Current Electricity

Electric Current1. Definition: flow of

electrically charged particles passing through a given area (movement of electrons in a wire)

2. Conventional Current – the flow of positive charge through a conductor

3. Why does current flow?a. The ends of an electric

conductor are at different electric potentials

b. Charge will flow from one end to another

c. There must be potential difference (voltage) applied to opposite ends of conductor

d. Flow will continue until each end reaches a common potential (no potential difference)

4. Measuring Electric Current (I)

a. Current measures the rate at which electrons flow

b. Current = amount of charge

timeI = q / t

c. Units: Ampere, A = Coulomb / second

Measuring Electric Current (I)(continued)

d. An ampere is the flow of 1 Coulomb of charge per second.

(6.25 x 1018 electrons per second)

e. No net charge on a conductor that carries a current (# protons in conductor = # electrons passing through)

If 10. coulombs of charge are transferred through an electric circuit in 5.0 seconds, then the current in the circuit is ….

A wire carries a current of 2.0 amperes. How many electrons pass a given point in this wire in 1.0 sec?

5.Factors effecting current a. Potential Difference (voltage)

i. Provides the “push” on charges to move them through the conductor (sometimes referred to as the electromotive force, emf)

ii. The greater the push on the charge, the greater the current

Examples of Voltage Sources

1. battery (converts chem. E to electric)

2. photovoltaic cell (light to electric)

3. generator (mechanical to electric)

b. Electric Resistancei. the conductor itself can have a

slowing effect on the current moving through it

ii. the resistance offered by a wire depends on the:

nature of the substance (resistivity)

length of the wire (long wire, more R)

width of the wire (thin wire, more R)

Resistance(continued)

iii.Resistance is temperature dependent

- The higher the temperature, the greater the resistance

iv. Resistance measured in Ohms (W)

6. Ohm’s Lawa. Current in a circuit is:

** directly proportional to the voltage across circuit** inversely proportional to the resistance of the circuit

b. Resistance = Voltage OR R = V Current Ias it appears on Reference Tables

b. Resistance = Voltage OR R = V Current Ias it appears on Reference Tables

Therefore, double the voltage, double the currentDouble the resistance, half the current

- Typical lamp cord has a resistance of less than 1 W

- Electric toaster oven offers 15 – 20 W of resistance

Practice ProblemWhat is the resistance of an electric

frying pan that draws 12 A of current when connected to 120-volt circuit?

Practice ProblemHow much current is drawn by a lamp

that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it?

Practice ProblemIf the resistance of your body were 100,000 W, what would be the current in your body when you touched the terminals of a 12-volt battery?

c. Resistance in a wire

Resistance = resistivity x lengthcross-sectional area

R = r L - resistivity can be found on Reference Tables

A - cross-sectional area will either be given or determined using A = pr2

Resistance (continued)

What is the resistance of .30 m length of copper wire that has a cross-sectional area of 5.0 x 10-5 m2?

7. Electric Powera. the charge moving in a circuit

expends energyi. typically results in the conversion of

electric energy to heat or mechanical energy

b. the rate at which energy is converted is electric power

Electric Power (c0ntinued)

c. Calculated using:Power = current x voltage

P = I VUnits: Watts = ampere x volt- this and variations of this formula can be found on your Reference Tables

In a circuit that consists of two lamps, the battery supplies a potential difference of 1.5 volts. Current in the circuit is 0.10 ampere, what rate does the circuit use energy?

8. Electric Energy (W)a. electric fields do work on charges

moving them through a circuitb. the charges can, in turn, can

convert the energy imparted to them

c. the work done by the charges is equal to the electrical energy used in the circuit

Electric Energy(continued)

d. Calculated using:Electric energy = Power x time

W = Pt- this and alternative versions

can be found in your Reference Tables

An electric iron operating at 120 volts draws 10. amperes of current. How much heat energy is delivered by the iron in 30. seconds?