Electricity, Ohms law and circuits

1. Electricity, Ohms law and circuits

2. 34 Current Electricity • the flow of electric energy or electrons (e-) • e- will only flow if there is a potential difference: • potential difference is called voltage • voltage drops along a circuit • created by doing work on the e- by a charge pump: ex. battery, generator, etc. • symbol = V • unit = volts (V) • more voltage = more current

3. Current • amount of charge (e-’s or their energy) that flows past a point in a second • current is the same throughout any one current path • symbol = I • unit = amperes (amps) (A) • more current means more voltage • I=Δq / Δt • q – charge, Coulombs (C) • t – time, seconds (s)

4. I = 11.4 mA = 0.0114 A • How many electrons per hour flow past a point in a circuit if it bears 11.4 mA of direct current? I = q / t q = Ne I = Ne / t N= I t / e N= 0.0114A∙ 3600s / 1.6 x 10-19 C = 2.57 x1020

5. Resistance • slows the flow of current • causes a voltage drop (potential difference) because it uses up the e-‘s energy • almost everything has some resistance Ex: light bulb, fan, resistor, motor, wire, etc. • better conductors have lower resistance • more resistance means more electrical energy is converted into heat, light, motion, etc.

6. symbol = R • units = Ohms () • lowering the resistance means more current can flow • R = ρ L / A • ρ – resistivity of the material, (Ω m) • L – length, m • A – area, m2

7. L = 4.0 cm R = 1000Ω A = 0.2 cm2 • What is the resistivity of a substance which has a resistance of 1000 Ω if the length of the material is 4.0 cm and its cross sectional area is 0.20 cm2? R = ρ L / A ρ = R A / L ρ = 1000Ω·0.2 cm2 / 4.0 cm = 50 Ω∙ cm 50 Ω∙ cm∙ 1m / 100cm = 0.5 Ω∙ m

8. Ohm’s Law V: potential difference: unit volts (V) I: current: unit amp (A) R: resistance: unit ohm () • Current affected by changes in V and R. • V and I are directly related. • R and I are inversely related. • The drop in potential occurs as electrical energy is transformed to other forms (heat, light) and work is done.

9. Power: Rate at which electrical energy is used up. • Unit: Watts (W) • Higher wattage appliances will use up more energy from the electrons – usually draw more current and cost more to run.

10. Example: A 60 Watt bulb running on household current can carry a current of 0.5 amps. What is the voltage? • What is the resistance?

11. Example: A portable generator can generate 25 amps of current at 120v. How much power does the generator produce? • How many 100 watt light bulbs with this generator be able to safely light? • What is the total resistance of the generator?

12. Drawing Circuits • Charge pump always goes on left. • Always use straight lines and 90 degree angles. • Space symbols out evenly. • Current always flows clockwise. • Symbols:

13. Battery • Resistor • Light bulb • Switch

14. ITotal ITotal • Series components are put together so that all the current must go through each one • SERIES: One path for current to flow. Three bulbs in series all have the same current.

15. Parallel components are put together so that the current divides, and each component gets only a fraction of it. • PARALLEL: Separate path for each bulb. I = 1/3 Itotal Itotal Itotal I = 1/3 Itotal I = 1/3 Itotal

16. Circuits: • Total voltage (Vtotal) and current (Itotal) are measured at the charge pump. • Each resistor is numbered – clockwise for series, left to right for parallel. • To solve a circuit: • Try to use Ohm’s Law • Use the rules (listed below) • Series Circuits: • ONE path for current to flow so: (1) the current is the same everywhere:

17. (2) each resistor uses up only part of the voltage supplied voltage drops across the resistors add up to the total voltage: (3) each resistor slows the current more so the total resistance is found by adding up all the Ohms present:

18. Parallel Circuits: • MULTIPLE paths for current to flow so: • the current splits into the different paths so current adds up to the total: • each resistor uses up all the energy of the electrons moving through it voltage drop is the same for each resistor:

19. (3) each resistor slows down only part of the current so the total resistance is lower than any one individual resistor: 1/Req = ∑(1/Ri ) Therefore,

20. 12V 5Ω 10Ω 5Ω (2) Example: Rtotal= _______ Itotal=_______ V1 = _______ V2 = _______V3 = ____________ I1 = _______ I2 = _______ I3 = _____________ 2Ω 6A 12V 12V 12V 2.4A 2.4A 1.2A

21. Kirchhoff's 1st Rule • Kirchhoff's 1st rule is also called the “junction rule”. • The sum of the currents entering a junction equals the sum of the currents leaving the junction. • This rule is based upon conservation of charge. • Find the current I4 (magnitude and direction). I2 = 3.0A I1 = 4.0A I4 = I1 +I2 – I3 I4 = ? I4 = 4.0A +3.0A – 1.5A = 5.5A I1 = 1.5A

22. Kirchhoff's 2nd Rule • Kirchhoff's 2nd rule is also referred to as the “loop rule”. • The net change in electrical potential in going around one complete loop in a circuit is equal to zero. • This rule is based upon conservation of energy.

23. Start Point - + • Use the loop rule to determine the potential drop across the light bulb. + V1 = 2.0V + - - V2 = 1.5V V5 = 9.0V - + - + V4 = ? V3 = 3.0V V1 + V2 + V3 + V4 - V5 = 0 V4 = V5 - V1 – V2 – V3 V4 = 9.0V – 2.0V – 1.5V – 3.0V = -2.5V

24. Start Point - + • Use the loop rule to determine the potential drop across the light bulb. + V1 = 2.0V + - - V2 = 9.0V V5 = 1.5V - + - + V4 = ? V3 = 3.0V V1 + V2 + V3 + V4 - V5 = 0 V4 = V5 - V1 – V2 – V3 V4 = 1.5V – 2.0V – 9.0V – 3.0V = -12.5V