Resistance• Factors• Equation (Ohm’s Law)• Interpreting graphs• Circuits and Resistance

Here’s the deal• electrons move through a conductor. • electrons can collide with the atoms of the

conductor. • This makes it more difficult for the current to

flow.

                     

         

Like a marathon…

• The roughness, difficulty and width of the road, including the obstacles along the way are some resistances to their forward motion.

• Length of wire• As length increases, resistance increases

• Diameter of wire• As diameter increases, resistance decreases

• Temperature• As temperature increases, resistance increases

So, in a conducting material, what factors affect the flow

of electrons?

Resistance•Measure of how a material or object opposes the flow of current•Ratio of pd across resistor and current in the resistor

Energy and Power•V = energy per unit charge•I = charge per unit time•Energy = VIt

• Unit: Joules, J•Power = energy per unit time•Power = VI

• Unit : Watts, W

Ohm’s Law

Volt(s)

Ampere(s)

Ohm(s)

Experiment 1

• Gradient is constant• Gradient = I/V = constant value• Resistance = 1/Gradient = constant value• Fixed Resistance• Fixed Resistor

What does the graph mean?

Experiment 2

• Gradient is changing decreasing• Gradient = I/V = decreasing value• Resistance = 1/Gradient = increasing value• Increasing Resistance• Filament bulb increases in temperature

What does the graph mean?

Interpreting Graphs

• What is the resistance when V=1 and V=3?

Interpreting Graphs• What is the

resistance when V=2, 5 and 5.5 volts?

Series Circuit

Itotal = I1 = I2 = In

Vtotal = V1 + V2 + Vn

Rtotal = R1 + R2 + Rn

Parallel Circuit

Itotal = I1 + I2 + In

Vtotal = V1 = V2 = Vn

= + +RnR2R1

1 1 1

Rtotal

1

• Series• The total resistance increases• The total current decreases

• Parallel• The total resistance decreases• The total current increases

What happens to the current in the circuit when you add more resistors?

• Series• More bulbs divide energy between themselves• Brightness decreases

• Parallel• Voltage or energy across each bulb is the same• Brightness is unaffected

What happens to the brightness of the bulbs when more are added?

• Series• There is only one path for current • Since bulb is busted, there is an open in the circuit• Other bulbs don’t light

• Parallel• The path for each bulb is independent• Other bulbs are lit; only busted bulb is unlit

What happens to the other bulbs in the circuit when one of the bulbs is busted?