INTRODUCTION
We know that an electric current-carrying wire carries on like a magnet.
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| Figure 1 Compass needle is avoided on passing an electric current through a metallic conductor |
We see that the needle is diverted. What does it cruel? It implies that the electric current through the copper wire has delivered a attractive impact. In this way able to say that power and attraction are connected to each other. At that point, what around the invert plausibility of an electric effect of moving magnets? In this Chapter we are going think about attractive areas and such electromagnetic impacts. We might moreover ponder approximately electromagnets and electric engines which include the attractive impact of electric current, and electric generators which include the electric impact of moving magnets.
Magnetic Field and field lines
We are commonplace with the reality that a compass needle gets diverted when brought close a bar magnet. A compass needle is, in reality, a little bar magnet. The closes of the compass needle point roughly towards north and south bearings. The conclusion indicating towards north is called north looking for or north post. The other conclusion that focuses towards south is called south looking for or south post. Through different exercises we have watched that like shafts repulse, whereas not at all like posts of magnets draw in each other.
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| Figure 2 Iron filings near the bar magnet align themselves along the field lines. |
The press filings organize themselves in a design as appeared Fig. 2. Why do the press filings orchestrate in such a design? What does this design illustrate? The magnet applies its impact within the locale encompassing it. Subsequently the press filings encounter a constrain. The constrain in this way applied makes press filings to orchestrate in a design. The locale encompassing a magnet, in which the constrain of the magnet can be recognised, is said to have a attractive field. The lines along which the press filings adjust themselves speak to attractive field lines.
Magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore it is taken by convention that the field lines emerge from north pole and merge at the south pole. Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curves.
The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger, that is, the force acting on the pole of another magnet placed is greater where the field lines are crowded.
No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
MAGNETIC FIELD DUE TO A CURRENT-CARRYING CONDUCTOR
we have seen that an electric current through a metallic conductor produces a attractive field around it. In arrange to discover the course of the field created let us rehash the action within the taking after way –
Activity
- Take a long straight copper wire, two or three cells of 1.5 V each, and a plug key. Connect all of them in series as shown in Fig.(a).
- Place the straight wire parallel to and over a compass needle.
- Plug the key in the circuit.
- Observe the direction of deflection of the north pole of the needle. If the current flows from north to south, as shown in Fig.(a), the north pole of the compass needle would move towards the east.
- Replace the cell connections in the circuit as shown in Fig. (b). This would result in the change of the direction of current through the copper wire, that is, from south to north.
- Observe the change in the direction of deflection of the needle. You will see that now the needle moves in opposite direction, that is, towards the west [Fig.(b)]. It means that the direction of magnetic field produced by the electric current is also reversed.
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| A basic electric circuit in which a straight copper wire is set parallel to and over a compass needle. The avoidance within the needle gets to be inverse when the course of the current is switched. |
Magnetic Field due to a Current through a Straight Conductor
What decides the design of the attractive field created by a current through a conductor? Does the design depend on the shape of the conductor? We should explore this with an action.
We should to begin with consider the design of the attractive field around a straight conductor carrying current.
Activity
- Take a battery (12 V), a variable resistance (or a rheostat), an ammeter (0–5 A), a plug key, connecting wires, and a long straight thick copper wire.
- Insert the thick wire through the centre, normal to the plane of a rectangular cardboard. Take care that the cardboard is fixed and does not slide up or down.
- Connect the copper wire vertically between the points X and Y, as shown in Fig. (a), in series with the battery, a plug and key.
- Sprinkle some iron filings uniformly on the cardboard. (You may use a salt sprinkler for this purpose.)
- Keep the variable of the rheostat at a fixed position and note the current through the ammeter.
- Close the key so that a current flows through the wire. Ensure that the copper wire placed between the points X and Y remains vertically straight.
- Gently tap the cardboard a few times. Observe the pattern of the iron filings. You would find that the iron filings align themselves showing a pattern of concentric circles around the copper wire
- What do these concentric circles represent? They represent the magnetic field lines.
- How can the direction of the magnetic field be found? Place a compass at a point (say P) over a circle. Observe the direction of the needle. The direction of the north pole of the compass needle would give the direction of the field lines produced by the electric current through the straight wire at point P. Show the direction by an arrow.
- Does the direction of magnetic field lines get reversed if the direction of current through the straight copper wire is reversed? Check it.
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| Figure (a) A design of concentric circles showing the field lines of a attractive field around a straight conducting wire. The bolts within the circles appear the heading of the field lines. (b) A near up of the design gotten. |
What happens to the diversion of the compass needle set at a given point in case the current within the copper wire is changed? To see this, vary the current within the wire. We discover that the diversion within the needle too changes. In reality, in case the current is expanded, the diversion moreover increments. It shows that the greatness of the attractive field delivered at a given point increments as the current through the wire increments.
What happens to the diversion of the needle in case the compass is moved absent from the copper wire but the current through the wire remains the same? To see this, presently put the compass at a more distant point from the conducting wire (say at point Q). What alter do you watch? We see that the diversion within the needle diminishes. In this way the attractive field delivered by a given current within the conductor diminishes as the remove from it increments. From Fig. (a) it can be taken note that the concentric circles speaking to the attractive field around a current-carrying straight wire ended up bigger and bigger as we move absent from it.
Right-Hand Thumb Rule
A helpful way of finding the heading of attractive field related with a current-carrying conductor is is given in Fig. 2
Envision merely are holding a current-carrying straight conductor in your right hand such that the thumb focuses towards the heading of current. At that point your fingers will wrap around the conductor within the heading of the field lines of the attractive field, as appeared in Fig. 2 Typically known as the right-hand thumb run the show*.
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| Figure-2 Right-hand thumb rule |
Example
A current through a horizontal power line flows in east to west direction. What is the direction of magnetic field at a point directly below it and at a point directly above it?
Solution
Ans. The current is in the east-west direction. Applying the right-hand thumb rule, we get that the magnetic field (at any point below or above the wire) turns clockwise in a plane perpendicular to the wire, when viewed from the east end, and anti-clockwise, when viewed from the west end.
Magnetic Field due to a Current through a Circular Loop
We have so distant watched the design of the attractive field lines created around a current-carrying straight wire. Assume this straight wire is bowed within the frame of a circular circle and a current is passed through it. How would the attractive field lines see like? We know that the attractive field created by a current-carrying straight wire depends contrarily on the separate from it. Essentially at each point of a current-carrying circular circle, the concentric circles speaking to the attractive field around it would gotten to be bigger and bigger as we move absent from the wire (Fig.3)
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| Figure-3 Magnetic field lines of the field created by a current-carrying circular loop |
By the time we reach at the middle of the circular circle, the curves of these enormous circles would show up as straight lines. Each point on the wire carrying current would grant rise to the attractive field showing up as straight lines at the center of the circle. By applying the proper hand run the show, it is simple to check that each segment of the wire contributes to the attractive field lines within the same course inside the circle.
*This rule is also called Maxwell’s corkscrew rule. If we consider ourselves driving a corkscrew in the direction of the current, then the direction of the rotation of corkscrew is the direction of the magnetic field.
We know that the attractive field created by a current-carrying wire at a given point depends straightforwardly on the current passing through it. In this manner, in the event that there’s a circular coil having n turns, the field created is n times as huge as that created by a single turn. This is often since the current in each circular turn has the same course, and the field due to each turn at that point fair includes up.
Objective based questions(MCQs)
Q1. A strong bar magnet is placed vertically above a horizontal wooden board. The magnetic lines of force will
be :
(a) only in horizontal plane around the magnet
(b) only in vertical plane around the magnet
(c) in horizontal as well as in vertical planes around the magnet
(d) in all the planes around the magnet
Q2. The magnetic field lines produced by a bar magnet :
(a) originate from the south pole and end at its north pole
(b) originate from the north pole and end at its east pole
(c) originate from the north pole and end at its south pole
(d) originate from the south pole and end at its west pole
Q3. Which of the following is not attracted by a magnet ?
(a) steel (b) cobalt (c) brass (d) nickel
Q4. The magnetic field lines :
(a) intersect at right angles to one another
(b) intersect at an angle of 45° to each other
(c) do not cross one another
(d) cross at an angle of 60° to one another
Q5. The north pole of earth’s magnet is in the :
(a) geographical south (b) geographical east
(c) geographical west (d) geographical north
Q6. The axis of earth’s magnetic field is inclined with the geographical axis at an angle of about :
(a) 5° (b) 15° (c) 25° (d) 35°
Q7. The shape of the earth’s magnetic field resembles that of an imaginary :
(a) U-shaped magnet (b) Straight conductor carrying current
(c) Current-carrying circular coil (d) Bar magnet
Q8. A magnet attracts :
(a) plastics (b) any metal (c) aluminium (d) iron and steel
Q9. A plotting compass is placed near the south pole of a bar magnet. The pointer of plotting compass will :
(a) point away from the south pole (b) point parallel to the south pole
(c) point towards the south pole (d) point at right angles to the south pole
Q10. The metallic pointer of a plotting compass gets deflected only when it is placed near a bar magnet because
the pointer has :
(a) electromagnetism (b) permanent magnetism
(c) induced magnetism (d) ferromagnetism
Q11. Which of the following statements is incorrect regarding magnetic field lines ?
(a) The direction of magnetic field at a point is taken to be the direction in which the north pole of a magnetic compass needle points.
(b) Magnetic field lines are closed curves
(c) If magnetic field lines are parallel and equidistant, they represent zero field strength
(d) Relative strength of magnetic field is shown by the degree of closeness of the field lines
Answers:- 1. (d) 2. (c) 3. (c) 4. (c) 5. (a) 6. (b)
7. (d) 8. (d) 9. (c) 10. (b) 11. (c)
