Current Electricity

Section 22.1 ObjectivesDescribe conditions that create current in an

electric circuit.Use Ohm’s law to calculate properties of an

electric circuit.Differentiate between power and energy in

an electric circuit.

Electric CurrentI=ΔQ/Δt

Example ProblemsThe current in a light bulb is 0.835 A. How

long does it take for a total charge of 1.67 C to pass through the filament of the bulb?

A total charge of 9.0 mC passes through a cross-sectional area of a nichrome wire in 3.5 s. What is the current in the wire?

ResistanceR = ΔV/IOhm’s law—the resistance is constant over a

wide range of applied potential differences

ResistorsCan change current by changing the

potential difference across the ends of the conductorI = ΔV/R

How can the current change if the potential difference stays the same?

Example ProblemsThe resistance of a steam iron is 19.0 Ω.

What is the current in the iron when it is connected across a potential difference of 120 V?

The current in a microwave oven is 6.25 A. If the resistance of the oven’s circuitry is 17.6 Ω, what is the potential difference across the oven?

Sources of CurrentPotential difference

Batteries

Generators

Electrical Components

Section 22.2 ObjectivesExplain how electrical energy is converted

into thermal energy.Calculate the amount of power dissipated

through resistors.Calculate the amount of energy used in

kilowatt-hours.

Electric PowerP = IΔV

Using Ohm’s law (ΔV = IR), we can express the power dissipated by a resistor by the following equations:P = I ΔV P = I2RP = (ΔV)2/R

Example ProblemAn electric space heater is connected across a 120 V outlet. The heater dissipates 1320 W of power in the form of electromagnetic radiation and heat. Calculate the resistance of the heater.

Example ProblemA 6.0 V battery delivers a 0.50 A of current to an electric motor connected across its terminals. What power is delivered to the motor?

The Kilowatt-HourAmount of electric energy used by a device is

its rate of energy consumption (J/s) times the number of seconds (s) used

Example ProblemAn electric space heater draws 15.0 A from a 120 V source. It is operated, on the average, for 5.0 h each day.

a) How much power does the heater use?b) How much energy in kWh does it

consume in 30 days?c) At $0.12 per kWh, how much does it

cost to operate the heater for 30 days?