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Ohms LawOhms Law
• V = IRV = IR
• V = voltage in volts (aka potential V = voltage in volts (aka potential difference)difference)
• I = Current in ampsI = Current in amps
• R = resistance in ohms (R = resistance in ohms (ΩΩ))
CurrentCurrent
• How would you define it?How would you define it?
• CurrentCurrent—the movement of electric —the movement of electric charge through a mediumcharge through a medium
Current, More PreciselyCurrent, More Precisely• CurrentCurrent—the —the raterate at which electric charge at which electric charge
flows through a given area. Described by the flows through a given area. Described by the letter Iletter I
• SI Unit: ASI Unit: Amps (amperes) “A”mps (amperes) “A”
• So picking a point, the amount of charge that So picking a point, the amount of charge that flows past that point in a given amount of flows past that point in a given amount of time.time.
+
++++ ++ +++
CurrentCurrent
I = I = Q / Q / tt• Current = Current = charge passing through areacharge passing through area
time intervaltime interval
• SI Unit: SI Unit: amperesamperes (almost always called (almost always called ampsamps), abbreviated A.), abbreviated A.
• 1 A = 1 C/s1 A = 1 C/s
Ex 1 HomeworkEx 1 Homework
• If the current in a wire of a CD player is If the current in a wire of a CD player is 5.00mA, how long would it take for 2.0C 5.00mA, how long would it take for 2.0C of charge to pass through a point?of charge to pass through a point?
If the current in a wire of a CD player is 5.00mA, 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 how long would it take for 2.0C of charge to pass through a point?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 CurrentConventional Current
• Current is the flow of any charge, so it Current is the flow of any charge, so it can be positive or negative flow.can be positive or negative flow.
• In conductors, what tends to be flowing?In conductors, what tends to be flowing?
Possible Charge FlowsPossible Charge Flows
• In conductive metals—electronsIn conductive metals—electrons
• In body fluids/solutions—ionsIn body fluids/solutions—ions
• In particle accelerators—protonsIn particle accelerators—protons
• All cause current! But direction All cause current! But direction described in relation to a positive charge.described in relation to a positive charge.
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.
Potential DifferenceWhat describes internal circuit?What describes external circuit?
• Sort of like Sisyphus.
0 PE
PE
Heat, SoundSquashed bugs
• All charges moving everywhere transmit the energy to the light bulb.
0 PE
PE
VoltageRise
VoltageDrop
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.
• 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
RReessiissttaannccee
• Resistance—the opposition of motion of charge through a conductor.
• determines how much current will flow in a circuit with known voltage source:
• Quantitatively:
R =
• The SI unit for resistance is the ohm ().
VI
• Materials that have a constant value for resistance over a large range of potential differences or voltages is said to be ohmic.
CCaauussiinngg RReessiissttaannccee
• In addition to the load the wire itself offers resistance
• “internal friction” due to collisions
• Lets examine the wire
CCaauussiinngg RReessiissttaannccee
1)1) LenLength of conductor
2) Cross sectional area of conductor
3) Temperature
4) Nature of the material
4 Factors effecting Resistance of the conductor
If the following are increased, resistance will:
– Length– increase– Cross-sectional area– decrease– Temperature – increase – Nature of the material
CCaauussiinngg RReessiissttaannccee
1)1) LenLength 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 “ ”
LLiiee DDeetteeccttoorrss
• Uses the natural resistance of the body.
• People have a general resistance of around 500 000 .
• Sweat causes dramatic change in resistance (as low as 100.)
• Machine measures the GSR.
SSuuppeerrccoonndduuccttoorrss
• 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
EExxaammppllee
Ex 3 The resistance of a steam iron is 19.0. What is the current in the iron when it is connected across a potential difference of 220.V?
EExxaammppllee
Ex 3 The resistance of a steam iron is 19.5. What is the current in the iron when it is connected across a potential difference of 220.V?
V = IRI = V/R
I = 220V/19I = 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 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• 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 106 J
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. 5• 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
Ex4. 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