1. Electric Current, Voltage, and Resistance

2. Electrical Pressure It is a flow of charged particles.
Charges flow from high voltage to low voltage
Voltage = electrical pressure that pushes charge
Voltage difference (V) is the push that causes charges to move
Voltage is measured in volts
Flow of current stops when the differences are equal.
How could you keep flow moving?

3. Flow of Charge
Charge will flow when there is a potential difference between the ends of a conductor

4. Voltage
Provides the “electric pressure” to move the e- between terminals in a circuit
If there is a voltage difference ACROSS a wire, then charge will flow THROUGH it.
If voltage difference increases, current increases

5. Batteries Dry Cell: Wet Cell:
Produces voltage difference between positive and negative terminals
The voltage difference between the terminals causes current to flow though a closed circuit
Wet Cell:
Contains two connected plates consisting of metallic compounds in a conducting solution

7. Resistance:
The tendency for a material to oppose the flow of electrons.
This happens because electrical energy is changed into thermal and light energy.
Causes electric current to lose energy as it moves through the filament
Resistance is measured in ohms (Ω)
Electrical conductors have a lower resistance than insulators.

8. Resistance (cont.)
The size, temperature, and length of the wire affects resistance
Electrons move more efficiently through thick wires than thin wires (the thicker the wire, the less resistance).
The longer the wire, the greater the resistance because more collisions occur.
The resistance increases as temperature increases.

9. Electric Resistance
Resistance decreases the amount of e- that can flow through a conductor
Increase resistance = decrease current
Factors affecting resistance:
Conductivity of material
Thickness of wire
Length of wire
temperature

10. Electric Current: The rate of flow of electric charge
Symbolized by: I (capital i)
Measured in coulombs per second
1 coulomb per second is an ampere (A)
1 C/s = 1 A
We can measure it with a device called an ammeter.

11. Closed Circuits A circuit is closed, conducting path
Electric Current is the flow of charges through a wire or any conductor
Current (I) is measured in Amperes (A)
Electrons are the charges that move because they are not locked in the nucleus but move around the atom

12. Ohm’s Law:
Current = Voltage Difference Resistance I (A) = V(V) R(Ω)

13. Ohm’s Law and Electric Shock
Touching surfaces of differing potential you become a pathway for current
Electric shock can overheat (cook) tissues or disrupt nerve functions = disrupt breathing…remove with non-conducting material and apply artificial respiration

14. Fuses:
When a fuse is blown, a small piece of metal melts if the current becomes too high.
This causes a break in the circuit, thus stopping the flow of current through the overloaded circuit.
Circuit breakers have a piece of metal that bends instead of melting and can be reset by changing it back to the on position.

15. Electrical Power
The rate at which electrical energy is converted to another form of energy
Power = current x voltage
Watts = Amps x volts

16. Calculating Energy: Depends on two things: Energy = power x time
Power required
How long they piece of equipment is used
Energy = power x time
E (kWh) = P (kW) x t(h)
Unit is kilowatt-hr (kWh)

17. Electric Circuits: Series: Parallel:
The current has only one loop to flow through.
Flashlights and some holiday lights
Amount of current is the same everywhere.
When any part of a series circuit is disconnected, no current flows through the circuit (open circuit).
Parallel:
Contain two or more branches for current to move through.
The current splits up to flow through the different branches.
Voltage difference is the same in each branch.
More current flows through the branches with lower resistance

18. Circuit Diagrams (schematics)
BATTERY/
VOLTAGE
SOURCE
SWITCH
RESISTOR

19. Problems
A 110 volt wall outlet supplies power to a strobe light with a resistance of 2200 ohms. How much current is flowing through the strobe light?
0.05 A
A CD player with a resistance of 40 ohms has a current of 0.1 amps flowing through it. Sketch the circuit diagram and calculate how many volts supply the CD player.
4.0 V
A 120-volt power source supplies a lamp with a resistance of 192 ohms. What is the current flow of the circuit?
0.625 A

20. Problems:
What is the resistance of the circuit conductors when the conductor voltage drop is 3 volts and the current flowing through the conductors is 100 amperes?
0.03 Ω

21. Problems:
In a series circuit with two 3-ohm light bulbs powered by a 6 V battery:
What is the resistance in the circuit?
What is the current in the circuit?
What is the voltage drop across each resistor?
Which bulb is brightest and why?
What would happen to the following if I added a 6-ohm light bulb after the second 3-ohm bulb?
The resistance
The current in the circuit
The voltage drop across each resistor
Which bulb is now brightest?

22. Problems:
Consider a circuit with a 2-ohm, 4-ohm and 8-ohm resistor in parallel to each other, being supplied by a 24V battery:
What is the voltage drop across each resistor?
What is the current in each branch?
2-ohm
4-ohm
8-ohm
Find the current in the circuit
What is the equivalent resistance of the circuit?