ohm’s law current, potential difference, and resistance
TRANSCRIPT
Ohm’s law
CURRENT, POTENTIAL DIFFERENCE, AND RESISTANCE
Current
Current (I): rate (how fast) the electrons are moving past one point in a circuit
Measured in amperes (A)
1 Ampere = 6.2 X 1018 electrons (called a Coulomb) per second.
Measured using an ammeter
Ammeters must be connected in series
Ammeters
• Black knob is the negative electrode• Red knobs are the positive electrodes
• In this picture, there are 3 red knobs (50 mA; 500 mA; 5A)
• Read the scale that ends in 50 mA if you attach wire to 50 mA knob
• Read the scale that ends in 500 mA if you attach wire to 500 mA knob
• Read the scale that ends in 5 A if you attach wire to 5 A knob
Electric Shock and the Body
A current of 0.001 A will tingle
The let-go threshold is 0.050 – 0.150 A and your muscles contract (squeeze) and convulse (shake)
Above this level, you can no longer let-go of the object that is shocking you
Your heart can stop beating if it receives a shock of 1 – 4.3 A.
A wall outlet can deliver up to 15 A, which is more than enough to kill a person!
Potential Difference
Potential Difference (Voltage) (V): the difference in electrical potential energy per unit charge at two points in a circuit
Electrons have electric potential energy when they leave the negative end of the source but some of this energy is used up when the electrons power the load (e.g. make the light bulb shine)
Electrons return to the positive electrode of the source with less energy
Electrons re-energize (get more energy) inside the source
Potential Difference
Potential difference = difference in potential energy
(J)
Charge (C)
1 J/C is called a volt (V)
Electric Potential Energy is measured in volts (V)
Measured using a voltmeter
Voltmeters must be connected in parallel
The negative side of the source needs to be connected to the negative side of the voltmeter
Voltmeters are read in the same way as ammeters
Resistance
Electrical Resistance (R): the ability of a material to oppose the flow of electric current in a circuit.
Measure resistance in ohms (Ω)
All materials have some resistance.
As the resistance increases, the current decreases and the temperature of the material increases
As electrons move through the material, they bump into the atoms of the material which converts some of the electrical energy into thermal energy
Factors That Effect Resistance
There are four major things that influence resistance
Type of material Cross-sectional area Length Temperature
Factors that Effect Resistance
Type of Material Conductors have a low
electrical resistance (ex. Copper); insulators have a high electrical resistance
Factors that Effect Resistance
Cross-Sectional Area The greater the diameter of a
wire, the less resistance it has.
When the wire is thicker, there is more room for the electrons to move
Factors that Effect Resistance
Length The longer the wire is, the
more internal resistance it has because there is more wire for the electrons to travel through
Factors that Effect Resistance
Temperature The warmer the
temperature is, the more collisions the molecules make with the electrons. This increases the resistance.
Resistance
Measuring the Resistance Measure with an ohmmeter Ohmmeters must be connected in
parallel with a load. The ohmmeter is powered and
provides an electric current through the load.
Resistors in Circuits
Resistor: Device that decreases the flow of electric current
many types of resistors Variable resistor: can be used
to increase/decrease volume, increase/decrease light intensity, etc.
Ohm’s Law
Ohm’s Law V=IR for most wires, the ratio of potential
difference (V) to current (I) is a constant.
The constant is called resistance (R).
Measured in Ohms (Ω) 1 Ω = 1V/1A
Ohm’s Law
Sometimes, certain loads do not obey Ohm’s Law because the ratio of potential difference to current is not constant.
These types of materials are called non-Ohmic conductors.
Ex. an incandescent bulb (as the bulb gets warmer, the resistance increases)