1. Chapter 34: Electric Current
Conceptual Physics
Bloom High School

2. 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

3. 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

4. 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)
Generators (car alternator)
Mechanical energy provides pressure
Voltage (V)- Potential energy per Coulomb
Electromotive force (emf)
“Electric pressure”
Charge- flows through a circuit

5. 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)

6. 34.4 Electrical Resistance
Resistance- prohibits the flow of charge
Measured in Ohms (W)
Higher resistance in thinner, longer, higher temp. wires

7. Resistor Chart Write down the 1st band, 2nd band, etc.
The final band represents the # of zeros
=25kW
=460kW
2-7-6=276W

8. 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

9. Examples Larger resistance, greater heat generated
Toaster W
Lightbulb- 100W
Little heat generated
Lamp cord- 1W

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. Calculations