Lecture One

Resistance, Ohm’s Law series and parallel

Resistance• The property of an electrical circuit that

opposes the flow of charges (electric current)• The SI unit for resistance (R) is the Ohm ()

What determines resistance?• Conductivity – how well it conducts or

what it is made out of• Length – long wires have more

resistance• Cross-section area – (thickness) Thick

wires are less resistant than thin wires

• Temperature – for most wires increased temperature means increased resistance.

Ohm’s Law

• Current is directly proportional to the voltage across the circuit an inversely proportional to the resistance of the circuit.

• Current = voltage/resistance• I = V/R

Problems for Practice

• What is the resistance of an electric frying pan that draws 12 amperes of current when connected to a 120 volt circuit?

• How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 30 volts is impressed across it?

Ohm’s Law and Simple Series Circuit

• Current has only one way to go through the resistors.

• The current through each resistor is the same.

• To get the total (or equivalent) resistance, add up the resistance of all the resistors.

• The current of the circuit is equal to the supply voltage divided by the total resistance.

+

-

V R2 R3R1

Parallel Circuit

Ohm’s Law and Simple Parallel Circuit

• The voltage is the same across each resistor.

• The current in each branch is inversely related to the resistance.

• The total current in the circuit equals the sum of the currents in the branches.

• As the number of parallel branches increases, the overall resistance decreases.

• 1/Requivalent = 1/R1 + 1/R2 + 1/R3 + 1/R4 + ….

+

-

V R2 R3R1

+

-

V

+

-

+

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V R2R2 R3R3R1R1

Lecture two!!

Complex circuits

R2

R3

R4

+

-

V

R1

Complex Circuit

Simplifying Complex Circuits

•  If two or more resistors are in series, combine them in an equivalent resistance.

• If two or more resistors are in parallel, combine them in an equivalent resistance.

• Repeat steps 1 and 2 until the circuit has been simplified into a single resistance.

• Determine the current of the simplest circuit.

AC/DC

Direct Current and Alternating Current

Direct Current

• Charge always flows in one direction.• Terminals on the battery have the

same sign as the charge.• Charge moves from repelling negative

to attracting positive terminal.

Alternating Current

• Household current is alternating (AC)• Current cycles between positive and

negative 60 cycles per second (60 hertz)

• 110 volts was chosen because it made light bulbs as bright as gas lamps

• Voltage is now officially 120 volts

240 volts

• Many big appliances like stoves, water heaters, and air conditioners need more electricity. They run on 240 volts

• 120 volts connects positive lead to ground

• 240 volts connects +120V to -120V

Alternating Current• Electrons alternate back and forth about

a relatively fixed position.• Occurs by alternating the polarity of

voltage at the source.• The frequency in North America is 60

hertz or 60 cycles per second. Typical voltage is 120 V.

• AC energy can be transmitted great distances without a lot of loss.

Lecture Three

Power

Electric Power

• The rate at which electrical energy is transformed into another form:– Mechanical energy– Heat– Light

• Electrical power = Current x voltage– 1 watt = 1 ampere x 1 volt

Problems and Formulas

The Basics for Direct Current Circuits

Current and Resistance

Current = charge/timeI = q/ t

Potential difference = current x resistanceV = IR

Household current in a circuit cannot generally exceed 15 A for safety reasons. What is the maximum amount of charge that could flow through this circuit in a house during the course of a 24.0-h day?

What is the resistance of the heating element in a car lock de-icer that contains a 1.5 V battery supplying a current of 0.5-A to the circuit?

Power

The amount of work done in a given time.

Power = work/elapsed timeP = E/ t = (qV)/ t = IV

The lighter in TJ’s car has a resistance of 4.0 . How much current does the lighter draw when it is run off the car’s 12 V battery? How much power does the lighter use?

Which has more resistance when plugged into a 120 V line, a 1400 W microwave oven or a 150 W electric can opener?