NIS – PHYSICAL SCIENCE

Lecture 52 – Lecture 53Electric Current

Ozgur Unal

1

2

Current and Voltage DifferenceThe 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 DifferenceWater 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 DifferenceIf 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.http://webphysics.davidson.edu/physlet_resources/bu_semester2/c09_current.html

5

BatteriesBatteries 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

ResistanceAs 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

ResistanceThe following formula can be used to calculate the resistance of a material:

R = ρ * L / AR: 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)http://phet.colorado.edu/sims/resistance-in-a-wire/resistance-in-a-wire_en.html

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 CircuitConsider 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?http://phet.colorado.edu/sims/ohms-law/ohms-law_en.htmlOhm’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 / RI: Current (Ampere, A)V: Voltage difference (Volts, V)R: Resistance (Ohms, Ω)

9

The Current in a Simple CircuitExample: 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.