Transcript Electric Current and Ohm`s Law

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
Resistance
 A 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 Difference
 Reminder potential energy is related to position
 Charges flow from a high to low potential energy
 Potential 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 Law
 Named after German scientist Georg Ohm
 Ohm is the unit of resistance
 Ohm’s Law
V = I x R or I = V/R
 Voltage (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 Law
 What is the voltage if the resistance is 3 ohms and the
current is 3 amps?
 V=IxR=
3 amps x 3 ohms = 9 volts
 Increasing the voltage increases the current
 Having the same voltage and increasing the resistance
will decrease the current
 Multimeters measure current, voltage and resistance