Last week … A simple electric circuit consists of -a current source, such as a car battery -a resistor, such as a light bulb, or heater Voltage drop = Resistance * Current Power = (Voltage drop) * Current A simple circuit: A battery creates an voltage V which pushes the current i through the resistor R. In the wires, voltage and current are constant. Units: Current in Amperes A Voltage in Volts V Resistance in Ohm Ω Honors credit paper: Due last day of finals

Magnets Context: Magnetic fields make an electro-motor turn. A compass needle points to the North pole of the earth. Definition: A magnet is a material or object that produces a magnetic field. A method to detect a magnetic field is to scatter iron filings and observe their pattern. magnetic field Discussion: -Each magnet has a North and a South pole. Opposite poles attract each other, equal poles repel each other. -A "hard" or "permanent" magnet is one that stays magnetized, such as the rock “loadstone” or iron (ferromagnet). -A "soft" or "impermanent" magnet is one that loses its memory of previous magnetizations. -A material without a permanent magnetic moment can, in the presence of magnetic fields, be attracted (paramagnetic: aluminum), or repelled (diamagnetic: graphite, super conductors).paramagneticdiamagnetic Iron filings that have oriented in the magnetic field produced by a bar magnet Lecture demos: -Load stoneLoad stone -Bar magnetBar magnet -Broken magnetBroken magnet

Electromagnets Context: Electric current creates a magnetic field. Definition: An electromagnet is a wire coil in which the magnetic field is produced by the flow of an electric current.magnetic fieldcurrent Discussion: - When the current is off, there is no magnetic field. When the current changes, the magnetic field changes. - An electromagnet creates a force and a torque on other magnets, including other electromagnets. This can be used to build electro motors. - A changing magnetic field induces a current, and thus a magnetic field in a second coil (magnetic brakes). -The direction of magnetic field in the second coil is opposite to the magnetic field in the first coil (repulsive force  E&M cannon). Iron filings that have oriented in the magnetic field produced by a coil carrying a current Lecture demos: - Helmholtz coilHelmholtz coil -E&M cannonE&M cannon -Faraday effectFaraday effect - Motor 1, Motor 2Motor 1Motor 2 - magnetic brakemagnetic brake

Transformers Context: High voltage power lines save energy. Definition: A transformer are two coils that transfer electrical energy from one circuit to another through magnetic coupling. A changing current in the first coil (the primary ) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second coil (the secondary).electrical energycircuitcurrentinduces Discussion: - The secondary induced voltage V 2 is scaled from the primary V 1 by a factor ideally equal to the ratio of the number of turns of wire in their respective windings (N 2, N 1 ): V 2 /V 1 = N 2 /N 1 -Power line: energy loss = resistance * current 2 Energy transferred = voltage * current  High voltage power lines are energy efficient A transformer (2 coils) reduces The voltage from 10000V to 110V. Lecture demos: -TransformerTransformer - E&M cannonE&M cannon - high voltage power linehigh voltage power line

Magnets & transformers A magnet is a material or object that produces a magnetic field. A method to detect a magnetic field is to scatter iron filings and observe their pattern. magnetic field An electromagnet is a wire coil in which the magnetic field is produced by the flow of an electric current.magnetic fieldcurrent A transformer are two coils that transfer electrical energy from one circuit to another through magnetic coupling. A changing current in the first coil (the primary ) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second coil (the secondary).electrical energycircuitcurrentinduces Iron filings that have oriented in the magnetic field produced by a bar magnet & a coil