Electric Current and Ohm’s Law

Electric Current

• Electric current is the continuous flow of electric charge

• Two types of current are direct and alternating

• Direct current (DC) is when the charge flows in one direction

• Alternating current (AC) is when the flow of electric charge regularly reverses direction

Electric Current

• An example of a direct current is a flashlight and most battery-operated devices

• Alternating current is in your home and school

• Current is defined as the direction in which the positive charges would flow

Conductors and Insulators• An electrical conductor is a

material through which charge can flow easily

• Metals such as copper, and silver are good electrical conductors

• An electrical insulator is a material through which charge cannot flow easily

• Wood, plastic, rubber and air are good electrical insulators

Resistance

• Resistance is the opposition to the flow of charges in a material

• The SI unit of resistance is the ohm• A material’s thickness, length and temperature

affect its resistance• Resistance is more in a longer wire• As temperature increases the resistance

increases since the electrons collide more often

ResistanceA superconductor is a material that has

almost zero resistance when it is cooled to low temperatures

Superconductors are trying to reduce the resistance to zero

The best superconductor has been cooled to 138K

Voltage

• In order for a charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy

• A flashlight will not work if there is no battery

Potential DifferenceReminder potential energy is related to

positionCharges flow from a high to low potential

energyPotential difference is the difference in

electrical potential energy between two places in an electric field

Potential Difference

• Potential difference is measured in joules per coulomb or volts

• Potential difference is also called voltage

Voltage Sources• Three common voltage sources

are batteries, solar cells and generators

• A battery is a device that converts chemical energy to electrical energy

• In a 9-volt battery the voltage drop is about 9 volts

Ohm’s LawNamed after German scientist Georg OhmOhm is the unit of resistanceOhm’s Law V = I x R or I = V/RVoltage (V) = current(I) x resistance (R)When the current is in amperes and the

resistance is in ohms the voltage is in volts

Ohm’s LawWhat is the voltage if the resistance is 3

ohms and the current is 3 amps?V = I x R =

3 amps x 3 ohms = 9 voltsIncreasing the voltage increases the currentHaving the same voltage and increasing the

resistance will decrease the currentMultimeters measure current, voltage and

resistance