Conceptual Physics

Chapter 23:

ELECTRIC CURRENT

Electric Potential

•Unit of measurement: volt,

• Example:• Twice the charge in same location has twice the

electric potential energy but the same electric potential.

• 3 times the charge in same location has 3 times the electric potential energy but the same electric potential

1 volt 1 coulomb

1 joule

Flow of Charge

• Why does heat flow from the end in flame to the end in your hand?

Flow of Charge

•Water flows from high pressure to low pressure until equilibrium is reached – then it ceases to flow.

Flow of Charge

• A continuous flow is possible if the difference in water levels—hence the difference in water pressures—is maintained with the use of a pump.

Electric Current

•Electric current is the flow of charged particles through conductors.

• Measured in Coulombs per Second or Amperes

Electric Current

•Terminology Matters!

•Charge flows through a circuit;

•Voltage is potential difference and so is established across a circuit.

Voltage Sources

•Voltage sources provide the electric potential DIFFERENCE that causes current to flow from higher potential to lower potential.

•Some voltage sources are:• Generators

• Batteries

•Unit of Electrical Potential Difference is the Volt (V)

Voltage Sources

•Electric potential difference (continued)Example: Water from a higher reservoir to a lower

one—flow continues until no difference

• No flow of charge occurs when potential difference is zero.

Voltage Sources

Voltage Sources

Electric Resistance

•How much current flows in a circuit depends on:• Voltage

• Electrical resistance (measured in Ohms)

Resistors are circuit elements that regulate current inside electrical devices.

Ω

Electric Resistance

Factors Affecting Electric Resistance

• Cross Sectional Area: Thin wires provide more resistance than thick wires

• Length: Doubling the length, doubles the resistance

• Material: Rubber provides much more resistance than copper of the same size

• Temperature: The higher the temperature, the more the resistance

Electric Resistance

•Resistance comes from any load on a circuit.

Electric Resistance

•Superconductors

•Materials with zero electrical resistance to the flow of charge.

•Flow of charge is without generation of heat.

Ohm’s Law

•Ohm’s law gives us the relationship between voltage, current, and resistance.

OHM’S LAW

The current in a circuit varies in direct proportion to the potential difference, or voltage, and inversely

with the resistance.

Ohm’s Law

In Equation Form:

Current resistancevoltage

Ohm’s Law

• Think!

• How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it?

Ohm’s Law

• Think!

• How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it?

Electric Shock

Damaging because of current passing through the body.

Caused by voltage

The amount depends on the body’s resistance

Electric Shock

Birds sit along high voltage wires. Why don’t they get shocked?

Direct and Alternating Current

•Direct current (dc) •Flows in one direction only.

•Electrons always move from the negativeterminal toward the positive terminal.

Direct and Alternating Current

•Alternating Current (ac) • Electrons in the circuit are moved first in one

direction and then in the opposite direction, alternating to and fro about fixed positions.

• This is accomplished by alternating the polarity of voltage.

Direct and Alternating Current

•Power transmission is more efficient at higher voltages.

• Europe adopted 220 V as its standard.

• U.S. continued with 120 V because so much equipment was already installed.

Direct and Alternating Current

•Converting from ac to dc• Household current is ac, but current in laptop is dc.

• The converter uses a diode, a tiny electronic device that acts as a one-way valve to allow electron flow in one direction only.

Direct and Alternating Current

• Converting from ac to dc

When input to a diode is ac, output is pulsating dc…

Direct and Alternating Current

• Converting from ac to dc

A capacitor is used in conjunction with the diode (or usually a pair of diodes) to smooth out the current.

Speed and Source of Electrons in a Circuit

How fast do electrons move in a circuit?

Speed and Source of Electrons in a Circuit

• When we flip the light switch on a wall and the circuit, an electric field is established inside the conductor.

• The electrons continue their random motions

while simultaneously being nudged by the electric

field.

Speed and Source of Electrons in a Circuit

Misconceptions about electric current:

“Current is propagated through the conducting wires by electrons bumping into one another.”

Speed and Source of Electrons in a Circuit

Misconceptions about electric current:

“Electrical outlets in the walls of the homes are a source of electrons.”

Electric Power

• Electric power is the rate at which electric energy is converted into another form.

• Power = current voltage

P = IV

• Units: Watts or Kilowatts

• Example: 100-watt lamp draws

0.8 ampere.

Electric Power

Think!

How much power is used by a calculator that operates on 8 volts and 0.1 ampere? If it is used for one hour, how much energy does it use?

Compact Fluorescent Lamps (CFLs)

• Compact fluorescent lamps

(CFLs) are a type of

fluorescent lamp that fits into

a standard lightbulb socket.

• For the same wattage, CFLs

emit much more light and

much less heat than

incandescent bulbs.

Light-Emitting Diodes (LEDs)

•Another light source even more long-lasting is the light-emitting diode (LED).

•Between CFLs and LEDs, common-use incandescent bulbs will soon be history.

Electric Circuits

• A circuit is any path along which electrons can flow from the negative terminal to the positive terminal

Electric Circuits

Two common ways to make a circuit:

• SERIES

• Forms a single pathway for electron flow between the terminals of the voltage source

• PARALLEL

• Forms branches, each of which is a separate path for the flow of electrons

Series Circuits

Series Circuits

Electric current through a single pathway.

Total resistance to current is the sum of individual resistances.

Current is equal to the voltage supplied by the source divided by the total resistance of the circuit.

The total voltage impressed across a series circuit divides among the individual electrical devices in the circuit so that the sum of the “voltage drops” across the resistance of each individual device is equal to the total voltage supplied by the source.

The voltage drop across each device is proportional to its resistance.

If one device fails, current in the entire circuit ceases.

Series Circuits

1. Current (I) is SAME throughout the whole circuit.

2. Voltage drops across each resistor will SUM to the battery voltage.

3. Equivalent Resistance – Find by adding up the resistance of every individual resistor.

REQ = R1 + R2 + R3

R1 R2 R3

V

Parallel Circuits

Parallel Circuits

Voltage is the same across each device and is equal to the voltage of the battery/source.

The total current in the circuit divides among the parallel branches. The amount of current in each branch is inversely proportional to the resistance of the branch.

The total current in the circuit equals the sum of the currents in its parallel branches.

As the number of parallel branches is increased, the overall resistance of the circuit is decreased.

A break in one path does not interrupt the flow of charge in the other paths.

Parallel Circuits

•As you open more checkout lanes, the traffic in each is decreased. This is how current behaves in a parallel circuit.

Parallel Circuits

1. Voltage (V) is SAME across each resistor, and equalto the battery voltage.

2. Current (I) through each resistor will SUM to the current through the battery.

3. Equivalent Resistance:

R1 R2 R3V

1 1 1 1

REQ = R1 + R2 + R3

When two identical lamps in a circuit are connected in parallel, the total resistance is

A. Less than the resistance of either lamp.

B. the same as the resistance of each lamp.

C. less than the resistance of each lamp.

D. None of the above.

Electric Circuits

CHECK YOUR NEIGHBOR

When two identical lamps in a circuit are connected in parallel, the total resistance is

• A. less than the resistance of either lamp.

• Explanation:Resistors in parallel are like extra lines at a checkout counter. More lines means less resistance, allowing for more flow.

Electric Circuits

CHECK YOUR ANSWER

Overloading Parallel Circuits

Electric Circuits

•Safety fuses are wires that melt when the given current is exceeded. In modern buildings, circuit breakers are used.

•A circuit breaker is an automatic switch that turns off when the current is excessive