# The Simple Circuit and Ohm’s Law

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The Simple Circuit and Ohm’s Law
Chapter 4 The Simple Circuit and Ohm’s Law Conductors • Switches • Switch Characteristics • Loads • Overcurrent • Overcurrent Protection Devices • Voltage and Current Measurements • DC Voltage Measurements • DC Current Measurements • Ohm’s Law • Determining Current • Determining Voltage • Determining Resistance • Determining Power

An electrical circuit consists of a voltage source, insulated conductors, a load, a switch, and a fuse. An electrical circuit is an assemblage of conductors and electrical devices through which electrons flow. See Figure 4-1. Electrical circuits consist of a complete path for the flow of electrons between two or more points. The most fundamental electrical circuit is a simple circuit in which a single energy source supplies current to a single load through electrical conductors.

In a schematic or wiring diagram, conductors are shown as lines
In a schematic or wiring diagram, conductors are shown as lines. Conductors that are connected often use a dot to indicate the connection. A conductor is a material that has a low electrical resistance and permits electrons to move through it easily. Conductors are generally a single wire, a group of wires, or another material suitable for carrying electric current. Most single conductors are enclosed in an insulated cover to protect the conductor, increase safety, and meet electrical code requirements. Some single conductors, such as ground wires, may be bare. The schematic symbol for a conductor is a line connecting two devices in an electrical circuit. This symbol is the same for insulated or bare conductors. See Figure 4-2.

Switches are control devices and are used to close and open circuits safely.
A switch is a mechanical, electronic, or solid-state electrical device that is used to start, stop, or redirect the flow of electrons in an electrical circuit. Switches are added to a circuit as a control device. See Figure 4-3. Turning a lamp ON and OFF by using a switch is safer and more convenient than connecting and disconnecting a conductor. The switch is connected in series with the voltage source and load. There is only one current path in the circuit, so when the switch is open, current does not flow to the lamp.

The position of the contacts, number of poles, number of throws, and type of break are used to describe switch contacts. Most switches are rated for the maximum current and maximum voltage they can safely handle. Large switches may also be rated in horsepower. All switches use contacts to start or stop the flow of electrons in a circuit. A contact is the conducting part of a switch that operates with another conducting part to make or break a circuit. The position of the contacts (normally open or normally closed), number of poles (single-pole, double-pole), number of throws (single-throw, double-throw), and type of break (single-break, double-break) are used to describe switch contacts. See Figure 4-4.

Switches are available in many shapes and are often designated according to their use.
Switches are available in many shapes. Switches can be activated manually, mechanically, or automatically. Once activated, the switch changes the position of the contacts. The contacts are used to start and stop the flow of electrons in a circuit. See Figure 4-5.

Common lighting circuit switches include two-way, three-way, and four-way switches.
Common lighting circuit switches include two-way, three-way, and four-way switches. The switch used to control a lamp depends on the number of different locations the lamp must be controlled from. See Figure 4-6.

Rotary switches are used to connect multiple positions to a single pole.
Another type of switch is the rotary switch (wafer). A rotary switch is a switch that has one or more poles that can be connected to several positions. See Figure 4-7. A rotary switch is typically used to switch between several signal sources. For example, a headphone user can choose between listening to a radio station, an audio cassette, a compact disc, or a digital video disc (DVD). More than one wafer controlled by a single shaft may be stacked to switch multiple circuits.

Loads convert electrical energy to another form of energy such as motion, light, heat, or sound.
A load is a device that converts electrical energy to motion, light, heat, or sound. Common loads include motors (electrical energy to motion), lamps (electrical energy to light), heating elements (electrical energy to heat), and speakers (electrical energy to sound). See Figure 4-8.

A short circuit has a resistance that is lower than the normal circuit resistance.
A short circuit is any circuit in which current takes a shortcut around the normal path of current flow. A circuit may contain a partial short that causes an increased electron flow (overcurrent) or a dead short. A partial short may or may not cause damage depending on the ratings of the circuit components. A dead short may develop that completely removes the resistance of the load from the circuit. See Figure 4-9.

Overcurrent protection devices such as fuses are used to protect a circuit from a short circuit or overcurrent that can cause circuit damage. An overcurrent protection device must be used to provide protection from short circuits and overloads to prevent the possible loss of property or life. An overcurrent protection device (OCPD) is a fuse or circuit breaker used to provide overcurrent protection in a circuit. Fuses and circuit breakers are OCPDs designed to automatically stop the flow of current in a circuit that has a short circuit or that is overloaded. See Figure 4-10.

Cartridge and plug fuses may be surrounded with glass or encased in a composite material to suppress an arc or flame. Fuse designs include cartridge and plug fuses. Fuses are available in various sizes and shapes. See Figure Glass cartridge fuses may have a single wire or flat conductor as the fuse element. The flat conductor has less conducting area in the middle where the fuse element melts. A glass delayed action cartridge fuse opens only when the current is greater than its rating for a predetermined amount of time. Delayed action cartridge fuses are used for loads that have an initial current surge when power is applied. Some glass fuses have pigtails attached to them so that they can be soldered into a circuit. Glass fuses allow the observation of the condition of the fuse element without using an ohmmeter.

A circuit breaker is an overcurrent protective device that does not need to be replaced each time the circuit current rating is exceeded. Circuit breakers may be thermally or magnetically operated. A circuit breaker is an overcurrent protective device with a mechanical mechanism that manually or automatically opens a circuit when a short circuit or overload occurs. See Figure Like fuses, circuit breakers are connected in series with circuit conductors. A circuit breaker opens and prevents current from flowing in a circuit when the current exceeds the rating of the circuit breaker. Circuit breakers contain a spring loaded electrical contact that opens the circuit. The spring opens and closes the contacts with a fast snap action. A circuit breaker does not have to be replaced each time the current rating is exceeded. Circuit breakers have replaced fuses in many applications. Circuit breakers have voltage, amperage, and interrupting ratings similar to fuses.

Thermal circuit breakers use a bimetallic strip attached to a latch mechanism to open the circuit when a short circuit or overload occurs. Circuit breakers may use thermal or magnetic methods to open a circuit when a short circuit or overload condition occurs. Thermal circuit breakers use a bimetallic strip attached to a latch mechanism. The bimetallic strip is made of two dissimilar metals that expand at different rates when heated. The bimetallic strip bends when heated and opens the contacts. See Figure The bimetallic strip may be heated directly by circuit current or indirectly by the rise in temperature caused by an increase in circuit current.

Magnetic circuit breakers use an electromagnet coil and armature to open the circuit when a short circuit or overload occurs. A magnetic circuit breaker uses an electromagnet coil and armature. In a magnetic circuit breaker, circuit current passes through the coil, producing a magnetic field. See Figure Normal circuit current does not affect the armature. However, when circuit current creates a magnetic force that exceeds the spring force on the armature and the friction of the latching mechanism, the armature is pulled to the electromagnet coil. The spring-loaded contact arm is released, breaking the current flow to the load instantaneously. Magnetic circuit breakers can be manually reset immediately.

Thermal overload relay contacts open when the current level is exceeded for a given period of time. The temperature rise in the metal frame of the motor is used to heat the bimetallic strip. Thermal overload relays operate by opening a bimetallic strip upon a rise in temperature. Most thermal overload relays can be reset using a pushbutton. Automatic-reset thermal overload relays do not have a spring action that requires manual resetting of the contacts. The contacts open when the current level is exceeded for a given period of time. When the bimetallic strip cools, the contacts snap closed, and the circuit is automatically energized. Automatic-reset thermal overload relays are often used on motors but can be a safety hazard. To protect personnel and property, automatic-reset thermal overload relays should only be used on circuits that must be kept running. See Figure 4-15.

In a standard motor control circuit, a relay coil controls a set of normally open contacts and a set of normally closed overload relay contacts. An overload relay may be incorporated into a standard start/stop motor control circuit. In a standard motor control circuit, a motor starter or contactor coil controls a set of normally open contacts, and a set of normally closed overload relay contacts. See Figure 4-16.

DC voltage measurements using a digital multimeter are taken by connecting the black test lead to the negative polarity test point and the red test lead to the positive polarity test point. DC voltage is measured with a DMM using a standard procedure. Always exercise caution when taking any circuit measurement. See Figure 4-17.

DC voltage is measured with an analog meter using standard procedures.
Voltage measurements can be made with an analog meter in a similar manner as with a digital multimeter. DC voltage is measured with an analog meter using the following standard procedure. Always refer to the instruction manual before using any meter. See Figure 4-18.

To measure current flow through a component, a meter must be connected so that the total electron flow is through the meter circuit. To measure current flow through a component, a meter must be connected so that the total electron flow is through the meter circuit. This means that the meter must be connected in series with the component so that only one path for electron flow exists. Meters set to measure current must have a very low resistance that does not substantially change the value of the current in the circuit. Direct current is measured with a DMM using the following standard procedure. Always turn the power to a circuit OFF before taking any measurements. See Figure 4-19.

DC current is measured with an analog multimeter using standard procedures.
Current measurements can also be taken using an analog multimeter. Direct current is measured with an analog multimeter using the following standard procedure. Always turn the power to the circuit OFF before taking any measurements. See Figure 4-20.

Clamp-on ammeters measure current by measuring the strength of the magnetic field around a single conductor. Clamp-on ammeters measure current in a circuit by measuring the strength of the magnetic field around a single conductor. Care should be taken to ensure that the meter does not pick up stray magnetic fields. Whenever possible, conductors under test should be separated from other surrounding conductors by a few inches. If this is not possible, several readings at different locations along the same conductor should be taken. Direct current is measured with a clamp-on ammeter or a DMM with a clamp on current probe accessory using the following standard procedure. See Figure 4-21.

Ohm’s law is the relationship between voltage, current, and resistance in an electrical circuit.
Ohm’s law is the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Using Ohm’s law, any value in this relationship can be found when the other two are known. The relationship between voltage, current, and resistance may be seen best in pie chart form. See Figure 4-22.

Current in a circuit increases with an increase in voltage and decreases with an increase in resistance. According to Ohm’s law, if the resistance in a circuit is held constant and the voltage varied, the current can be determined for each value of voltage. The voltage/current curve is linear, which means that a specific change in voltage causes a specific change in current. A resistor is referred to as a linear load because of this straight line curve. See Figure 4-23.

Voltage in a circuit increases with an increase in current and increases with an increase in resistance. According to Ohm’s law, if the resistance in a circuit is held constant and the current varied, the voltage can be determined for each value of current. The current/voltage curve is linear, which means that the voltage drop across a resistor is directly proportional to the current flowing through it. See Figure 4-24.

Resistance in a circuit increases with an increase in voltage and decreases with an increase in current. According to Ohm’s law, if the current in a circuit is held constant at 2 A and the voltage across it varied from 0 V to 6 V, the resistance value can be determined for each value of voltage. The voltage/resistance curve is linear, which means that with a constant current flow through a resistor, the value of the resistor must be increased to increase the voltage drop across it. See Figure 4-25.

The power formula is the relationship between power, voltage, and current in an electrical circuit.
The power formula is the relationship between power, voltage, and current in an electrical circuit. The power formula is often referred to as Watt’s law. Any value in this relationship may be found when the other two values are known. The relationship between power, voltage, and current may be seen best in pie chart form. See Figure 4-26.

Power in an electrical circuit is calculated by multiplying current by voltage.
Power is directly related to voltage and current. Current flow through an electrical component either generates energy (a power source) or dissipates energy (a resistance, such as a lamp). Power is the rate at which energy is generated or consumed. The power supplied to a circuit must be consumed; therefore, power consumption must be equal to the power dissipated by a circuit. If the voltage in a circuit is varied from 0 V to 6 V and the resistance is held constant at 2 Ω, the current in the circuit is dependent on the value of the applied voltage. In this circuit, the current varies from 0 A to 3 A as calculated by Ohm’s law. Power is calculated by multiplying current times voltage. The voltage/current/power curve is nonlinear. See Figure

Power in an electrical circuit calculated by multiplying current squared by resistance.
If the voltage in a circuit is fixed at 6 V, and resistance is varied from 0 Ω to 6 Ω, current also changes. In this circuit, the resistance/current/power curve is nonlinear. See Figure Note that with voltage applied, and the resistance at 0 Ω, the current becomes undefined. The power consumed at this point is 0 W. Only resistance can consume power in an electrical circuit. Since no resistance is present, no power is consumed.

Power in an electrical circuit can be calculated by dividing voltage squared by resistance.
If both the circuit voltage and resistance are varied to maintain a constant current, the resistance/voltage/power curve becomes linear. See Figure At any point on the graph, if the voltage is divided by the resistance, the current is 1 A. It is only when the current is allowed to vary with the voltage that the power curve is nonlinear and varies at a square rate.