Chapter 34: Electric Current

Conceptual Physics

Bloom High School

34.1 Flow of Charge

• Heat flow- difference in temperature exists• Potential difference- when the ends of a conductor

are at different electric potentials– Voltage– Continues until both ends are at the same potential– DPE/q (Volts=Joules/Coulomb)

• Potential- PE/q• Water as an analogy (Fig. 34.1)

– To keep the water flowing, we need a pump• Keeps the difference in pressure different

– More water pumping is like higher pressure water

34.2 Electric Current

• Electric current- the flow of electric charge– Ampere (A)- 1 Coulomb per second

• Coulomb (C)- 6.25x1018 electrons

– For a current of 5 amps, 5 C/s flows through it• The net charge on a wire is always zero

– The number of electrons moving in equals the number moving out, regardless of voltage

– Think of water flowing through a hose

34.3 Voltage Sources

• Voltage source- an electric “pump” is required to keep electrons moving– Established across a circuit– Dry cells (regular batteries)

• Chemical reaction provides pressure

– Wet cells (car batteries)• Chemical reaction provides pressure

– Generators (car alternator)• Mechanical energy provides pressure

• Voltage (V)- Potential energy per Coulomb– Electromotive force (emf)– “Electric pressure”

• Charge- flows through a circuit

Home electricity

• Potential difference of 120 volts between the two holes in the outlet– 120 Joules of energy applied to each Coulomb

• Charge flowing through vs across– Charge flow through (like water in a hose)– Voltage across circuit is constant (like water

pressure in the hose)

34.4 Electrical Resistance

• Resistance- prohibits the flow of charge

• Measured in Ohms (W)– Higher resistance in

thinner, longer, higher temp. wires

Resistor Chart

• Write down the 1st band, 2nd band, etc.

• The final band represents the # of zeros

• 2-5-000=25kW• 4-6-0-000=460kW• 2-7-6=276W

34.5 Ohm’s Law

• Ohm- discovered that current (amps) is proportional to the voltage (volts) and inversely proportional to the resistance (W)– Current = Voltage/Resistance (I=V/R)– Amperes = Volts/Ohms

• Greater resistance leads to less current• Greater voltage leads to greater current

Examples

• Larger resistance, greater heat generated– Toaster- 15-20W– Lightbulb- 100W

• Little heat generated– Lamp cord- 1W

34.6 Ohm’s Law and Electric Shock

• Volts or Amps?– Human body offers resistance of 100W-500kW– 12V on dry skin isn’t felt– 12V on wet skin is painful– See Table 34.1

• Birds can rest on a wire because there is negligible potential difference between their feet

• If the resistance is 1000W, and you touched a 24V battery, how much current is drawn?– 24V/1000W=0.024A (possibly fatal)

• Third prong- acts as a ground for higher voltage devices

34.7 Direct Current and Alternating Current

• Direct current (DC)- charge always flow in one direction– From negative terminal toward

positive terminal• Alternating current (AC)- charge

moves back and forth– In North America, cycles at 60 Hz

120V– In Europe, 240V

High Voltage in the home

• Some appliances require 240VAC– Clothes dryer, electric furnace, tanning bed– Every home has three lines: +120VAC, 0VAC, -

120VAC– 120VAC comes from +120V connected to 0V

line(neutral)• Potential difference of 120VAC

– 240VAC comes from +120V connected to -120V line• Potential difference of 240VAC

34.8 Converting AC to DC

• Diode- allows current to flow in one direction only– Takes the back-and-forth of AC and makes it “forth”

only– Because it removes half of the charge, a capacitor

is used to smooth out the charge• Capacitor- acts as a short-term battery

– Smoothes out the pulses from a diode

34.9 The Speed of Electrons in a Circuit

• When a voltage is applied (potential difference), the speed of the electrons is slow– The random motion of the electrons is pushed along

by the voltage• Net speed (drift speed)- 1 cm/s in DC• In AC, drift speed is 0 m/s

34.10 The Source of Electrons in a Circuit

• When you purchase a hose, it doesn’t come with water installed

• When you purchase a lamp, it does come with electrons already!

• Electric utilities sell the energy needed to oscillate the electrons, not the electrons themselves

34.11 Electric Power

• Moving charges expend energy and results in heat

• Electric power- the rate at which mechanical energy is converted to electrical energy– Power = Current x Voltage (P=I V)– 1 Watt = Ampere x Volts

• 100 W bulb = 120V x 1A• 60W bulb = 120V x 0.5A

Calculations