Week 9 and 10 – SS3 First Term Physics Notes

WEEK 9 & 10
ELECTROMAGNETIC FIELD
CONTENT

• Patterns of magnetic field
• Magnetic field around a straight conductor carrying current
• Force on a current carrying conductor in a magnetic field
• Force between conductors carrying current

   

PATTERNS OF MAGNETIC FIELDS
Magnetic field pattern can easily be observed using iron fillings. The magnetic is put on paper and the iron fillings are sprinkled lightly on the paper around the magnet. The paper is tapped gently and the iron fillings are found to turn and set to in definite direction.
MAGNETIC FIELD AROUND A STRAIGHT CONDUCTOR CARRYING CURRENT
A straight conductor carrying current can be shown that it has magnet filed around it. Allow a thick isolated copper wire to pass vertically through a hole in a card board shit. As shown below, sprinkle some iron fillings uniformly on the cardboard around the vertical wire connect the ends of the wire to a battery, switch on the current and place some compass needles around the wire. Note the direction to which the compass needle point. Switch on the current and note the swing of the needles and how they point.
It will be observed that when current is switch on and the card board is gently tapped, the fillings arrange themselves in a series of concentric circles about the wire as centre. Also as soon as the current is switch on, the needles will swing around and form a circle with the wire as centre. The direction of the filed depends on the direction of flow of the current. Such a direction can always be obtained by applying the Right Hand Grip Rule.

 

                                        Compass needle

 Card board
Sprinkled with iron fillings

 
 FORCE ON A CURRENT CARRYING CONDUCTOR IN A MAGNETIC FIELD
A conductor carrying an electric current, when placed in the magnetic field experiences a mechanical force. This can be demonstrated by using two metal rails fixed on each side of a powerful horse-shoe magnet. A copper rod is placed across the rays. When the current is passed through this copper rod, it is observed that the copper rod rolls along the rays, toward the right. If by adjusting the rheostat, more current is made to flow through the rod. One will notice that the rod moves faster, thus the force on the rod increases when the current increases.
If the direction of flow of current is reversed by reversing the connections at the battery terminals, the rod will be observed to move towards the left, opposite to the previous direction of motion.
If one turns the magnet such that the magnetic field is parallel to the length of the rod as shown below, it will be observed that the current carrying the rod remains stationary no matter the amount of current hat pass through. There is therefore no force on the rod.
ELECTROMAGNETIC FIELD II

 CONTENT

• Electromagnetic field
• Fleming left hand rule
• Applications of electromagnetic field

 ELECTROMAGNETIC FIELD: This is a field representing the joint interaction of electric and magnetic forces. It is exerted on charged particles. The force on a charge q moving with velocity v less than the velocity of light is given by     
        F = q (E + v x B)
A conductor carrying an electric current when placed in a magnetic field experiences a mechanical force. It can be demonstrated by using two metal rails fixed on each side of a powerful horse-shoe magnet. A copper rod is placed across the rails. When we pass current through this copper rod, it is observed that the copper rood rolls along the rails, towards the right. If by adjusting the rheostat, we cause more current to flow through the rod, we will observe that the rod moves faster. Thus, the force on the rod increases when the current increases.
Direction of the force – The direction of force on a current carrying conductor placed perpendicular to the magnetic field is given by Fleming’s left-hand rule which is stated as follows:
If the thumb, forefinger and middle finger are held mutually at right angles to one another with the fore-finger pointing in the direction of magnetic field, and the second finger in the direction of Current, then the thumb will point in the direction of the force producing motion.

 APPLICATIONS OF ELECTROMAGNETIC FIELD
1.    Electric Motor – The electric motor is a device for converting electrical energy into chemical energy. It consists:
(i) A rectangular coil of insulated wire, known as armature
(ii) A powerful magnetic field in which the armature turns is provided by two curve pole pieces of a powerful magnet
(iii) A commutator consisting of a split copper ring, two halves of which are insulated from each other.
(iv) Two carbon brushes which are made to press lightly against either side of the split-ring commutator
ii. Moving Coil Galvanometer – This galvanometer is one of the most sensitive and accurate methods for detecting or measuring extremely small currents or potential differences. It consist essentially of
1. A light rectangular vertical coil ABCD pivoted in jeweled bearings such that it can move in a vertical plane2. Two curved pole piece of a horse shoe magnet and a soft iron core or cylinder inserted between the pole pieces.
3. Two spiral non-magnetic control springs of phosphor bronze, each of which is attached to the jeweled bearing or spindle. Current enters or leaves the rectangular coil through these spiral springs. The springs also provide the control couple.

 CLASSWORK

1. What do you understand the term electromagnetic field?
2. What is a transformer?
3. The transformer in a disc video decoder is used to step down 240V supply to 12V. If there are 2400 turns in the secondary coil find: (i) the turn ratio of the transformer (ii) the number of turns in the primary coil
4. State Fleming’s left hand rule

 ASSIGNMENT
SECTION A

1. Induced current depends the: (i) the number of turns in the coil (ii) strength of the magnet (iii) speed with which the magnet is plunged into the coil (a) I only (b) II only (c) III only (d) none of the above
2. To convert an alternating current dynamo into a direct current dynamo, the (a) the number of turns in the coil is increased (b) strength of the field magnet is increased (c) slip rings are replaced with split ring commutator (d) coil is wound on a soft iron armature
3. If a current-carrying coil is mounted on a metal frame, the back e.m.f. induced in the coil causes (a) inductance (b) eddy current (c) electromagnetism (d) dipole moment
4. A transformer with 5500 turns in its primary is used between a 240V a.c. supply and a 120V kettle. Calculate the number of turns in the secondary (a) 2750 (b) 460 (c) 11,000 (d) 232
5. The direction of induce current in a straight wire balanced in a magnetic field is determined by using ___________ (a) Fleming’s right hand rule (b) Maxwell’s screw rule (c) Faraday’s law (d) Len’s law
6. Energy losses dues to eddy currents are reduced by using (a) low resistance wires (b) insulated soft iron wires (c) few turns of wire (d) high resistance wires
7. From the generating station to each substation, power is transmitted at a very high voltage so as reduce (a) eddy current (b) hysteresis loss (c) heating in the coils (d) magnetic flux leakage
8. A devise used to prevent wearing away of the make-and-break contacts of an induction coil is called a/an (a) fuse (b) electroscope (c) resistor (d) capacitor
9. The current of a primary coil of a transformer is 2.5A. If the primary coil has 50 turns and the secondary 250 turns. Calculate the current in the secondary coil [neglect energy losses in the transformer] (a) 0.2A (b) 0.5A (c) 2.5A (d) 5.0A
10. The voltage and the current in the primary of a transformer are 2000V and 2A respectively. If the transformer is used to light ten 12V, 30W bulbs, calculate its efficiency (a) 100% (b) 90% (c) 50% (d) 75%

    SECTION B
    

    1. (a) Draw a simple labeled diagram illustrating the principle of a step down transformer and explain how it works (b) state three ways y which energy is lost in a transformer and how they can be minimized (c) if a transformer is used to light a lamp rated at 60W, 220V from a 4400V a.c. supply, calculate (i) ratio of the number of the turns of the primary coil to the secondary coil in the transformer (ii) current taken from the mains circuit if the efficiency of the transformer is 95%
       
    2. (a) State the law of magnetic induction (b) explain how one of the laws illustrate the principle of conservation of energy (c)(i) draw a well labeled diagram of a simple d.c. electric motor and explain how it works (ii) state two reasons why the efficiency of an electric motor is less than 100%
       
    3. (a) state Faraday’s law of electromagnetic induction (b) draw a labeled diagram of an induction coil and explain how it works (c) state the reason why a capacitor should be included in the primary circuit of the coil (d) how is the effect of eddy current minimized in the coil? (e) state three uses of induction coil