Magnetic effect of a current | ICSE Class 10 Physics Notes

This note covers Oersted’s experiment, magnetic fields and field lines, current in straight wires and circular loops, electromagnets, the three direction rules, the direct-current motor, electromagnetic induction, the alternating-current generator, household supply frequency and transformers.

What does Oersted’s experiment show about electric current?

Electric current is the rate of flow of electric charge. The SI unit of current is the ampere, written A. SI means the International System of Units.

A conductor is a material through which electric charge can move. When current passes through a metallic wire, it produces a magnetic effect in the space around the wire.

A compass contains a small magnetic needle that can turn freely. Away from other nearby magnetic influences, its ends point approximately north and south. Its north-seeking end is called its north pole; the other end is its south pole.

How is the effect demonstrated?

1. Connect a straight copper wire to cells and a key, meaning a switch used to open or close the circuit.
2. Place the straight wire parallel to and above a compass needle. Observe the needle before closing the key.
3. Close the key so that current flows. The compass needle is deflected from its previous direction.
4. Reverse the cell connections. The current reverses and the needle deflects in the opposite direction.

Oersted’s experiment shows that electricity and magnetism are related. The needle responds because the current produces a magnetic field, a region where magnetic force can be detected. Reversing the current changes the direction of that field, rather than merely changing how strongly the wire heats up.

In the arrangement with the wire above the compass, current from north to south deflects the needle’s north pole towards the east. Reversing the current deflects it towards the west. The position of the wire relative to the compass matters when stating these directions.

Note: A compass detects a magnetic effect; it does not directly measure the current in the wire. Describe what the needle does first, then explain what that observation shows.

What are magnetic fields and magnetic field lines?

Definition: A magnetic field is a region in which magnetic force can be detected. The symbol B represents the magnetic field, which has both magnitude, meaning size or strength, and direction. The SI unit of magnetic field is the tesla, with symbol T.

Magnetic field lines are lines used to represent a magnetic field. At a point, the field direction is the direction in which the north pole of a small compass needle points. Arrows on field lines record this direction.

Outside a bar magnet, field lines run from the north pole to the south pole. Inside the magnet, they run from south to north. They therefore form closed curves, rather than beginning and ending at the poles.

What do spacing and direction tell us?

Closely crowded field lines indicate a stronger magnetic field. Where the lines are farther apart, the field is weaker. This is a way of comparing field strength in a drawing, not a claim that physical threads fill the space around a magnet.

No two magnetic field lines cross. A crossing would assign two different field directions to the same point. A compass placed there cannot point in both directions at once. This makes non-intersection an essential feature of a correct field diagram.

Iron filings reveal a field pattern when sprinkled around a magnet and gently tapped. A compass adds information about direction. The filings’ arrangement and the compass needle’s orientation provide different pieces of evidence about the same magnetic field.

How does a straight current-carrying wire produce a magnetic field?

The magnetic field lines around a straight current-carrying wire form concentric circles, meaning circles with a common centre. The wire passes through their centre. These circles lie in planes perpendicular, or at right angles, to the wire.

How can the pattern be observed?

1. Pass a straight copper wire vertically through a horizontal sheet of cardboard.
2. Connect the wire in a circuit with a battery, a key and a variable resistance that allows the current to be adjusted. Resistance is opposition to the flow of current.
3. Sprinkle iron filings evenly on the cardboard, close the key and tap the cardboard gently.
4. Observe the circular pattern. Place a compass at different points to identify the direction around each circle.

For a fixed position near the wire, increasing the current increases the magnetic field strength. If the current remains the same, moving farther from the wire gives a weaker field. The circles grow larger as the distance from the wire increases.

Reversing the current reverses the direction of the circular field. It does not turn the circles into straight lines. Separate the shape of the field pattern from its direction and strength when describing a change to the circuit.

The circuit drawing shows a vertical wire through cardboard, circular field arrows, a battery, a key, an ammeter and variable resistance. An ammeter measures electric current. Photograph: “A close up of the pattern obtained.” It shows iron filings arranged around the straight wire passing through the cardboard.

See Fig. 12.6 in your NCERT textbook

The compass is placed beside the wire on the cardboard, not along the wire’s length. Its north pole indicates the local direction around the circle. A complete description therefore identifies the wire, the circular pattern and the direction indicated by the compass.

How do we apply the right-hand thumb rule?

The right-hand thumb rule gives the direction of the magnetic field around a straight conductor when the direction of current is known. It relates current along the wire to field direction around it.

Conventional current means current in the direction in which positive charge would move. In a metallic wire, this is opposite to the direction of electron motion. Use conventional current when applying the direction rules.

1. Imagine gripping the straight wire with your right hand.
2. Point the right thumb along the direction of conventional current.
3. Allow the fingers to curl naturally around the wire.
4. The curled fingers indicate the direction of the magnetic field lines.

Why must the viewpoint be stated?

Clockwise and anticlockwise depend on which end of a wire you look from. For a horizontal power line carrying current from east to west, the circular field looks clockwise when viewed from the east end and anticlockwise when viewed from the west end.

These descriptions represent the same field viewed from opposite ends. The field has not changed simply because the observer has moved. First establish the current arrow, then the viewing direction, and finally describe the sense of the field.

Use the thumb for current and the curled fingers for the magnetic field. This rule does not give the force on a wire in an external magnetic field. It also differs from Fleming’s right-hand rule, which concerns the direction of current produced when the magnetic field linked with a circuit changes.

Note: A statement such as “the field is clockwise” is incomplete unless the viewing direction is clear. An arrow on the wire and a stated viewpoint prevent an otherwise correct rule from giving an ambiguous answer.

What field is produced by a circular loop and a solenoid?

A circular loop is a wire bent into a circle. A coil contains turns of wire. When current flows through a circular loop, each part of the wire contributes to the magnetic field.

Near the wire, the field lines curve around it. Towards the centre of the loop, the arcs of these circles appear as straight lines. Applying the right-hand thumb rule to different parts of the wire shows that their fields contribute in the same direction within the loop.

For a circular coil with the same current in each turn, the contributions of the turns add together. Increasing the number of turns increases the field strength. Increasing the current also increases the field at a given point.

The drawing shows a circular current-carrying loop with curved field lines around the wire. The lines through the central region are drawn nearly straight, with arrows showing their direction.

See Fig. 12.8 in your NCERT textbook

How is a solenoid related to a bar magnet?

A solenoid is a coil of many closely wound circular turns of insulated wire in the shape of a cylinder. Insulation is a covering that prevents direct electrical contact between neighbouring turns. A current-carrying solenoid has a field pattern resembling that of a bar magnet.

One end behaves as a north pole and the other as a south pole. Inside the solenoid, the field lines are parallel straight lines, representing a uniform magnetic field. A core is a piece of material placed inside a coil. A soft-iron core can be magnetised by the solenoid’s field.

The drawing shows a cylindrical coil connected to a battery. Field lines pass along its interior and curve back around its exterior, resembling the field pattern of a bar magnet.

See Fig. 12.10 in your NCERT textbook

How do electromagnets compare with permanent magnets?

An electromagnet is a magnet produced using electric current. A common arrangement consists of a soft-iron core surrounded by a coil of insulated copper wire. Current in the coil produces the magnetic field that magnetises the core.

A permanent magnet retains its magnetism for a long period after the magnetising influence has been removed. Soft iron is suitable for an electromagnet because it becomes magnetised in an applied field and loses its magnetisation when that field is removed.

Which properties make an electromagnet useful?

Electromagnetic cranes use controllable magnetic attraction to lift iron scrap and release it when the current is switched off. Electric bells use an electromagnet to attract an iron armature, meaning the movable iron part on which the magnetic attraction acts.

The important link is between the property and the use. A lifting device benefits from attraction that can be switched. A compass benefits from a needle that remains magnetised. Calling one type “strong” and the other “weak” does not explain this difference.

For the same coil, raising the current strengthens its magnetic field. Reversing that current reverses its polarity. Strength and polarity are separate properties, so changing one should not be confused with changing the other.

How does Fleming’s left-hand rule describe magnetic force?

A current-carrying conductor placed in an external magnetic field can experience a force. External magnetic field means the field supplied by another source, such as a magnet, rather than the wire’s own field.

In a demonstration, an aluminium rod is suspended between the poles of a magnet and connected to a battery. When current passes through the rod, it is displaced. Reversing the current reverses the displacement; reversing the magnetic field also reverses it.

How are the three directions related?

Fleming’s left-hand rule gives the direction of force when the current and magnetic field are perpendicular. Stretch the thumb, forefinger and middle finger of the left hand so that each is perpendicular to the other two.

The force is perpendicular to both current and field. Its magnitude is greatest when current is at right angles to the magnetic field. A correct use of the rule therefore begins by identifying the field and current directions separately.

The conductor does not have to be made of iron for this effect to occur. The aluminium rod demonstrates a force on a conductor carrying current. This is different from the attraction of an unpowered piece of iron to a magnet.

Note: The right-hand thumb rule finds the field made by a current. Fleming’s left-hand rule finds the force on a current in an external field. Decide which physical effect the question describes before choosing a rule.

Quantitative extension: how is the magnetic force calculated?

For a straight wire in a uniform external field, the force magnitude is $F=IlB\sin\theta$. Here $I$ is current in amperes, $l$ is wire length in metres, $B$ is field strength in tesla and $\theta$ is the angle between current and field.

For a wire perpendicular to the field, $F=IlB$. The SI unit of force is the newton, written N. Fleming’s left-hand rule supplies the direction, while the formula supplies the magnitude.

Worked example 1. A straight wire of mass 200 g and length 1.5 m carries 2 A. A uniform horizontal magnetic field perpendicular to the wire holds it suspended. Find the field magnitude, taking gravitational acceleration as $9.8\,\mathrm{m\,s^{-2}}$.

Formula: $F=mg$ and $B=\frac{mg}{Il}$, because the upward magnetic force balances the weight.

Substitute: $m=200/1000=0.200\,\mathrm{kg}$, so $F=0.200\times9.8=1.96\,\mathrm{N}$. Then $B=\frac{1.96}{2\times1.5}=0.653\,\mathrm{T}$.

Answer: The required field is approximately 0.65 T, giving an upward magnetic force of 1.96 N to balance the wire’s weight.

Worked example 2. A wire carries 8 A at an angle of 30° to a uniform magnetic field of 0.15 T. Calculate the magnetic force per unit length.

Formula: $\frac{F}{l}=IB\sin\theta$.

Substitute: $\frac{F}{l}=8\times0.15\times\sin30^{\circ}=8\times0.15\times0.5$.

Answer: The force per unit length is 0.60 N/m. The angle factor matters because the wire is not perpendicular to the field.

Worked example 3. A 3.0 cm wire carrying 10 A lies inside a solenoid, perpendicular to its axis. The field inside the solenoid is 0.27 T. Find the magnetic force on the wire.

Formula: $F=IlB$, since the solenoid’s internal field is along its axis and therefore perpendicular to the wire.

Substitute: $l=3.0/100=0.030\,\mathrm{m}$, giving $F=10\times0.030\times0.27=0.081\,\mathrm{N}$.

Answer: The magnetic force has magnitude 0.081 N.

What are the main parts and energy transfer of a DC motor?

A direct current, abbreviated DC, flows in one direction. A DC electric motor uses an electrical supply to produce rotation. Its energy transfer is from electrical energy to mechanical energy, meaning energy associated with motion or mechanical work.

The motor uses the force on a current-carrying conductor in a magnetic field. A coil is arranged between magnetic poles so that the forces on its sides can produce a turning effect. Fleming’s left-hand rule identifies the force direction on a suitable side.

Which parts belong in the simple sketch?

Draw a rectangular coil between north and south magnetic poles. Connect its ends to the separate halves of a split ring. Show a brush touching each half and connect the brushes to a DC source. Label the coil, magnet, split-ring commutator and brushes.

The split-ring commutator and the brushes have different roles. The ring rotates with the coil, whereas the brushes provide stationary contacts. Naming both parts is necessary because neither label can substitute for the other.

A motor receives electrical energy and delivers mechanical motion. It should therefore be identified by its energy input and useful output, as well as by its parts. The presence of a magnet and a coil alone does not distinguish a motor from a generator.

What is electromagnetic induction and how is it demonstrated?

Electromagnetic induction is the production of an electromotive force when the magnetic flux linked with a circuit changes. Electromotive force, abbreviated emf, is the energy supplied per unit charge by a source.

The SI unit of electromotive force is the volt, written V. Magnetic flux describes how much magnetic field passes through a surface, taking its orientation into account.

If the conducting circuit is closed, an induced emf can produce an induced current. A galvanometer is a sensitive instrument used to detect small electric currents and their directions through pointer deflections.

What does a moving magnet show?

1. Connect the ends of a coil to a galvanometer, forming a closed conducting circuit.
2. Move the north pole of a bar magnet towards the coil. The galvanometer pointer deflects while the magnet moves.
3. Hold the magnet stationary relative to the coil. The pointer shows no deflection in this arrangement.
4. Pull the magnet away. The pointer deflects in the opposite direction, indicating reversal of the induced current.

Moving the magnet faster produces a larger deflection. Moving the south pole towards the coil gives the opposite current direction to moving the north pole towards it. The direction of the change matters, as well as the fact that a change occurs.

The drawing shows a bar magnet with its north pole facing a coil labelled C₁. The coil is connected to a galvanometer labelled G. An arrow indicates the magnet’s motion towards the coil, and magnetic field lines pass through the coil.

See Fig. 6.1 in your NCERT textbook

Here C₁ is simply the label of the coil and G labels the galvanometer. Moving the coil towards a stationary magnet produces the same kind of effect. Relative motion means motion of one object compared with the other.

Relative motion is one way to change the linked field, but it is not essential in every induction arrangement. Switching the current in a nearby coil on or off can induce a momentary current in a second, stationary coil.

Quantitative extension: how are flux and induced current calculated?

For a uniform field through a flat loop, magnetic flux is $\Phi=BA\cos\theta$, where $A$ is area and $\theta$ is the angle between the field and the perpendicular to the loop. The SI unit of magnetic flux is the weber, written Wb.

For one turn, the induced emf magnitude is $\varepsilon=\frac{|\Delta\Phi|}{\Delta t}$ when flux changes steadily over time $\Delta t$. In a loop of resistance $R$, the induced current magnitude is $I=\frac{\varepsilon}{R}$. Resistance is measured in ohms, written Ω.

Worked example 4. A square loop of side 10 cm and resistance 0.5 Ω stands vertically in the east-west plane. A uniform field of 0.10 T points north-east and decreases steadily to zero in 0.70 s. Find the induced emf and current magnitudes.

Formula: $A=a^2$, $\Phi=BA\cos\theta$, $\varepsilon=\frac{|\Delta\Phi|}{\Delta t}$ and $I=\frac{\varepsilon}{R}$.

Substitute: The side is $a=0.10\,\mathrm{m}$, so $A=0.010\,\mathrm{m^2}$. The field makes 45° with the loop’s perpendicular. Initially, $\Phi=0.10\times0.010\times\cos45^{\circ}=7.071\times10^{-4}\,\mathrm{Wb}$; finally, the flux is zero.

Thus $\varepsilon=\frac{7.071\times10^{-4}}{0.70}=1.010\times10^{-3}\,\mathrm{V}$ and $I=\frac{1.010\times10^{-3}}{0.5}=2.020\times10^{-3}\,\mathrm{A}$.

Answer: To the precision used here, the induced emf is 0.0010 V and the induced current is 0.0020 A. The steady terrestrial field contributes no flux change.

Derivation: how does a moving rod develop an emf?

Consider a straight conducting rod of length $l$ moving at speed $v$ perpendicular to a uniform magnetic field $B$. The rod is perpendicular to both the motion and the field. The following force-and-work argument gives the motional emf magnitude.

1. A charge of magnitude $q$ moving with the rod experiences a magnetic force of magnitude $F=qvB$.
2. The work associated with moving the charge through the rod’s length is $W=Fl=qvBl$.
3. Emf is work per unit charge, so $\varepsilon=\frac{W}{q}=\frac{qvBl}{q}$.

Result: $\varepsilon=Blv$. This magnitude applies to the perpendicular arrangement described above; the direction rule determines which end is at higher potential.

How does Fleming’s right-hand rule relate to an AC generator?

Fleming’s right-hand rule gives the direction of induced current when a conductor moves across a magnetic field. Stretch the right thumb, forefinger and middle finger mutually perpendicular to one another.

The forefinger represents the magnetic field, the thumb represents the conductor’s motion, and the middle finger gives the induced conventional current. Motion is supplied to the conductor; current is the result. This distinguishes the situation from the motor effect.

What are the generator’s main parts?

An alternating current, abbreviated AC, changes direction periodically. A simple AC generator converts mechanical energy into electrical energy by electromagnetic induction. Its coil is turned mechanically in a magnetic field, changing the magnetic flux linked with it.

The drawing shows a coil between magnetic poles labelled N and S, meaning north and south. It labels the axle, slip rings and carbon brushes. The output connections are labelled alternating emf.

See Fig. 6.13 in your NCERT textbook

Slip rings are separate complete rings; a split-ring commutator has separated halves. Keep this distinction visible in labelled sketches. The generator’s mechanical input and electrical output are the reverse of the motor’s electrical input and mechanical output.

For a direction question, identify what is given and what is required. Known current and field with force required indicate Fleming’s left hand. Known motion and field with induced current required indicate Fleming’s right hand.

Quantitative extension: what determines a generator’s maximum voltage?

For a coil rotating uniformly in a uniform field perpendicular to its rotation axis, the peak emf is $\varepsilon_0=NBA\omega$. Here $N$ is the number of turns, $A$ is the area of each turn and $\omega=2\pi f$ is angular speed, with rotation frequency $f$.

Worked example 5. Kamla pedals a stationary bicycle connected to a 100 turn coil of area 0.10 m². The coil rotates at half a revolution per second in a uniform field of 0.01 T perpendicular to its rotation axis. Find the maximum generated voltage.

Formula: $\omega=2\pi f$ and $\varepsilon_0=NBA\omega$.

Substitute: $f=0.5\,\mathrm{Hz}$, so $\omega=2\times3.14\times0.5=3.14\,\mathrm{rad\,s^{-1}}$. Hence $\varepsilon_0=100\times0.01\times0.10\times3.14=0.314\,\mathrm{V}$.

Answer: The maximum generated voltage is 0.314 V. This is the peak value, rather than a constant voltage throughout the rotation.

What does household AC frequency mean, and why is AC useful?

Frequency is the number of complete cycles occurring each second. A cycle is one complete repetition of a periodic change. The SI unit of frequency is the hertz, written Hz. One hertz means one complete cycle per second.

The frequency of household AC supply in India is 50 Hz. This means that the alternating supply completes fifty cycles in each second. A full cycle includes the changes in both directions; one direction reversal is not itself a complete cycle.

How do AC and DC differ?

Voltage means potential difference, the energy transferred per unit charge between two points. An advantage of AC is that its voltage can be changed easily and efficiently using transformers. This is useful in transmitting electrical energy over long distances.

For the same power transferred, using a higher transmission voltage allows a smaller current. Power means the rate of energy transfer. The SI unit of power is the watt, written W. A smaller current reduces heating losses in the transmission wires, whose electrical resistance opposes current flow.

The voltage is raised for transmission and reduced near consumers. Raising voltage does not create extra energy. It changes the conditions under which energy is transferred and reduces losses along the route. The transformer is therefore central to explaining this advantage of AC.

How do step-up and step-down transformers differ?

A transformer changes an alternating voltage to a higher or lower alternating voltage. It contains two insulated coils on a soft-iron core. The coils are electrically insulated from each other, while the magnetic field in the core links them.

The primary coil is the coil connected to the input supply in this arrangement. The secondary coil provides the output. Alternating current in the primary produces a changing magnetic field, which induces an emf in the secondary. This is mutual induction, induction in one coil caused by changing current in another.

A load is a device or circuit receiving the output. An ideal transformer has no energy losses. A winding is a coil of wire.

What are the characteristics of each type?

The current comparisons refer to the ideal description; actual transformers have small energy losses.

Thicker wire is used for the winding carrying the larger current to reduce heating loss. A laminated core, made of thin insulated layers of iron, helps reduce heating caused by currents induced within the core itself.

The drawing labels a soft iron core, primary and secondary. One arrangement has the coils wound over one another; the other has them on separate limbs of the core.

See Fig. 7.16 in your NCERT textbook

Draw a soft-iron core carrying separate labelled primary and secondary coils. For a step-up transformer, show more secondary turns; for a step-down transformer, show fewer. Label the AC input and output, keeping the two windings electrically separate.

A transformer transfers energy between electrical circuits through a changing magnetic field. It does not turn AC into DC. A steady primary current produces no continuing change of field, so it cannot sustain an induced secondary voltage in the way an alternating supply does.

Quantitative extension: how are transformer voltage and current related?

For an ideal transformer whose windings link the same changing flux, $\frac{V_s}{V_p}=\frac{N_s}{N_p}$. The letters $p$ and $s$ identify primary and secondary quantities; $N$ is the number of turns. Compare corresponding voltage amplitudes or effective values.

Derivation: why does stepping up voltage reduce current?

1. With no energy losses, input power equals output power: $P_p=P_s$.
2. Using the ideal transformer power relation gives $V_pI_p=V_sI_s$. Rearranging, $\frac{I_s}{I_p}=\frac{V_p}{V_s}$.
3. The voltage-to-turns relation gives $\frac{V_p}{V_s}=\frac{N_p}{N_s}$. Substitution therefore relates the currents directly to the winding turns.

Result: $\frac{I_s}{I_p}=\frac{N_p}{N_s}$. More secondary turns increase voltage but decrease current in the ideal transformer. The comparison uses corresponding current and voltage values.

Worked example 6. An ideal transformer has 100 primary turns and 200 secondary turns. Its input is 220 V at 10 A. Calculate the secondary voltage and current.

Formula: $V_s=\frac{N_s}{N_p}V_p$ and $I_s=\frac{N_p}{N_s}I_p$.

Substitute: The voltage ratio is $N_s/N_p=200/100=2$, so $V_s=2\times220=440\,\mathrm{V}$. The current ratio is $N_p/N_s=100/200=0.5$, so $I_s=0.5\times10=5.0\,\mathrm{A}$.

Answer: The output is 440 V at 5.0 A. The power check gives $P_p=220\times10=2200\,\mathrm{W}$ and $P_s=440\times5.0=2200\,\mathrm{W}$, consistent with the ideal assumption.

Glossary

• Magnetic field — A region where magnetic force can be detected, described by both strength and direction.
• Magnetic field line — A line whose direction follows the north pole of a small compass placed in the field.
• Conventional current — The direction of current defined as the direction in which positive electric charge would move.
• Solenoid — A cylindrical coil made from many closely wound circular turns of insulated conducting wire.
• Electromagnet — A magnet produced by electric current, commonly using an insulated coil surrounding a soft-iron core.
• Permanent magnet — A material that retains its magnetism for a long period after the magnetising influence is removed.
• Commutator — A device that reverses current connections; the simple DC motor uses a split-ring form.
• Brushes — Stationary conducting contacts that connect an external circuit to rotating rings in a motor or generator.
• Electromagnetic induction — Production of an electromotive force through a change in the magnetic flux linked with a circuit.
• Galvanometer — A sensitive instrument whose pointer deflection detects small electric currents and indicates their direction.
• Alternating current — Electric current that changes direction periodically as its repeating cycle progresses with time.
• Frequency — The number of complete cycles of a periodic change occurring in each second.
• Transformer — A device that raises or lowers alternating voltage using magnetically linked, electrically insulated coils.
• Primary coil — The transformer winding connected to the input supply in the arrangement being considered.
• Secondary coil — The transformer winding in which an output voltage is induced by the changing linked field.

Common errors and misconceptions

• Misconception: A straight wire produces straight magnetic field lines along its length. Correct: Its field lines are concentric circles centred on the wire, in planes perpendicular to it.
• Misconception: Field lines cross where a magnetic field is particularly strong. Correct: Stronger fields are shown by closer lines. A crossing would incorrectly assign two field directions to one point.
• Misconception: All right-hand rules give the same information. Correct: The thumb rule gives the field around a current; Fleming’s right-hand rule gives induced current from motion across a magnetic field.
• Misconception: A nearby stationary magnet necessarily produces a continuing current in a stationary coil. Correct: In the magnet-and-coil experiment, induction depends on changing the linked magnetic field.
• Misconception: The simple AC generator uses the motor’s split-ring commutator. Correct: The simple AC generator uses separate complete slip rings. The DC motor uses a split-ring commutator.
• Misconception: A motor converts mechanical energy into electrical energy. Correct: That is the generator’s energy transfer. A motor takes electrical energy and provides mechanical motion.
• Misconception: A step-up transformer increases both voltage and current. Correct: In the ideal transformer supplying a load, increased secondary voltage is accompanied by smaller secondary current, not creation of energy.
• Misconception: A 50 Hz supply makes fifty direction reversals each second. Correct: The number refers to complete cycles per second. A complete alternating cycle includes changes in both directions.

Exam-style questions with model answers

Q1. A straight wire is placed parallel to and above a compass needle. State the deflection of the needle’s north pole when current flows from north to south, and when that current is reversed. [2 marks]

1. With the wire above the compass and current flowing from north to south, the needle’s north pole deflects towards the east.
2. When the current is reversed to flow from south to north, the needle’s north pole deflects towards the west.

Q2. Name the rule used in each situation and state what it determines: current in a straight wire is known; current in a conductor perpendicular to an external field is known; a conductor’s motion across a field is known. [3 marks]

1. The right-hand thumb rule determines the magnetic field direction around the straight wire. The thumb follows current and the curled fingers follow the field.
2. Fleming’s left-hand rule determines the force direction on the conductor carrying current perpendicular to the external magnetic field.
3. Fleming’s right-hand rule determines the induced conventional current direction when the conductor moves across the magnetic field.

Q3. For a long straight current-carrying wire, describe the field pattern and predict these separate changes: increasing current at a fixed observation point; moving the observation point farther away while current remains unchanged. [3 marks]

1. The field lines form concentric circles centred on the wire. Their planes are perpendicular to the direction of the straight wire.
2. Increasing the current increases the field strength at the fixed observation point, provided the wire and observation position remain unchanged.
3. Moving farther away while keeping the current unchanged decreases the field strength at the observation point; the circles there have larger radii.

Q4. Describe a soft-iron electromagnet: give its construction, explain the core material, state how its strength can be changed, state how its polarity can be reversed, and explain one use requiring switching. [5 marks]

1. An electromagnet can be made by winding insulated copper wire around a soft-iron core and passing electric current through the coil.
2. Soft iron becomes magnetised in the coil’s magnetic field and loses its magnetisation when that magnetising field is removed.
3. Its magnetic strength can be increased by increasing the current through the same coil, with the other conditions unchanged.
4. Its polarity can be reversed by reversing the current through the coil, which reverses the direction of the magnetic field.
5. An electromagnetic crane can lift iron scrap with the current switched on and release the scrap when the current is switched off.

Q5. State the purpose of each of these parts in a simple DC motor: coil, magnet, split-ring commutator and brushes. [4 marks]

1. The coil carries electric current and can rotate under the turning effect of magnetic forces acting on its sides.
2. The magnet supplies the external magnetic field in which the current-carrying coil is placed.
3. The split-ring commutator rotates with the coil and reverses the current connections to the coil through its insulated conducting halves.
4. The brushes provide stationary conducting contacts between the external DC supply and the rotating commutator.

Q6. A coil forms a closed circuit with a galvanometer. Predict the observations when a bar magnet’s north pole is moved towards it, held still, withdrawn, approached faster, and finally held still while the coil moves towards it. Keep the same connections throughout. [5 marks]

1. As the north pole approaches the coil, the galvanometer deflects, showing that an electric current has been induced during the motion.
2. When the magnet is held still relative to the coil, the pointer shows no deflection because this arrangement has no changing linked field.
3. When the north pole is withdrawn, the pointer deflects in the opposite direction to its deflection during approach.
4. Approaching faster produces a larger pointer deflection than the slower approach, indicating a larger induced current in the same circuit.
5. Moving the coil towards the stationary north pole makes the galvanometer pointer deflect in the same direction as when that pole approaches the stationary coil. Relative approach changes the magnetic flux linked with the coil and induces current.

Q7. For a simple AC generator, state the energy transfer, the induction principle, the purpose of slip rings and the purpose of brushes. [4 marks]

1. The generator transfers mechanical energy supplied to rotate its coil into electrical energy delivered to an external circuit.
2. Rotation changes the magnetic flux linked with the coil, producing an induced electromotive force by electromagnetic induction.
3. Separate complete slip rings connect to the coil ends and rotate with them, providing connections for the alternating output.
4. Stationary conducting brushes touch the rotating rings and connect them to the external circuit while allowing the coil to rotate.

Q8. An ideal step-up transformer supplies a load from an alternating input. Compare its secondary with its primary in voltage, turns, current and required wire thickness. State one use. [5 marks]

1. The secondary voltage is higher than the primary voltage, which is the defining change made by a step-up transformer.
2. The secondary has more turns than the primary. The greater number of secondary turns provides the higher induced output voltage.
3. The secondary current is smaller than the primary current in the ideal transformer supplying a load; increased voltage does not create energy.
4. The primary carries the larger current and uses thicker wire to reduce heating loss; the secondary carries the smaller current.
5. A step-up transformer raises voltage before long-distance power transmission, allowing smaller transmission current for the same power and reducing heating loss in the wires.

Key takeaways

• Oersted’s experiment links electricity and magnetism: a current-carrying wire deflects a compass, and reversing current reverses the deflection.
• Magnetic field lines form closed curves, never cross and indicate stronger fields where the lines are more closely crowded.
• A straight current-carrying wire produces concentric circular field lines; the right-hand thumb rule supplies their direction.
• A solenoid’s field resembles a bar magnet’s field, and a soft-iron core provides a controllable electromagnet.
• Fleming’s left-hand rule gives magnetic force direction; Fleming’s right-hand rule gives induced current direction from conductor motion.
• A DC motor transfers electrical energy into mechanical energy, while an AC generator transfers mechanical energy into electrical energy.
• Induction depends on a change in linked magnetic flux; relative motion of a magnet and coil is one way to produce it.
• Household AC in India has frequency 50 Hz; transformers raise voltage for transmission and lower it near consumers.
• A step-up transformer has more secondary turns and higher secondary voltage; its ideal secondary current is smaller when supplying a load.

Test yourself

Why can two magnetic field lines not intersect?

An intersection would give two different field directions at the same point, but a compass north pole can indicate only one local field direction.

What happens to a straight wire’s field when the current increases at a fixed observation point?

The magnetic field becomes stronger at that point. Its concentric circular pattern remains, provided the wire’s shape and position remain unchanged.

Which part of the right hand gives current direction in the right-hand thumb rule?

The thumb points along conventional current, while the fingers curling around the wire indicate the direction of its magnetic field.

Why is soft iron suitable for an electromagnet?

Soft iron becomes magnetised in the coil’s field and loses its magnetisation when that field is removed, allowing controllable magnetic attraction.

What must change to induce an emf in a stationary coil?

The magnetic flux linked with the coil must change. Changing current in a nearby coil can produce this change without moving either coil.

How do the rings of a simple DC motor and AC generator differ?

The DC motor uses a split-ring commutator with insulated halves. The AC generator uses two separate complete slip rings connected to the coil ends.

What does a household AC frequency of 50 Hz mean?

It means the alternating supply completes fifty full cycles each second. A complete cycle includes the changes in both directions.

What distinguishes the secondary coil of a step-down transformer?

It has fewer turns and a lower voltage than the primary. In the ideal transformer supplying a load, its current is larger.