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.

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 DifferenceWhat describes internal circuit?What describes external circuit?

• Sort of like Sisyphus.

0 PE

PE

Heat, SoundSquashed bugs

Potential Difference

0 PE

PE

VoltageRise

VoltageDrop

• 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.

Resistance

Viva la 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

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 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

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