Physics Final- Chapter 23: Electric Current

The relationship among voltage, current, and resistance is summarized by a statement called Ohm's law

. Ohm discovered that the current in a circuit is directly proportional to the voltage established across the circuit and is inversely proportional to the resistance of the circuit.

The circuit shown in Figure 23.18 illustrates the following major charac- teristics of parallel connections:

1. Each device connects the same two points A and B of the circuit. The voltage is therefore the same across each device. 2. The total current in the circuit divides among the parallel branches. Since the voltage across each branch is the same, the amount of current in each branch is inversely proportional to the resistance of the branch-Ohm's law applies separately to each branch. 3. The total current in the circuit equals the sum of the currents in its par- allel branches. This sum equals the current in the battery or other voltage source. 4. As the number of parallel branches is increased, the overall resistance of the circuit is decreased. Overall resistance is lowered with each added path between any two points of the circuit. This means the overall resistance of the circuit is less than the resistance of anyone of the branches.

The circuit shown in Figure 23.17 illustrates the following important characteristics of series connections:

1. Electric current has but a single pathway through the circuit. This means that the current passing through the resistance of each electrical device along the pathway is the same. 2. This current is resisted by the resistance of the first device, the resistance of the second, and that of the third also, so the total resistance to current in the circuit is the sum of the individual resistances along the circuit path. 3. The current in the circuit is numerically equal to the voltage supplied by the source divided by the total resistance of the circuit. This is in accord with Ohm's law. 4. 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. This characteristic follows from the fact that the amount of energy supplied to the total current is equal to the sum of energies supplied to each device. 5. The voltage drop across each device is proportional to its resistance- Ohm's law applies separately to each device. This follows from the fact that more energy is dissipated when a current passes through a large resistance than when the same current passes through a small resistance.

Series circuit

An electric circuit in which electrical devices are connected along a single wire such that the same electric current exists in all of them.

Parallel circuit

An electric circuit in which electrical devices are connected in such a way that the same voltage acts across each one and any single one completes the circuit independently of all the others.

Direct current (dc)

Electrically charged particles Rowing in one direction only.

Alternating current (ac)

Electrically charged particles that repeatedly reverse direction, vibrating about relatively fixed positions. In the United States, the vibrational rate is commonly 60 Hz.

Alternating current (ac) acts as the name implies.

Electrons in the circuit are moved first in one direction and then in the opposite direction, alternating to and fro about relatively fixed positions.

Protons, on the other hand, do not move because they are bound inside the nuclei of atoms that are more or less locked in fixed posi- tions.

In conducting fluids, however-such as in a car battery-positive ions typically compose the flow of electric charge.

The rate of electrical flow is measured in amperes.

One ampere is a rate of flow equal to 1 coulomb of charge per second.

Electric current

The Row of electric charge that trans- ports energy from one place to another. Measured in amperes, where 1 A is the Row of 6.25 X 1018 electrons per second, or 1 coulomb per second.

Potential difference

The difference in electric potential between two points, measured in volts. When two points of different electric potential are connected by a conductor, charge flows so long as a potential difference exists. (Synonymous with voltage difference.)

Electrical resistance

The property of a material that resists electric current. Measured in ohms (D).

Electric power

The rate of energy transfer, or the rate of doing work; the amount of energy per unit time, which electrically can be measured by the product of current and voltage.

Ohm's law

The statement that the current in a circuit varies in direct proportion to the potential difference or voltage across the circuit and inversely with the circuit's resistance.

A simple parallel circuit is shown in Figure 23.18.

Three lamps are connected to the same two points A and B. Electrical devices connected to the same two points of an electrical circuit are said to be connected in parallel. The pathway for current from one terminal of the battery to the other is completed if only one lamp is lit.

Electrical resistance is measured in units

called ohms. The Greek letter omega, n, is commonly used as the symbol for the ohm.

Power =

current X voltage (j/s)

Just as water current is the flow of H20 molecules,

electric current is simply the flow of electric charge.

The rate at which electric energy is converted into another form, such as mechanical energy, heat, or light, is called

electric power.

How much current exists depends not only on the voltage but also on the

electrical resistance the conductor offers to the flow of charge.

Unless it is in a superconductor, a charge moving in a circuit expends

energy. This may result in heating the circuit or in turning a motor.

In circuits of metal wires, electrons make up the flow of charge. This is because one or more electrons from each metal atom are

free to move throughout the atomic lattice. These charge carriers are called conduction electrons.

the flow ceases when both ends reach the same temperature. Similarly, when the ends of an electrical conductor are at different electric potentials-when there is a potential difference-charge flows

from one end to the other. 1 The flow of charge persists for as long as there is a potential difference. Without a potential difference, no charge flows.

When connected in parallel, t

hey form branches, each of which is a separate path for the flow of electrons. Both series and parallel connections have their own distinctive characteristics.

Copper wire has ... resistance than steel wire of the same size

less

Thick wires have .... resistance than thin wires. .

less

Longer wires have ... resistance than short wires.

more

Electric power is equal to the

product of current and voltage.

These devices are commonly connected in a circuit in one of two ways,

series or parallel.

Any path along which electrons can flow is a circuit. For a continuous flow of electrons,

there must be a complete circuit with no gaps. A gap is usually pro- vided by an electric switch that can be opened or closed to either cut off or allow energy flow.

When connected in series,

they form a single pathway for electron flow between the terminals of the battery, generator, or wall socket (which is simply an extension of these terminals).

The resistance of a wire depends both on the

thickness and length of the wire and on its particular conductivity.

Current =

voltage / resistance (ohms law)

Electric power is measured in

watts (or kilowatts), where 1AX 1V= 1W.

By dc, we mean direct current,

which refers to the flowing of charges in one direction. A battery produces direct current in a circuit because each terminal of a battery always has the same sign: the pos- itive terminal is always positive, and the negative terminal is always negative.