Basics of Electric Circuits, Magnets, and Transformers

1. Last week … • A simple electric circuit consists of • a current source, such as acar 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

2. Magnets • Context: Magnetic fields make an electro-motor turn. A compass needle points to the North pole of the earth. • Definition: • A magnetis 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. • 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). Iron filings that have oriented in the magnetic field produced by a bar magnet • Lecture demos: • Load stone • Bar magnet • Broken magnet

3. Electromagnets • Context: Electric current creates a magnetic field. • Definition: • An electromagnetis a wire coil in which the magnetic field is produced by the flow of an electric current. • 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 coil • E&M cannon • Faraday effect • Motor 1, Motor 2 • magnetic brake

4. 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). • Discussion: • The secondary induced voltage V2 is scaled from the primary V1 by a factor ideally equal to the ratio of the number of turns of wire in their respective windings (N2, N1): V2/V1 = N2/N1 • Power line: energy loss = resistance * current2 • Energy transferred = voltage * current •  High voltage power lines are energy efficient A transformer (2 coils) reduces The voltage from 10000V to 110V. • Lecture demos: • Transformer • E&M cannon • high voltage power line

5. Magnets & transformers A magnetis 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. An electromagnet is a wire coil in which the magnetic field is produced by the flow of an electric current. 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). Iron filings that have oriented in the magnetic field produced by a bar magnet & a coil