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Magnetic Effects of Electric Current - Q&A

1. A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
(a) towards south
(b) towards east
(c) downward
(d) upward

Answer: (d) upward
Explanation: We can find the direction using Fleming's Left-Hand Rule.
1. Point your forefinger in the direction of the Magnetic Field (which we need to find).
2. Point your middle finger in the direction of the Current. (Since the alpha particle is positively charged, the current direction is the same as its motion, i.e., West).
3. Point your thumb in the direction of the Force/Deflection (given as North).
If you align your left hand this way (Middle finger West, Thumb North), your forefinger points Upward. Therefore, the magnetic field is upward.

[Image of Fleming's Left Hand Rule diagram]

1. Which of the following correctly describes the magnetic field near a long straight wire carrying current?
(a) The field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.

Answer: (d) The field consists of concentric circles centred on the wire.
Explanation: When current flows through a straight wire, the magnetic field lines form closed circles around the wire. The centre of these circles lies on the wire itself. You can verify this using the Right-Hand Thumb Rule.

2. At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.

Answer: (c) increases heavily.
Explanation: A short circuit occurs when the live wire comes in direct contact with the neutral wire. This creates a path of very low resistance. According to Ohm's Law (I = V/R), when resistance (R) becomes very small, the current (I) increases drastically to a very high value.

3. State whether the following statements are true or false.
(a) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(b) A wire with a green insulation is usually the live wire of an electric supply.

Answer:
(a) True. (At the centre of a circular loop, the magnetic field lines are straight and parallel, indicating a uniform magnetic field).
(b) False. (Green insulation is used for the Earth wire. The live wire usually has Red or Brown insulation).

4. List two methods of producing magnetic fields.

Answer:
1. Using a Permanent Magnet: Bar magnets or horseshoe magnets naturally produce a magnetic field around them.
2. Using an Electric Current (Electromagnetism): Passing electric current through a straight conductor, a circular loop, or a solenoid produces a magnetic field.

5. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?

Answer: The force is largest when the direction of the current is perpendicular (at 90 degrees) to the direction of the magnetic field.

6. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?

Answer: The direction of the magnetic field is downward.
Explanation: Using Fleming's Left-Hand Rule:
1. Current: Electrons move from back to front. Since electrons are negative, conventional current is opposite, i.e., from Front to Back (towards you). Point your middle finger towards yourself.
2. Force: The deflection is to the Right. Point your thumb to the right.
3. Field: Your forefinger will point Downward.

7. State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.

Answer:
(i) Right-Hand Thumb Rule: Imagine holding the current-carrying wire in your right hand with your thumb pointing in the direction of the current. The direction in which your fingers wrap around the wire gives the direction of the magnetic field lines.
(ii) Fleming's Left-Hand Rule: Stretch the thumb, forefinger, and middle finger of your left hand perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb points in the direction of the force (motion).
(iii) Fleming's Right-Hand Rule: Stretch the thumb, forefinger, and middle finger of your right hand perpendicular to each other. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of the conductor, then the middle finger will show the direction of the induced current.

[Image of Fleming's Right Hand Rule diagram]

8. When does an electric short circuit occur?

Answer: An electric short circuit occurs when:
1. The Live wire and the Neutral wire come in direct contact with each other.
2. This can happen if the insulation of the wires is damaged or if there is a fault in the electrical appliance.
3. In this situation, the resistance of the circuit becomes almost zero, causing a dangerously high current to flow.

9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?

Answer:
Function: The earth wire provides a low-resistance path for electric current to flow into the ground in case of leakage.
Necessity:
1. Metallic appliances (like iron, toaster, fridge) conduct electricity.
2. If the live wire accidentally touches the metallic body of the appliance, the appliance becomes "live" (electrically charged).
3. Without earthing, anyone touching it would get a severe electric shock.
4. Earthing ensures that this leakage current flows to the ground instead of through the user's body, acting as a safety measure.

Quick Navigation:
Quick Review Flashcards - Click to flip and test your knowledge!
Question
Which scientist discovered that a compass needle is deflected by an electric current in 1820?
Answer
Hans Christian Oersted
Question
In honour of Hans Christian Oersted, what physical quantity is measured in the unit 'oersted'?
Answer
Magnetic field strength
Question
Term: Compass needle
Answer
Definition: A small bar magnet whose ends point approximately towards the north and south directions.
Question
What is the specific name given to the end of a magnet that points towards the north?
Answer
North seeking pole (or North pole)
Question
How do like poles of two different magnets interact with each other?
Answer
They repel each other.
Question
What is the term for the region surrounding a magnet where its force can be detected?
Answer
Magnetic field
Question
The paths along which iron filings align themselves around a magnet represent _____.
Answer
Magnetic field lines
Question
By convention, in which direction do magnetic field lines emerge from a bar magnet?
Answer
From the North pole
Question
By convention, at which pole do magnetic field lines merge into a bar magnet?
Answer
The South pole
Question
Inside a bar magnet, what is the direction of the magnetic field lines?
Answer
From the South pole to the North pole
Question
Why are magnetic field lines described as 'closed curves'?
Answer
They emerge from the North pole, enter the South pole, and continue from South to North inside the magnet.
Question
How is the relative strength of a magnetic field represented visually using field lines?
Answer
By the degree of closeness of the field lines.
Question
Where is the magnetic field of a bar magnet at its strongest?
Answer
At the poles, where the field lines are most crowded.
Question
Why is it physically impossible for two magnetic field lines to cross each other?
Answer
It would imply that a compass needle at the intersection points in two directions simultaneously.
Question
What pattern do magnetic field lines form around a long, straight current-carrying conductor?
Answer
Concentric circles
Question
How does increasing the current through a straight wire affect the magnitude of the produced magnetic field?
Answer
The magnitude of the magnetic field increases.
Question
What happens to the magnetic field strength produced by a wire as the distance from the wire increases?
Answer
The magnetic field strength decreases.
Question
As one moves further away from a current-carrying straight wire, how does the visual pattern of concentric circles change?
Answer
The circles become larger and larger.
Question
According to the Right-hand thumb rule, if the thumb points in the direction of the current, what do the wrapped fingers represent?
Answer
The direction of the magnetic field lines.
Question
What is the alternative name for the Right-hand thumb rule?
Answer
Maxwell’s corkscrew rule
Question
A current flows from East to West in a horizontal power line; in what direction is the magnetic field directly below it when viewed from the East?
Answer
Clockwise
Question
Describe the appearance of magnetic field lines at the very centre of a current-carrying circular loop.
Answer
They appear as straight lines.
Question
How does the magnetic field produced by a circular coil with $n$ turns compare to a single-turn coil?
Answer
The field is $n$ times as large.
Question
Why does a circular coil with multiple turns produce a stronger magnetic field than a single turn?
Answer
The current in each turn has the same direction, so their individual magnetic fields add up.
Question
Term: Solenoid
Answer
Definition: A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
Question
Which common permanent magnet has a magnetic field pattern nearly identical to that of a current-carrying solenoid?
Answer
A bar magnet
Question
Describe the nature of the magnetic field lines inside a current-carrying solenoid.
Answer
Parallel straight lines
Question
What does the presence of parallel straight field lines inside a solenoid indicate about its magnetic field?
Answer
The magnetic field is uniform (the same at all points) inside the solenoid.
Question
Term: Electromagnet
Answer
Definition: A magnet formed by placing a core of magnetic material, like soft iron, inside a current-carrying solenoid.
Question
Which French scientist suggested that a magnet exerts a force on a current-carrying conductor?
Answer
Andre Marie Ampere
Question
How does reversing the direction of current in a conductor placed in a magnetic field affect the force acting upon it?
Answer
The direction of the force is reversed.
Question
Under what specific orientation between current and magnetic field is the magnitude of the force on a conductor the highest?
Answer
When the current is at right angles ($90^{\circ}$) to the direction of the magnetic field.
Question
In Fleming's left-hand rule, which finger is aligned with the direction of the magnetic field?
Answer
The forefinger (first finger)
Question
In Fleming's left-hand rule, what does the middle finger (second finger) represent?
Answer
The direction of the electric current
Question
According to Fleming's left-hand rule, what does the direction of the thumb indicate?
Answer
The direction of motion or the force acting on the conductor.
Question
List three common devices that utilise current-carrying conductors and magnetic fields.
Answer
Electric motors, electric generators, and loudspeakers.
Question
What is the conventional direction of electric current relative to the motion of electrons?
Answer
Opposite to the direction of electron motion.
Question
Which two organs in the human body produce significant magnetic fields due to nerve impulses?
Answer
The heart and the brain
Question
What medical diagnostic technique relies on the magnetic fields produced inside the human body?
Answer
Magnetic Resonance Imaging (MRI)
Question
What is the standard potential difference between the live and neutral wires in UK/Indian domestic supply?
Answer
$220\text{ V}$
Question
What is the standard frequency of the AC electric power supplied to houses in the User Source Material?
Answer
$50\text{ Hz}$
Question
Identify the standard insulation colour for the 'live' wire in a domestic circuit.
Answer
Red
Question
Identify the standard insulation colour for the 'neutral' wire in a domestic circuit.
Answer
Black
Question
Identify the standard insulation colour for the 'earth' wire in a domestic circuit.
Answer
Green
Question
What is the primary function of the earth wire in domestic appliances with metallic bodies?
Answer
It provides a low-resistance path for leakage current to prevent electric shocks.
Question
Why are domestic appliances connected in parallel rather than in series?
Answer
To ensure each appliance receives the same potential difference and has a separate switch.
Question
Term: Short-circuiting
Answer
Definition: An abrupt increase in circuit current occurring when live and neutral wires come into direct contact.
Question
Term: Overloading
Answer
Definition: A situation where the total current drawn exceeds the circuit's capacity, often due to connecting too many appliances to one socket.
Question
Which safety device protects a circuit from damage caused by short-circuiting or overloading?
Answer
Electric fuse
Question
What physical process causes a fuse to break the circuit during an overcurrent event?
Answer
Joule heating melts the fuse wire.
Question
What is the typical current rating for a domestic circuit intended for high-power appliances like geysers?
Answer
$15\text{ A}$
Question
What is the typical current rating for a domestic circuit intended for low-power appliances like bulbs?
Answer
$5\text{ A}$
Question
If an electron beam moves from a back wall to a front wall and is deflected to the right, what is the direction of the magnetic field?
Answer
Upward
Question
When a proton moves freely in a magnetic field, which two properties of the proton can change?
Answer
Velocity and momentum
Question
How does the deflection of a compass needle change as the current through a nearby wire is increased?
Answer
The deflection increases.
Question
Which property of a solenoid allows it to be used to magnetise a steel rod placed inside it?
Answer
The presence of a strong, uniform magnetic field inside the coil.
Question
What happens to the direction of force on a current-carrying rod if the magnetic field poles are interchanged?
Answer
The direction of the force is reversed.
Question
Why don't the magnetic field lines inside a solenoid ever intersect?
Answer
Because the magnetic field is uniform and a single point cannot have two field directions.
Question
What safety measure ensures that a user does not get a shock from a refrigerator's metallic body?
Answer
Connecting the metallic body to the earth wire.
Question
A positively charged alpha-particle projected West is deflected North; what is the direction of the magnetic field?
Answer
Upward