Magnetic Effects of Electric Current

• Hans Christian Oersted (1777-1851) Oersted showed that electricity and magnetism are related to each other. His
research later used in radio, television etc.

• The unit of magnetic field strength is name Oersted in his honour.

Oersted Experiment

• On passing the current through the copper wire XY in the circuit, the compass needle which is placed near the conductor gets deflected. If we reverse the direction of current, the compass needle deflect in reverse direction. If we stop the flow of current, the needle comes at rest.

• Hence, it conclude that electricity and magnetism are linked to each other. It shows that whenever the current will flow through the conductor, then magnetic field around.

Magnetic Field

• It is the region surrounding a magnet, in which force of magnet can be detected. It is a vector quantity, having both direction and magnitude.

Compass needle

• It is a small bar magnet, whose north end is pointing towards north pole and south end is pointing towards south pole of earth.

Magnetic field lines

• When a bar magnet is placed on a card board and iron fillings are sprinkled, they will arrange themselves in a pattern as shown below.

• The lines along which the iron filling align themselves represent magnetic field lines. Hence, magnetic field line is a path along which a hypothetical free north pole tend to move towards south pole.

Characteristics of Magnetic field lines

• The direction of magnetic field lines outside the magnet is always from north pole to south pole of bar magnet and are indicated by an arrow.

• Inside the magnetic, the direction of field lines is from its south pole to north pole.Thus magnetic field lines are closed curve

• The strength of magnetic field is expressed by the closeness of magnetic field lines. Closer the lines, more will be the strength and farther the lines, less will be the magnetic field strength.

• No two field lines will intersect each other. If they intersects, then at point of intersection the compass needle will show two direction of magnetic field which is not possible.

Right Hand Thumb Rule

• It is a convenient way of finding the direction of magnetic field associated with current carrying conductor. Hold the straight were carrying current in your right hand such that thumb points towards the direction of current, then your folded fingers around the conductor will show the direction of magnetic field.

• This rule is also called Maxwell’s corkscrew rule.

Solenoid

• A Coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called solenoid.

Magnetic field due to a current in a solenoid

• Using R.H. Thumb Rule, we can draw the pattern of magnetic field lives around a current carrying solenoid.

• One end of the solenoid behaves as a magnetic north pole, white the other end behave as the South Pole.

• The filed lines inside the solenoid are in form of parallel straight lines, that implies that magnetic field inside the solenoid is same at all points i.e. Field is uniform.

Electromagnet

• Strong magnetic field inside the solenoid can be used to magnetise a magnetic material for example soft iron, when it is placed inside the coil. The magnet so formed is called electromagnet.

Force on a current carrying conductor in a magnetic field.

• Andre Marie Ampere (1775-1836) suggested that the magnet also exert an equal and opposite force on the current carrying conductor.

• We will observe that the rod will displace i.e. the rod will experience a force, when it is placed in magnetic field, in a perpendicular direction to its length.

• The direction of the exert force will be reversed if the direction of current through the conductor is reversed.

• If we change the direction of field by inter changing the two poles of the magnet, again the direction of exert force will change.

• Therefore the direction of exerted force depends on

(1) direction of current
(2) direction of magnetic field lines.

Left Hand Fleming Rule

• According to this rule, stretch thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular to each other.

• If fore finger represent direction of magnetic field and middle finger represent direction of current, then thumb will point in the direction motion or force acting on the conductor.

• Functioning of electric motor is based on this rule. It convert electrical energy into mechanical energy.

Michael Faraday

• Gave the law of Electromagnetic Induction.

Galvanometer

• It is an instrument that can detect the presence of a current in a circuit. If pointer is at zero (the centre of scale) the there will be no flow of current.

ElectroMagnetic Induction

• Can be explained by two experiments

(a) FIRST EXPERIMENT “SELF INDUCTION”

• In this experiment, when the north pole of bar magnet is brought closes to the coil or away from the coil, we see momentary deflection in the needle of galvanometer on either side of null point. First right and then left.

• Similarly, if we keep the magnet stationary and coil is made to move towards or away from the north pole of magnet. Again we will observe deflection in the needle of galvanometer.

• If both bar magnet and coil kept stationary, there will be no deflection in galvanometer.

• This experiment can also be done with the south pole of magnet, we will observe the deflection in galvanometer, but it would be in opposite direction to the previous case.

• It concludes that motion of magnet with respect to coil or vice-versa, changes the magnetic field. Due to this change in magnetic field lines, potential difference is induced in the same coil, which set up an induced current in the circuit.

(b)SECOND EXPERIMENT – Mutual Induction

• In this experiment plug in the key that is connect coil with battery and observe the deflection in galvanometer. Now plug out the key that is disconnect the coil-1 from battery and observe the deflection in galvanometer, which will be in reverse direction.

• Hence, we conclude that potential difference is induced in secondary coil (coil-2), whenever there is a change in current, in primary coil (coil-1) (by on and off of key).

• This is because, whenever there is change in current in primary coil.

• Magnetic field associated with it also changes.

• Now, magnetic field lines around the secondary coil (coil-2) will change and induces the electric current in it (observed by the deflection of needle of Galvanometer in secondary circuit).

• This process, by which changing of strength of current in primary coil, induces a current in secondary coil is called Electromagnetic Induction”

• The induced current is found to be highest when the direction of motion of coil is at right angles to the magnetic field.

Fleming’s Right Hand Rule

• Three of them perpendicular to each other.

• Rule can be defined at–

Stretch thumb , forefinger and middle finger of right hand, so that they are perpendicular to each other. The forefinger indicates direction of magnetic field, thumb shows the direction of motion of conductor, then the middle
finger will shows the direction of induced current.

• Electrical generator is based on the principle of electromagnetic induction. It convert mechanical energy into electrical energy.

Advantages of Alternate Current (AC) over Direct Current (DC)

• Electric power can be transmitted to longer distances without much loss of energy. Therefore cost of transmission is low.

• In India the frequency of AC is 50 Hz. It means after every 1/100 second it changes its direction.

Domestic Electric Circuits

• In our homes, the electric power supplied is of potential difference V = 220 V and frequency 50 Hz.

• It consist of three wires

(1) Wire with red insulation cover – LIVE WIRE (POSITIVE) Live wire is at high potential of 220 V.

(2) Wire with black insulation cover – NEUTRAL WIRE (NEGATIVE) Neutral wire is at zero potential Therefore, the potential difference between the two is 220 V.

(3) Wire with Green insulation cover – EARTH WIRE it is connected to a copper plate deep in the earth near house.
The metallic body of the appliances is connected with the earth wire as a safety measure.

• Function

Earth wire provide a low resistance to the current hence any leakage of current to the metallic body of the appliances, keep its potential equal to that of earth. That means zero potential and the user is saved from severe
electric shock.

• Point to be noted in domestic circuit

(1) Each appliance has a separate switch of ON/OFF.

(2) In order to provide equal potential difference to each appliance, they should be connected parallel to each other. So that they can be operated at any time.

(3) We have two electric circuit in our home One consist of current of 15 A for high power appliances Other circuit consist of current 5 A for low power appliances.

Short Circuiting

• Due to fault in the appliances or damage in the insulation of two wires, the circuit will offer zero or negligible resistance to the flow of current. Due to low resistance, large amount of current will flow. According to Joule’s law of heating effect (HaI2) heat is produced in live wire and produces spark, damaging the device and wiring.

• Overloading

Overloading can be caused by
(1) Connecting too many appliances to a single socket or (2) accidental rise in supply voltage if the total current drawn by the appliances at a particular time exceeds the bearing capacity of that wire, it will get heated up. This is known as overloading.

• Fuse a safety device can prevent the circuit from overloading and short circuiting.

Important Questions

1. List the properties of magnetic lines of force.

Ans. Properties of magnetic lines of force are listed below :

(i) Outside a magnet, the magnetic field lines are directed from N-pole of magnet to the S-pole. However, inside a magnet, the field lines are directed from S-pole to N-pole. Thus, magnetic lines are closed curves.

(ii) A magnetic field line always points in the direction of magnetic field.

(iii) The relative strength of magnetic field lines is given by degree of closeness of the field lines. The magnetic field is stronger in the region where the lines are crowded.

(iv) No two magnetic field lines can ever intersect with each other.

2. Why don’t two magnetic lines of force intersect each other ?

Ans. No two magnetic field lines can ever intersect each other. If they do, then it would mean that at the point of intersection there are two directions of magnetic field, which is not possible.

3. A current -carrying conductor is placed in a magnetic field. Now answer the following :

(i) List the factors on which the magnitude of force experienced by conductor depends.

(ii) When is the magnitude of this force maximum ?

(iii) State the rule which helps in finding the direction of motion of conductor.

(iv) If initially this force was acting from right to left, how will the direction of force change if :

(a) direction of magnetic field is reversed ?

(b) direction of current is reversed ?

Ans. (i) The magnitude of force experienced by current-carrying conductor when placed in a magnetic field depends on (a) current flowing(I), (b) length of conductor (l), (c) the strength of magnetic field (B) , and (d) orientation of conductor in magnetic field.

(ii) Magnitude of force is maximum when current-carrying conductor is placed at right angles to the direction of magnetic field.

(iii) Fleming’s left-hand rule states that stretch the forefinger, the central finger and the thumb of your left-hand in mutually perpendicular directions. If the forefinger shows the direction of the magnetic field and the central finger that of the current, then the thumb will point towards the direction of motion of the conductor.

(iv) (a) Direction of force is reversed that is now the force acts from left to right.

(b) Direction of force is reversed that is now the force acts from left to right.

4. Explain the activity to show that a current-carrying conductor experiences a force when placed in a magnetic field.

Ans. A current-carrying conductor experiences a magnetic force due to a magnetic field. The force is maximum when the conductor is placed perpendicular to the direction of magnetic field. To demonstrate the force we perform the following experiment :

Experiment : Take a small sized aluminium rod AB of length about 5 cm. Suspend the rod AB horizontally form a rigid stand using two connecting wires.

Place a strong horse-shoe magnet NS is such a way that the rod AB lies between the poles of the magnet. Let the north pole N of magnet lies below the rod and the south pole S lies directed in vertically upward direction is present.

Complete the electrical circuit of aluminium rod AB by connecting it in series with a battery, key K and a variable resistance.

Put the plug in key K so that an electric current begins to flow through the aluminium rod from end B to end A. We observe that on passing current the rod AB is displaced towards the left. It shows that the aluminium rod is experiencing a mechanical force due to which it is being deflected. Direction of deflection is given by Fleming’s left hand rule.

Effect of change in direction of current : Reverse the connections of battery so that on applying plug in key K, current flows in the aluminium rod form A to B. We now find that the rod AB is deflected towards the right. Thus, by reversing the direction of current, direction of force is also reversed. It is in accordance with Fleming’s left-hand rule.

Effect of change in direction of magnetic field : Now reverse the direction of magnetic field to vertically downward direction. For this purpose suspend, the horse-shoe magnet in such a way that N-pole of magnet lies above the aluminum rod AB and S-pole of magnet lies below the rod. If now current in aluminium rod is flown from B to A, the rod is deflected towards right. If current in the rod is flowing from A to B, then the rod is deflected towards left. It shows that direction of deflection is reversed on reversing the direction of magnetic field.

5. One of the major causes of fire in office buildings is short-circuiting. List three reasons which may lead to short-circuiting. How can it be prevented ?

Ans. Three possible reasons of short-circuiting of an electrical circuit are as follows :

(i) The insulation of electrical wirings is damaged.

(ii) The electrical appliance used in the circuit is defective.

(iii) An appliance of higher power rating is being run on an electrical line of lower power rating.

short-circuiting can be prevented by the use of electrical fuse of appropriate capacity.

6. Describe four important features of domestic electric supply lines.

Ans. Important features of domestic electric supply lines are as given below :

(i)  Different circuits and different appliances in each circuit are connected in parallel.

(ii) Each appliance is provided with an independent on/off switch which is always joined to the live wire.

(iii) Two separate circuits are used, one of 15 A for appliances with higher power ratings and another of 5 A for bulbs, tubes etc.

(iv) Electric fuses of appropriate capacities are used ahead of each electric circuit as a safety measure. Moreover, proper earthing must be ensured.

7. Describe an activity to explain how a moving magnet can be used to generate electric current in a coil.

Ans. Take a coil AB of insulated copper wire having a number of turns. Connect the ends of coil to a sensitive galvanometer G. Now take a bar magnet NS and rapidly bring the magnet towards the end B of coil . The galvanometer gives momentary deflection in on direction. Now take the magnet away from the coil, the galvanometer again gives momentary deflection but in the opposite direction. It clearly shows that motion of magnet induces, a current in the coil.

Again keep the magnet fixed and gently move the coil AB either towards the magnet of away from the magnet. We get deflection in galvanometer even now. Thus, an induced current is produced in a coil whenever there is relative motion between the coil and the magnet. This phenomenon is known as the electromagnetic induction.

8. A metallic conductor is suspended perpendicular to the magnetic field of a horse-shoe magnet. The conductor gets displaced towards left when a current is passed through it. What will happen to the displacement of the conductor if the :

(i) current through it is increased ?

(ii) horse-shoe magnet is replaced by another stronger horse-shoe magnet ?

(iii) direction of current through it is reversed ?

Ans. (i) On increasing the current flowing through metallic conductor, the force experienced by it is proportionately increased because F is directly proportional to l

(ii) On using a stronger horse-shoe magnet the magnetic force increases because F is directly proportional to B.

(iii) On reversing the direction of current the direction of force is reversed and conductor is displaced towards right instead of left direction.

9. What are magnetic field lines? How is the direction of magnetic field at a point determined ? Mention two important properties of magnetic field lines.

Ans. Magnetic field lines are used to represent a magnetic field. A field line is the path along which the north pole of a small compass tends to move. The direction of the magnetic field at a point is given by the direction in which north pole of compass placed at that point would take.

10. What happen if a domestic electric circuit is short-circuited ?

Ans. As a result of short-circuiting, the resistance of the circuit decreases to a very small value and consequently the current increases enormously. It results in heating the live wires and producing a spark at the place of short circuit, which may even cause fine in the building.

11. What precautions should be taken to avoid the overlaoding of domestic electric circuits ?

Ans. Following precautions should be taken to avoid the overloading of domestic electric circuits :
(i) too many electrical appliances should not be operated using a single socket.
(ii) Too many large power rating appliances should not be switched on at a time.

12. What is short-circuiting ? What is its possible cause ?

Ans. Short-circuiting means that the two wires, live and neutral, of the domestic electric circuit have come in contact with each other. Short-circuiting may take place either due to their insulations having been damaged or due to a fauld in the appliances.

13. What is the role of fuse, used in series with any electrical appliance ? Why should a fuse with defined rating not be replaced by one with a larger rating ?

Ans. The fuse is a safety device used to prevent any damage to an electrical appliance either due to short-circuiting or overloading of the electrical circuit.IF a fuse, with defined rating, is replaced by one with a larger rating then the fuse wire will nor burn even when a current even greater amount than the safe limit is flowing. As a result the electrical appliances will be damaged.

14. What is the usual colour code followed for connecting live, neutral and earth wires ? Why is it important ?

Ans. As per usual colour code followed the live wire is of red coloured insulation, neutral wire of black coloured insulation and earth wire is of green coloured insulation. If the colour code is followed then any person would identify the correct connecting wire and would be able to handle them properly.

15. Mention the provision of two different current ratings in our domestic circuits. Explain, with reason, the advantage of such a provision.

Ans. In our domestic circuits provision is made for two different current ratings :
(i) a rating of 5 A for bulbs, tubes, CFL’s and fans etc.
(ii) a rating of 15 A for electric iron, gyser, refrigerator etc
Advantage of this provision is that we may use fitting and line wire of appropriate power rating for appliances.

16. Distinguish between a direct current and alternating current.

Ans. Direct current(D.C.)
(i) Direct current always flows in one direction only
(ii) The magnitude of current may or may not remain constant
(iii) Current obtained from a battery and D.C generator is D.C.

Alternating Current (A.C.)
(i) Alternating current reverses its direction periodicly.
(ii) Magnitude of current continuously changes with time.
(iii) Current obtained from an A.C. generator and current in our domestic circuit are A.C.

17. Distinguish between Electric Motor and Electric Generator

Ans. Electric Motor
(i) It is a device which converts electrical energy into mechanical energy.
(ii) It works on the principle of Fleming’s left-hand rule.

Electric Generator
(i) It is a device which converts mechanical energy into electrical energy.
(ii) It works on the principle of Fleming’s right-hand rule.

18. State two ways by which the strngth of an electromagnet can be increased.

Ans. the strength of an electromagnet can be increased by
(i) increasing the number of turns of windings of the solenoid coil wrapped around the electromagnet.
(ii) increasing the amout of electric current flowing through the coil.