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ELECTRICITY 2

ELECTRICITY 2. Background. Circuit - a connection including a power source, wiring, and one or more resistors Resistor – something that uses electrical energy (or “lowers the potential energy”) (ie: bulb, motor, buzzer, appliance, etc.) Power Source – a battery or outlet. Schematics.

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ELECTRICITY 2

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  1. ELECTRICITY 2

  2. Background • Circuit- a connection including a power source, wiring, and one or more resistors • Resistor – something that uses electrical energy (or “lowers the potential energy”) (ie: bulb, motor, buzzer, appliance, etc.) • Power Source – a battery or outlet

  3. Schematics

  4. Types of Circuits

  5. Ohm’s Law : V = IR

  6. Power and Charge

  7. The Water Analogy - CURRENT (read, don’t copy) • When a hose is attached to a faucet and the valve is opened, water flows from the faucet through the hose. If you used a stopwatch and a graduated container, you could measure the rate of flow of the water in, for example, liters per second. • Similarly, we can measure the number of e-’s flowing past a given point in a unit of time. • Current is expressed as e-’s per second and is measured in amps. • 1 amp = 1 C of electrons per second • 1C = 6.25 E 18 e-’s

  8. Factoids About Amps • “It’s not the volts that kill you, it’s the amps!” • Plug: ground (removes excess charges from an a.c. object

  9. The Water Analogy - VOLTAGE (read, don’t copy) • Water flows through a hose because there is a driving force behind the water. We can increase the rate of flow of the water by increasing the driving force. (Add pumps to the line) • The driving force in a circuit is called the “electromotive force” (emf) and is a “difference in potential” that causes e-’s to move. • EMF is also known as voltage and is measured in volts • Think of “potential” the way we say something has a lot of “potential energy” when raised to a great height

  10. The Water Analogy – RESISTANCE (read, don’t copy) • Water molecules, moving through a pipe, rub against the walls of the pipe and slow down. The walls of the pipe oppose or offer resistance to the flow of water. Longer and narrower pipes offer more resistance to the flow of water than shorter and wider pipes.

  11. Factoids About Ohms

  12. What Impedes Electricity? There are 4 factors that affect resistance in wires: 1. Length – longer conductors offer greater resistance 2. Temperature – R h as T h (for most metals) However, R i as T h for C and semiconductors) Superconductors are certain metals which become excellent conductors at extremely low temperatures 3. Type of Material – every substance has its own electrical resistance (ie: Cu conducts better than Al) 4. Diameter of Wires – thick wires have less resistance than thin wires; resistance varies inversely with the cross-sectional area of the conductor

  13. The Water Analogy - SUMMARY

  14. Why Does Electricity Flow? Electricity flows whenever there is an electric “potential difference” No flow – No potential difference b/w bird’s feet Zap! One wire with one V is connected by balloon fabric to another wire with another voltage No flow – No potential difference b/w person’s hands

  15. Types of Current • DC – “Direct Current” • Batteries are a source of DC current • Electricity travels in one direction (- to +) • All energy is stored (chemical) and can be used up quickly • AC – “Alternating Current” • Current coming from outlets • First goes in one direction, then in the opposite at a rate of 60 cycles per second (aka: 60 Hz)

  16. Remember: • Electrons don’t actually travel through the wire, they wiggle and their energy transfers • The power company sells you energy; you supply the electrons • Can be transmitted long distances with voltage step-ups • AC has a lower heat loss in wires

  17. Safety Factoid • Electricians work “with one hand in their pocket” when there’s danger of a hot wire since if they used 2 hands the current can travel across their chest (the current doesn’t necessarily want to travel through the person; it just wants to get to the ground since the ground has relatively few e-‘s)

  18. Calculations Involving Circuits

  19. Example 1 - Series If VT = V1 + V2…then 100V = V1 + 60V So…V1 = 40 V V1 = 40 V If V = IR,……..then 40V = I1 ( 20W) So…I1 = 2 A I1 = 2 A (and since it’s a series circuit and I is the same everywhere, then…) To solve for R2, V = IR, and 60V = 2A(R2) So…R2 = 30W 40 V 30 W 2 A

  20. You have both R’s. If 1/RT = 1/R1 + 1/R2 Then 1/RT = 1/3 + 1/6 So RT = 6/3 or 2W Example 2 - Parallel Now you have a V and a T. If V = IR, then 6V = I(2W) So I = 3A Since, for a Parallel Circuit, the voltage is the same in each branch, V1 must be 6V and V2 must be 6V RT = 2W V1 = 6 V IT = 3A V2 = 6 V In a Parallel Circuit, the current is not (necessarily) the same in each branch. V1 = I1R1; 6 = (I)3; I1 = 2A V2 = I2R2; 6 = (I)6; I2 = 1A I1+I2=IT 6V 2A 1A 6V 2W 3A

  21. Example 3 - Combined You have two parallel resistors and you know their values. Start there. 1/RT = 1/5 + 1/20 1/RT = 4/20 + 1/20 = 5/20 so R2,3 = 4W So it is as if… R2,3 = 4W And to find RT, simply add R1 and R2,3 So you get 9W Since we know that the IT is 4A but we know that in a parallel branch the current splits up, it would only be true that at I1 the current would be 4A. 9W 1

  22. Kirchoff’s Rules • For complex circuits

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