Circuits. Now put the round bulb in a socket and set up the following circuit. The bulb should light up.

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1 Name: Partner(s): 1118 section: Desk # Date: Purpose Circuits The purpose of this lab is to gain experience with setting up electric circuits and using meters to measure voltages and currents, and to introduce the magnetic effects of currents. Introduction and Theory In this section you will compare the brightness of light bulbs in various circuits. Because the batteries you are using are not ideal voltage sources, you may see slight brightness changes where you would not with an ideal voltage source. If you see only a slight change in brightness when changing the circuit, then you should conclude the brightness is the same. Experiments: Obtain needed equipment. Light a using one battery and 2 connecting wires. Do not put the bulb in a socket yet! In the diagram below, draw lines to show where you connected the wires to make the bulb light up. Your lines SHOULDN T cross each other, as this would indicate that the wires are connected where the lines cross. Now put the in a socket and set up the following circuit. The bulb should light up Circuits (Handout) - 1 Saved: 11/15/18, printed: 11/15/18

2 In metals, such as the wires and light bulb filament, the positive charges (protons) are bound in the atoms and unable to move. Some of the negative charges (electrons), however, move easily throughout the metal. Draw arrows on the lines in the left hand diagram below to represent the direction electrons are moving in the circuit. By convention, though, conventional current (I) is defined as the movement of positive charges. Draw arrows on the right hand diagram showing the conventional current. From here on, we will be referring to conventional current whenever we talk about current and flow of charge in a circuit. Direction of electron motion Direction of conventional current, I Connect batteries in parallel: Add another battery parallel to the first one as shown below. Compare the brightness of the to its brightness when connected to a single battery.(remembering that small changes are due to imperfect batteries). Connect batteries in series, same direction: Connect the 2 batteries in series as in the following diagram. Is the brighter, dimmer, or the same as before? no light dimmer same brighter 1118 Circuits (Handout) - 2 Saved: 11/15/18, printed: 11/15/18

3 Connect batteries in series, opposite directions: Swap the polarity of one of the batteries, as displayed in the following diagram. What happened to the? no light dimmer same brightness brighter How should you connect two batteries to achieve the maximum voltage? Series connection of light bulbs Connect two different bulbs in series as below, note the symbols of the batteries. or long bulb long bulb Are both bulbs lit? Yes No If not, which bulb is lit? long bulb none Switch the order of the bulbs; do you see any difference in which bulb is lit? Yes No Unscrew the unlit bulb. What happens to the other bulb? no light dimmer same brightness brighter You will explain these behaviours on Page Circuits (Handout) - 3 Saved: 11/15/18, printed: 11/15/18

4 Parallel connection of light bulbs Now connect the 2 bulbs in parallel. See the diagram below. Are both bulbs lit? Yes No long bulb Unscrew the. What happens to the long bulb? no light dimmer same brighter Screw the back. Unscrew the long bulb. What happens to the? no light dimmer same brighter Based on our experiments on two connections, how should the wiring be done in your home? Compare from two aspects below: Can all appliances work properly when you want them to? Will turning one appliance on/off affect the others? wired in parallel wired in series As a summary, will you wire your home in parallel or in series? parallel series In the space below, draw the circuit to obtain the maximum amount of light with 2 batteries and 3 bulbs. Same connection will work for more bulbs, but it may cause a problem (see Page 7) Circuits (Handout) - 4 Saved: 11/15/18, printed: 11/15/18

5 Measure the series and parallel circuits We have been using the brightness of bulbs as a qualitative measure of the number of electrons passing through the light bulb. Now, we will use an ammeter to measure the current flowing through the light bulb. Set up a series circuit as shown in the next diagram, using the. Note that the ammeter is always connected in series with its connection towards the connection on the battery. You must break the circuit at some point in order to insert the ammeter. It is a good idea to set up the basic circuit without any meters first, then decide where you are going to break the circuit. This point should not have branches so that you are sure which current you are measuring. Record below the conventional current I in A (amperes), together with the uncertainty (use half of the smallest division). Note that the ammeter MUST be in series to work: it needs to be inserted into the circuit. ammeter The current is I = (Scale: 1A ). (Remember the units and the uncertainty!) Now measure the current on the other side of the light bulb as shown with the dotted arrow. Is it approximately the same? Yes No Should it be the same? Explain: Yes No In the circuit above, add a in series. What happens to the first? no light dimmer same brighter What do you think has happened to the current? Why? 1118 Circuits (Handout) - 5 Saved: 11/15/18, printed: 11/15/18

6 We will now measure the voltages in volts (V) across the light bulbs with a voltmeter. The voltmeter is connected with its terminal towards the terminal of the battery as well. However, it is connected in parallel with the light bulb being measured. Again, connect the circuit without any meters first, then, (without breaking the circuit) connect the two terminals of the voltmeter as probes to each side of the light bulb. Note that the voltmeter must be in parallel with the bulb do not break the circuit when you connect the voltmeter voltmeter round bulb long bulb Measure the voltages and currents for the series circuit above. (Always remember the units and the uncertainties!) Vbatteries = (Scale: ) Vround = (Scale: ) Vlong = (Scale: ) Iround = (Scale: ) Write an equation relating the 3 voltages: Write an equation relating the 2 currents: Ilong = (Scale: ) For components connected in series, the voltage is (the same/shared), while the current is (the same/shared). Why doesn t the appear to be lit? Which light bulb has higher resistance? How can you tell? 1118 Circuits (Handout) - 6 Saved: 11/15/18, printed: 11/15/18

7 Measure the currents and voltages for the parallel circuit shown on page 4. Ibatteries = (Scale: ) Iround = (Scale: ) Ilong = (Scale: ) Vround = (Scale: ) Write an equation relating the 3 currents: Write an equation relating the 2 voltages: Vlong = (Scale: ) For components connected in parallel, the voltage is (the same/shared), while the current is (the same/shared). Which light bulb has higher resistance? How can you tell? Is this consistent with the answer from the series circuit? Yes No From this exercise and the ones on Page 4, we see that the parallel connection allows multiple appliances to work together, but the total current increases as more appliances are connected. High current through a wire is a fire hazard. That is why we need fuses or circuit breakers in our home, which stops the current if it is too high. Strictly speaking, one cannot say parallel appliances work independently: if you connect too many appliances, the total current will be too high, and the jumper will disconnect power to every appliance. Magnetic effects of currents Place a compass on your desk where it is away from any steel objects (legs and edges of the desk, battery holders, etc.) The red end of the compass should point in the direction of the magnetic field or North. If it is not the case, ask for another compass. For the circuits in this section, the battery will be short circuited, so the circuits need to be connected and disconnected quickly, else your battery will be depleted very quickly! Connect both ends of a single wire to the battery and then hold the wire right above the compass, so that the conventional current will flow North over the compass. Now, while looking at the compass, quickly connect and then disconnect the other end of the wire to the battery. (This can be done by just quickly touching the end of the wire to the battery.) Record the direction the compass needle deflects (either East or West). West East 1118 Circuits (Handout) - 7 Saved: 11/15/18, printed: 11/15/18

8 Switch the connections to the battery so the current flows South. Now connect the circuit again. Does the compass needle swing East or West? West East Now place the wire below the compass, so that the wire is lined up directly under the compass needle. Complete the circuit in such a way that current in the wire is flowing North underneath the compass. Does the compass needle swing East or West? West East Switch the connections to the battery so the current flows South. Now connect the circuit again. Does the compass needle swing East or West? West East The compass needle points in the direction of the net magnetic field, which is the vector sum of the Earth s magnetic field BE and the wire s magnetic field Bwire. If the two magnetic fields are perpendicular to each other and have equal magnitude, as shown in the diagram to the right, how many degrees is the angle? B E θ B wire = Degrees The current in the wire creates a magnetic field around the wire. Hold your right hand in front of you so that your thumb is pointing up (the same direction the current is flowing in wire A). With your thumb still pointing up, pretend to grab the wire: i.e., let your fingers curl around the wire. Notice the direction your fingers are curling around the wire. This is the direction of the magnetic field lines. What you have just done is made use of the Right Hand Rule (RHR) for finding the magnetic field around a current carrying wire. I In the figure on the right, draw an arrow around wire A on the circle to show the direction of the magnetic field. Use the RHR again to check and mark the direction of the magnetic field around wire B: point your thumb in the direction of the current (which is down) and pretend to grab the wire. Again, draw an arrow around wire B on the circle to show the direction of the magnetic field. I A Direction of the magnetic field due to current I B 1118 Circuits (Handout) - 8 Saved: 11/15/18, printed: 11/15/18