Electricity and Magnetism – Exercise Questions
STEP 2: Very Short Questions
a. What is the name of the device shown below?
The device shown in the diagram is an electric motor.
b. What is DC?
Direct current is a type of electric current that flows steadily in one direction without changing its strength or direction over time.
c. When the terminals of the battery, connected to the DC motors were reversed, the motor rotated in the opposite direction. Why?
When you reverse the battery terminals, the direction of current flow changes. This causes the magnetic force acting on the coil to reverse, making the motor spin in the opposite direction.
d. If a magnetic compass is placed near position A, which pole of its needle will face position A? Which pole of the needle of the magnetic compass will be pulled if the direction of the current is reversed?
Using the right-hand grip rule, position A behaves like a magnetic North pole. Since opposite poles attract, the South pole of the compass needle will point toward it. If the current direction is reversed, position A becomes a South pole, so the North pole of the needle will be pulled toward it.
e. There is a solenoid connected to the galvanometer. A bar magnet is pushed into the solenoid. Now the solenoid also acts as a magnet. Why?
Pushing a bar magnet into the solenoid changes the magnetic field passing through it. This change creates an induced electric current in the solenoid, which then produces its own magnetic field, making the solenoid behave like a magnet.
f. What is meant by the frequency of electricity being 50 Hz in Nepal?
A frequency of 50 Hz means that the alternating current completes fifty complete cycles every second, or changes its direction one hundred times per second.
g. Name an instrument based on the magnetic effect of current.
An electric motor works on the principle of the magnetic effect of electric current, where a current-carrying coil experiences a force when placed in a magnetic field.
h. Why does a magnetic needle deflect near the current-carrying conductor?
The magnetic needle deflects because the electric current flowing through the conductor creates its own magnetic field, which interacts with the needle’s magnetic field and causes it to move.
i. The current time graph of DC is parallel to the x-axis. What does it mean?
A horizontal line parallel to the time axis shows that the current remains constant and does not change as time passes.
STEP 3: Short Questions
a. Why the current produced from a solar panel cannot run a refrigerator?
Solar panels produce direct current, which flows steadily in one direction. A refrigerator requires high-voltage alternating current to operate properly. The direct current from solar panels cannot be stepped up to higher voltages using a transformer because transformers only work with alternating current. Therefore, the low voltage from a solar panel is not sufficient to run a standard refrigerator without converting the direct current to alternating current first.
b. Differences between an AC Generator and an Electric Motor
AC Generator:
∙ Converts mechanical energy into electrical energy
∙ Works on the principle of electromagnetic induction
∙ Uses slip rings to deliver alternating current to the external circuit
∙ Requires mechanical force (input) to rotate the coil and produces electricity (output)
Electric Motor:
∙ Converts electrical energy into mechanical energy
∙ Works on the principle of motor effect (magnetic effect of current)
∙ Uses a commutator (split rings) to reverse current direction and maintain rotation
∙ Requires electrical power supply (input) to produce mechanical rotation (output)
c. How can a drop in the voltage be prevented while transmitting electricity over long distances?
We can prevent voltage drops by using a step-up transformer at the power station to transmit electricity at very high voltage and low current. This method reduces energy loss because less heat is generated in the transmission wires when the current is low.
d. Write down any two methods to increase emf produced from the given device.
We can increase the generator’s output by increasing the total number of wire turns in the armature coil, or by rotating the armature or magnet at a faster speed.
e. ‘Use of alternating current would be limited in absence of transformer’. Justify this statement.
Transformers are electromagnetic devices that increase or decrease the voltage of alternating current. Without transformers, we couldn’t step up voltage for efficient long-distance transmission from power stations to cities. High-voltage transmission is crucial because it minimizes energy loss as heat in long wires. Additionally, household appliances need specific low voltages that can only be safely provided by stepping down the high transmission voltages. Without transformers, distributing electricity over large areas would be extremely inefficient and impractical.
f. When the battery connected to the electric motor was replaced with an LED bulb and the armature was forcefully rotated, the bulb glowed. The electric motor is not a source it cannot glow the bulb. Ram is surprised. Clarify what’s happening in this experiment for him.
The motor and generator have similar internal structures with a coil inside a magnetic field. When you forcefully rotate the armature, the device acts as a generator, converting mechanical energy into electricity through electromagnetic induction. As the coil moves through the magnetic field, the changing magnetic flux induces an electric current. This induced current flows through the circuit and provides enough energy to light the LED bulb, even though the device was originally designed as a motor.
g. An alternating current of 220 V is available in our home. If we have to light a bulb of 6V, which type of transformer should we use? Show it in a figure.
To light a 6V bulb from a 220V supply, you need a step-down transformer. The transformer should have many more turns of wire in the primary coil connected to the 220V supply than in the secondary coil connected to the 6V bulb. This arrangement reduces the voltage from 220V to 6V safely.
h. A dynamo is connected to Sushma’s bicycle. The bulb connected to her cycle glows with the help of the dynamo. The brightness of the bulb increases when she paddles the bicycle faster. Explain.
As you pedal faster, the magnet inside the dynamo rotates more quickly, which increases how fast the magnetic field changes through the coil. According to electromagnetic induction principles, a faster change in the magnetic field produces a larger induced voltage and current. This increased current flowing to the bulb makes it glow brighter.
STEP 4: Long Questions
a. Describe the structure of the dynamo. Draw its well-labelled diagram.
A bicycle dynamo contains a cylindrical permanent magnet that rotates and a stationary coil of insulated wire wrapped around a soft iron core. The magnet is connected to the bicycle wheel through an axle. As the wheel turns, the magnet spins, causing the magnetic field passing through the coil to change continuously. This changing magnetic field induces an electric current in the coil based on electromagnetic induction. The current flows out through output terminals to power the bicycle’s lights.
Diagram should show: The axle connected to the wheel, the rotating permanent magnet, the stationary coil on soft iron core, and the output terminals.
b. Observe the given diagram and answer the following questions.
i. Which principle is being tested in the above experiment?
The experiment tests the principle of electromagnetic induction.
ii. When the magnet is dropped through the coil, the current is induced and deflection is observed in the needle of the machine. What does it mean?
The deflection shows that moving the magnet changes the magnetic field through the coil, which induces an electric current in the circuit.
iii. If the magnet is at rest inside the coil, current will not be produced. Why?
No, current will not be produced because there is no relative motion between the magnet and coil, so the magnetic field through the coil remains constant.
iv. What can we do to deflect the needle of the galvanometer wider?
We can make the needle deflect more by using a stronger magnet or by moving the magnet through the coil more quickly.
c. Describe the working mechanism of an electric motor and draw its schematic diagram.
An electric motor has a rectangular coil called an armature placed between the north and south poles of a strong permanent magnet. When electric current flows into the coil through carbon brushes and a commutator, the coil creates its own magnetic field. This field interacts with the permanent magnet’s field, producing a force that makes the coil rotate. The commutator is a split ring that reverses the current direction every half rotation, ensuring the coil continues spinning in the same direction continuously.
Diagram should label: The permanent magnets showing north and south poles, the armature coil, the commutator or split rings, the carbon brushes, and the battery or power source.
d. A transformer is shown in the figure.
i. Label the parts A and B.
A is the Primary Coil and B is the Secondary Coil.
ii. Which type of transformer is it?
This is a step-up transformer because the secondary coil has more turns of wire than the primary coil.
iii. The use of alternating current (AC) is limited without a transformer.’ Justify it.
Alternating current forms the foundation of our electrical grid because its voltage can be easily transformed to meet different needs. Step-up transformers allow electricity to be transmitted at very high voltages, which dramatically reduces energy lost as heat during long-distance travel through wires. Without this ability, power plants could only serve small local areas due to massive energy losses. Additionally, step-down transformers safely reduce high voltages to levels suitable for household appliances and lighting. Therefore, the flexibility and widespread use of alternating current depend completely on transformers.
Step: 5 Numerical Problems
(i) The p.d. of the power supply to the primary coil of a transformer is 220 V and the turns in the primary coil are 2000. What must be the number of turns in the secondary coil to run a radio of 11 V from the transformer?
Solution:
Given:
∙ Primary voltage, Vₚ = 220 V
∙ Primary turns, Nₚ = 2000
∙ Secondary voltage, Vₛ = 11 V
∙ Secondary turns, Nₛ = ?
Formula:
For a transformer: Vₛ/Vₚ = Nₛ/Nₚ
Calculation:
Nₛ/Nₚ = Vₛ/Vₚ
Nₛ = (Vₛ × Nₚ)/Vₚ
Nₛ = (11 × 2000)/220
Nₛ = 22000/220
Nₛ = 100
Answer: The number of turns in the secondary coil is 100 turns.
(ii) A transformer has 600 turns in the primary coil and 1200 turns in the secondary coil. The potential difference across the primary coil is 110 V. What is the potential difference induced across the secondary coil?
Solution:
Given:
∙ Primary turns, Nₚ = 600
∙ Secondary turns, Nₛ = 1200
∙ Primary voltage, Vₚ = 110 V
∙ Secondary voltage, Vₛ = ?
Formula:
For a transformer: Vₛ/Vₚ = Nₛ/Nₚ
Calculation:
Vₛ/Vₚ = Nₛ/Nₚ
Vₛ = (Nₛ × Vₚ)/Nₚ
Vₛ = (1200 × 110)/600
Vₛ = 132000/600
Vₛ = 220 V
Answer: The potential difference induced across the secondary coil is 220 V.
(iii) The number of turns in the primary coil of a certain transformer is 440 times more than that in the secondary coil. Calculate the input voltage in the primary coil if the e.m.f. generated in the secondary coil is 220 V.
Solution:
Given:
∙ Nₚ = 440 × Nₛ
∙ Secondary voltage, Vₛ = 220 V
∙ Primary voltage, Vₚ = ?
Formula:
For a transformer: Vₚ/Vₛ = Nₚ/Nₛ
Calculation:
Vₚ/Vₛ = Nₚ/Nₛ
Vₚ/Vₛ = (440 × Nₛ)/Nₛ
Vₚ/Vₛ = 440
Vₚ = 440 × Vₛ
Vₚ = 440 × 220
Vₚ = 96800 V
Answer: The input voltage in the primary coil is 96800 V.
(iv) In a transformer, the ratio of the secondary turns to primary turns is 2:3. Which type of transformer is it? Find the secondary voltage when the transformer is connected to a power supply of 220 V.
Solution:
Given:
∙ Nₛ : Nₚ = 2 : 3
∙ Primary voltage, Vₚ = 220 V
∙ Secondary voltage, Vₛ = ?
Type of transformer:
Since Nₛ < Nₚ (2 < 3), this is a step-down transformer.
Formula:
For a transformer: Vₛ/Vₚ = Nₛ/Nₚ
Calculation:
Vₛ/Vₚ = Nₛ/Nₚ
Vₛ/220 = 2/3
Vₛ = (2 × 220)/3
Vₛ = 440/3
Vₛ = 146.667 V
Answer: This is a step-down transformer and the secondary voltage is 146.667 V.