Resistance and Ohm’s Law

Electron flow vs. Conventional current There are two ways to explain the way current

moves: Electron flow Conventional current

Conventional current was the first system used to explain electron flow (some electricians still use this)

Electron flow is the newer system used to explain electron flow

Both systems are used today

The difference between Conventional Current and Electron Flow Conventional current: Electric current runs

from positive to negative terminal Electron flow: Electrons flow from negative

to positive terminal The only difference between the two systems

is how they name the charges. Scientifically, the same thing is happening: electrons flow from one terminal to another terminal

http://www.mi.mun.ca/users/cchaulk/eltk1100/ivse/ivse.htm#

Measuring electricity: Resistance #1: Current

#2: Potential Difference

#3: Resistance You will now be learning resistance

4.5 V

Resistance Resistance: the degree to which a substance

opposes the flow of electric current through it All substances resist electron flow to some

extent Conductors, like metals, have low resistance Insulators have high resistance Resistance is measured in ohms (Ω)

Resistance

Give an example of a load A load adds resistance to a circuit. It gains energy

from the electrons that pass through it. For this reason, they can get hot.

Resistors A load can act as a resistor However, small resistors are

added to a circuit to help reduce the current.

A wide variety of resistors are made for different applications, especially in electronics. For example, televisions contain dozens of different resistors.

Resistors Let’s use this Gizmo to see what happens

when we add resistors to a circuit http://www.explorelearning.com/index.cfm?

method=cResource.dspView&ResourceID=398

What does resistance do? Why do you think resistors are used in a circuit?

Many electric devices can’t handle a high current, so resistors help reduce the flow of electrons

What does increasing resistance do to a circuit? It reduces the flow of electrons

What does decreasing resistance do to a circuit? It increases the flow of electrons

Since increasing resistance reduces current, potential difference (voltage) increases. Look at the diagram of the water pipe and explain why.

Factors that affect resistanceFactor How it affects resistance

Material Metals that are good conductors that decrease resistance. Silver is the most conductive metal, therefore it has the least resistance.

Temperature As temperature increases, conductivity decreases. Therefore, the hotter the wire, the higher the resistance.

Length The longer the wire, the higher the resistance. Therefore, if the length of a wire is doubled, the resistance is doubled.

Cross-sectional area Thick wires conduct more electricity than thin wires. Therefore, the thinner the wire, the higher the resistance.

Georg Ohm A German man who studied

mathematics and then electricity He devoted much of his life to the

study of potential difference, current and resistance.

His discoveries have led us know much of what we understand today about electricity.

Ohm’s Law Ohm’s Law establishes the relationship

between potential difference (V), current (I) and resistance (R).

The symbol for resistance is called the Ohm ( ) in honour of Georg Ohm’s work. Remember a “Law” in Science is a conclusion

after a great number of experiments and much collection of evidence. It is accepted by the whole science community.

Ohm’s Law: VIR Triangle The V I R triangle can be used to calculate all formulations

of Ohm's law. Use a finger to hide the value to be calculated. The other

two values then show how to do the calculation.

Helpful acronym: Very Interesting Rumour

Ohm’s Law Continued:

(potential difference)

Let’s Try a Few Problems using GRASP:11.1: A current of 4.0A flows through a 40

resistor in a circuit. What is the voltage?11.2: A 30V battery generates a current through

a 15 resistor. How much current does the battery generate?

11.3: An electric stove is connected to a 240V outlet. If the current flowing through the stove is 20A, what is the resistance of the heating element?

Homework: Complete the Practice Problems on the

handout Complete questions p. 447 #8, 9, 10, 13