteries in series?

 

 

If the bulbs shine more brightly, then more current is flowing.  If it shines less brightly, then less current is flowing.

 

Q5)  When two batteries were connected in series ________ (more/less) current flowed than when a single cell was connected across the light bulb.

 

Make sure that the circuit is disconnected. Use the digital multimeter to measure the potential difference across each battery.  Record your results.

 

D6)  Battery 1 : _________

Battery 2: ___________

 

Connect the + terminal of one battery to the – terminal of the other and then measure the potential difference across the combination.  Record the result.

 

D7)  Series Combination: ________

 

Q8)  How did the series combination compare to each battery?  (I.e. same, less, more).

 

Q9)  How does the sum of the two potential differences compare to the series combination?

 

Q10)  Referring back to Q5), was the current greater when the light bulb was connected across a bigger or smaller potential difference?

 

 

Q11)  What does this suggest about the relationship between the potential difference and the current for things like light bulbs?

 

 

Part 3 Resistance

      First, again connect a single light bulb across the battery so that you have an idea of how bright the light bulb is. 

      Next, connect two light bulbs one after another across the battery.  When you connect circuit elements one after another, this is called connecting in series.  We can make a circuit schematic to show the circuit we are making.  We use a symbol

 


like                                 to represent a battery

 


and we use                                      to represent a light bulb or a resistor in general.                 

 

The circuit you will construct then can be shown as

 

 

 

 

 

 

 

 

 


Construct this circuit and have your instructor check it once you are done.

 

Q12)  When you connected the light bulbs in series, did they each shine more brightly or less brightly than the single bulb?

 

 

 

If the bulbs shine more brightly, then more current is flowing.  If they shine less brightly, then less current is flowing.

 

Q13)  When two light bulbs were connected in series ________ (more/less) current flowed than when a single light bulb was connected.

 

Light bulbs are examples of what are called resistors.  The more resistors connected in series, the more resistance to current flow.

 

      We can measure the resistance of a light bulb with a digital multimeter.  Resistance is measured in units of ohms, which is denoted by the Greek letter omega, Ω.  Turn the switch on the DMM to the 200 Ω position.  Make sure that the battery is disconnected from the circuit.  Place the leads on either side of the light bulb and hold them firmly in place until you get a reading.  Record the resistance and then repeat for the other light bulb.

D14)  Resistance of light bulb 1: ________________

Resistance of light bulb 2: ________________

 

Now measure the resistance of the two light bulbs connected in series.

 

D15)  Record the value of the series combination of resistors: ______________

 

Q16)  How does the resistance of the series combination of resistors compare to the individual resistances?  (i.e. greater, less, the same)

 

 

Q17)  How does the resistance of the series combination of resistors compare to the sum of the resistances of the light bulbs?

 

 

Q18)  Referring back to Q13) was the current in the circuit greater or less when the resistance was greater?

 

 

Q19)  What does this suggest about the relationship between current and resistance for things like light bulbs?

 

 

Q20)  Fill in the following blanks with either direct or inverse.  There is a ________ relationship between the current and the potential difference if the resistance changed, and there is a _________ relationship between the current and the resistance if the potential difference isn’t changed.  This result is known as Ohm’s law and we will explore it more next time.

 

Part 4  Short Circuits

      A short circuit is a serious problem in a home since it can lead to a fire.  Reconnect the circuit with two light bulbs in series across a battery.  Observe the brightness of the bulbs.  Now connect the alligator clip cable across one of the bulbs.  The alligator clip cable is now “shorting out” that light bulb.

 

Q21)  What happens to the brightness of the bulb across which the alligator cable is connected?

 

 

 

Q22)  What happens to the brightness of the other bulb?

 

 

 

Q23)  Does more, less, or the same amount of current flow in the circuit when the light bulb is shorted out?  Give specific evidence for your answer.

 

 

A short circuit can cause a lot of current to flow.  This will heat the wires and the can become very hot possibly causing a fire.