Download Electric Circuits

Electricity
Current
• Electricity is a flow of electrons (negative charge) through wires
and components.
• The rate at which electrons flow is called current.
• Current is measured in Amperes (Amps).
• You can measure the current in a circuit with an
ammeter.
Current Equation
Electric current = amount of charge
amount of time
Item
I=q
t
Symbol
Unit
Abbreviation
Current
I
Ampere
(Amps)
A
Charge
q
Coulomb
C
Time
t
Seconds
s
Current Practice Problems (I = q/t)
1. What is the current if 3600 C of charge passes
a point in 17 min?
* What do you have to change? (HINT – Time)
WTF
W=> Charge = 3600 C
Time = 17 minutes x (60 s/min) = 1020 seconds
T => Current =?
F => I = q/t
Current Practice Problems (I = q/t)
1. What is the current if 3600 C of charge passes
a point in 17 min?
* What do you have to change? (HINT – Time)
I = q/t
= 3600 C/1020 s
I = 3.53 Amps
Current Practice Problems (I = q/t)
2. For what period of time must a current of 0.22
A, flow if 7.5 E18 electrons are passed through
it?
*What do you have to change? (HINT – Total Charge)
WTF
W=> Charge = (7.5 E18)(-1.6 E-19) = -1.2 = 1.2 C
Current = .22 A
T => Time =?
F => I = q/t => t= q/I
Current Practice Problems (I = q/t)
2. For what period of time must a current of 0.22
A, flow if 7.5 E18 electrons are passed through
it?
*What do you have to change? (HINT – Total Charge)
t= q/I
t = 1.2 C / .22 A
t = 5.45 s
Current Practice Problems (I = q/t)
3. What is the charge of a current that has 25 A
and takes 5 seconds to round the circuit?
WTF
W => Time = 5 seconds
=> Current = 25 A
T=> Charge = ?
F => I = q/t => q = I t
Current Practice Problems (I = q/t)
3. What is the charge of a current that has 25 A
and takes 5 seconds to round the circuit?
q = It
q = 25 A x 5 s
q = 125 C
Electricity: Review & Intro to Circuits
Series Circuit
• A simple loop for the current
to flow round
• Components are connected
end-to-end, one after the
other.
• If one bulb ‘blows’ it
breaks the whole circuit
and all the bulbs go out.
Parallel Circuits
• Multiple loops for the
current to flow round
• Components are connected
& at some point the current
splits & then comes back
together. These locations
are called Junctions.
• If one bulb ‘blows’ there is
still a complete circuit so the
other bulb stays alight.
Junctions
Direct Current (DC)
• DC = Direct Current
– Supported by Thomas Edison
– current flows in one direction
• Example: Battery
Edison
Alternating Current (AC)
• AC = Alternating Current– Supported by Nikola Tesla
– current reverses direction many times
per second. This suggests that AC devices
turn OFF and ON.
• Example: Wall outlet
Tesla
Edison vs. Tesla
What do you think?
• Between Edison & Tesla, who do you think
contributed the most to society & why?
• Why do we know more about Edison than
Tesla?
Voltage (V)
• Voltage is the difference
between two charges.
– also called Potential Difference
& Electric Potential.
• Batteries have a + charge on
one side and a - charge on the
other.
– The difference between those 2
charges is the voltage of the
battery.
Voltage
- Voltage use a voltmeter to
measure the voltage of a
battery or the voltage drop
of a device.
- Voltage drop is how much
voltage is lost from one side
of a device to the other side
of the device.
Current
• Electricity is a flow of electrons (negative charge) through wires
and components.
• The rate at which electrons flow is called current.
• Current is measured in Amperes (Amps).
• You can measure the current in a circuit with an
ammeter.
Resistance (R)
• Resistance (R) is the restriction of electron
flow.
- As current travels through a device it slows down
because the electrons are attracted to the
protons in the device.
- Resistance is a measure of the change in current.
- The unit for resistance is the OHM, W
Voltage, Current & Resistance Review
𝑽 = 𝑰𝑹
Ohm’s Law
“The voltage (potential difference) is directly related to
the current, when the resistance is constant”
Item
Symbol
Unit
Abbreviation
Voltage
V
Volts
V
Current
I
Amperes
A
Resistance
R
Ohm’s
W
Voltage Sample Problem
1. What is the voltage of a circuit with 15 amps of
current and a light with 7 ohms of resistance ?
W
I = 15 A
R=7W
V = (15 A)(7 W)
T
F
V?
V = IR
V = 105 V
Voltage Practice Problems (V=IR)
What is the voltage of a circuit that has a
current of 75 A with a resistance of 10 W ?
WTF
W=> Current = 75 A
Resistance = 10 W
T => Voltage = ?
F => V = IR
Voltage Practice Problems (V=IR)
What is the resistance of a circuit that has a
current of 150 A with a voltage of 35 V?
WTF
W=> Current = 150 A
Voltage = 35 V
T => Voltage = ?
F => V = IR
R = V/I
Voltage Practice Problems (V=IR)
What is the current of a circuit that has a
resistance of 50 W and a voltage of 30 V?
WTF
W=> Resistance = 50 W
Voltage = 30 V
T => Current = ?
F => V = IR
I = V/R
Voltage Practice Problems (V=I/R)
1. V = IR
= 75 A x 10 W
= 750 V
2. V = IR
R=V/I
= 35 V / 150 A
= .23 W
3. V = IR
I = V/R
= 30 V/50 W
= .2 A
𝑷 = 𝑰𝑽
Power (P)
• POWER is the rate at which work (energy) is
done.
• Joules/Second or Watts.
• Electricity in Kilowatt hours (kW/h).
Item
Symbol
Unit
Abbreviation
Power
P
Watts
W
Current
I
Amperes
A
Voltage
V
Volts
V
Power Sample Problem
• Calculate the power rating of a home
appliance (in kilowatts) that uses 8 amps of
current when plugged into a 120-volt outlet.
W
I=8A
V = 120 V
P = (8A)(120V)
T
F
P?
P = IV
P = 960 W
.96 kW
Power Practice Problems (P=IV)
1. What is the power of a circuit that has a
current of 80 A with a voltage of 20 V?
2. What is the current of a circuit that has a a
voltage of 35 V & it’s power is 770 W?
3. What is the voltage of a circuit that has a
current of 50 A and 650 W of power?
Power Practice Problems (P=IV)
1. P=IV
P = (80 A)(20 V)
P = 1600 W
2. P = IV
I = P/V
I = 770 W / 35 V
I = 22 A
3. P = IV
V = P/I
V = 650 W / 50 A
V = 13 V
Energy
(E = Pt)
• Energy is the amount of Power produced in a
certain time
– (Watts)(seconds) or Joules
Item
Symbol
Unit
Abbreviation
Energy
E
Joules
J
Power
P
Watts
W
time
t
Seconds
s
Energy Sample Problem
• How much energy does a light bulb use if it
uses 150 watts of power over 30 seconds?
W
P = 150 W
t = 30 s
E = (150 W)(30 s)
T
F
E?
E = Pt
E = 4500 J
Energy Practice Problem
1. Calculate the Energy of a 75 watt light bulb
that burns for 15 seconds.
2. Find the Power of a circuit that has 1.5 x 103
Joules of energy that runs for 50 sec.
3. What is the time that is needed to produce
1000 J with the power of 250 W?
Energy Practice Problem
1. E = Pt
= (75 W)(15 sec)
E = 1125 J
2. E = Pt
P = E/t
= 1.5 x 103 J / 50 s
P = 30 W
2. E = Pt
t = E/P
t = 1000 J/250 W?
t=4s