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 /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  ceID=398 ceID=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 resistance FactorHow 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