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DC Circuits
Circuits can be very simple……
• Or complex …………
Key idea:
Electric Current.
Electric current is a flow of electrons,
(usually in a metal wire.)
Electric Field
-
+
• The electrons are in an electric field, so
they experience a force that pushes them
along the wire.
Electric Field
-
+
Force
• For a current to flow you need two things:
• 1) A source of electrical energy
• For a current to flow you need two things:
• 1) A source of electrical energy
• For a current to flow you need two things:
• 1) A source of electrical energy
2) …… AND a closed
circuit
• The drift velocity of electron in a wire is a
few mm per sec.
• Why does the lamp turn on as soon as the
switch is closed?
Electric Current
The size of the current flowing is the
amount of charge passing a point in one
second.
• In a torch, this is about six million million
million electrons in one second.
• This is called One Amp of current
• Remember six million million million
electrons is called One Coulomb of charge.
• So One Amp is One Coulomb flowing every
second
1Coulomb
1Amp 
1sec
ch arg e
Current 
time
q
I=
t
• Key idea:
• Voltage:
This is the amount of energy gained by each
coulomb of charge in a battery (or other
source)
Or
The amount of energy lost by each coulomb
of charge in a lamp (or other user)
• What happens when the voltage is
increased?
Electrons have more energy and they travel
faster
• What happens when the resistance is
increased?
Electrons have same energy but they travel
slower
Ohms Law
voltage
current=
resistance
V
I
R
V  I R
V  I R
V
V
R
I
I
• What does the graph show?
• Which resistor has highest resistance?
V
I
• This is an ohmic resistor. The resistance
is constant.
Non Ohmic Resistors
Some things don’t have constant resistance
V
R
I
R=….
voltage
R=….
R=….
R=….
current
• nb, the resistance is NOT the slope of the
graph in this case.
• Because the resistance increases with
temperature, V increases as the current
I
increases
• Diode
I
• Diode is forward biased, resistance is LOW
• Diode
Diode is reverse biased, resistance is HIGH
voltage
V
R
I
R
R
R
R
current
Series and Parallel Circuits
2 Amps
2 Amps
6 Volts
Current in series circuit is the same at any point.
Voltage gained in battery = Voltage lost in lamp
Series Circuit
6V
1 Amp
0V
1 Amp
3V
3V
Current is now only 1 Amp. Resistance is double)
Current is the same at any point. (not same as before)
Voltage gained in battery = Voltage lost in both lamps
4Amp
Current through each lamp
is 2 Amp
Current from battery splits,
and is shared by the lamps
2Amp
2Amp
Voltage gained in a loop = Voltage lost in the loop
Voltage gained in battery = Voltage lost in each lamp
• examples
• Find the size of the current (I)
3A
I
• 3A
• Find the size of the voltage (V)
4V
3V
V
• 7V
• 1A
• Find the size of the resistance
15Ω
6Ω
• 21Ω
15Ω
6Ω
• What happens to the total resistance if another
resistor is added in series
15Ω
6Ω
6Ω
• increases
• Find the size of the current (I)
V
4V
6Ω
10 V
I
• 1A
• Find the size of the current (I)
3A
I
6A
• 9A
• Find the size of the current (I)
2A
6A
I
• 4A
• Find the size of the voltage (V)
4V
V
• 4V
• Find the size of the resistance
3Ω
6Ω
• 2Ω
• What happens to the total resistance if another
resistor is added in parallel
• decreases
• Find the size of the current (I)
12 V
I
6Ω
• 2A
• Find the size of the current (I)
12Ω
I
3A
6Ω
• 4.5A
• Find the size of the total resistance
12Ω
10Ω
6Ω
• 14Ω
• Find the size of the voltage across R
4V
12Ω
R
6Ω
5V
• 1V
• Find the size of the voltage across R
R
5V
6Ω
7V
• 2V
• Find the size of the voltage across the 6Ω
12Ω
10Ω
3A
6Ω
• 18 V
• Find the size of the voltage across the 10Ω
12Ω
10Ω
3A
6Ω
• 45 V
12 V
16 Ω
4V
8Ω
• Calculate the current in each resistor
• Calculate the unknown resistance
• 0.5A
• 1A
• R = 4/1.5
• Now for a couple of longer ones…………………
12 V
100 Ω
50 Ω
200 Ω
300 Ω
• Calculate the current in each resistor
12 V
100 Ω
50 Ω
200 Ω
300 Ω
• SOLUTION
•
•
•
•
Calculate
Calculate
Calculate
Calculate
the
the
the
the
resistance of the parallel bit
total resistance.
total current (same as I50)
currents in the parallel bit
240 V
100 Ω
150 Ω
200 Ω
• Calculate the power output of the 200Ω resistor
with the switch closed
• Explain what happens to the current in the 150 Ω
resistor if the switch is opened
240 V
100 Ω
150 Ω
200 Ω
•
•
•
•
•
•
Calculate the
Calculate the
Calculate the
Calculate the
Use P = I2R
SOLUTION
resistance of the parallel bit
total resistance.
total current (same as I150)
currents in the parallel bit
240 V
100 Ω
150 Ω
200 Ω
• Explain what happens to the current in the 150 Ω
resistor if the switch is opened
SOLUTION
• A parallel resistor is removed.
• Total resistance increases
• Total current decreases (I= V/R battery
voltage unchanged)
• Current through 150 Ω resistor decreases