Magnetic Effects of Electric Current | CBSE Class 10 Science Notes

This note covers the magnetic effect of electric current, magnetic fields and field lines, straight conductors, the right-hand thumb rule, circular coils, solenoids, electromagnets, force on a current-carrying conductor, Fleming’s left-hand rule, magnetism in medicine, domestic circuits, earthing and electric fuses.

How does an electric current produce a magnetic effect?

Electric current is the rate of flow of electric charge. A conductor is a material through which charge can flow, such as a copper wire. A continuous, closed conducting path is an electric circuit.

A wire carrying current behaves like a magnet. This can be demonstrated using a compass, whose needle is a small bar magnet. Its ends point approximately north and south. The north-seeking end is its north pole; the south-seeking end is its south pole.

What does the compass experiment show?

1. Connect a straight, thick copper wire in a circuit. Label its ends X and Y, and keep this part of the wire perpendicular to the plane of the paper.
2. Place a small compass horizontally near the wire. Observe the initial position of its needle.
3. Insert the plug key, the switch that completes the conducting path, so that current passes through the circuit.
4. Observe the deflection, or change in direction, of the compass needle. The current in the wire has produced a magnetic effect.

The compass responds to the magnetic influence around the wire. This observation links electricity and magnetism: current flowing through a metallic conductor produces a magnetic field around it. A magnetic field is a region in which magnetic force can be detected.

What did Oersted discover?

Hans Christian Oersted lived from 1777 to 1851. In 1820, he accidentally observed that a compass needle deflected when current passed through a nearby metallic wire. The observation established a connection between electrical and magnetic phenomena.

Oersted’s research later contributed to technologies such as radio, television and fibre optics. The unit of magnetic field strength called the oersted bears his name. The compass experiment provides a direct way to recognise the magnetic effect without seeing the field itself.

What do magnetic field lines show, and how can they be traced?

A magnetic field has both magnitude, meaning its strength, and direction. The direction at a point is the direction taken by the north pole of a compass placed there. Like magnetic poles repel one another, while unlike poles attract.

Definition: Magnetic field lines represent a magnetic field. A field line shows the path along which a hypothetical free north pole would tend to move.

How do iron filings reveal a pattern?

Fix white paper to a drawing board, place a bar magnet at its centre, and sprinkle iron filings uniformly around it. Gently tap the board. The filings experience magnetic force and arrange themselves along lines representing the field.

The pattern reveals the region influenced by the magnet. A compass adds directional information: its north pole indicates the direction of the field at its position. The filings and compass therefore provide related ways of studying the same magnetic field.

How is a field line traced with a compass?

1. Place a bar magnet on paper fixed to a board and mark its boundary. Put a small compass near the magnet’s north pole.
2. The compass needle’s south pole points towards the magnet’s north pole. Mark the positions of both ends of the needle.
3. Move the compass until its south pole occupies the previous position of its north pole. Mark the new positions and repeat towards the magnet’s south pole.
4. Join the marked points with a smooth curve. Repeat from other starting positions to obtain several field lines.

The bar has its south pole on the left and north pole on the right. Curved lines loop around it, with arrows outside pointing from north towards south. The lines crowd near the poles.

See Fig. 12.4 in your NCERT textbook

If two lines crossed, the compass would have to point in two directions at their intersection. This is impossible. Field lines do not intersect. Their spacing describes relative strength, while their arrows describe direction; both features matter when reading a field diagram.

What magnetic field surrounds a straight current-carrying wire?

The field around a straight current-carrying wire forms concentric circles, meaning circles with a common centre. The wire passes through their centre. Changing the shape of the conductor changes the pattern of the magnetic field produced by its current.

How is the circular pattern demonstrated?

The apparatus includes a 12 V battery, a rheostat, an ammeter with a range of 0 to 5 A, a plug key, connecting wires, cardboard and a long, straight, thick copper wire. Here V denotes the unit volt, and A denotes the unit ampere.

Potential difference, measured in volts, is the work done per unit charge moved between two points. Current is measured in amperes. An ammeter measures current; a rheostat changes resistance, the opposition to charge flow, to regulate the current.

1. Pass the thick copper wire vertically through the centre of a fixed rectangular cardboard. Keep the wire perpendicular to the cardboard.
2. Connect the wire, battery, key, rheostat and ammeter in series, meaning along one conducting path. Keep the rheostat setting fixed and note the current.
3. Sprinkle iron filings uniformly over the cardboard, close the key and tap the cardboard gently. The filings form concentric circles around the wire.
4. Place a compass on one circle. Its north pole gives the field direction there, which can be marked using an arrow.

Part (a) shows a vertical wire through cardboard, circular field lines, a compass and the connecting circuit. Part (b) is a photograph showing the circular iron-filings pattern around the wire.

See Fig. 12.6 in your NCERT textbook

What changes the strength and direction?

For the compass-reversal experiment, place a straight wire parallel to and above the compass needle. Current from north to south deflects the needle’s north pole towards the east. Reversing the current from south to north makes it deflect towards the west.

The wire’s position relative to the compass is part of this observation. Keep that arrangement clear when describing the result. A change in current magnitude, a change in observation distance and a reversal of current are different experimental changes.

How does the right-hand thumb rule determine field direction?

The right-hand thumb rule connects the direction of current in a straight conductor with the direction of its surrounding magnetic field. It answers a direction question once the current direction is known.

1. Imagine holding the straight current-carrying conductor in your right hand.
2. Point your thumb in the direction of current through the conductor.
3. Let the remaining fingers curl around the conductor.
4. The direction in which the fingers curl is the direction of the magnetic field lines.

Reversing the current reverses the thumb’s direction and hence the curling direction of the fingers around the wire. This agrees with the opposite compass deflections observed when the battery connections are reversed.

Maxwell’s corkscrew rule expresses the same relationship. Imagine driving a corkscrew forward in the direction of current. Its direction of rotation gives the direction of the magnetic field around the conductor.

How is the rule applied to an east-to-west power line?

Worked example 1. Current flows from east to west through a horizontal power line. Find the direction of its magnetic field at points directly below and directly above the wire.

Answer: Point the right thumb westwards. The magnetic field points towards the south directly below the wire and towards the north directly above it. Viewed from the east end, the magnetic field circulates clockwise in a plane perpendicular to the wire. Viewed from the west end, the same circulation appears anticlockwise.

This is the viewing-direction description of the field at points around the line, including those above and below it. Clockwise and anticlockwise must be tied to a stated viewpoint: changing the viewing end changes how the same circulation appears.

Note: In the right-hand thumb rule, the thumb represents current and the curled fingers represent magnetic field. The thumb does not represent force in this rule.

How does a circular coil produce a magnetic field?

A circular loop is a wire bent into a circle. When current passes through it, each section of the wire contributes to the magnetic field. Near the wire, the field pattern contains circular curves around the conducting sections.

As the distance from a section of wire increases, the circles representing its field become larger. Near the centre of the loop, the arcs of these large circles appear as straight lines. Keep this distinction between the shape near the wire and the appearance near the centre.

Applying the right-hand rule to successive sections of the loop shows that each section contributes to the field in the same direction within the loop. Their contributions therefore add together rather than cancel one another there.

What happens when there are several turns?

Let n mean the number of turns in a circular coil. For the same current and corresponding coil arrangement, the magnetic field from n turns is n times the field from one turn. The current in every turn flows in the same sense around the coil.

The field of a current-carrying wire at a given point depends directly on the current through it. The number of turns and the current therefore both matter when describing a coil’s magnetic effect. Do not confuse the number of turns with the size of the current.

How can the coil pattern be observed?

1. Make two holes in a rectangular cardboard. Insert a circular coil with many turns through them, with the coil’s plane perpendicular to the cardboard.
2. Connect the ends of the coil in series with a battery, a plug key and a rheostat.
3. Sprinkle iron filings uniformly over the cardboard and close the key to pass current.
4. Tap the cardboard gently several times and observe the pattern in which the iron filings arrange themselves.

The activity makes the field pattern visible, while the right-hand rule supplies its direction. Together they explain how bending a wire into a loop changes the pattern without changing the underlying connection between electric current and magnetism.

What are solenoids and electromagnets?

Definition: A solenoid is a coil of many circular turns of insulated copper wire wound closely into the shape of a cylinder.

Insulation is the non-conducting covering of the wire. The field pattern around a current-carrying solenoid resembles the field pattern around a bar magnet. One end behaves as a magnetic north pole and the other as a magnetic south pole.

What does the field inside a solenoid look like?

Inside the solenoid, the field lines are parallel straight lines. They represent a uniform magnetic field: the field is the same at all points inside the solenoid. The curved lines outside link the two ends in a pattern similar to a bar magnet’s field.

Closely wound turns form a horizontal coil connected to a circuit. Straight field lines run through the coil, while curved lines loop around the outside. Arrowheads mark the direction of the field.

See Fig. 12.10 in your NCERT textbook

When comparing the two field patterns, distinguish the material producing the field from the pattern itself. A bar magnet and a current-carrying solenoid have similar magnetic patterns even though one is a magnet and the other is a coil carrying current.

How is an electromagnet formed?

A strong magnetic field inside a solenoid can magnetise a piece of magnetic material, such as soft iron, placed inside it. The magnet formed in this way is an electromagnet. Its core is the material around which the insulated copper coil is wrapped.

The sequence is current in the coil, magnetic field within it, and magnetisation of the core. This connects the magnetic effect first detected with a compass to the formation of a magnet using an electric circuit.

How is force on a current-carrying conductor determined?

A current-carrying conductor produces a magnetic field that can exert force on a nearby magnet. Andre Marie Ampere, the French scientist who lived from 1775 to 1836, suggested that the magnet must exert an equal and opposite force on the conductor.

What does the suspended-rod experiment show?

Suspend a small aluminium rod, about 5 cm long, horizontally using connecting wires. Here cm means centimetre. Label the rod’s ends A and B. Place it between the poles of a strong horse-shoe magnet, with the north pole below and south pole above.

This makes the magnetic field point vertically upwards. Connect the rod in series with a battery, a key and a rheostat. In this arrangement, current from end B to end A makes the rod move towards the left.

Displacement means the change in the rod’s position. Its movement demonstrates that the magnetic field exerts a force on the current-carrying rod. The direction of that force depends on both the current direction and the field direction.

How is Fleming’s left-hand rule used?

For current perpendicular to the magnetic field, the force is perpendicular to both. Fleming’s left-hand rule gives this direction. Stretch the thumb, forefinger and middle finger of the left hand so that all three are mutually perpendicular, or at right angles to one another.

• The forefinger points along the magnetic field.
• The middle finger points along the electric current.
• The thumb gives the direction of force or motion.

Devices using current-carrying conductors and magnetic fields include electric motors, electric generators, loudspeakers, microphones and measuring instruments. The rule keeps the three relevant directions distinct when reasoning about magnetic force.

How is the rule applied to an electron?

An electron is a negatively charged particle. Conventional current is taken opposite to the direction in which electrons move. First identify the electron’s motion, then reverse that direction to set the current finger.

Three horizontal arrows point right and are labelled magnetic field. A vertical arrow labelled electron points downwards, perpendicular to the field arrows.

See Fig. 12.14 in your NCERT textbook

Worked example 2. An electron moves down the page into a magnetic field directed to the right in the plane of the page. Is its magnetic force directed right, left, out of the page or into the page?

Answer: The conventional current direction is upwards, opposite to the electron’s motion. Point the left forefinger rightwards and middle finger upwards. The thumb points into the page, so the correct choice is into the page.

How are electric currents and magnetism connected in the body?

Electric currents produce magnetic fields even when the currents are very weak. Ions, which are electrically charged particles, carry weak currents along nerve cells in the body. These currents also produce magnetic fields.

When we touch an object, nerves carry an electric impulse, an electrical signal, towards the muscles needed for the response. This impulse produces a temporary magnetic field. The magnetic effect is therefore linked to an electrical event within the body.

How strong are these fields?

The fields associated with these nerve impulses are very weak, about one-billionth of the earth’s magnetic field. The word “about” matters: this is an approximate comparison of magnitudes. A weak field is still a magnetic field produced by electric current.

The heart and brain are two main organs where the magnetic field produced is significant. The magnetic field inside the body is connected with obtaining images of different body parts through Magnetic Resonance Imaging, abbreviated MRI.

Analysis of these images helps medical diagnosis, meaning the identification of a medical condition. The example extends the connection between current and magnetic fields from wires and coils to living systems, and shows an important use of magnetism in medicine.

How are domestic electric circuits arranged?

The mains is the main electrical supply to a house. It may arrive through overhead wires supported on poles or through underground cables. Domestic circuits distribute this supply to appliances in separate parts of the house.

Which quantities and units are used in circuit calculations?

For a charge $Q$ passing in time $t$, current is $I = Q/t$. Potential difference is $V = W/Q$, where $W$ denotes work done. Charge is measured in coulombs, time in seconds and work in joules.

Ohm’s law gives $V = IR$, provided the conductor’s temperature remains the same. Here $R$ is resistance. Rearranging gives $I = V/R$ and $R = V/I$. These relations connect the voltage across a resistor with the current through it.

What kind of current does the supply provide?

The domestic supply is alternating current, or AC, which reverses direction after equal intervals of time. In contrast, direct current, or DC, flows in one direction. The domestic AC supply has a frequency of 50 Hz.

Frequency is the number of complete cycles per second; Hz is the symbol for hertz. A frequency of 50 Hz means 50 complete cycles each second. The current reverses direction every 1/100 second, with two reversals per complete cycle.

An advantage of AC over DC is that electric power can be transmitted over long distances without much loss of energy. This distinguishes the type of current from its voltage: the household supply is described by both 220 V and 50 Hz.

Which wires and connections are involved?

The live wire usually has red insulation. The neutral wire has black insulation. The potential difference between live and neutral in the domestic supply described here is 220 V. This is a voltage between two wires, not a value of current.

At the meter-board, the supply wires pass through a main fuse into the electricity meter. The main switch connects them to the house’s line wires, which feed separate circuits.

Often, two separate circuits are used. A 15 A circuit serves appliances with higher power ratings, such as geysers and air coolers. A 5 A circuit serves bulbs and fans. A current rating states the specified current for the circuit.

Electric power is the rate at which electrical energy is consumed or dissipated. Its unit is the watt, written W. A kilowatt, written kW, equals 1000 watts. Power ratings and current ratings describe different electrical quantities.

The schematic labels live, neutral and earth wires, the electricity board’s fuse, an electricity meter, and a distribution box containing the main switch and circuit fuses. Branch connections are drawn across the supply, labelled 220 V.

See Fig. 12.15 in your NCERT textbook

Why are appliances connected in parallel?

In a parallel connection, appliances are connected across the same live and neutral wires. Each appliance therefore has the same potential difference across it. Each also has its own switch to turn its current on or off.

Trace the arrangement in order: incoming supply, main fuse, meter, main switch, and separate household circuits. Within a circuit, individual appliance branches connect across live and neutral. This separates the supply route from the individual switching arrangements.

The earth wire has green insulation and provides a safety connection. It is usually connected to a metal plate buried deep in the earth near the house. Its protective role is particularly relevant to appliances with metallic bodies.

Derivation: Equivalent resistance of parallel branches

Let three resistors have resistances $R_1$, $R_2$ and $R_3$, measured in ohms. Each has the same potential difference $V$ in volts. Their branch currents are $I_1$, $I_2$ and $I_3$, measured in amperes.

1. The total current is the sum of the branch currents: $I = I_1 + I_2 + I_3$.
2. For equivalent resistance $R_p$, Ohm’s law gives $I = V/R_p$. In each branch, $I_1 = V/R_1$, $I_2 = V/R_2$ and $I_3 = V/R_3$.
3. Substitute these expressions into the current sum: $V/R_p = V/R_1 + V/R_2 + V/R_3$. Divide by the common non-zero voltage.

Result: $1/R_p = 1/R_1 + 1/R_2 + 1/R_3$. Parallel branches divide the current, allowing appliances with different resistances to draw different currents at the same supply voltage.

How do earthing and fuses protect domestic circuits?

Earthing means connecting the metallic body of an appliance to the earth wire. Appliances for which this protection is especially useful include an electric press, toaster, table fan and refrigerator. The connection provides a low-resistance conducting path for leakage current.

If current leaks to the metallic body, this path keeps the body at the potential of the earth, and the user may not get a severe electric shock. The safety explanation depends on the conducting connection between the appliance’s body and earth.

What causes short-circuiting and overloading?

Short-circuiting occurs when live and neutral wires come into direct contact, for example because their insulation is damaged or an appliance has a fault. The current then rises abruptly. This can overload the circuit and damage the circuit or appliances.

Overloading involves an unduly high current in a circuit. It can also result from an accidental rise in supply voltage. Sometimes it is caused by connecting too many appliances to a single socket. Avoiding that connection helps prevent this source of overloading.

How does a fuse interrupt excessive current?

1. An electric fuse is connected in series with the circuit or device it protects.
2. An unduly high current flows during overloading or short-circuiting.
3. The current produces Joule heating, meaning heat generated by current in the fuse wire.
4. The fuse wire melts, breaking the conducting path and stopping the current before possible damage continues.

A fuse and an earth wire serve different protective functions. The fuse interrupts an excessive current; the earth wire connects an appliance’s metallic body to earth through a low-resistance path. Both are important features of domestic electrical safety.

Derivation: Joule’s law of heating

Consider a steady current $I$ in amperes through a purely resistive circuit of resistance $R$ in ohms, at potential difference $V$ in volts, for time $t$ in seconds. The energy supplied is dissipated as heat $H$, measured in joules.

1. Moving charge $Q$ through potential difference $V$ requires work $W = VQ$. The source supplies this energy to the circuit.
2. Since $Q = It$, the supplied energy becomes $W = VIt$. In the purely resistive circuit this gives $H = VIt$.
3. Substitute Ohm’s law, $V = IR$, into the heat expression: $H = (IR)It = I^2Rt$.

Result: $H = I^2Rt$. Heating increases with the square of current for a fixed resistance and time. This explains why excessive current can melt a fuse wire.

Power and energy: $P = VI$ gives the energy supplied each second, in watts. Over time $t$, the electrical energy supplied is $E = Pt$, in joules when time is in seconds. For a resistor, $P = I^2R = V^2/R$.

What happens when a high-power oven uses a low-current circuit?

Let P represent electric power in watts, V potential difference in volts, and I current in amperes. In the power relation P = VI, V is a quantity symbol; after a number, V is the unit symbol for volt.

Worked example 3. An electric oven rated 2 kW is operated at 220 V on a domestic circuit rated 5 A. Use P = VI and 1 kW = 1000 W to determine whether it overloads the circuit.

Answer: P = 2000 W. Rearranging the power relation gives I = P/V = 2000/220 A, approximately 9.09 A. This exceeds 5 A, so the circuit is overloaded. A suitable 5 A fuse melts and breaks the circuit.

How are branch currents and total resistance calculated?

Worked example 4. Resistors of 5 Ω, 10 Ω and 30 Ω are connected in parallel across a 12 V battery. Find each branch current, the total current and the equivalent resistance.

Formula: $I_1 = V/R_1$, $I_2 = V/R_2$, $I_3 = V/R_3$, $I = I_1 + I_2 + I_3$ and $R_p = V/I$.

Substitute: $I_1 = 12/5 = 2.4\,\mathrm{A}$, $I_2 = 12/10 = 1.2\,\mathrm{A}$ and $I_3 = 12/30 = 0.4\,\mathrm{A}$.

Answer: The branch currents are 2.4 A, 1.2 A and 0.4 A. Adding gives $I = 2.4 + 1.2 + 0.4 = 4\,\mathrm{A}$. Hence $R_p = 12/4 = 3\,\Omega$, so the total current is 4 A and equivalent resistance is 3 Ω.

How can heating measurements determine voltage?

Worked example 5. A 4 Ω resistor produces 100 J of heat each second. Find the potential difference across it.

Formula: $H = I^2Rt$, so $I = \sqrt{H/(Rt)}$, followed by $V = IR$.

Substitute: Use $H = 100\,\mathrm{J}$, $R = 4\,\Omega$ and $t = 1\,\mathrm{s}$. Then $I = \sqrt{100/(4\times1)} = 5\,\mathrm{A}$, and $V = 5\times4 = 20\,\mathrm{V}$.

Answer: The current is 5 A and the potential difference is 20 V. The heating relation first determines current; Ohm’s law then determines voltage.

How much heat does an electric iron produce?

Worked example 6. An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 s.

Formula: $H = I^2Rt$. Substitute: $H = 5^2\times20\times30 = 25\times20\times30 = 15000\,\mathrm{J}$.

Answer: The heat developed is 15000 J. Square the current before multiplying by the resistance and time.

How is a bulb’s power calculated?

Worked example 7. An electric bulb connected to a 220 V generator carries a current of 0.50 A. Find its power.

Formula: $P = VI$. Substitute: $P = 220\times0.50 = 110\,\mathrm{W}$.

Answer: The power is 110 W, equivalent to an energy consumption of 110 joules per second.

How much energy does an electric motor consume?

Worked example 8. An electric motor takes 5 A from a 220 V line. Determine its power and the energy consumed in 2 h.

Formula: $P = VI$ and $E = Pt$. Substitute: $P = 220\times5 = 1100\,\mathrm{W}$. Convert the time to seconds: $t = 2\times3600 = 7200\,\mathrm{s}$.

Answer: The power is 1100 W. The energy consumed is $E = 1100\times7200 = 7920000\,\mathrm{J}$, or 7920000 J. Equivalently, $E = 1.1\times2 = 2.2\,\mathrm{kWh}$, so the motor consumes 2.2 kWh.

Glossary

• Magnetic field — The region around a magnet or current-carrying conductor in which magnetic force can be detected.
• Magnetic field line — A line representing the path along which a hypothetical free north pole would tend to move.
• Compass — An instrument with a small magnetic needle whose north pole indicates magnetic field direction at its position.
• Concentric circles — Circles sharing a common centre, representing the magnetic field pattern around a straight current-carrying wire.
• Right-hand thumb rule — A rule relating current direction along a straight wire to the direction of its surrounding magnetic field.
• Solenoid — A cylindrical coil made from many closely wound circular turns of insulated copper wire.
• Uniform magnetic field — A magnetic field that is the same at all points, represented by parallel straight field lines.
• Electromagnet — A magnet formed by placing a magnetic core, such as soft iron, within a current-carrying coil.
• Fleming’s left-hand rule — A rule using mutually perpendicular left-hand fingers to connect magnetic field, current and force directions.
• Rheostat — A device used to change resistance in an electric circuit and thereby regulate its current.
• Earthing — Connecting an appliance’s metallic body to earth through a low-resistance conducting path for leakage current.
• Short-circuiting — Direct contact between live and neutral wires that causes an abrupt increase in circuit current.
• Overloading — A condition in which an unduly high electric current flows through a circuit.
• Electric fuse — A protective device whose wire melts under excessive current, breaking the circuit and stopping current flow.

Common errors and misconceptions

• Misconception: Magnetic field lines end at a magnet’s south pole. Correct: They continue inside the magnet from south to north, forming closed curves.
• Misconception: Field lines cross wherever the magnetic field is strong. Correct: Crowding indicates strength; crossing would require two field directions at one point.
• Misconception: Increasing distance from a straight wire strengthens its field. Correct: For the same current, the field decreases with increasing distance.
• Misconception: A circular coil’s turns produce fields that cancel inside it. Correct: Their fields add because the current in each turn has the same direction.
• Misconception: The right-hand thumb rule and Fleming’s left-hand rule assign the same quantity to the thumb. Correct: The right thumb represents current; the left thumb represents force or motion.
• Misconception: An electron’s motion gives the current direction for the left-hand rule. Correct: Conventional current is opposite to electron motion.
• Misconception: The green wire is the live wire, and earthing replaces a fuse. Correct: Green identifies the earth wire; earthing and fuses perform different protective functions.
• Misconception: A 2 kW oven can operate within a 5 A circuit rating at 220 V. Correct: It requires approximately 9.09 A, exceeding that rating and overloading the circuit.

Exam-style questions with model answers

Q1. Why can two magnetic field lines not intersect? [2 marks]

1. The north pole of a compass indicates one definite magnetic field direction at any given point.
2. Intersecting lines would require the needle to point in two directions at their intersection, which is impossible.

Q2. A compass is placed at a fixed point near a straight current-carrying wire. Explain what happens when current increases, when the compass is moved farther away at unchanged current, and when current is reversed. [3 marks]

1. Increasing the current increases the compass deflection at the fixed point. This indicates that the magnetic field at that point has become stronger.
2. Moving the compass farther away while keeping the current unchanged decreases its deflection. The magnetic field becomes weaker as distance from the wire increases.
3. Reversing the current reverses the magnetic field direction around the wire. The north pole of the compass consequently deflects in the opposite direction.

Q3. Current flows east to west through a horizontal power line. State the rule for finding its magnetic field direction and describe the circulation viewed from each end. [3 marks]

1. Use the right-hand thumb rule: point the right thumb along the current, towards the west, so the curled fingers indicate the magnetic field direction.
2. Viewed from the east end, the field circulates clockwise in a plane perpendicular to the power line.
3. Viewed from the west end, the same field circulates anticlockwise. Stating the viewing end is necessary when describing this circular direction.

Q4. Define a solenoid, describe its poles and internal field, and explain how it forms an electromagnet using soft iron. [4 marks]

1. A solenoid is a cylindrical coil formed by many closely wound circular turns of insulated copper wire.
2. When it carries current, one end behaves as a magnetic north pole and the other as a magnetic south pole.
3. The field lines inside it are parallel straight lines, indicating a uniform magnetic field that is the same at all points inside.
4. Place soft iron inside the coil. The strong magnetic field magnetises this core, forming an electromagnet.

Q5. An electron moves vertically down the page into a magnetic field directed horizontally right in the page. Determine the current direction and magnetic force direction using Fleming’s left-hand rule. [3 marks]

1. An electron is negatively charged, so conventional current is opposite to its motion. Downward electron motion therefore corresponds to an upward current direction.
2. Hold the left-hand fingers mutually perpendicular. Point the forefinger rightwards along the magnetic field and the middle finger upwards along the current.
3. The left thumb then points into the page. This gives the force direction, which is perpendicular to both the field direction and the current direction.

Q6. Describe how to demonstrate the magnetic field around a straight current-carrying wire using cardboard, iron filings and a compass. Include the circuit, procedure, pattern and direction. [5 marks]

1. Pass a long, straight, thick copper wire vertically through the centre of a fixed rectangular cardboard, ensuring that the wire is perpendicular to it.
2. Connect the wire in series with a battery, a plug key, a rheostat and an ammeter. Keep the rheostat setting fixed and note the current.
3. Sprinkle iron filings uniformly on the cardboard, close the key to pass current, and tap the cardboard gently several times to reveal the pattern.
4. The filings arrange in concentric circles centred on the wire. These circles represent the magnetic field lines around the straight current-carrying conductor.
5. Place a compass at a point on a circle. Its north pole indicates the magnetic field direction at that point, which can be shown by an arrow.

Q7. An oven rated 2 kW operates at 220 V on a 5 A domestic circuit. Using P = VI, where P is power, V is voltage and I is current, and 1 kW = 1000 W, calculate its current and explain the expected effect on a suitable 5 A fuse. [3 marks]

1. Convert the oven’s power to watts: 2 kW equals 2000 W. Rearranging P = VI gives I = P/V for the current drawn at the given voltage.
2. Substitute the supplied values: I = 2000/220 A, approximately 9.09 A. This is greater than the circuit’s stated rating of 5 A, so the circuit is overloaded.
3. The excessive current heats the suitable 5 A fuse wire until it melts. The circuit breaks and current stops flowing.

Q8. Explain five features of domestic electrical safety and use: parallel appliance connections, separate switches, earthing, fuse action and avoiding too many appliances at one socket. [5 marks]

1. Appliances are connected in parallel across live and neutral wires. This arrangement gives each appliance the same potential difference rather than dividing the supply between successive appliances.
2. Each appliance has a separate switch in its branch. This allows the current through that appliance to be turned on or off individually.
3. Earthing connects a metallic appliance body to earth through a low-resistance path. If current leaks to the body, the connection keeps its potential at that of the earth.
4. A fuse protects the circuit by interrupting excessive current. Joule heating melts its wire during overloading or short-circuiting, breaking the conducting path.
5. Too many appliances connected to one socket can cause overloading. Avoiding this arrangement helps prevent excessive current arising from that particular cause.

Key takeaways

• A current-carrying metallic wire produces a magnetic field, demonstrated by the deflection of a nearby compass needle.
• Magnetic field lines form closed curves, never intersect, and crowd together where the magnetic field is stronger.
• The field around a straight wire forms concentric circles; increasing current strengthens it and increasing distance weakens it.
• The right-hand thumb rule gives the field direction around a straight wire from the direction of current.
• A solenoid has a bar-magnet-like field and a uniform internal field; a soft-iron core can become an electromagnet.
• Fleming’s left-hand rule relates field, current and force directions when the current and magnetic field are perpendicular.
• Domestic appliances are connected in parallel across live and neutral wires, with individual switches controlling their current.
• Earthing provides a protective path for leakage current, while a fuse interrupts excessive current caused by overloading or short-circuiting.

Test yourself

What did Oersted observe in 1820?

A compass needle deflected when current passed through a nearby metallic wire, showing a link between electricity and magnetism.

What direction do field lines take inside a bar magnet?

Inside the bar magnet, the field lines run from its south pole towards its north pole, completing closed curves.

What indicates a stronger magnetic field in a field-line diagram?

Closer spacing of magnetic field lines indicates a stronger field and a greater force on another magnetic pole.

What does the right thumb represent in the right-hand thumb rule?

The right thumb points along the current in the straight conductor; the curled fingers give the magnetic field direction.

Why do the fields from successive turns of a circular coil add within it?

Current flows in the same sense around each turn, so the turns contribute magnetic fields in the same direction within the coil.

When is the force on a current-carrying conductor in a magnetic field largest?

The force is largest when the direction of current is at right angles to the direction of the magnetic field.

Why must electron motion be reversed before setting the current finger?

Electrons carry negative charge, and conventional current is taken in the direction opposite to their motion.

What happens to current when live and neutral wires come into direct contact?

The current rises abruptly, causing short-circuiting; a suitable fuse can melt and interrupt the excessive current.