SPH 3 U Unit 5 Electricity Magnetism Unit

SPH 3 U – Unit #5 Electricity & Magnetism

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

Electric Power & Energy • The ______ is a unit of power. • _______ is the rate at which energy moves or is used. • Since energy is measured in _______, 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 _______ is equal to 1, 000 watts.

Reviewing Terms

Defining Power

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 100 W – 120 V light bulb if it was connected to a 240 V system? What would happen to the light bulb?

Practice Pg. 655#41, 42 Pg. 658#43 -45

Another Relationship for Power

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 108 V?

Practice Pg. 662#46 -50

Energy Consumption • A seemingly unusual unit for energy is used when talking about electrical energy consumption – ___________ • One ______ is the energy transformed by 1000 W in one hour (3. 6 x 10^6 J). • A typical charge by an energy company for consumed energy is roughly ______. That means that for 7 cents you can buy enough energy to lift 360 kg a vertical distance of more than 1 km!

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. 07 per Kw-h, how much would it cost to operate the television for 30 days?

Practice Pg. 664#51 -53

Series Circuits • In series circuits, current can only take one path. • The amount of _____ is the same at all points in a series circuit.

Resistance in Series • Each resistance in a series circuit adds to the total resistance of the circuit. Rtotal = R 1 + R 2 + R 3. . .

Resistance in Series • Light bulbs, resistors, motors, and heaters usually have much greater ______ than wires and batteries.

Example #1 • How much current flows in a circuit with a 1. 5 -volt battery and three 1 ohm resistances (bulbs) in series?

Voltage in a Series Circuit • Each separate resistance creates a ______ as the current passes through. • As current flows along a series circuit, each type of ______transforms some of the electrical energy into another form of energy. • ______ is used to calculate the voltage drop across each resistor.

Kirchoff’s Voltage Law

Parallel Circuits • In parallel circuits the current can take _________ • Because there are multiple branches, the ____ is not the same at all points in a parallel circuit.

Parallel Circuits • Sometimes these paths are called ______. • The current through a branch is also called the ________. • 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 __________.

Kirchoff’s Current Law

Kirchoff’s Current Law

Voltage & Current in a Parallel Circuit • In a parallel circuit the _____ is the same across each branch because each branch has a low resistance path back to the battery. • The amount of ______ in each branch in a parallel circuit is not necessarily the same. • The resistance in each branch determines the current in that branch.

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.

Advantages of Parallel Circuits • Parallel circuits have two big advantages over series circuits: .

Short Circuit • A short circuit is a parallel path in a circuit with ____________. • 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 ________.

Resistance in Parallel Circuits • Adding resistance in parallel provides another path for current, and ____current flows. • When more current flows for the same voltage, the total resistance of the circuit ___________. • This happens because every new path in a parallel circuit allows more current to flow for the same voltage.

Resistance in Parallel Circuits

Example #3 • A circuit contains a 2 ohm resistor and a 4 ohm resistor in parallel. Calculate the total resistance of the circuit.

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 ________.

The 3 Circuit Laws

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___________. Use multiple steps to combine series and parallel resistors.

Solving Circuits Problems 4. If you know the total resistance and current, use _________ 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 ________ and ________ as necessary.

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?

Combined Circuits Key Question: How do we analyze network circuits?

Combined (Network) Circuits • In many circuits, resistors are connected _______________. • Such a circuit is called a ______. • 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.

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?