Last lecture … Work = displacement * force Work = work done by a force on an object

1. Last lecture … • Work = displacement * force • Work = work done by a force on an object • Displacement = displacement of the object • Force = component of a force on the object in the direction of the displacement • Work-Energy Theorem • - If the total work done on some object by all forces is positive, the object speeds up. • If the total work done on some object by all forces is negative, the object slows down.

2. Simple electrical circuits • Context: How do I make a light bulb light up? • Definition: • A simple electric circuit consists of • a current source, such as acar battery • a resistor, such as a light bulb, or heater • Example: • A simple circuit: A battery creates an voltage V which pushes the current i through the resistor R. • Lecture demo: • Battery and light bulb • - Blow a fuse

3. Discussion: - Conventional current was defined early in the history of electrical science as a flow of positive charge.In solid metals, like wires, the positive charge carriers are immobile, and only the negatively charged electrons flow. Because the electron carries negative charge, the electron current is in the direction opposite to that of conventional (or electric) current. In other conductive materials, the electric current is due to the flow of charged particles in both directions at the same time. Electric currents in solutions (such as salt water) are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. Electric currents in plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing protons constitute the electric current. • Lecture demos: • Salt water circuit • Pickle circuit • Plasma ball • Unit for current: • Amperes A

4. Current Sources • Batteries (two chemicals –red&gray) • Capacitors (two metallic objects) • Rubbing of two different materials • Squeezing • Thermo couples (temperature difference) • Generator • Electrical outlet batteries

5. Wires • No voltage drop along a wire, • constant voltage along a wire • No charge is lost, • constant current along a wire

6. Resistors • Definition: A resistoris a two-terminal electrical component that opposes an electric current by producing a voltage drop between its terminals. • Examples: light bulbs, long thin wires, non-metallic materials • Discussion: • Resistance depends on temperature • Resistors produce heat & light • Power = (voltage drop) * Current • Non-metallic resistor • Lecture demos: • Resistor model • Resistors produce hat & light • low temperature • small resistance • Light bulbs in parallel • and in series • Charges flow like a liquid: • current = volume/time • voltage = pressure Ohm’s law: Voltage drop = Resistance * Current Unit of resistance: Ohm

7. Simple electrical circuits • 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