Electromagnetic Induction. Chapter 37

Transcription

1 Electromagnetic Induction Chapter 37

2 Wire moves past magnetic field Field moves past wire a voltage is produced.

3 Electromagnetic induction Magnetism is not the source of voltage the wire is not the source of the voltage. Voltage is induced by the work done in pushing the magnet through the coil.

4 Faraday s Law The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops. Last chapter we found out that magnetism is caused by electricity, can it be the other way around? Yes!

5 More coils will require more effort, work, but you get a greater current! This is a form of energy conservation! Power = Voltage * Current Interestingly, if the coil isn t connected to anything (circuit or meter) it takes no work to plunge the magnet through the coil. We get a voltage, but not a current and therefore no work is done.

6 Generator demos Hand crank generator with light bulb demo motor as a generator with demo galvanometer Electromagnetic jump rope

7 Generator

8 Original Westinghouse Generators at Niagara Falls, Oct. 27, 1953: "The three generators in the foreground began operating fifty-eight years ago in the world s first large power installation The Edward Dean Adams power station of Niagra Mohawk Power Corporation. Built by the Westinghouse Electric Corporation and rated at 5,000 electrical horsepower..."

9 Generators and Alternating Current 1 second AC in US is 60 complete cycles per second As the loop rotates, the magnitude and direction of the induced voltage (and current) change. 60 Hertz or 60 Hz One complete rotation of the loop produces one complete cycle in voltage (and current).

10 Magnetic forces current carrying wires in a magnetic field. moving charged particles

11 Magnetic forces on moving charged particles (in wires) Magnetic Force A beam of electrons is perpendicular to the magnetic field, and the resultant force is perpendicular to both the charged particles and the magnetic field.

12 Motor and generator comparison Converts electrical energy into mechanical energy Motor Effect Converts mechanical energy into electrical energy Generator Effect Be able to discuss similarities and differences with a diagram!

13 Transformer When the switch of the primary is opened or closed the galvanometer in the secondary registers a current. But when the switch remains closed, no current is registered on the galvanometer of the secondary Our laboratory experience, Output voltage Input voltage Why? Faraday s law has shown that the magnetic field must change within the loops of wire to induce voltage.

14 An iron core increases the magnetic field

15 Voltages voltage in can primary be stepped coil up or stepped voltage down in secondary in a transformer. coil turns in primary coil = turns in secondary coil V p = N p V N s s A higher voltage in the secondary is called a step up transformer A lower voltage in the secondary is called a step down transformer

16 What is the primary function of a transformer? To change the magnitude of voltage in an alternating current circuit

17 Conservation of energy V p = N p V N s s Power in = Power out I V = p p I s V s

18 Current and voltage in a transformer I V = I s s p V p V = s N N s p V p

19 For a transformer with 10 turns in the primary and 20 turns in the secondary If a voltage of 100 V is put across the primary, what will be the voltage output in the secondary? The secondary is connected to a light with a resistance of 50 ohms. What will be the current in the secondary circuit? What is the power in the secondary coil? What is the power in the primary coil? What is the current drawn by the primary coil?

20 For a transformer with 10 turns in the primary and 20 turns in the secondary The voltage has been stepped up and the current has been stepped down. Ohm s law says that increased voltage will produce increased current. Is there a contradiction here or does Ohm s law not apply to transformers?

21 Transmission of power A hydroelectric plant in Niagara Falls sends 3000 V at 400 Amps, to the transformer in a substation that steps it up to 120,000 V for transmission to homes in New York City. What is the current in the secondary? IpV p = I svs

22 Power transmission 30 kv Power plant 138 kv 765 kv High voltage transmission lines 240V to your home power poles Transmission substation Stepped up 138 kv 765 kv Power substation Stepped down 33 kv to 115 kv Transmission drum Stepped down 100V - 600V

23 Induction of electric and magnetic fields A magnetic field is created in any region of space in which an electric field is changing with time. The magnitude of the created magnetic field is proportional to the rate at which the electric field changes. The direction of the created magnetic field is at right angles to the changing electric field. James C. Maxwell This is the definition of Light [Maxwell s work is the] most profound and the most fruitful that physics has experienced since the time of Newton. Albert Einstein

24 Electromagnetic waves The fields are perpendicular to each other, and perpendicular to the direction of travel.

25 END