Download L45-kirchhoff- Jan13-ch5

What we are Doing Today ?
1) Question(s) of Day
2) Kirchhoff’s Laws & Magnetism Notes
3) Continue Lab 41 & 42 at the back
4) Make sure you finished the HW p.173 of
the textbook questions 10-18
5) Lab is due on Thursday
6) Tutorial after school today. Room 308
Circuits Series and Parallel
Circuits
Kirchoff’s Laws

Kirchoff’s Laws show the relationship
between current and voltage

Kirchoff’s First Law…current (junction
rule)
At any junction, the sum of the currents entering
the junction equals the sum of the currents leaving

Current in a series circuit:
 Any junction has one path in and one path
out…..current is the same everywhere

Current in a parallel circuit:

Sum of the currents entering equals the
sum of the currents leaving

Kirchoff’s Second Law…potential
difference (loop rule)
The sum of the drops in potential (voltages)
equals the potential (voltage) at the source

Potential difference in a series circuit:
What are the missing voltages in the above
circuits?
Voltages add up to the voltage at the source

Potential difference in a parallel circuit:
What are the voltages in bulb 1
and bulb 2?
Voltage drop in each loop is the same
as the voltage at the source.
Series Circuits
Series Circuits

Current
One path - all current flows through
each resistor (remember the junction
rule?)
Equation:
I = I1 = I2 = I3 ....

Voltage
The sum of the drops in potential difference equals the
potential difference at the source (Remember the loop
rule?)
Equation:
V = V1 + V2 + V3 ...

Resistance
V = V1 + V2 + V3 ...
But V=IR, so…..
IR = I1R1 + I2R2 + I3R3 ....
Since
I = I1 = I2 = I3 ....
Current cancels in the equation, and…..
Req = R1 + R2 + R3 ....
Series Circuit Example:

In the circuit below: R1=4Ω, R2=2Ω, and R3=6Ω. If
the current in the circuit is 0.5 A, what is the voltage
at the source?
Method 1

Find total resistance in the circuit:
R = R1+R2+R3 = 4Ω + 2Ω + 6Ω = 12Ω
Then use Ohm’s Law to find total voltage:
V = IR = 0.5 A x 12 Ω = 6V
Method 2

Use Ohm’s Law to find voltage drop in
each resistor
Then add all the voltages:
V = V1 + V2 + V3 = 2V + 1V + 3V = 6V
Parallel Circuits
Parallel Circuits

Current
Sum of currents entering a junction
equals the sum of the currents leaving
(remember the junction rule?)
Equation:
I = I1 + I2 + I3 ....
Example:
 I1=2A
 I2=1A
 I4=6A
 What is I3?
I4 = I1 + I2 + I3
6A = 2A + 1A + ?
I3 = 3A

Voltage
The sum of the drops in potential difference equals the
potential difference at the source (Remember the loop
rule?) The voltage in each loop is the same as the
source of potential:
Equation:
V = V1 = V2 = V3 ...

Resistance
I = I1 + I2 + I3 ....
But I=V/R, so…..
Since
V V1 V2 V3
= +
+ ...
R R1 R2 R3
V = V1 = V2 = V3 ...
Voltage cancels in the equation, and…..
1
1
1
1
= +
+ ...
Req R1 R2 R3
Parallel Circuit Example:

R1=2Ω, R2=2Ω, and
R3=4Ω. If the voltage
at the source is 16V,
what is the total
current in the circuit?
Add a voltmeter to
show the voltage drop
across R3.
Method 1

Find total resistance in the circuit:
1
1
1
1
= +
+
Req R1 R2 R3
Need a common denominator
Then use Ohm’s Law to find total voltage:
Method 2

Use Ohm’s Law to find current in each
resistor
Then add all the currents:
I = I1 + I2 + I3 = 8A + 8A + 4A = 20A
What happens to total current
in a parallel circuit when you
add more appliances?






Additional appliances - more resistance
More paths…total resistance DECREASES
Less resistance, more current
More current, more thermal energy (heat)
Short circuits, fires…….
Solution?
What devices do you have in your home
to protect you from too much current?

1. Fuses - short wire melts, breaks circuit
(thickness of wire determines current
rating)
 2. Circuit breakers - switch opens when
current reaches a preset value
 3. GFI (ground fault interrupter) - found
in kitchen and bathroom outlets - detects
differences in current caused by extra
current paths - works like a circuit breaker
For a Summary
Look
at table 5.28 on page 163
of the textbook
What to do now?
1)
2)
3)
Homework p. 173-174 questions 19-20
Finish lab 41 & 42 at the back
Homework check p.173 questions 10-18