Transformers Not Robots in Disguise Standards: 5f Students know magnetic materials and electric currents (moving electric charges) are sources of magnetic fields and are subject to forces arising from the magnetic fields of other sources. 5h Students know changing magnetic fields produce electric fields thereby inducing currents in nearby conductors.

The Two Coil Problem and Simple Transformer Current in secondary coil when turned on or turned off but not when kept on or off. Changing magnetic field from primary is intercepted by secondary coil inducing a voltage and current. Can have a constantly changing voltage by using AC Question: When will there be a current in the secondary coil for the two coil system shown below? How come? How can we make this induced voltage and current permanent? Activities: Demonstrate the circuit discussed and show that current only occurs when the primary coil is first turned on, first turned off but not kept on or off. Discuss how during the above times a changing magnetic field is being generated by the primary coil and that the secondary coil intercepts this charge inducing a voltage and current according to Faraday’s law. Ask what would happen if we connected coil to AC voltage source. Would have an ever changing current and magnetic field generated by primary producing an ever changing voltage and current in the secondary. The voltage in the secondary could be changed by changing the number of secondary coils. This is a device called a transformer.

The Modern Transformer Iron core directs changing magnetic field of primary through secondary. Step up has more turns on secondary coil and steps up voltage, opposite for step down transformer Math Equations Power in Primary = Power in secondary (IpVp = IsVs) T p / T s = Vp/Vs = Is/Ip Ohm’s law can be applied to primary or secondary values Question: How does the modern transformer differ from the simple transformer shown in the previous slide. What is the difference between a step up and step down transformer? Write the equations showing the relationships between: a) the power in the primary and secondary coils b) the turns, voltage and current ratios of a transformer c) how is Ohm’s law used for transformers? Activities: Show picture or actual transformer. Have students note that both coils are wrapped around iron core that directs magnetic field lines from primary through secondary coil. Distinguish between step up and down transformers in terms of turns. State that step up and down refer to voltage NOT current. A step up transformer steps up voltage but steps down current so power remains constant. List three math relationships starting with power, then turns and voltage ratios, current ratios and finally Ohm’s law. Ohms law good only for exclusively primary or secondary values. No mix and matching.

Check Question A transformer has 100 turns on its primary and 10 turns on its secondary. There 100 A with an energy of 120 V of AC on the primary. What is the voltage on the secondary? What is the power of the primary? What is the power of the secondary? What is the current in the secondary? What is the resistance of the secondary?

The Power Grid Chose AC over DC Safer since not at 120 V entire time Allows stepping up and down of voltage with transformers Chose High Voltage, low current over opposite Lower heat loss in wires (P=I2R) Can use thinner, higher R wires (CHEAPER) Question: Why was AC chosen over DC for the power grid? Why was high voltage, low current chosen over the opposite for the power grid. What quantity remains the same all along the grid on both sides of a transformer? Activities: Explain that AC was chosen over DC because since the voltage is not 120 V the entire time there is a greater chance of surviving electrocutions. Explain AC also chosen because it allows use of transformers which allows voltage to be stepped up and down. For DC you could use a resistor to step voltage down (voltage drop) but not upward. Explain that high voltage and low current means lower heat in wires and less loss. But it also means that can use thinner, higher resistance wires. THIS IS CHEAPER because you use less metal and fewer towers. Take class through different steps of power transmission. Ask whether step up or down transformers are used. Show that power stays same throughout process.

Electromagnetic waves Maxwell realizes that can rewrite Oerstead’s and Faraday’s discoveries in terms of fields. Changing magnetic field causes a changing electric field Changing electric field causes changing magnetic field Maxwell discovers that two fields will regenerate only if they go 1 speed (300 000 km/s) This means that light is a regenerating electric and magnetic field (1885). Question: How did Maxwell change Oerstead’s and Faraday’s laws. At what speed do the electric and magnetic fields need to move to regenerate one another? What does this mean about light? Activites: Bring up Maxwell’s changes to Oerstead’s and Faraday’s observations and laws, namely you don’t need the wires and you can just think of the induced voltage and magnetism as fields. Make changes to the laws on the board. Point out that the result of one law is the start of the other. Draw arrows indicating a cycle between the two laws. State that Maxwell calculated that the magnetism and voltage would regenerate one another at only one speed. Too fast and the fields would move faster for ever, too slow and they would die out. This speed turned out to be 300 000 km/s (the speed of light). At this moment Maxwell realized that light was an electromagnetic wave. Thompson device and light or radio demo Penelope and James go for a walk in the garden. “You are with the only man that knows why the stars shine so bright.”

Autobots, Roll Out!