Physics Unit E

1. Physics Unit E Circuit Types

2. Circuits • Need a source of energy, a pathway for electrons, and something that uses energy. • Ex) Sources: Battery, 120 V plug, AC generator, DC generator • Pathway: Wires (copper or silver) • Loads (use energy): light bulbs, motors, heater, MP3 player • A switch is used to control a circuit.

3. Switches/fuses • An open switch interrupts the path and causes an open circuit. • A closed switch allows electrons to flow thus turning the circuit on. (Light switch at home, power button on Blu-ray player) • A fuse is also used to stop too much current from flowing through a circuit which could damage loads. • A fuse creates an open circuit when it melts and is part of the circuit. • A circuit breaker at home works similarly, except you can re-use it.

4. Circuit Symbols • 1.5 V Cell • DC Generator • AC Generator • Open Switch • Closed Switch – + DC AC

5. Circuit Symbols • Resistor • Lamp • Fuse • Motor M

6. Cells • Symbol for one cell consists of a short line (negative terminal) and a long line (positive terminal). • Adding cells together in series increases the voltage. • Electrons always flow away from the negative terminal and around a circuit to the positive one. • Note: Conventional current is used by engineers or electricians and is opposite to electron flow (current). (Conventional current flows into a negative terminal). – +

7. Cells in series • Cells connected in series are added together to give total voltage. (one cell = 1.5 V) • Two cells in series add to 3.0 V

8. Cells in parallel • Cells connected in parallel have the same voltage applied on each path. • Lasts longer, but delivers same voltage. • Increases charge, but not potential. 1.5 V

9. Series Circuit layout • In a circuit, if there is one path: it is a series circuit.

10. Series circuits • The current is the same through the loads. • All loads are in one path, so one switch is needed to control the whole circuit. • Unscrewing a light bulb will create a open circuit. • Voltage is divided up among the loads based on the resistance. • A higher resistance load will use more voltage. • All the voltage gains are equal to the voltage losses for the one path.

11. Parallel circuit layout • A circuit with more than one path is a parallel circuit. Combinations also occur.

12. Parallel circuits • Current passes through separate circuits to each load. • Each separate circuit is called a branch circuit. • Unscrewing a light bulb will stop current flow in that path only. • The current splits up to each path based on resistances of each path. • A path with lower resistance will have more current.

13. Parallel circuits • All the current that leaves a battery recombines again back at the battery. Therefore, the currents for each branch add up to the total current. • Along each path, the voltage is the same. (All the voltage gains are equal to the voltage losses for the one path.)

14. Circuit Diagrams • Draw a circuit with a DC Power source, two resistors in parallel, an ammeter measures total current and a voltmeter measures the voltage drop across one resistor.

15. Circuit Diagrams • Draw a circuit diagram with 2 bulbs in series, a switch that controls the circuit, a fuse and 3 cells connected in series.

16. Circuit Diagrams • Draw a circuit with 6.0 V in cells, controlled by a switch, in series with a light bulb and a resistor. • What is the voltage at the light bulb and resistor if they have the same resistance? • If 0.2A of current flows in this circuit, what current flows through the bulb and the resistor? • Determine the resistance for the two loads using Ohm’s Law.

17. Class/home work • Page 554 #1-7 • Page 575 #1-7