Download Activity 1.2a – Ohm`s Law and Kirchoff`s Law Purpose: To study the

Activity 1.2a – Ohm’s Law and Kirchoff’s Law
Purpose:
1. To study the mathematical relationship between voltage, resistance, and current.
2. To learn to use Ohm’s Law to make accurate predictions.
3. To construct electrical circuits and measure voltage, current and resistance using
electronic test equipment
Equipment:
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2 Fischertechnik Bulbs
2 Switches
Connecting wires
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Power supply
Control Unit with USB cord
Multi-meter
Procedure:
1. Complete the table below for the three variables of electricity:
Name
Symbol
Your Definition
Unit
Voltage
Current
Resistance
2.
Label the Ohm’s Law formula wheel and determine the formulas below:
Use values in AMPS, VOLTS, and OHMS with
no prefixes (i.e. millivolts, megaohms, etc.)
E or V=
I=
R=
3. Using the Ohm meter (the multi-meter set to read resistance), measure the resistance
of one of the Fischertechnik bulbs. Remember, it has to be disconnected. Record the
resistance here:
Rb1 =
4. Using the volt meter (the multi-meter set to read voltage) measure the voltage of the
power supply and record it here:
Vs =
5. Using the voltage and resistance measured, calculate the total current, It, your circuit
should draw. Start with Ohm’s law, rearrange to derive the formula for current, then
substitute values and calculate. Show your work neatly and completely.
6. Hook the bulb to the power supply. Measure the voltage drop across the bulb. Is the
power supply voltage the same as what you measured with the bulb disconnected?
Vs =
The voltage is lower (about 1 volt) because the power supply is small. It heats up
when it is being used, which causes the voltage to drop a bit.
7. Disconnect the power. Using the ammeter (the multi-meter set to read current)
measure the current. Do this by wiring the ammeter in series with the bulb. Record
the current. Is the current what you predicted it would be? Why or why not?
It =
8. Disconnect the power to the circuit. Wire two bulbs in series. Draw a schematic of
the circuit here. Do it neatly so that it resembles a CAD drawing. Yes, I’m serious.
9. Predict the total resistance (call it RT) using Kirchoff’s Law. Using this RT, calculate
current (call it IT) for the circuit. Start with Ohm’s law, derive the formula for current,
then substitute values and calculate. Show your work neatly and completely.
10. Disconnect the series circuit from the power supply and measure the resistance. Does
it match the prediction?
RT =
11. Connect the power to the circuit. Measure the voltage drop across each bulb while it
is lit. Record the voltage drop here. What is the relationship of the drop across each
bulb to the total voltage?
Vb1 =
Vb2=
Vtotal=
12. Disconnect the circuit and place the ammeter in the circuit. Reconnect the power and
measure the current. Does it make any difference where you measure the current?
Explain your answer in terms of the properties of current in a series circuit.
13. Disconnect the power. Wire two bulbs in parallel. Draw a schematic of the circuit
here. Do it neatly so that it again resembles a CAD drawing. Yes, I’m serious about
making decent sketches because it’s important.
14. Predict the total resistance RT for the circuit using Kirchoff’s Law. Then use RT and
Ohm’s Law to calculate current, IT. Show your math.
RT =
It =
15. Measure the resistance across the two bulbs. Does it match the prediction? If not ask
Mr. Bayer if your prediction was valid. Or, research “parallel resistors” for the equation
needed to improve your estimate in Question 14.
Rt =
16. Connect the power to the circuit. Measure the voltage drop across each bulb. Record
the voltage drop here. What is the relationship of the drop across each bulb to the total
voltage? What if there was three bulbs?
Vb1 =
Vb2=
Vtotal=
17. Disconnect the circuit and place the ammeter in the circuit. Reconnect the power and
measure the current. Does it make any difference where you measure the current?
Explain your answer in terms of the properties of current in a parallel circuit.
Conclusion:
1. Is there a difference in the brightness of the bulbs if they are wired in series or
parallel? Explain why or why not.
2. As the electrical energy forces its way through the bulbs and wire, what conversions
does the electrical energy go through? Is the energy “used up”? Why or why not?
3. Of the three ways you wired the lights (single, series, and parallel) which gave the
most light? Which provided the least? Explain why.
4. DC motors speed up or slow down when the voltage powers them is raised or lowered.
If you had two permanent magnet DC motors, and you wanted them to both run at full
speed, how should you wire them?
5. Compare and contrast the advantages and disadvantages of parallel and series
circuits. Make a 2 row x 3 column table to document advantages, disadvantages, and
at least three real-world examples/or ideal situations for each type of circuit.