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)