Ohms Law V = IR V = voltage in volts (aka potential difference) I = Current in amps R = resistance in ohms (Ω)
Current How would you define it? Current the movement of electric charge through a medium
Current, More Precisely Current the rate at which electric charge flows through a given area. Described by the letter I SI Unit: Amps (amperes) A + + + + + + + + So picking a point, the amount of charge that flows past that point in a given amount of time.
Current I = DQ / Dt Current = charge passing through area time interval SI Unit: amperes (almost always called amps), abbreviated A. 1 A = 1 C/s
Ex 1 Homework If the current in a wire of a CD player is 5.00mA, how long would it take for 2.0C of charge to pass through a point?
If the current in a wire of a CD player is 5.00mA, how long would it take for 2.0C of charge to pass through a point? t = Q/I t = 2 C/.005 A 400 sec
Where do the charges come from? Charge carriers are the electrons in the conducting wire. They transmit the energy
Conventional Current Current is the flow of any charge, so it can be positive or negative flow. In conductors, what tends to be flowing?
Possible Charge Flows In conductive metals electrons In body fluids/solutions ions In particle accelerators protons All cause current! But direction described in relation to a positive charge.
Why does current flow?
What is potential energy? stored energy due to an object s position or condition in a field of force.
Think: Where is the electrical potential energy of a positive test charge (q+) higher, at the point A or B? Why? Point A. Because of it s location, it is not where it wants to be. It took work to get it there!
Describe: High energy location for a positive test charge is furthest from (-)source charge; near a (+)source charge low energy location for a positive test charge is near a (-)source charge; far from (+)source charge
Circuits This explains why conventional current flows opposite to the flow of electrons From positive terminal to negative terminal Describes flow in relation to + test charge If a charge moves in an electric field it s potential energy changes Charge flows from Hi potential to Low potential, if there is a conducting path.
Internal vs External Circuit
Potential Difference What describes internal circuit? What describes external circuit? Sort of like Sisyphus. PE 0 PE Heat, Sound Squashed bugs
Potential Difference PE Voltage Rise Voltage Drop 0 PE
All charges moving everywhere transmit the energy to the light bulb. PE Voltage Rise Voltage Drop 0 PE
example 6 V battery. Internal Circuit (the electrochemical cell) Provides chemical energy to move test charge from low energy negative terminal internally to high energy positive terminal External Circuit (Wires attached to terminal) Test charge: natural movement Away from positive, towards negative As + test charge moves from + terminal to terminal, it loses 6V of potential energy for every coulomb of charge Since energy can t be destroyed, it is transformed (light turns on)
VOLTAGE Battery is the Voltage source Referred to as Voltage Potential Difference Electrical Potential Electromotive Force Electrical pressure battery voltage is the amount of work (energy) done per Coulomb of charge (J/C =1 V) Voltage sources only maintain the difference in potential in the circuit. SI Unit: Volts (V) or J/C
FYI: Electrical Potential Energy vs Electrical Potential Electrical Potential Energy: Joules Dependent on magnitude of charge and location of charge Describes work to move a charge Electrical Potential: Voltage or Joules/Coulomb is the Joules of potential energy per charge Describes affect of field at a particular location 1J/C = 1 Volt
Most alkaline dry cells contain a zinc casing which serves as the negative terminal. The zinc is oxidized to Zn+2 ions during the chemical reactions. This oxidation process produces two electrons per zinc atom that accumulate at the negative terminal. There is a carbon rod that is inserted in the middle of the cell that serves as the positive terminal of the cell. The carbon rod is not consumed in the chemical reactions.
Viva la resistance. Resistance
When a light bulb is connected to a battery, what effects how much current flows through the circuit? Voltage (provides energy work to move the charge) Direct
Resistance the opposition of motion of charge through a conductor. determines how much current will flow in a circuit with known voltage source: Quantitatively: DV R = I The SI unit for resistance is the ohm (W).
Materials that have a constant value for resistance over a large range of potential differences or voltages is said to be ohmic.
In addition to the load the wire itself offers resistance internal friction due to collisions Lets examine the wire
4 Factors effecting Resistance of the conductor 1) Length of conductor 2) Cross sectional area of conductor 3) Temperature 4) Nature of the material
If the following are increased, resistance will: Length increase Cross-sectional area decrease Temperature increase Nature of the material
1) Length of conductor 2) Cross sectional area of conductor 3) Conducting material 4) Temperature
The calculations; Ohms Law R = V/I or more commonly V = IR Where V = Voltage in Volts V I = Current in Amps A R = Resistance in Ohms W
Uses the natural resistance of the body. People have a general resistance of around 500 000 W. Sweat causes dramatic change in resistance (as low as 100W.) Machine measures the GSR.
We said that temperature affects resistance. Some materials have zero resistance below a certain temperature (called their critical temperature) Materials that display such properties are called superconductors.
Ex 2: What is the potential difference required for 20 amps of current to flow through a 5 ohm resistor? What are you solving for? V = IR V = (20 A) (5Ω) V = 100 V
Ex 3 The resistance of a steam iron is 19.0W. What is the current in the iron when it is connected across a potential difference of 220.V?
Ex 3 The resistance of a steam iron is 19.5W. What is the current in the iron when it is connected across a potential difference of 220.V? V = IR I = V/R I = 220V/19W I = 11.28 A
Something to think about What is the function of a battery? How is current flow described? What is the unit for voltage, current, resistance, and power? What factors affect resistance in a conductor? What is the relationship between voltage, current, and resistance?
What is the difference between a 40W light bulb and a 75W light bulb?
What is power? In general The amount of work done per time period. How have we discussed the idea of work in relation to electricity? Work is done by the electric field on electrons (or other charged particles) as they move from one area to another due to potential difference.
Electric Power Electric power is the rate at which charge carriers do work.
Electric Power P = W/t or P = E/t = J/s Where W is Work and E is Energy Power is measured in Watts (W). Given this relationship: P = IV, how is this still watts? P= (C/s)(J/C) = J/s
Ex. 4 A 1000.-watt heater operates at 115 V. Calculate the current, resistance, and energy generated in 1.00 hour.
Ex. 4 A 1000.-watt heater operates at 115 V. Calculate the current, resistance, and energy generated in 1.00 hour. I = P/V I = 1000W/115V = 8.70A R = V/I R = 115V /8.70A =13.2Ω E = Pt E = (1000W)(3600s) = 3.6 x 10 6 J
Ex. 4 A 1000.-watt heater operates at 115 V. Calculate the current, resistance, and energy generated in 1.00 hour. I = P/V I = 1000W/115V = 8.70A R = V/I R = 115V /8.70A =13.2Ω E = Pt E = (1000W)(3600s) = 3.6 x 10 6 J
Ex 5 The Nutri Ninja Junior offers a resistance of 16 ohms when a current of 7.5A is flowing through the circuit. What is the power rating of the Nutri Ninja delicious smoothies? ASK YOURSELF WHAT YOU KNOW R = 16Ω I = 7.5 A P =? Solve for V using V = IR 7.5A x 16Ω = 120V Solve for P using P = IV 7.5A x 120V = 900W
Ex 6 Compare the resistance of a 40 Watt light bulb to a 75 Watt light bulb when connected to a 120 V circuit. Which bulb has a higher resistance? How would we calculate this?
Ex 6 Compare the resistance of a 40 Watt light bulb to a 75 Watt light bulb when connected to a 120 V circuit. Let s calculate this. Knowns: P = 40W P = 75W V = 120V R =? I have volts and power, how do I find resistance? I know V = IR and P = VI Rearrange V = IR to solve for I, substitute this into P = VI Thus: I = V/R therefore P = V x V/R or P = V 2 /R Rearrange P = V 2 /R to solve for R R = V 2 /P R = (120V) 2 /40W = 360Ω R = (120V) 2 /75W = 192Ω The higher the Wattage the lower the resistance!!!!!!
Electric Power So what do we pay for on our electric bill? Rate of energy usage in Watts More specifically KW/hr instead Watts (which is J/s)
Ex. 7 How much does using a 1500W hair dryer cost each month (30 days) if you use it for 5 minutes each day, and the price for 1 kilowatt hour is $0.10? SOLVE
Ex 7. How much does using a 1500W hair dryer cost each month (30 days) if you use it for 5 minutes each day, and the price for 1 kilowatt hour is $0.10? Figure out total time (in hours since final answer is based on kw per hour) Convert Watt to kilowatts Determine Energy Usage per hour Multiply Energy Usage times cost
Cost Cost = Price x kwhr Cost = ($0.10/kWHr) (3.75 kwhr) = $0.38 Ex 7. How much does using a 1500W hair dryer cost each month (30 days) if you use it for 5 minutes each day, and the price for 1 kilowatt hour is $0.10? Time in Hours: (30 days x 5min/day x 1hr/60min) = 2.5hr kilowatts 1500W x 1kW/1000W = 1.5 kw Energy per hour E = Pt (1.5 kw) (2.5hr) = 3.75 kwhr