SPH3U1 Lesson 10 Magnetism. If the wire through a magnetic field is bent into a loop, the loop can be made to turn up to 90 0.

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3 SPH3U1 Lesson 10 Magnetism GALVAOMETERS If the wire through a magnetic field is bent into a loop, the loop can be made to turn up to otice how the current runs in the opposite directions on opposite sides of the loop leading to opposite forces. This is what causes the turn. Check out the directions of the forces with the right-hand rule for the motor principle. 3

4 SPH3U1 Lesson 10 Magnetism A loop like above can be turned into a simple device called a galvanometer. The single loop is strengthened with several more loops. A needle is attached to act as a pointer and a return spring is used to make the loop turn back to its start point when the current is. Galvanometers are used to make ammeters and voltmeters. The addition of a high resistance resistor in series makes it into voltmeter with a scale that is determined by the resistance of the resistor. Adding a very small resistor in parallel makes this an ammeter the size of the resistor determines the range. DC MOTORS:HOW A MOTOR WORKS The purpose of a motor is to turn electrical energy (current) into mechanical energy (spinning motion). Answer the questions below to see how a motor works. FIRST WAY TO LOOK AT IT 1. Use the diagram below to label the stator, armature, brushes and commutator on diagram #1 on the next page. In the diagram to the left, the armature spins. The commutator is attached to the armature, so it spins too. The brushes don t move. 2. On the diagram to the left, use the + and on the battery to label the direction of the current. 3. Use your right-hand rule #3 (for the motor principle) on each side of the wire to verify the direction of the force. 4. On diagram #1 on the next page, label the direction of the current. Then use right-hand rule #3 to determine the direction of the force on each side of the wire. Label the forces. Does the armature spin clockwise, or counter-clockwise? 5. Repeat question #4 for diagram #2. (Hint: it won t be much different) 6. Look at diagram #3. IF the current in the darker half of the armature was still going into the page (like in the first two diagrams), why would the armature get stuck here? 7. Look at diagram #4. Label the direction of the current in the armature. Is the direction of the current in the dark half of the armature the same or opposite to diagram #2? Why did the current in the armature switch direction? 8. Use right-hand rule #3 to determine the direction of the magnetic force on both sides of the armature in diagram #4. 4

5 SPH3U1 Lesson 10 Magnetism

6 SPH3U1 Lesson 10 Magnetism 9. Label the direction of current flow and use right-hand rule #3 to determine the direction of the magnetic force on both sides of the armature in diagram # Does the direction the motor rotates ever change? If so, where? 11. Does the direction of the current in the armature ever change? If so, where? 12. Does the direction of the current from the source (battery) every change? If so, where? 13. Why is this motor called a DC motor? AOTHER WAY TO LOOK AT IT In the diagram below, the wire coiled around the iron armature causes the armature to have a south and a north pole. The poles of the armature are attracted and/or repelled by the poles of the permanent magnets which makes the motor spin. 1. For each diagram, label the direction of the current, the north and south poles of the armature and show which way the motor turns. 2. Write a brief explanation of how this motor works. HOMEWORK 1. page 566 #1-3, page 571 # Go the course moodle/shared folder/website. Read the DC motor assignment. 6

7 Blackline Master AME: DATE: The Right-Hand Rule for the Motor Principle Examine the diagrams below and use the right-hand rule for the motor principle to answer the questions. 1. Draw an arrow showing the direction of the force on the wire in the diagram below. 2. Label the north and south poles of the magnets in the diagram below. The force on the wire is directed out of the page. 3. Label the direction of the conventional current in the diagram below. The arrow shows the force on the wire, directed out of the page. BLM

8 SPH3U Lesson 11 Electromagnetism USIG MAGETISM TO IDUCE A ELECTRIC CURRET LEARIG GOALS Students will: State and apply both Faraday s Law of Induced current and Lenz s Law THE DISCOVERY OF ELECTROMAGETIC IDUCTIO The production of electricity by magnetism is called. Michael Faraday first demonstrated that magnetism can produce electricity. Faraday showed that when a magnet approaches a coil, a current is in the coil. The of induced current depends on the pole of the magnet that the coil. A stationary magnet will not induce current. There must be motion of the coil or magnet to induce current. (Word Bank: direction, induced, electromagnetic induction, approaches) CREATIG CURRET FROM A MAGETIC FIELD FARADAY S LAW OF ELECTROMAGETIC IDUCTIO When a conductor is located near a moving or changing magnetic field, an electric potential difference is induced over the conductor causing an induced current to flow. The induced potential difference (voltage) is called an electromotive force or emf. Although there is a slight difference between potential difference and emf, this difference is not important to us at this point. A galvanometer is a sensitive current detector. This diagram illustrates electromagnetic induction. As the magnet is moved into the wire coil, current is generated in the coil. Faraday discovered that three factors influence the magnitude of emf and induced current in the wire: The of the wire the higher the velocity, the greater the emf and current. The of the magnetic field the stronger the magnetic field, the greater the emf and current. The of the wire in the magnetic field the longer the wire, the greater the emf and current. A segment of a closed loop of wire moves through a magnetic field. ote that the wire must be perpendicular to the magnetic field in order for current and emf to be induced.

9 SPH3U Lesson 11 Electromagnetism THE DIRECTIO OF IDUCED CURRET LEZ S LAW Lenz s law states that induced current is in a direction that opposes the change or motion that produced it. Lenz s law means that induced current creates a magnetic force that acts on the wire. This is expected as the wire moves in a magnetic field a current is induced in the wire the induced current in the wire creates a magnetic field around the wire the two magnetic fields interact creating a force This force always opposes the motion of the wire. Lenz s law thus obeys the law of conservation of energy it takes work to produce energy in a different form. USIG LEZ S LAW FIDIG DIRECTIO OF CURRET Lenz s Law can be looked at in three ways. I. A bar magnet approaching a loop of wire This is like the simulation you just did. Use this idea to draw on the direction of the induced magnetic field and the induced current and fill in the blanks The magnet is approaching the loop so the induced current must create a magnetic field that is to the inside the loop. Therefore the current must be The magnet is moving away from the loop so the induced current must create a magnetic field that is to the inside the loop. Therefore the current must be

10 SPH3U Lesson 11 Electromagnetism II. A bar magnet approaching a solenoid In this case, the induced current will create a north and a south pole in the solenoid (RHR #2). The induced poles will oppose the motion of the bar magnet. A north pole of a permanent magnet is moving away from the solenoid as shown below: a) To oppose the moving away, a pole must be induced on the left side of the solenoid to attract the magnet back b) This south pole is created by an induced current that flows (label it) c) Label the induced current and poles on the diagram below S III. A wire moving in a magnetic field In this diagram, a segment of a closed loop of wire moves upwards through a magnetic field. ote that the wire must be perpendicular to the magnetic field in order for current and emf to be induced. In this case, the induced current will flow a way that creates a force (RHR #3) opposite to the direction of motion. a) In the diagram above, the wire is moving up. The opposite of up is. b) The induced force will be directed. c) Use RHR #3 to figure out the direction of the current. Label it. Check your answer. WHY DOES THIS ALL THIS MAKE SESE? It s conservation of energy read the steps below... the wire moves in a magnetic field a current is induced in the wire the induced current in the wire creates a magnetic field around the wire the two magnetic fields interact creating a force This force always opposes the motion of the wire. Lenz s law thus obeys the law of conservation of energy it takes work to produce energy in a different form.

11 SPH3U Lesson 11 Electromagnetism PRACTICE In each situation, use the following steps to determine the direction of the current in the helix. 1. Determine which pole and in what the direction the magnet is moving (e.g. pole moving towards coil) 2. Determine what pole ( or S) would oppose that motion. Write this on the electromagnet. 3. Determine what direction the current must be flowing in order to create the necessary pole. a. S S b. c. d. S S v e. Text page 594 #1-3 AD summarize Lenz s Law and Drop-Tower Rides

12 SPH3U Lesson 13 Electromagnetism ELECTRICAL GEERATORS LEARIG GOALS Students will: Compare and contrast a DC motor and a DC generator Describe the modification needed to turn a DC generator into an AC generator Use Lenz s law and Faraday s law to describe how a generator produces electricity MOTORS VS. GEERATORS Motors and generators are essentially the same device. In an electric motor, (kinetic or electrical) energy is put into the motor and (kinetic or electrical) energy is produced. In a generator, (kinetic or electrical) energy is put into the generator and (kinetic or electrical) energy is produced. Describe some methods we use to turn generators: AC VS. DC GEERATORS DC = direct current: current flows continuously in one direction. AC = alternating current: current that periodically changes direction. In orth America, AC current is generated with a frequency of 60 Hz so that it changes direction 120 times per second. DC GEERATORS If a DC motor is connected to some source that causes it to spin, it will act as a DC generator. The commutator that caused the current to flip every half turn in the motor does the opposite when the motor is used as a generator. 1. As the coil rotates, the black half moves downwards through the magnetic field (see diagram to the right). 2. Lenz`s law says a force must act upwards on the wire to oppose the motion. 3. Apply the right hand rule for the motor principle to determine the current direction in the black wire. Mark this current on the diagram. 4. Repeat 1 3 for the white half of the loop.

13 SPH3U Lesson 13 Electromagnetism As the loop rotates, the current induced in the loop reverses every half turn (see diagram to the left). a) Indicate the direction of the IDUCED force on the loop in each diagram. b) Why is there no induced current in diagrams A and C? c) The current in the loop switched direction between diagrams and d) Why does the signal to the meter always have + on the left even though the current in the loop switches? AC GEERATORS To make an AC generator is simple. The current in the armature is already AC. It is the split-ring commutator that undoes the alternating. We need to ensure the same half of the loop remains in contact with the same brush; this is accomplished with the setup in the diagram to the right. These two separate rings make up what is called a slip-ring commutator. ow we can see that the current coming out of the generator is alternating (see the galvanometer in the diagram to the left).

14 SPH3U Lesson 13 Electromagnetism AOTHER PERSPECTIVE In this AC generator, the armature is being FORCED to turn clockwise by an external force (like a windmill). The top brush is always in contact with the inner ring of the commutator and the lower brush is always in contact with the outer ring. a) Use Lenz s Law to determine what pole the dark end of the armature will be and what direction the induced current will be. b) When does the current in the solenoid change direction? CLASS WORK P604 Q1-4

15 SPH3U Lesson 13 Electromagnetism TRASFORMERS LEARIG GOALS Students will: Understand why AC electricity generation was chosen over DC Calculate currents, voltages and powers from both the input and output sides of a transformer. WHY TRASFORMERS ARE IMPORTAT FOR POWER TRASMISSIO Generators at power plants produce large amounts of electrical energy at low voltage. The voltage is increased to allow efficient transmission of the electrical energy over long distances to end users High voltages in the lines are reduced by power substations and transformers to an appropriate voltage (240 V or 120 V) WHY IS A HIGHER VOLTAGE MORE EFFICIET FOR TRASMISSIO? Say you have 10 kw of power to transmit through power lines that have a resistance of 1.0 Ω. Electrical power can be calculated in two different ways: P = IV or P = I 2 R a) Say the power is transmitted at a voltage of 200 V. a. Determine the amount of current in the power lines (Hint: P = IV) b. With this current, calculated the amount of power that is dissipated from the resistance of the wire (Hint: P = I 2 R) b) Repeat the procedure in (a) but with a voltage of 2000 V Based on your calculations above, explain why power is actually transmitted at an extremely high voltage (around V) a. 50 A b W b) 5 A; 25 W

16 SPH3U Lesson 13 Electromagnetism HOW DO TRASFORMERS WORK? The power supply sends current into the primary coil This current creates a magnetic field around the wires of the primary coil which moves through the iron core The magnetic field induces a current in the secondary coil 1. Why does the core of a transformer have to be made of iron? 2. Why does the primary signal have to be alternating current (Hint: think of Faraday s experiments) CALCULATIG THE OUTPUT VOLTAGE AD CURRET Because energy cannot be created or destroyed, the electrical energy of the primary signal (input), must be equal to the electrical energy of the secondary signal (output) which also means that the primary power must be equal to the secondary power: P p = P s Use the equation P = IV to determine which of the following ratios is correct: = = Hence, if the voltage increases, the current and vice versa. THE UMBER OF TURS I A TRASFORMER It is the number of turns on both sides of the transformer that determine the secondary voltage and current. The primary or secondary voltage can be determined by the following equation: = where p is the number of turns on the primary (input) coil and s is the number of turns on the secondary (output) coil. 1. A transformer that increases the voltage is called a step-up transformer and one that decreases the voltage is called a step-down transformer. Look at the two diagrams at the top of the page. Which is a step-up transformer and which is a step-down transformer? Explain how you can tell. (Answer on last page) 2. Use the two equations above to finish the following equation in terms of p and s : =

17 SPH3U Lesson 13 Electromagnetism 3. Answer the following: a. A transformer has more turns in the primary coil than the secondary coil. How does it change the voltage of the input signal? How will it change the current? b. A transformer that doubles the voltage of the primary signal will the current. 4. A transformer is designed to operate a 12 V door chime from a 120 V supply. If the transformer has 800 turns on the primary coil and the door chime draws 1.5 A of current, find a. the number of turns on the secondary coil b. the current supplied to the primary coil HOMEWORK: P 609 #1-5,7-9 Answers: 1. the top diagram is the step-up transformer because there are more turns on the secondary coil 2. = 3. (a) It will decrease the voltage and increase the current (b) half 4. (a) 80 turns (b) 0.15

18 Blackline Master AME: DATE: Transformer Equations The transformer equations relate the number of windings,, the voltage V, and the current, I, in a transformer. The subscript p refers to the primary coil, and the subscript s refers to the secondary coil. Use the equations to solve each of the following problems. Transformer Equations Vp p I s p Vs s I p s Vp V s I I s p 1. A step-down transformer has a primary voltage of 65 V. The number of windings in the primary coil is 80, and the number of windings in the secondary coil is 16. Determine the voltage of the secondary coil. V s = 2. A step-up transformer has a secondary voltage of 240 V. The number of windings in the primary coil is 15, and the number of windings in the secondary coil is 60. Determine the voltage of the primary coil. V p = 3. A step-down transformer has a primary voltage of 120 V and a secondary voltage of 30 V. The primary current is 0.12 A. Determine the current in the secondary coil. I s = 4. A step-up transformer has a primary current of 0.96 A and a secondary current of 0.12 A. The number of windings in the primary coil is 592. Determine the number of windings in the secondary coil. s = 5. A step-down transformer has a primary current of 0.18 A and a secondary current of 0.72 A. The primary voltage is 240 V. Determine the voltage of the secondary coil. V s = 6. A step-up transformer has 38 windings in the primary coil and 152 windings in the secondary coil. The primary current is 0.72 A. Determine the current in the secondary coil. I s = 7. A step-down transformer has a secondary current of 0.91 A and a primary current of 0.13 A. The secondary voltage is 38 V. Determine the voltage of the primary coil. V p = BLM

19 Blackline Master 13.5-A ame: Date: 13.5-A Transformers in the Distribution of Electrical Energy 1. Which transformers are step-up transformers and which ones are step-down? How is this indicated in the diagram even if the electric potential differences were not labelled? 2. Which two transformers are usually farthest apart? Describe the voltage and electric current between these two transformers and explain why they have these properties. 3. Why is the pole transformer located close to homes instead of out in the country? 4. How many different (separate) circuits are shown in the diagram? Discuss how the electric current changes in the circuits, starting from the generating station and ending at a home. BLM 13.5-A Copyright 2011 by elson Education Ltd.