Download Ohms Law and Basic Circuit Theory

Ohm’s Law and Basic Circuit Theory
Ohm’s Law: Click on the following link to open the simulation.
http://phet.colorado.edu/sims/ohms-law/ohms-law_en.html
When the simulation opens you should see the following
As you can see the basic equation associated with Ohm’s Law is V = IR. V is the
voltage, I is the current and R is the resistance. The circuit that we apply this to is a
DC circuit. That is a circuit that is carrying current in only one direction as is
produced by a battery.
Q1) On your worksheet sketch the circuit. Set the resistance to 140 ohms. Complete
the table on your worksheet. As you increase the voltage the number of batteries will
increase in 1.5-volt increments. Note hat the simulation shows current flow not
electron flow. Current flows from positive to negative and the electrons flow from
negative to positive. Current flow is an historical aberration. There is no physical
material flowing as is shown for the current. What is really occurring is an electron
flow. Nevertheless current flow is still spoken of.
Resistance (Ohms)
140
140
140
140
140
140
Voltage (Volts)
1.5
3.0
4.5
6.0
7.5
9.0
Current (milliamps)
Q2) Complete the following, as the voltage across the resistor is doubled the current
that flows through the resistor is ...
Q3) What did you notice about the physical size of the V and the I in the equation as
this increase in voltage occurred
Q4) What therefore can be said about the relationship between current and voltage
given a constant resistance value in the circuit as shown?
Q5) Using the data from the table above open an Excel spreadsheet and copy the data
for voltage and current into the spreadsheet. Plot the voltage on the y-axis and the
current on the x-axis. Make sure you use a scatter plot. Add a trend line to the graph
that is plotted and make sure that you display the equation of the line on the graph.
Q6) What is the equation of this line and what is its significance?
Q7) Set the voltage to 4.5 volts and change the resistance in increments of 100 ohms.
Remember that the voltage that is supplied by the batteries is then applied across the
resistor. Complete the following table.
Voltage (volts)
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
Resistance (Ohms)
100
200
300
400
500
600
700
800
900
1000
Current (mA)
Q8) What conclusion can be drawn regarding the current that flows in a circuit as the
resistance is increased with constant voltage applied. Be specific with your answer.
Q9) The equation for Ohm’s Law can be written in three different forms each with a
different quantity as the subject of the equation. Write each of these in the space
provided.
Q10) Write a word statement summarising Ohm’s Law by completing the following
statement:
The current that flows through a resistor in a circuit is .......
Circuit Theory:
To investigate circuits in more detail we will use another simulation. Click on the
following link,
http://phet.colorado.edu/simulations/sims.php?sim=Circuit_Construction_Kit_Virtual
_Lab_Version_DC_Only
Click on the
box.
What opens should look similar to the following
a) Using Voltmeters and Ammeters:
Set up a circuit that looks similar to the following,
A
B
Attach the voltmeter terminals to the points marked A and B as shown. Set the wire
resistivity to a value as shown below.
Q11) What values are displayed on the voltmeter and the ammeter? Remember
voltmeters measure energy difference of the electrons between two points and
ammeters measure current or electron flow.
Q12) The voltmeter placed as it was is essentially part of the circuit. Electrons must
therefore pass through the voltmeter. What conclusion can be drawn about placing
voltmeters in circuits in this fashion with respect to the circuits ability to allow
electron flow?
Set the wire resistivity to almost none. This approximates real wires. Modify your
circuit so it looks similar to the following.
B
A
Attach the voltage probes to positions A and B. Close the switch. Click the Show
Values dialog box. Record the values in the table.
Device
Measurement (add appropriate units
for each measurement)
Ammeter
Light Globe
Voltmeter
Battery
Switch
Q13) The battery increases the energy of the electrons. Given that 1-volt represents
an energy gain of 1 joule per coulomb what energy gain does this battery supply?
Q14) The light bulb uses the energy of the electrons. What energy loss does the light
bulb produce in the electrons?
Q15) The resistance of the switch is stated as zero ohms. Connect the voltmeter
across the switch. How much energy is lost per coulomb of electrons as they pass
through the switch.
Ohm’s Law was discussed previously. Apply this law to the light bulb. You know its
resistance and you know the voltage drop across it. The current in the circuit is the
same everywhere. You can check this by adding more ammeters to the circuit as
shown.
Q16) Determine the current flowing through the light bulb by using the appropriate
form of the Ohm’s Law equation. How does the value that you calculated compare to
the value as shown on the simulation?
Set up another circuit as shown below.
A
B
Note that this circuit has two light globes.
Q17) Measure the voltage across the each light globe and then across the two light
globes. Record the values in the table.
Voltage
Voltage across A only
Voltage across B only
Voltage across AB
Q18) Summarise the results above.
Q19) The brightness of the bulb is shown by yellow lines surrounding it. Compare
the brightness of the bulb(s) when the circuit has one bulb and when it has two bulbs.
Series and Parallel Circuits:
For this activity you will follow the directions found on the Experiment 20 – Series
and Parallel Circuits handout. Rather than doing an actual experiment you will use
the circuit simulation software that you were using for the previous activity on
ammeters and voltmeters.
The first circuit can be set up as shown in the following diagram.
You will note that right-clicking on any of the resistors or batteries allows you to alter
their values according to the values specified in Experiment 20.
Make sure that you read the background material at the beginning of the sheet very
carefully. It is essential that you work through this simulation thoroughly as your
investigative practical requires you to understand many of the principles discussed.
The parallel circuit can be set up to look similar to the following,
Resistivity:
For this activity you click on the following link.
http://phet.colorado.edu/sims/resistance-in-a-wire/resistance-in-a-wire_en.html
Work through the following activities based on this simulation. Make sure that you
answer each question on the sheet provided.
1. Investigate the controls and determine which symbols stand for what.
2. Increase the resistivity to 0 .75Ω cm. What happens to the resistance? What
about the wire at the bottom of the screen? What do you think the black dots
represent? Explain.
3. Return the resistivity to 0 .5Ω cm and increase length to 12cm. What happens
to resistance and the wire? Why does the resistance increase?
4. Return the length to 10cm and increase area to 6.98cm². What happens to
resistance and the wire? Why?
5. What is the relationship between A and R? ρ and R? L and R?
6. On the answer sheet you will find three tables as shown below
R
l
A
ρ
R
l
A
ρ
R
l
A
ρ
Set all variables to the numbers given at the start and only change the number for the
corresponding variable (one table where L changes, one where A changes, and one
where ρ changes).
In each table, change each variable three to four times and record. Graph the data that
you have recorded using Excel. Sketch the graphs on your answer sheet.