1. Electricity
7-2 Electric Current

2. What is an Electric Current?
The flow of charges through a wire of conductor is called electric current
Current is usually the flow of electrons
Electric current is measured in amperes (A)
Charges flow from high voltage to low voltage (potential difference – must be present!)

3. Electric Current
A voltage difference is the push that causes charges to move.
Voltage difference is measured in volts (V) .

4. For charges to flow, the wire must always be connected in a closed path, or circuit.
If the circuit is broken by removing the battery, or the lightbulb, or one of the wires, current will not flow.

5. Batteries
To keep an electric current continually flowing in the electric circuit a voltage difference needs to be maintained in the circuit.
A battery can provide the voltage difference that is needed to keep current flowing in a circuit.
Current flows as long as there is a closed path that connects one battery terminal to the other battery terminal.

6. Dry-Cell Batteries
A cell consists of two electrodes surrounded by a material called an electrolyte.
The electrolyte enables charges to move from one electrode to the other.

7. Dry-Cell Batteries
One electrode is the carbon rod, and the other is the zinc container.
The electrolyte is a moist paste containing several chemicals.
The cell is called a dry cell because the electrolyte is a moist paste, and not a liquid solution.

8. Sources of Electricity
A dry cell battery produces a voltage difference between its zinc container and its carbon suspension rod, causing current to flow between them

9. Sources of Electricity
A wet cell battery contains two connected plates made of different metals in a conducting solution

10. A wet-cell battery contains several wet cells connected together.

11. Sources of Electricity
Wall sockets have a voltage difference across the two holes of an electrical outlet, and a generator at a power plant provides this voltage difference

12. What is Resistance? Resistance opposition the flow of electrons
electrical energy is converted to thermal energy & light
all materials have some electrical resistance
measured in ohms ()
Tungsten - high resistance
Copper - low resistance

13. Resistance
As the electrons flow through the filament in a lightbulb, they bump into the metal atoms that make up the filament.
In these collisions, some of the electrical energy of the electrons is converted into thermal energy.

14. Resistance
Eventually, the metal filament becomes hot enough to glow, producing radiant energy that can light up a dark room.

15. Resisting the Flow of Current
Resistance is the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light.
With the exception of some substances that become superconductors at low temperatures, all materials have some electrical resistance.
Resistance is measured in ohms ().

16. What is Resistance? Resistance depends on… the conductor
wire thickness
more resistance in thinner wires
wire length
more resistance in longer wires
temperature
more resistance at high temperatures

17. V = I × R Ohm’s Law V= potential difference (V) I= current (A)
Ohm’s Law – the current in a circuit equals the voltage difference divided by the resistance
V = I × R
V= potential
difference (V)
I= current (A)
R= resistance ()
Voltage increases when current increases.
Voltage decreases when resistance increases.

18. Ohm’s Law Problem R V I R = 160  I = V ÷ R V = 120 V
A lightbulb with a resistance of 160  is plugged into a 120-V outlet. What is the current flowing through the bulb?
GIVEN:
R = 160 
V = 120 V
I = ?
WORK:
I = V ÷ R
I = (120 V) ÷ (160 )
I = 0.75 A I V R

19. Section Check
Question 1
__________ is the net movement of electric charges in a single direction.
A. An open circuit
B. Electric current
C. Proton flow
D. Voltage

20. Section Check 2
Answer
The answer is B. Electric forces in a material cause electric current to flow.

21. Question 2 Answer What is a volt?
Section Check 2
Question 2
What is a volt?
Answer
The unit of measurement for voltage difference, which is related to the force that causes electric charges to flow.

22. Section Check 2
Question 3
What is the tendency for a material to oppose the flow of electrons called?
Answer
The tendency for a material to oppose the flow of electrons is called resistance, and is measured in ohms.

23. What property of electric current causes light bulbs to give light?
Resistance
As electrons flow through the filament, the filament resists their flow and changes electrical energy into thermal energy and light.