Transcript Ohms PPT

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)
2)
3)
4)
Length of conductor
Cross sectional area of conductor
Temperature
Nature of the material
If the following are increased,
resistance will:
– Length
– increase
– Cross-sectional area
– decrease
– Temperature
– increase
– Nature of the material
1)
2)
3)
4)
Length of conductor
Cross sectional area of conductor
Conducting material
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
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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 106 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 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
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?
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
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?
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
Cost
Cost = Price x kWHr
Cost = ($0.10/kWHr) (3.75 kWHr) = $0.38