Electrical ResistancePhysics ACurrent #3

Targets:I can explain resistance and its function in a circuit.I can calculate resistance in series and parallel circuits.I can relate current to electric potential difference and resistance.

Resistance An electron traveling through the wires and loads

of the external circuit encounters resistance. Resistance is the hindrance to the flow of charge.

While the electric potential difference established

between the two terminals encourages the

movement of charge, it is resistance that

discourages it. The rate at which charge flows from terminal to

terminal is the result of the combined effect of these two quantities.

Factors That Affect Resistance The flow of charge through wires is often

compared to the flow of water through pipes.

The resistance to the flow of charge in an electric circuit is analogous to the frictional effects between water and the pipe surfaces as well as the resistance offered by obstacles that are present in its path.

It is this resistance that hinders the water flow and reduces both its flow rate.

Like the resistance to water flow, the total amount of resistance to charge flow within a wire of an electric circuit is affected by some clearly identifiable variables.

Factors That Affect Resistance The total length of the wires will affect the amount of

resistance.

The longer the wire, the more resistance that there will be.

There is a direct relationship between the amount of resistance encountered by charge and the length of wire it must traverse.

After all, if resistance occurs as the result of collisions between charge carriers and the atoms of the wire, then there is likely to be more collisions in a longer wire.

More collisions mean more resistance.

Factors That Affect Resistance The cross-sectional area of the wires will affect the

amount of resistance.

Wider wires have a greater cross-sectional area.

Water will flow through a wider pipe at a higher rate than it will flow through a narrow pipe.

This can be attributed to the lower amount of resistance that is present in the wider pipe.

In the same manner, the wider the wire, the less resistance that there will be to the flow of electric charge.

When all other variables are the same, charge will flow at higher rates through wider wires with greater cross-sectional areas than through thinner wires.

Factors That Affect Resistance A third variable that affects the resistance to charge

flow is the material that a wire is made of.

Some materials are better conductors than others and offer less resistance to the flow of charge. Silver is one of the best conductors but is never used

in wires of household circuits due to its cost. Copper and aluminum are among the least

expensive materials with suitable conducting ability to permit their use in wires of household circuits.

Factors That Affect Resistance The conducting ability of a

material is often indicated by its resistivity.

The resistivity of a material is dependent upon the material's electronic structure and its temperature.

For most (but not all) materials, resistivity increases with increasing temperature.

The table lists resistivity values for various materials at temperatures of 20 degrees Celsius.

Ohm’s LawΔV = I • R

This equation indicates the two variables that would affect the amount of current in a circuit.

Ohm’s LawI = ΔV / R

How are current and electric potential difference related?

The greater the battery voltage (i.e., electric potential difference), the ___________ the current.

Ohm’s LawI = ΔV / R

How are current and total resistance related?

The greater the resistance, the ________ the current.

Charge flows at the greatest rates when the battery voltage is increased and the resistance is decreased.

Check for Understanding

Which circuit has the greatest resistance?

Check for Understanding1. Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply.

a. decrease the voltage

b. decrease the resistance

c. increase the voltage

d. increase the resistance

Check for Understanding2. A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the following would cause the bulb to shine less brightly? Choose all that apply.a. increase the voltage of the battery

(add another cell)

b. decrease the voltage of the battery

(remove a cell)

c. decrease the resistance of the circuit

d. increase the resistance of the circuit

Check for Understanding3. You have likely been warned to avoid contact with electrical appliances or even electrical outlets when your hands are wet. Such contact is more dangerous when your hands are wet (vs. dry) because wet hands cause ____.

a. the voltage of the circuit to be higher

b. the voltage of the circuit to be lower

c. your resistance to be higher

d. your resistance to be lower

e. the current through you to be lower  

Check for Understanding4. If the resistance of a circuit were tripled, then the current through the circuit would be ____.

a. one-third as much

b. three times as much

c. unchanged

d. ... nonsense! There would be no way to

make such a prediction.

Check for Understanding5. If the voltage across a circuit is quadrupled, then the current through the circuit would be ____.

a. one-fourth as much

b. four times as much

c. unchanged

d. ... nonsense! There would be no way to

make such a prediction. 

Check for Understanding6. A circuit is wired with a power supply, a resistor and an ammeter (for measuring current). The ammeter reads a current of 24 mA (milliAmps). Determine the new current if the voltage of the power supply was ...

a. ... increased by a factor of 2 and the resistance was held constant.

b. ... increased by a factor of 3 and the resistance was held constant.

c. ... decreased by a factor of 2 and the resistance was held constant.

Check for Understanding6. A circuit is wired with a power supply, a resistor and an ammeter. The ammeter reads a current of 24 mA. Determine the new current if the voltage of the power supply was ...

d. ... held constant and the resistance was increased by a factor of 2.

e. ... held constant and the resistance was increased by a factor of 4.

f. ... held constant and the resistance was decreased by a factor of 2.

Check for Understanding6. A circuit is wired with a power supply, a resistor and an ammeter. The ammeter reads a current of 24 mA. Determine the new current if the voltage of the power supply was ...

g. ... increased by a factor of 2 and the resistance was increased by a factor of 2.

h. ... increased by a factor of 3 and the resistance was decreased by a factor of 2.

i. ... decreased by a factor of 2 and the resistance was increased by a factor of 2.

Check for Understanding7. An electrical device with a resistance of 3.0 Ω will allow a current of 4.0 amps to flow through it if a voltage drop of ________ Volts is impressed across the device.

Check for Understanding8. When a voltage of 120 V is impressed across an electric heater, a current of 10.0 amps will flow through the heater if the resistance is ________ Ω.

Check for Understanding9. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60 Ω will have a current of ________ Amps.

10. Use the Ohm's law equation to determine the missing values in the following circuits.