Magnetic Effects of Current

 

Magnetic Field Due To Current Carrying Conductor

Magnetic field is generally interpreted as a region where the force of magnetism acts. This force of magnetism is generally produced as a result of moving charge or some magnetic material. The origins of this interpretation date back to 19th century. During the early years of 19th century, a scientist named H. C. Oersted discovered that a current carrying conductor produces magnetic effect around it. Although science and technology were not as developed as they are today, based on observations, it was already known that the effect of lightning striking a ship caused the malfunctioning of compass needles, disrupting the navigation system. People back then knew that lightning was a form of electricity and also the fact that the working of a compass is based on the earth’s magnetic field. This suggested a relationship between the two, the moving electric charge (current) and the magnetic field.

Current is generally defined as the rate of flow of charge. We already know that stationary charges produce an electric field which is proportional to the magnitude of the charge. The same principle can be applied here, moving charges produce magnetic fields which are proportional to the current and hence a current carrying conductor produces magnetic effect around it. This magnetic field is generally attributed to the sub-atomic particles in the conductor, for e.g. the moving electrons in the atomic orbitals.

Magnetic field has both magnitude and direction and hence it is a vector quantity and is denoted by B. Magnetic field due to a current carrying conductor depends on the current in the conductor and distance of the point from the conductor.  The direction of the magnetic field is perpendicular to the wire. If you wrapped your right hand fingers around the wire with your thumb pointing in the direction of the current, then the direction in which the fingers would curl will give the direction of the magnetic field. This will be clearer with the diagram shown below where the red lines represent the magnetic field lines.

 

The magnetic field produced due to a current carrying conductor has the following characteristics:

  • It encircles the conductors.
  • It lies in a plane perpendicular to the conductor.
  • Reversal in direction of current flow reverses the direction of the field.
  • Strength of the field is directly proportional to the magnitude of current.
  • Strength of the field at any point is inversely proportional to the distance of the point from the wire.

It's difficult to comprehend the role of magnetism in our lives as we can’t see them. Take a look around and the realization of its importance will not be as difficult. The motors that are used so extensively around the world whether it's a toy car or a bullet train or an aircraft or a spaceship they all use the same magnetic effect.  

Fleming's Left Hand Rule

Whenever a current carrying conductor is placed in a magnetic field, the conductor experiences a force which is perpendicular to both the magnetic field and the direction of the current. According to Fleming's left hand rule, if the thumb, forefinger and middle finger of the left hand are stretched to be perpendicular to each other as shown in the illustration at left, and if the forefinger represents the direction of magnetic field, the middle finger represents the direction of current, then the thumb represents the direction of force. Fleming's left hand rule is applicable for motors.

How to Remember Fleming's Left Hand Rule?

Method 1: Relate the thumb with thrust, fore finger with field and center-finger with current as explained below. 

  • The Thumb represents the direction of Thrust on the conductor (force on the conductor).
  • The Fore finger represents the direction of the magnetic F
  • The Center finger (middle finger) the direction of the C

Method 2: Relate the Fleming's left-hand rule with FBI (wait! NOT with the Federal Bureau of Investigation). Here, F for Force, B is the symbol of magnetic flux density and I is the symbol of Current. Attribute these letters F,B,I to the thumb, first finger and middle finger respectively. 

Fleming's Right Hand Rule

Fleming's right hand rule is applicable for electrical generators. As per Faraday's law of electromagnetic induction, whenever a conductor is forcefully moved in an electromagnetic field, an emf gets induced across the conductor. If the conductor is provided with a closed path, then the induced emf causes a current to flow. According to the Fleming's right hand rule, the thumb, fore finger and middle finger of the right hand are stretched to be perpendicular to each other as shown in the illustration at right, and if the thumb represents the direction of the movement of conductor, fore-finger represents direction of the magnetic field, then the middle finger represents direction of the induced current. 

How to Remember Fleming's Right Hand Rule?

You can follow the same methods mentioned above for Fleming's left hand rule. In this case, you just have to consider your right hand instead of the left hand.

Electromagnetic induction (or sometimes just induction) is a process where a conductor placed in a changing magnetic field (or a conductor moving through a stationary magnetic field) causes the production of a voltage across the conductor. This process of electromagnetic induction, in turn, causes an electrical current - it is said to induce the current.

Discovery of Electromagnetic Induction

Michael Faraday is given credit for the discovery of electromagnetic induction in 1831, though some others had noted similar behaviour in the years prior to this.

The formal name for the physics equation that defines the behaviour of an induced electromagnetic field from the magnetic flux (change in a magnetic field) is Faraday's law of electromagnetic induction.

The process of electromagnetic induction works in reverse as well, so that a moving electrical charge generates a magnetic field. In fact, a traditional magnet is the result of the individual motion of the electrons within the individual atoms of the magnet, aligned so that the generated magnetic field is in a uniform direction. (In non-magnetic materials, the electrons move in such a way that the individual magnetic fields point in different directions, so they cancel each other out and the net magnetic field generated is negligible.)

Maxwell-Faraday Equation

The more generalized equation is one of Maxwell's equations, called the Maxwell-Faraday equation, which defines the relationship between changes in electrical fields and magnetic fields.

It takes the form of:

∇×E = – B / ∂t

Where the ∇× notation is known as the curl operation, the E is the electric field (a vector quantity) and B is the magnetic field (also a vector quantity). The symbols ∂ represent the partial differentials, so the right-hand of the equation is the negative partial differential of the magnetic field with respect to time.

Both E and B are changing in terms of time t, and since they are moving the position of the fields are also changing.

DIRECT AND ALTERNATING CURRENT

DIRECT CURRENT: An electric current that flows continuously in a single direction is called a direct current, or DC. The electrons in a wire carrying direct current move slowly, but eventually they travel from one end of the wire to the other because they keep plodding along in the same direction.

The voltage in a direct-current circuit must be constant, or at least relatively constant, to keep the current flowing in a single direction. Thus, the voltage provided by a flashlight battery remains steady at about 1.5 V.

The positive end of the battery is always positive relative to the negative end, and the negative end of the battery is always negative relative to the positive end. This constancy is what pushes the electrons in a single direction.

ALTERNATING CURRENT: Another common type of current is called alternating current, abbreviated AC. In an alternating-current circuit, voltage periodically reverses itself. When the voltage reverses, so does the direction of the current flow.

In the most common form of alternating current, used in most power distribution systems throughout the world, the voltage reverses itself either 50 or 60 times per second, depending on the country.

The electrons in an AC circuit don’t really move along with the current flow. Instead, they sort of sit and wiggle back and forth. They move one direction for 1/60th or 1/50th of a second depending on frequency system, and then turn around and go the other direction for 1/60th or 1/50thof a second. The net effect is that they don’t really go anywhere.

Alternating current is used in nearly all the world’s power distribution systems, for the simple reason that AC current is much more efficient when it’s transmitted through wires over long distances. All electric currents lose power when they flow for long distances, but AC circuits lose much less power than DC circuits.

Advantages of AC electricity

There are distinct advantages of AC over DC electricity. The ability to readily transform voltages is the main reason we use AC instead of DC in our homes.

Transforming voltages

The major advantage that AC electricity has over DC electricity is that AC voltages can be readily transformed to higher or lower voltage levels, while it is difficult to do that with DC voltages.

Since high voltages are more efficient for sending electricity great distances, AC electricity has an advantage over DC. This is because the high voltages from the power station can be easily reduced to a safer voltage for use in the house.

Changing voltages is done by the use of a transformer. This device uses properties of AC electromagnets to change the voltages.

Tuning circuits

AC electricity also allows for the use of a capacitor and inductor within an electrical or electronic circuit. These devices can affect the way the alternating current passes through a circuit. They are only effective with AC electricity.

A combination of a capacitor, inductor and resistor is used as a tuner in radios and televisions. Without those devices, tuning to different stations would be very difficult.

Domestic Electric Circuits

Alternating current (AC) voltage is relied on to provide power flawlessly on a daily basis to Domestic Electric Circuits. Simply put, AC voltage is capable of converting voltage levels with just a transformer, making it far easier to transport across great distance than DC, whose conversion requires more complex electronic circuitry.

Electric charge in AC periodically changes direction, causing the voltage level to reverse. As a result, AC voltage needs to step up if transmitted over a large distance, but this does not affect the speed of the transition process. Such ease in conversion allows for AC also to appear in electric generators, motors, and power distribution systems. Requiring only a transformer to convert its voltage levels is perhaps the greatest advantage AC has over DC, as direct current may only create magnetic fields, preventing it from working with transformers at all.