Download SPH3U – Unit #5 - Learning with Mr. Langlois

SPH3U – Unit #5
Electricity & Magnetism
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Unit Overview
•
Electric Energy & Circuits
•
•
•
•
•
•
•
•
•
•
Magnetism
•
•
•
•
•
Electric Charge & Electrical Structure of Matter
Electric Potential
Elementary Charge (The Millikan Experiment)
Electric Current
Resistance
Electric Power & Energy
Series Circuits
Parallel Circuits
Complex Circuits
Natural Magnetism
Magnetic Fields
Electromagnetism (RHR#1 & 2)
The Motor Principle (RHR#3)
Electromagnetic Induction
•
•
•
Lenz’s Law
The Generator Effect
Transformers
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Electric Power & Energy
• The watt (W) is a unit of power.
• Power is the rate at which energy moves or is
used.
• Since energy is measured in joules, power is
measured in joules per second.
• One joule per second is equal to one watt.
• One watt is a pretty small amount of power.
• In everyday use, larger units are more convenient
to use.
• A kilowatt (kW) is equal to 1,000 watts.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Reviewing Terms
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Defining Power
V
EQ
Q
EQ  VQ
P  t  VQ
VQ
P
t
P  VI
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Example #1
• In North America, the standard electric
outlet has a potential difference of 120 V.
In Europe, it is 240 V. What would be the
power output of a 100W – 120 V light bulb
if it was connected to a 240 V system?
What would happen to the light bulb?
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Practice
Pg. 655#41, 42
Pg. 658#43-45
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Another Relationship for Power
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Example #2
An electric kettle is rated at 1500 W for a
120 V potential difference.
a) What is the resistance of the heating
element of the kettle?
b) What will be the power output if the
potential difference falls to 108V?
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Practice
Pg. 662#46-50
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Energy Consumption
• A seemingly unusual unit for energy is used
when talking about electrical energy
consumption – The kilowatt-hour (KW-h).
• One kilowatt-hour is the energy transformed by
1000 W in one hour (3.6x10^6J).
• A typical charge by an energy company for
consumed energy is roughly $0.07 per Kw-h.
That means that for 7 cents you can buy enough
energy to lift 360 kg a vertical distance of more
than 1 km!
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Example #3
• A family has its television set on for an
average of 4.0 h a day. If the television set
is rated at 80 W and energy costs
$0.07per Kw-h, how much would it cost to
operate the television for 30 days?
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Practice
Pg. 664#51-53
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Series Circuits
• In series circuits, current can only take one path.
• The amount of current is the same at all points in a
series circuit.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Resistance in Series
• Each resistance in a series circuit adds
to the total resistance of the circuit.
Rtotal = R1 + R2 + R3...
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Resistance in Series
• Light bulbs, resistors, motors, and heaters
usually have much greater resistance than
wires and batteries.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Example #1
• How much current flows in a circuit with a
1.5-volt battery and three 1 ohm resistances
(bulbs) in series?
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Voltage in a Series Circuit
• Each separate resistance creates a voltage
drop as the current passes through.
• As current flows along a series circuit, each
type of resistor transforms some of the
electrical energy into another form of
energy.
• Ohm’s law is used to calculate the voltage
drop across each resistor.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Kirchoff’s Voltage Law
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Parallel Circuits
• In parallel circuits the current can take more than one
path.
• Because there are multiple branches, the current is not
the same at all points in a parallel circuit.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Parallel Circuits
• Sometimes these paths are called branches.
• The current through a branch is also called the branch
current.
• When analyzing a parallel circuit, remember that the
current always has to go somewhere.
• The total current in the circuit is the sum of the currents
in all the branches.
• At every branch point the current flowing out must equal
the current flowing in.
• This rule is known as Kirchhoff’s current law.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Kirchoff’s Current Law
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Kirchoff’s Current Law
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Voltage & Current in a Parallel
Circuit
• In a parallel circuit the voltage is the
same across each branch because
each branch has a low resistance path
back to the battery.
• The amount of current in each branch
in a parallel circuit is not necessarily
the same.
• The resistance in each branch
determines the current in that branch.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Example #2
• Two bulbs with different resistances are
connected in parallel to batteries with a total
voltage of 3 volts.
• Calculate the total current supplied by the
battery.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Advantages of Parallel Circuits
•
Parallel circuits have two big
advantages over series circuits:
1. Each device in the circuit sees the full
battery voltage.
2. Each device in the circuit may be
turned off independently without
stopping the current flowing to other
devices in the circuit.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Short Circuit
• A short circuit is a parallel path in a circuit with zero or
very low resistance.
• Short circuits can be made accidentally by connecting a
wire between two other wires at different voltages.
• Short circuits are dangerous because they can draw
huge amounts of current.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Resistance in Parallel Circuits
• Adding resistance in parallel provides
another path for current, and more current
flows.
• When more current flows for the same
voltage, the total resistance of the circuit
decreases.
• This happens because every new path in
a parallel circuit allows more current to
flow for the same voltage.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Resistance in Parallel Circuits
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Example #3
• A circuit contains a 2 ohm resistor and
a 4 ohm resistor in parallel.
Calculate the total resistance of the
circuit.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Analysis of Circuits
• All circuits work by manipulating currents
and voltages.
• The process of circuit analysis means
figuring out what the currents and voltages
in a circuit are, and also how they are
affected by each other.
• Three basic laws are the foundation of
circuit analysis.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
The 3 Circuit Laws
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Solving Circuits Problems
1. Identify what the problem is asking you to find.
Assign variables to the unknown quantities.
2. Make a large clear diagram of the circuit. Label
all of the known resistances, currents, and
voltages. Use the variables you defined to
label the unknowns.
3. You may need to combine resistances to find
the total circuit resistance. Use multiple steps
to combine series and parallel resistors.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Solving Circuits Problems
4. If you know the total resistance and current,
use Ohm’s law as V = IR to calculate voltages
or voltage drops. If you know the resistance
and voltage, use Ohm’s law as I = V ÷ R to
calculate the current.
5. An unknown resistance can be found using
Ohm’s law as R = V ÷ I, if you know the current
and the voltage drop through the resistor.
6. Use Kirchhoff’s current and voltage laws as
necessary.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Example #4
• A bulb with a resistance of 1Ω is to be used
in a circuit with a 6-volt battery. The bulb
requires 1 amp of current. If the bulb were
connected directly to the battery, it would
draw 6 amps and burn out instantly. To limit
the current, a resistor is added in series
with the bulb. What size resistor is needed
to make the current 1 amp?
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Combined Circuits
Key Question:
How do we
analyze
network
circuits?
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Combined (Network) Circuits
• In many circuits, resistors are connected
both in series and in parallel.
• Such a circuit is called a network circuit.
• There is no single formula for adding
resistors in a network circuit.
• For very complex circuits, electrical
engineers use computer programs that
can rapidly solve equations for the circuit
using Kirchhoff’s laws.
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com
Example #5
• Three bulbs, each with a resistance of 3Ω, are
combined in the circuit in the diagram
• Three volts are applied to the circuit.
• Calculate the current in each of the bulbs.
• From your calculations, do you think all three bulbs will
be equally bright?
SPH3U – Physics 11 University Preparation – Unit 5 – Electricity & Magnetism
Created by: Mr. D. Langlois – GECDSB – www.mrlanglois.wordpress.com