83B - Electric Circuits
83B1 - Current, Voltage and Resistance

Here you will learn to:

Define current, voltage and resistance,

Distinguish between AC and DC current,

Explain factors that affect resistance, and

Use Ohm's law to calculate voltage, current or resistance in a series circuit.

In order to talk meaningfully about electricity, and especially how we can use electricity, we need to be able to measure its fundamental properties. There are three primary characteristics that describe the nature of current electricity and they are unified by Ohm's law (V = IR).

Current

Electric current has the symbol I and is measured in amperes or amps (A). Current refers to how much electricity is flowing, or more specifically, how many electrons are moving through a circuit in a unit of time. Physical currents in electrical circuits consist of electrons in the wire moving along a conductor under the influence of a potential difference (voltage).

There are a very large number of electrons flowing past a point in a circuit in one second so it is not convenient to talk about current in terms of the number of electrons so we talk instead in terms of the unit of charge. As you already know, there are 6.25 x 1018 electrons in a coulomb of charge so we talk of current in terms of coulombs of electrons per second. One amp of current is the equivalent of one coulomb of electrons passing a point in the circuit in one second so 1 A = 1 Cs-1.

Ammeters are devices used to measure the current in a circuit. Because current flows through resistors, an ammeter is placed in series with the resistor so that the total current passes through both devices. In order to minimize the effect of the ammeter on the dynamics of the circuit, the resistance of the ammeter is extremely small. Hence, the circuit current is relatively unaffected by the presence of the low resistance ammeter.

AC v DC Current

There are two different ways that electricity is produced, and they are used in most cases for very different purposes. They can also be converted from one form to another.

The first and simpler type of electricity is called direct current, abbreviated DC. This is the type of electricity that is produced by batteries and lightning for example. The AC current produced flows directly in only one direction from the positive to negative terminal of a battery. In the circuit, the electrons are moving in one direction only (towards the positive) and flow at a specific rate. When you use a flashlight, pocket radio, portable CD player or virtually any other type of portable or battery-powered device, you are using direct current. Most DC circuits are relatively low in voltage; for example, a car's battery is approximately 12 V and that's about as high a DC voltage as anybody ever needs.

The other type of electricity is called alternating current or AC. This is the electricity that you get from the electricity grid and that you use to power most of your electrical appliances. For AC current, the positive and negative potentials are switching back and forth at a constant rate and this causes the electrons to rapidly change direction and move backwards and forwards at the same rate in the conductor. The other key characteristic of AC is its frequency, measured in cycles per second (s-1) or more commonly, hertz (Hz). This number describes how many times in a second the current alternates from positive to negative and back again, completing one cycle. In Australia, the standard is 50 Hz, meaning 50 cycles from positive to negative and back again in one second. In other parts of the world the standard is 60 Hz.

Why does standard electricity come only in the form of alternating current? There are a number of reasons, but one of the most important is that a characteristic of AC is that it is relatively easy to change voltages from one level to another using a transformer, while transformers do not work for DC. This capability allows the companies that generate and distribute electricity to do it in a more efficient manner, by transmitting it at high voltage for long lengths, which reduces energy loss due to the resistance in the transmission wires. Another reason is that it is easier to mechanically generate alternating current electricity than direct current electricity. Computers, for example, use only direct current, which means that the alternating current provided by your utility must be converted to direct current before use. This is the primary function of your power supply.

Voltage

If a force is used to move a negative charge towards another negative charge, it does work on that charge and the moved charge gains potential energy. This is analogous to the definition of the gravitational potential energy which is the work done by the force of gravity in moving a mass through a certain distance (height). The units of potential difference, or electric potential, are joules per coulomb (JC-1) or volts (V). Physically, potential difference has to do with how much work the electric field does in moving a coulomb of charge from one place to another. A 12 V battery will give 12 joules of energy to each coulomb of electrons that move through the circuit. Batteries are rated by the potential difference across their terminals. In a 12 V battery the potential difference between the positive and negative terminals is precisely 12 V. On the other hand, the potential difference across the power outlet in the wall of your home is 240 V.

Voltage is sometimes called potential difference or electromotive force (EMF) and it refers to the amount of potential energy the electrons have in an object or circuit. You can think of this as the amount of energy the electrons have and carry around the circuit. The more energy the electrons have, the higher the voltage. If we draw an analogy to a waterfall, the voltage would represent the height of the waterfall: the higher it is, the more potential energy the water has by virtue of its distance from the bottom of the falls and the more kinetic energy it will have as it hits the bottom.

The energy carried by electrons in a circuit is transformed into another type of energy in a resistor. Resistors do electrical work by transforming energy from one form into another. The voltage (energy) possesed by the electrons as they leave a power sourceor battery is dissipated in each resistor so that when the electrons return to the power source they have no energy left.

Voltmeters, also called potentiometers, measure the voltage drop between two points in a circuit. If you were to meaure the voltage across a resistor, you are measuring the energy difference or the loss of energy for the electrons passing through it. The voltmeter is placed in parallel across a resistor in order to measure the voltage drop or potential difference. To minimize the effect of the voltmeter on the dynamics of the circuit, the net resistance of the voltmeter is designed to be very high. As a result, the current through the voltmeter is extremely small and can be ignored in the equations, leaving the circuit relatively unaffected.

Resistance

The third primary characteristic of electricity is resistance, normally abbreviated R and is measured in ohms (Ω). Resistance refers to how much the material that is conducting electricity opposes the flow of electrons. The higher the resistance, the harder it is for the electrons to pass through the resistor.

Electrical resistance is caused by collisions between moving electrons and the atoms of a conductor. When electrons collide with the atoms of the conductor they lose some of their energy and cause the conductor's atoms to vibrate more vigorously. This additional kinetic energy represents the heating effect observed for resistors as energy is dissipated in the resistor. More vigorous vibration also causes more collisions with electrons as the atoms are essentially taking up more space.

Factors Affecting Resistance

The factors that affect the resistance of a conductor are the type of metal, the temperature of the conductor, the length of the conductor and the cross sectional area. Each is described as follows:

  • Type of Metal - The type of metal from which the conductor is made affects resistance. Different conductors hold their electrons with different degrees of force. If the electrons don’t flow freely, then the resistance is high.

  • Temperature of the Conductor - As the temperature increases so does the resistance of most metals in a linear fashion within the bounds of normal temperatures encountered in electrical devices. In non-metals, the resistance generally decreases with increased temperatures.

  • Length of the Conductor - For a uniform thickness and composition, conductor resistance increases linearly with the length of the conductor.

  • Cross-Sectional Area - As the cross-sectional area of a uniform composition conductor increases, the resistance decreases.

Ohm's Law

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83B2 - Series and Parallel Circuits

In this section you will learn to:

 

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