Resistance and Ohm’s Law The Livermore Centennial bulb was first installed in 1901 and has only been turned off a handful of times since then. It holds the record for being the oldest known working light bulb. They don’t make them like the used to! http://www.centennialbulb.org/photos.htm Livermore California Lightbulb 110 years old – only turned off a handful of times (located at Firehouse). Reached 1,000,000 hours in 2015! Resistance and Ohm’s Law

Recall that Voltage is…. how much electrical energy is carried by each charged particle. The higher the voltage (potential difference) the greater the push between electrons and the greater the tendency of electrons to flow! (like water pressure in a hose). Circle the picture showing greater voltage!

Current

What is Current? The flow of electrical charge through a circuit. Which tube has more current passing through it?

Resistance

What is resistance? Resistance is how hard it is for an electric current to flow through a material. Electrical resistance ( a load) slows down current and transforms electrical energy into other forms of energy. Which tube has more resistance?

Why does a bulb light up? Resistance in the filament transforms electrical energy into heat and light.

What is a resistor? A device in a circuit that slows down current thus lowering the voltage through parts of the circuit.

Ohm’s Law V = I R Georg Ohm

How is voltage related to current? The amount of current that flows through a wire depends on the voltage. The greater the voltage the faster the charges flow. V = IR If ↑ V then ↑ I If ↓ V then ↓ I

How is Current related to Resistance? Amount of current also depends on how the wire resists the flow of the charge. V = IR or I = 𝑽 𝑹 If ↓ R then ↑ I If ↑ R then ↓ I

V = I R Summary The unit for resistance is called the Ohm (Ω) The relationships between voltage(V), current(I), and resistance(R): V = I R in Volts (V) in Amperes (A) in Ohms (Ω)

Ohm’s Law Problem A vacuum cleaner is plugged into a 120 V outlet. The vacuum cleaner is rated at 12000 mA. What is the resistance of the motor and circuitry of the vacuum?

Doing the Work Thinking Steps Identify the known variables. Voltage (V) = 120 V Current (I) = 12000 mA Identify the known variables. Identify all the unknown variables. Adjust formula to solve for unknown. Convert all values to standard units. Volts (V)–Ohms () - Amperes(A) Substitute the numbers into the formula and solve. Resistance (R) = ? V = IR  𝐼 𝑅 𝐼 = 𝑉 𝐼 →𝑅 = 𝑉 𝐼 12000 mA X 1 𝐴 1000 𝑚𝐴 = 12 A 𝑅= 𝑉 𝐼 = 120𝑉 12𝐴 =10 

DO THIS! Textbook pages 293, 294 #1-3 on both! Text: Pg. 301 #8 – 13 (omit 11) Text: Pg. 303 #26 – 31 (omit 30) Calculating Resistance Problems! For marks!

Calculations using Ohm’s Law V=IR I (amps) V (volts) R (ohms) 12 75 15 240 5.5 20

Calculations using Ohm’s Law I (amps) V (volts) R (ohms) I=V/R =12/75=0.16 A 12 75 15 240 R=V/I =240/15=16  5.5 V=IR =5.5 x 20=110 V 20