Electricity and Magnetism Module 2

Background

When water flows through a garden hose, we can characterize the rate of flow as the volume of water passing any cross section of the hose per time. Units for this flow could be m³/s. Similarly, for a conducting wire electric charge can flow down the wire. We call the rate of flow of electric charge the current, which is the charge Q passing a cross section of the wire per time t. In SI units this is C/s. 1 C/s is also called an ampere, A. Conventionally the current is given the symbol I, so the definition of current is:

\(I \equiv \frac{\Delta Q}{\Delta t}\)

In order for water to flow in a hose a source of pressure is required. Similarly, for a current to flow in a wire a source of voltage is required. Common voltage sources are batteries, electric generators, and power supplies. In this Module we will be using a battery.

Note: the battery you will be using in the Activities is filled with acid. Do not lay it on its side or turn it upside down.

 

    Activity 1

Mounted on a plastic frame is a light bulb and two banana sockets. On the bottom of the light bulb are two metal contacts which are connected to wires. The other ends of the wires are connected to the banana sockets, which are a convenient way to attach wires with a corresponding plug. The figure on the previous page shows the bulb, wires and sockets.

 

The figure to the right traces the conductors from the banana sockets through the light bulb. If you are viewing this in color, the conductors are in red.

 

 

 

1. Examine the mounted light bulb and identify the parts that are indicated in the above figures. Connect a wire from each terminal of the battery to each of the banana sockets. The light bulb should light. It is good practice to use a red wire to connect to the red terminal of the battery marked +, and a black wire to connect to the black terminal of the battery marked –.

2. Here are four possible models for how the current flows in the wires when the light bulb is lit:
   
   
   
   
   Which case is most correct? Why?

3. You are supplied with a clamp meter, which measures the current in a wire that goes between the jaws of the clamp. Appendix 1 describes how to use this meter. Use the meter to measure the current in one place along one of the wires. As you slightly move the position of the clamp the measured current will change a bit. Quantify this by guessing the uncertainty \(u_I\) to one significant figure.¹

4. Use the clamp meter to check your prediction of Part B. Were you correct?

Please disconnect all the wires and turn off the meter when you are done with this Activity.

 

    Activity 2

Instead of drawing a picture of an electric circuit, we can schematically represent it with a circuit diagram. Here are a few elements of circuit diagrams.

Wire
Wires that are joined
Wires that are not joined
Light Bulb
Battery

For the Battery shown above, the positive terminal is on the left and the negative terminal is on the right . Here is a mnemonic for remembering this: a + symbol has more line in it than –, and the longer line of the battery is the + terminal.

Draw a circuit diagram of the circuit of Activity 1.

 

     Activity 3

In Activity 1 the light bulb had two conducting contacts on the bottom. Most light bulbs only have a single contact on the bottom, and use the conducting side of the base for the other contact.

Using the supplied unmounted flashlight light bulb, the battery, and only one wire can you make the light bulb light? You may not cut the wire.

 

    Activity 4

1. A knife switch is comprised of a thin strip of conducting metal which rotates about a pivot on one end and allows for an electrical connection with a conductive slot on the other end. Connect the knife switch as shown between one terminal of the battery and one terminal of the mounted light bulb as shown. What happens when the “knife” is pressed down in between the slot (switch is closed)? Explain the behaviour of the circuit as a result of closing the switch. Notice the brightness of the bulb when the switch is closed.
   
   
   
    

 

You are supplied with a common light switch which is mounted with banana sockets for easy connection. A photograph of the insides of such a switch appears to the right.


 

 

Explain how this switch works.
 

2. Place the light switch in the circuit in place of the knife switch that you used in Part A. Now try toggling the switch on and off. Do you notice a difference between the common light switch and the knife switch in terms of their interaction with the circuit and/or the light bulb?

3. The circuit diagram symbol for a switch is shown to the right.
   
   
   
   
   The electrical circuit diagrams of parts A and B are shown on the right. In the circuit shown, the switch is wired in series with the battery and the light bulb. Note that although the components are laid out differently from the figure in Part A, the two representations are completely equivalent.
    

 

 

4. Here is a circuit in which the two switches are wired in parallel to each other. Predict the brightness of the light bulb if both switches are closed at the same time. What if only one switch is closed. Explain your predictions.
   
   Wire the circuit to test your prediction. You will find it convenient to note that the banana plugs can be connected on top of one another for better cable management.

Please disconnect all wires from all the circuit elements when you have completed this Activity.

 

    Activity 5

Devices that measure electrical currents are called ammeters. Unlike clamp meters, conventional ammeters must be inserted in series into the circuit, just as the single switch was inserted into the circuit in Parts A and B of Activity 4.

The circuit diagram symbol for a conventional ammeter is shown to the right.

 

 

Here is the circuit diagram of an ammeter in series which allows for the measurement of the current in a circuit comprised of a battery and a light bulb.

 

You are supplied with multimeters which can be used as conventional ammeters. For details on DC current measurement using a multimeter, please refer to Appendix 2.A.

 

Create the circuit and measure the current in the wire. Repeat your measurement using the clamp meter. Do your measurements yield the same results for the magnitude and direction of the current? How do the values compare to the results of Part D of Activity 1? Would your results be different if you had measured the current on the other side of the light bulb? Why or why not?

Please disconnect all wires from all the circuit elements and turn off the meters when you have completed this Activity.

 

    Activity 6

As the name suggests, Voltmeters measure voltages across different circuit elements.

The circuit symbol for a voltmeter is shown on the right.

 

 

Voltmeters are typically wired in parallel. The circuit diagram that measures the voltage of the battery while powering a light bulb is shown on the diagram to the right.

 

Instructions on how to use a multimeter as a voltmeter are given in Appendix 2.B. Use a multimeter to measure the voltage of the battery. Compare your measurement with the rated voltage written on the front of the battery. Explain your finding.

Disconnect the battery from the circuit and use the voltmeter to measure its voltage. How does it compare with the voltage when it was in the circuit?

 

Please disconnect all wires from all the circuit elements and turn off the meter when you have completed this Activity.

 

    Activity 7

Assemble the circuit that lights the 6V 6W light bulb with the ammeter in series with the circuit. In the circuit diagram to the right we have indicated a number of points in the circuit. Use the voltmeter to measure the voltage difference between 1 and 2, 2 and 3, 1 and 3, 4 and 5, etc. If the meter reads a very small voltage difference between two points, you should decrease the scale of the reading by rotating the upper knob: when the scale is too small the meter will read -1; in this case increase the scale of the reading.

 

Do you see a pattern? What is the voltage “drop” across the light bulb? What about across the ammeter? One of the wires? Explain and summarize your findings. Why did we use the word “drop”?

Please disconnect all wires from all the circuit elements and turn off the meters when you have completed this activity.

 

 

 

Appendix 1 – The Clamp Meter

A clamp meter measures the current in a wire that passes through the jaws of the circular clamp. For now we will treat how the meter does this as “magic”; in a later Module we will return to investigate how it works.

The jaws may be separated by pressing on the Clamp Opening Handle.

When the current is flowing in the direction shown, the reading will be positive; if the current is flowing in the opposite direction to that shown the reading will be negative. There is a small arrow on the inside of the jaws of the clamp indicating the current direction shown in the figure.

Here is a close-up of the controls of the meter. The Function Select knob has three positions:

1. Off

2. 400A

3. 40A

We will be using the 40A function.

After turning the meter on it must be zeroed.

1. Place the meter close to the part of the wire whose current will be measured and orient the meter as it will be when it is clamped around the wire.

2. Press ZERO: The display will read ZERO.

3. Press on the Clamp Opening Handle to separate the jaws of the clamp, place the clamp around the wire, and release the handle.

The meter will now read the current in the wire in amperes.

To measure the current at a different location or with the meter at a different orientation:

1. Move the meter close to where it will do the new measurement, oriented as it will be when clamped around the wire.

2. Press on ZERO; the display will no longer read ZERO.

3. Press the ZERO button again: the display will read ZERO.

4. Clamp the meter around the wire and read the current on the display.

If it is difficult to see the display because of the orientation of the meter:

1. Press on the HOLD button. This will cause the reading to be held and the display will read HOLD.

2. Remove the meter and read the current on the display.

3. Press on the HOLD button to return the meter to normal operation. The display will no longer read HOLD.

 

Appendix 2 – The Multimeter

This module uses multimeters, which are devices capable of a number of different electrical measurements. With the flexibility of this instrument comes a price: at first glance there is a bewildering array of controls and inputs. This Appendix will guide you through this complexity to learn how to use the meter to measure DC currents and voltages.

Just as for the clamp meter, for now we will treat how the instrument actually works as “magic”.

Note: typically a multimeter is capable of measuring currents and voltages in a number of different ranges. You should always begin measurements with the largest available range. This is because if the voltage or especially the current is much larger than the selected range you can easily blow a fuse inside the meter.

Note: particularly for current measurements, turn off the power to the circuit or disconnect the battery before putting the meter in the circuit.

Note: please conserve the batteries inside the meter. When you are finished using the meter turn it OFF.

Note: if the meter starts to beep, something is wrong! Please turn off the meter and the power to the circuit and investigate further.

 

2.A – Measuring DC Currents

The figure below shows the multimeter set up to measure a DC current with a maximum value of 10 amperes.

Note that:

1. The red wire is connected to the 10A input on the meter.

2. The black wire is connected to the COM input on the meter. This is the common input which is paired with the various other inputs depending on what you are trying to measure.

3. The rotary knob points to 40m/10A. When the red wire is connected to the 10A input as here, this measures a 10 ampere maximum DC current. The icon to the right is the symbol used by the meter for DC currents.

If the current flows through the meter from the 10A input to the COM one, the display will be a positive number. If the current is flowing the other way, it will be negative.

If you are sure that the current that you wish to measure is less than 300 mA, then you may set up the meter as shown on the next page. If you are not sure, first do a 10A measurement as just described.

Note that:

1. The red wire is connected to the mA A input on the meter.

2. The black wire is connected to the COM input on the meter. This is the common input which is paired with the various other inputs depending on what you are trying to measure.

3. The rotary knob points to 300m. This selects a maximum current of 300 mA.

You should always begin with the 300m range selected. If the display reads a current less than 40 mA, you can then select the 40m range. Similarly, the 4m range has a maximum of 4 mA, and the 400 range has a maximum of 400 A.

If the current flows through the meter from the mA A input to the COM one, the display will be a positive number. If the current is flowing the other way, it will be negative.

 

2.B – Measuring DC Voltages

The figure below shows how to set up the meter to measure DC voltages.

Note that:

1. The red wire is connected to the V  input. This input can measure either voltage or resistance. Here we will measure DC voltage.

2. The black wire is connected to the COM input. This is the common input which is paired with the various other inputs depending on what you are trying to measure.

3. The rotary knob points to 1000. This selects a maximum range of 1000 V. The icon shown to the right is the symbol used by the meter to indicate DC voltages, and the 1000 that is selected with the rotary knob in the above figure is in the group of ranges indicated by this symbol.

You should always begin with the 1000 range selected. If the voltage is less than 400 V, you can then turn the rotary knob to the 400 position. Similarly, there are ranges for 40 V, 4 V, and 400 mV.

If the voltage at the V  input is positive relative to the voltage of the COM input, the display will be a positive number. If the V voltage is negative relative the COM one, the display will be a negative number.

 

 

This Guide Sheet was written by David M. Harrison, Dept. of Physics, Univ. of Toronto in November 2007. Activity 1 draws on material from Priscilla W. Laws et al., Workshop Activity Guide, Module 4, Unit 22.6, (John Wiley, 2004), pg. 604.

The photograph of the interior of a light switch is by Scott Erhardt, and placed in the public domain at http://en.wikipedia.org/wiki/Image:Light_switch_inside_explained.jpg.