1. Lecture 52 – Lecture 53 Electric Current Ozgur Unal
NIS – PHYSICAL SCIENCE
Lecture 52 – Lecture 53
Electric Current
Ozgur Unal

2. Current and Voltage Difference
The net movement of electric charges in a single direction is an electric current.
In a metal wire, or any material, electrons are in constant mottion in all directions  no net movement.
However, when an electric current flows, electrons continue their random movement, but they also drift in the direction that the current flows.

3. Current and Voltage Difference
Water flows from high pressure to low pressure  pressure difference.
Similarly, electric charge flows when there is a voltage difference.
A voltage difference is related to
the force that causes electric
charges to flow.
Voltage difference is
measured in Volts.

4. Current and Voltage Difference
If you want the water flow continuously, a pump should provide a pressure difference in a loop of water  Figure 12!!
Similarly, for electric charges voltage difference is provided by batteries.
In order for electric current to flow continuously, the path of the the current must be closed  circuit.
A closed path that electric current
flows is a circuit.

5. Batteries
Batteries provide a voltage difference for the charges to flow in a circuit.
Dry-cell batteries are called dry because the material inside the battery is not liquid.
When the two terminals of a dry-cell battery are connected in a circuit, a reaction occurs inside the battery.
As a result of the reaction,
electrons accumulate on one
terminal of the battery.
The voltage difference
between the terminals
causes current to flow.

6. Resistance As the electrons flow through the filament, 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.
Eventually the metal filament becomes hot enough to glow.
Resistance is the tendency of a material to oppose the flow of electrons, changing the electrical energy to thermal energy.
Example: The resistance of the filament causes it to glow as the current passes through the filament.
All materials have electrical resistance.
SI unit of resistance is ohms (Ω).

7. Resistance
The following formula can be used to calculate the resistance of a material:
R = ρ * L / A
R: Resistance of the material (Ω)
ρ: resistivity -depends on the type of the material- (Ω*m)
L: length of the material (m)
A: Area of the material (m2)
Example: Calculate the resistance of a copper wire with a length and surface area of 10 cm and 0.02 cm2, respectively. The resistivity of copper is 1.68 x 10-8 Ω*m.
Metals (gold, iron, nickel) are good conductors, but why is copper used instead of any other metal in household wiring?

8. The Current in a Simple Circuit
Consider the simple circuit we have constructed before.
How do you think the current (I) change if there is little voltage difference (V) between the terminals of a battery?
If I increase the resistance (R) of the light bulb, do you think it will be easier or harder for the current (I) to flow?
Ohm’s Law states the relationship between voltage (V), resistance (R) and current (I).
According to Ohm’s law, the current in a circuit equals the voltage difference divided by the resistance:
I = V / R
I: Current (Ampere, A)
V: Voltage difference (Volts, V)
R: Resistance (Ohms, Ω)

9. The Current in a Simple Circuit
Example: A 1.5 V battery is connected to a circuit with a light bulb. If the current through the light bulb is 0.2 Amperes, what is the resistance of the filament of the light bulb?
Consider a flashlight..
Draw the electric circuit inside the flashlight.
Make sure to include all the circuit components in your drawing.
If the resistance of the bulb is 20 ohms,
calculate the current that flows in the circuit.