WELCOME TO PERIOD 21 CAUTION: Keep your phones, tablets, calculators, watches, etc. off of the table. Homework Exercise #20 is due today. Midterm 2 grade ranges: Average = 26.5 A: 30 – 33 B: 26 – 29 C: 19 – 25 D: 15 – 18 E: 0 – 14 PHYSICS 1103 – PERIOD 21 •How do electrical transformers change voltage and current? •How do transformers use induced current and magnetism? •How can transformers make electricity transmission safer and more efficient? •Put away your phones and electronics. The large magnets will damage them. Electrical transformers Transformers trade high voltage for low current or low voltage for high current. A step-up transformer increases voltage and decreases current. A step-down transformer decreases voltage and increases current. Note: Transformers work only with alternating (AC) current. They do not work with the direct (DC) current provided by batteries or capacitors. Induced magnetism and current 1) A changing current in a coil of wire induces a changing magnetic field around the coil. 2) This changing magnetic field can induce a changing current (AC) in another nearby coil of wire. Transformers • A transformer consists of two coils of wire. • The wire coils are wrapped around iron cores that concentrate the magnetic fields. Iprimary Isecondary Current enters the primary coil Current leaves from the secondary coil An alternating current in the primary coil induces a changing magnetic field around the primary. This changing magnetic field induces a changing (AC) current in the secondary coil of wire. Primary and secondary coils of copper wire. Transformers • Transformers trade high voltage for low current or low voltage for high current. • The power into the primary equals the power out of the secondary (except for small joule heating losses). Power into the primary coil = Iprimary x Vprimary Power out of the secondary coil = Isecondary x Vsecondary Power = I V Ip Vp = Is Vs Step-up and step-down transformers • The number of turns of wire in the primary (Np) and secondary (Ns) coils determines the voltages. • More turns of wire equals higher voltage. Step down reduces voltage Np = 5 Step up increases voltage Ns = 3 Np = 3 Np Ns Vp Vs Ns = 5 Transmitting electricity • Large amounts of electricity cannot be easily stored. • Therefore, electricity must be generated as it is needed and transmitted to consumers. • What is the most efficient way to transmit electricity and reduce waste due to joule heating? Joule heating Joule heating occurs when a wire is heated as current flows through it. Pjoule = I2 R Pjoule I R = power (in watts) = current (in amps) = resistance (in ohms) High joule heating during the transmission of electricity is dangerous and wastes power. Reducing joule heating How can joule heating be reduced? Should electricity be transmitted at.. • High voltage or low voltage? • Very large currents or smaller currents? Which circuit will transmit more power (have brighter bulbs)? Circuit #1 Circuit #2 Step Down Step Down Transmitting electricity U.S. power grid: 300,000 km of lines operated by about 500 power companies BEFORE THE NEXT CLASS… Read textbook chapter 22 Complete Homework Exercise 21 Bring a blank Activity Sheet 22 to class.