Experiment 6: Kirchhoff's Laws
To check whether Kirchhoff’s Laws actually work, you will set up the circuit shown, predict what
the currents ought to be, then measure the currents to see if your prediction was right.
Meters: The wires from a voltmeter should go to the two
points you are finding the potential difference between; that
is, put it in parallel. An ammeter should be in series, so the
current to be measured flows through it. Never connect an
ammeter in parallel; its resistance is practically zero, so this
would create a “short circuit” and overload the meter.
Set up this circuit, using two power supplies and three
resistors between 100 Ω and 1000 Ω. Set two of the
multimeters to read DC volts. Leave out the ammeters until
later; just have wires where they will go. Measure the
potential differences across the power supplies with the circuit fully assembled. (Putting a load on
the power supplies changes their voltages.) Adjust E1 and E2 to be different from each other and in
the range 1 V to 5 V. Record all voltages and resistances. You can then remove the meters as long
as you do not touch the knobs on the power supplies again.
Use Kirchhoff’s laws to calculate I1, I2 and I3. Hints: There will be two loop equations and a point
equation, a system of three equations in three unknowns. There are several ways to work through
this system of equations. Do it another way if you want, but this approach is about as easy as any:
- Look for a loop equation containing just I1 and I2. (That would come from a loop containing
R1 and R2.) Rearrange it into the form I2 = something involving I1.
- Look for a loop equation containing just I1 and I3. (From a loop containing R1 and R3.)
Rearrange it into the form I3 = something involving I1.
- Write an equation from the point rule. Substitute your first equation into this to eliminate I2.
Substitute your second equation into it to eliminate I3. Solve for I1.
- Put your answer for I1 into the first two equations to get I2 and I3.
Calculating the uncertainties in the answers would be complicated. As a rough estimate, let’s say
each I might be off by 5% or .01 mA, whichever is larger.
Insert the three meters into the circuit, set to read milliamps, to measure I1 measure I2 and I3.
Connect each one by disconnecting a wire at a resistor and inserting the meter in series, so the
current to be measured flows through it. (Never connect an ammeter in parallel.) Get this approved
by the instructor before turning the circuit on. A current flowing in the direction opposite that
shown in the picture should be recorded as negative. Don't take the circuit apart before the
instructor accepts your paper, so you can double-check data if necessary.
In your conclusion, state whether what theory predicts matched observation.
PHY 132
Report on experiment 6: Kirchhoff's laws
DATA:
E 1 = ______________ , E 2 = ______________
R1 = ______________ , R2 = ______________ , R3 = ______________
Calculate the currents:
Measured I1 = ______________ , I2 = ______________ , I3 = ______________