16.1 Electrical Current Electric Current
Electric Current
When the ends of an electric conductor are at different electric potentials, charge flows from one end to the other Flow of Charge
. Charge flows when there is a potential difference, or difference in potential (voltage), between the ends of a conductor. The flow continues until both ends reach the same potential.
. When there is no potential difference, there is no longer a flow of charge through the conductor.
. To attain a sustained flow of charge in a conductor, one end must remain at a higher potential than the other.
. The situation is analogous to the flow of water. Flow of Charge
Water flows from higher pressure to lower pressure. The flow will cease when the difference in pressure ceases. Flow of Charge
Water continues to flow because a difference in pressure is maintained with the pump. The same is true of electric current. Electric Current
. Electric current is the flow of electric charge.
. In solid conductors, electrons carry the charge through the circuit because they are free to move. Electric Current Measuring Current
. Electric current is measured in amperes, symbol A.
. An ampere is the flow of 1 coulomb of charge per second.
. When the flow of charge past any cross section is 1 coulomb (6.25 x 1018 electrons) per second, the current is 1 ampere. Electric Current Formula:
Current = (charge / time) I = q / t
I = Current (amps) q = amount of charge (coulombs) t = time (seconds)
Units:
coulombs / second = Amperes or Amps
1 C / 1 s = 1 A Electric Current
Most small currents are expressed in: milli-amperes (mA): 1 mA = 1 x 10-3 A micro-amperes (µA): 1 µA = 1 x 10-6 A Voltage Sources
Voltage Sources . Charges do not flow unless there is a potential difference.
. Something that provides a potential difference is known as a voltage source.
. Batteries and generators are examples of voltage sources that are capable of maintaining a continuous flow of electrons. Voltage Sources
Steady Voltage Sources . In a battery, a chemical reaction releases electrical energy.
. Generators—such as the alternators in automobiles— convert mechanical energy to electrical energy.
. The electrical potential energy produced is available at the terminals of the battery or generator. Voltage Sources
The potential energy per coulomb of charge available to electrons moving between terminals is the voltage.
The voltage provides the “electric pressure” to move electrons between the terminals in an electric circuit. Voltage Sources
Low High Potential Potential
Electron Pump
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. + - Voltage Sources
Distinguishing Between Current and Voltage
There is often some confusion between charge flowing through a circuit and voltage being impressed across a circuit.
Consider a long pipe filled with water:
• Water will flow through the pipe if there is a difference in pressure across the pipe or between its ends. • Water flows from high pressure to low pressure. What are two voltage sources used to provide the energy that allows charges to move steadily?
Batteries and Generators Types of Current
There are two types of current:
1. DC Current 2. AC Current Types of Current
DC Current By DC, we mean direct current, which refers to a flow of charge that always flows in one direction.
• A battery produces direct current in a circuit because the terminals of the battery always have the same sign of charge. • Electrons always move through the circuit from the negative terminal toward the positive terminal. Types of Current
AC Current Alternating current (AC), as the name implies, is electric current that repeatedly reverses direction.
• Electrons in the circuit move first in one direction and then in the opposite direction. • They alternate back and forth about relatively fixed positions. Which way does current flow?
Conventional Current Flow of current from positive terminal to the negative terminal.
Electron Current Flow of current from the negative terminal to the positive terminal. Direction of Current
Direction of Current: Current in Electric Circuits
For current to flow, an electric circuit needs to be a closed loop.
Basic electric circuits consist of three things:
• Electron pump (Battery) • Device that reduces potential (User) • Conducting connection (Wires) What affects current flow?
1. Potential Difference (Voltage)
2. Resistance (Slowing or reduction of the flow of current ) Assessment Questions
Example #1 A charge of 30 Coulombs passes through a 24 Ω resistor in 6 seconds. What is the current through the resistor? Assessment Questions
Example #2 Charge flowing at a rate of 2.5 x 106 elementary charges per second is equivalent to a current of: Assessment Questions
Example #3 The current though a light bulb is 2 amps. How many coulombs of electric charge pass through the light bulb in one minute? Assessment Questions
Example #4 The current going from the cathode to the screen in the television picture tube is 5.0 x 10-5 amperes. How many electrons strike the screen in 5 seconds? Assessment Questions
Example #5 A current of 5 A flowed in a copper wire for 20 seconds.
a. How many coulombs of charge passed through the wire at this time? b. How many electrons flowed through this wire at this time? Assessment Questions
Example #6 Electric charge will flow in an electric circuit when:
a. electrical resistance is low enough. b. a potential difference exists. c. the circuit is grounded. d. electrical devices in the circuit are not defective. Assessment Questions
Example #7 The electric current in a copper wire is normally composed of: