Electric Current, Voltage and Resistance

Electric Circuit

Schematic Symbols

Diagram of Electric Circuit

How is current produced?

• When a high potential is connected by a conductive material to a low potential.

– Flow of electrons is called electric current.

– Units: C/s = Amperes (Amps)

High Potential

Low Potential Equal

PotentialEqual Potential

t

qI

• When the potentials are equal, the current stops flowing – To increase the potential of the electrons

an electron pump must convert (do work) another form of energy into electrical potential energy.

• Electron pump examples: – Voltaic or galvanic cell (dry cell)– Several cells connected (Battery)– Photovoltaic cell (solar cell)– Generator

High Potential

Low Potential

Electron Pump

Remember: Electric Potential Energy- Two Unlike Charges

Higher Potential Energy

Lower Potential Energy

+

-

•To cause movement of a charge, there must be a potential difference.

Two types of current

• Direct Current – (D.C.) Flow of electrons in only one direction

– Battery

• Alternating Current – (A.C.) Flow of electrons at first in one direction and then the other direction.

– Generator– In your House– 60 Hertz

• Direct Current

• DC• Provided by

batteries

• Alternating Current

• AC• Provided by power

companies

Electric Current:

• The flow of electric charges.

Electric Current, I

I = q

t

• Rate

• Unit: Coulomb / sec = Ampere (A)

• Andre Ampere (1775-1836)

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Conventional current has the direction that the (+) charges

would have in the circuit.

Conventional Current• By tradition,

direction in which “positive charges” would flow.

• Direction is opposite of electron flow.

Ammeter• Measures electric current.

• Must be placed in series.

What affects current flow?

• Potential Difference – Voltage

Closing the switch establishes a potential difference (voltage) and an electric field in the circuit.

• Electrons flow in a net direction away from the (-) terminal.

High PotentialLow

Potential

Battery (Chemical Cell):

• A device that converts chemical energy to electricity.

• A battery provides a potential energy difference (voltage source).

A Battery Provides Energy

Electric Circuit• The battery “pumps” positive charges from

low (-) to

high (+) potential.

Voltaic Cell• Alessandro Volta (1800’s)• Battery

Voltmeter

• Measures the voltage between two points in an electric circuit.

• Must be connected in parallel.

Ohm’s Law (Georg Ohm, 1787-1854)

V = IR

• The voltage , V, across a resistor is proportional to the current, I, that flows through it.

• In general, resistance does not depend on the voltage.

Ohmic Resistor

• A device that obeys Ohm’s Law, who’s resistance does not depend on the voltage.

What affects current flow?

• Potential Difference – Voltage

• Resistance - slowing or reduction of the flow of current

– Resistance depends on the material, length of material, and the cross sectional area of the material through which the current is moving.

A

LR R is resistance, is resistivity,

L is length, A is the cross sectional area

ResistanceResistance is the amount that a substance or load is opposing the flow of electrons

Symbol for resistance is R

Measured in ohms

Units are

R = V I

“how hard it is for electrons to flow”

Material

FOUR FACTORS AFFECTING THE RESISTANCE OF A WIRE

Resistance

Resistance is the amount that a substance or load is opposing the flow of electrons

Symbol for resistance is R

Measured in ohms

Units are

R = V I

“how hard it is for electrons to flow”

Material

Cross-sectional Area

Length

Temperature

Gold is a better conductor than copper

has less resistance than

The thicker the wire, the less resistance it has.

FOUR FACTORS AFFECTING THE RESISTANCE OF A WIRE

more wire = more resistance

less wire = less resistance

Electronics work better in the cold because wires have more resistance when they are hot.

A 50 metre extension cord has more resistance than a 10 metre extension cord.

Resistance

Resistance is the amount that a substance or load is opposing the flow of electrons

Symbol for resistance is R

Measured in ohms

Units are

R = V I

“how hard it is for electrons to flow”

Material

Cross-sectional Area

Length

Temperature

Gold is a better conductor than copper

has less resistance than

The thicker the wire, the less resistance it has.

FOUR FACTORS AFFECTING THE RESISTANCE OF A WIRE

more wire = more resistance

less wire = less resistance

Electronics work better in the cold because wires have more resistance when they are hot.

A 50 metre extension cord has more resistance than a 10 metre extension cord.

Resistance

Resistance is the amount that a substance or load is opposing the flow of electrons

Symbol for resistance is R

Measured in ohms

Units are

R = V I

“how hard it is for electrons to flow”

Material

Cross-sectional Area

Length

Temperature

Gold is a better conductor than copper

has less resistance than

The thicker the wire, the less resistance it has.

FOUR FACTORS AFFECTING THE RESISTANCE OF A WIRE

more wire = more resistance

less wire = less resistance

Electronics work better in the cold because wires have more resistance when they are hot.

A 50 metre extension cord has more resistance than a 10 metre extension cord.

Resistance

• Resistance of an object to the flow of electrical current.

• R= V / I

• Resistance equals the ratio of voltage to current.

• Unit: Ohm (Ω)

Resistor

• An object that has a given resistance.

Resistors use up Energy

Electric Circuit• A resistor uses up energy.

• When the current goes through the resistor it goes to a lower potential.

What We Know...

Definition Units Symbol

Current The rate at which electrons move from one place to another

Ampere (A) I

Voltage (Potential Difference)

Measures the amount of energy given to electrons to move them

Volt (V) V

Resistance Degree to which a substance opposes the flow of electric current through it

Ohms (Ω) R

Ohm’s Law Practice Problems1. Find the current through a 12-ohm resistive

circuit when 24 volts is applied.

2. Find the resistance of a circuit that draws 0.06 amperes with 12 volts applied.

3. Find the applied voltage of a circuit that draws 0.2 amperes through a 4800-ohm resistance.

4. Find the applied voltage of a telephone circuit that draws 0.017amperes through a resistance of 15,000 ohms.

Ohm’s Law Practice Problems5. A 20-volt relay has a coil resistance of

200 ohms. How much current does it draw?

6. A series circuit has 1200-ohms of total resistance with 12 V as the power supply. What is the total current of this circuit?

7. A transformer is connected to 120 volts. Find the current if the resistance is 480-ohms?

Voltage, Current, and Resistance

In Series and Parallel Circuits:

Pearson Investigating Science 9 11.1

Potential Difference = Voltage

At each point in a circuit where the electrons need to be pushed, they use up their voltage.

By the time the electricity gets back to the battery, the voltage is all used up and is 0 V.

Voltage is a comparison between the pushing force of electrons between two points in a circuit.

Symbol for voltage is V

Measured in volts

Units are V

V = I R

“the pushing force”

Voltage in Series and Parallel

• Series Circuits– Voltage decreases as electrons cross each

electrical load (and lose energy)– Voltage across the source equals the sum of the

voltages across each electrical load

– Vs = V1 + V2 + V3

• Parallel Circuits– If the loads are identical, the voltage is the same

everywhere

– Vs = V1 = V2 = V3

Pearson Investigating Science 9 11.1

Current

In a series circuit the current is the same at every point in the circuit.

Current is the measure of the amount of charge moving past a point in a circuit every second.

Symbol for current is I

Measured in amperes

Units are A

I = V R

“the number of moving electrons”

Current in Series and Parallel

• Series Circuits– Current is the same everywhere

– Is = I1 = I2 = I3

• Parallel Circuits– Current is not the same everywhere– Current entering or leaving the source equals the

sum of the current passing through each electrical load

– Is = I1 + I2 + I3

Series Circuits

• Only one path for current to follow• User followed by another user (ie.

Resistor followed by a light)

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…

Series Circuit Rules

• Current is the same for each user

• The effective resistance is the sum of all resistors in the series

–

• The sum of the voltage drop is equal to the total voltage drop

–

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...321 VVVVequ

Parallel Circuits

• Two or more paths for current to follow

Parallel Circuit Rules

• Total current in the circuit is the sum of the current in all its paths (branches)

–

• The equivalent resistance decreases with more parallel resistors

–

• Voltage is the same in each path.

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...1111

321

RRRRequ

Shopping Mall Analogy – SeriesAs a class, we are going to the mall on a field trip.

Each student (electron) starts at the ATM (source) to take out $3 (voltage – 3V).

One store (load): the whole class (current) goes to 1 store and each student spends their $3. Everyone wants to spend all of their money because it’s a field trip! Current stays the same, voltage is the same as the source

Two stores (loads): the whole class goes to each store, spending some of their money at each. Everyone wants to spend all of their money, but also wants to buy

something at each store! Current is the same, voltage of loads will add up to V of source

Everyone meets back at the ATM because they’re broke (V = 0) and have no more money to spend. This is the end of the field trip!

Shopping Mall Analogy – ParallelAs a class, we are going to the mall on a field trip.

Each student (electron) starts at the ATM (source) to take out $3 (voltage – 3V).

One store (load): same as before! Current stays the same, voltage is the same as the source

Two stores (loads): half of the class (current) goes to one store spending their $3, while the other half goes to the other store, spending their $3. Everyone wants to spend all of their money because it’s a field

trip! Current of loads adds up to A at source, voltage is the same

Everyone meets back at the ATM because they’re broke and have no more money to spend. This is the end of the field trip!

Resistance in Series and Parallel

• Series Circuits– Equivalent (total) resistance equals the sum of the

individual electrical loads

– RE = R1 + R2 + R3

• Parallel Circuits– Equivalent (total) resistance of the circuit is smaller

than the smallest resistance