Download File - The Physics Doctor

A –Level Physics:
Electrical Quantities:
Kirchhoff’s Laws
Objectives:
Additional skills gained:
• Deriving equations
• Accurate definition of quantities
Starter:
Write an evaluation of the strengths and
weaknesses of a ski slope model of an electric
circuit including discussion of:
• Representation of the charge carriers
• Analogies of electrical energy
• Ski lift as a cell
• Snowpark obstacles as components
• Differing speeds by different skiers
• How each quantity is represented
You have 15mins ONLY
Extension: Draw a diagram of the model with
annotations
Gustav Kirchhoff
• Gustav Kirchhoff was a
German physicist who
worked in the middle of the
19th century
• He made many significant
contributions to science,
specifically in the fields of
spectroscopy,
thermodynamics and
electricity
• There are two main circuit
rules that bear his name:
• The junction rule
• The closed loop rule
Junction Rule
This rule states that the algebraic sum of all the currents
entering a junction is equal to zero
ΣI=0
In order to conserve electrical charge (law of conservation of
charge), the sum of all currents arriving at a point (a
junction), must be equal to those leaving it!
If a current is arriving we give it a positive value and if it is
leaving we give it a negative value
Junction Rule- In action
Let’s look at the figure below and consider our point of interest
(or junction) as the resistor.
There is 0.8A of current flowing into the resistor (I4) and 0.8A
flowing out (I5). Remember if it flows away from a point then
we consider the value as a negative, so it is -0.8A going away.
The current in + current out = 0
I4 + I5 = 0
0.8A + -0.8A =0
So it proves the rule!
Prove the rule again
for the junction
indicated in pink
Closed Loop Rule
In order for energy to be conserved in any loop of a circuit
(not necessarily the whole circuit), the sum of EMFs must
equal the sum of the potential differences around a loop.
Σε= ΣV
Note: The potential differences are a result of ohms law
calculations based upon the current and the resistance. For
this reason, the P.d. may have a negative or positive value
dependent on the direction of the current
Closed Loop Rule
So in the figure below let’s look at the large loop consisting of a cell,
a fixed resistor and a variable resistor.
Starting at the cell and moving clockwise, we can see the only EMF is
the cell so that gives us a total EMF of 3.0V.
We can tell that V3 must equal 0.85V as the p.d for any component in
parallel is the same (and V4) is parallel. Continuing we know that the
next in the loop is the variable resistor which has a p.d of 2.15V
Closed Loop Rule
As the current is flowing the same way through all components, we
can consider them all to have a positive value.
So thus far:
Σε= ΣV
V1= V3+ V2
3.0V = 0.85V + 2.15V
3.0V = 3.0V
The rule is proven!
Closed Loop Rule
Now let’s consider another loop. This time between A and B, including
the bulb!
Try to prove the rule for this loop. Remember, you consider
it completely separate from the rest of the circuit
There are no EMFs here in this loop so the total p.ds must = 0!
Starting a point A and going clockwise, the p.d through the bulb is
0.85V (V3). Continuing back to A, the p.d. through the resistor is going
against the flow of current so is considered negative, so -0.85V (V3)
Σε=
So:
ΣV
0= V3+ V2
0V= 0.85V + -0.85V
0V = 0V
Proven!
Closed Loop Rule- Practice
15 mins
fig D
Simple rules
Use the printouts to make basic notes on the following:
1. Current and Voltages in series circuits
2. Current and Voltages in parallel circuits
3. Resistance in series and parallel circuits
Draw the summary table on page 93 (sheet), in your
books as this will be an easy key to check each time.
Make it obvious and neat in your notes
Answer the questions on page 93
20mins
Levelled Practice Questions
You have been handed practice questions for Kirchhoff’s
laws (and linked calculations).
Normal= C grade
Higher= B grade
Extension= High A grade
These must be completed by next monday