Class 10 › Science › Chapter
Magnetic Effects of Electric Current
Oersted's discovery, magnetic fields and field lines, fields due to a straight wire, a circular loop and a solenoid, electromagnets, the force on a current-carrying conductor and Fleming's left-hand rule, and domestic electric circuits with their safety devices.
1. The Magnetic Effect of Electric Current
- Current produces a magnetic effect: in Activity 12.1, a compass placed near a straight copper wire is deflected when current is passed through the wire. The deflection shows that the current-carrying wire behaves like a magnet. So electricity and magnetism are linked.
- The reverse question: if electric current has a magnetic effect, can moving magnets have an electric effect? This chapter studies magnetic fields and such electromagnetic effects, and electromagnets.
- Earlier chapter: the previous chapter ("Electricity") dealt with the heating effect of current; here we study the magnetic effect.
Hans Christian Oersted (1777–1851)
A leading 19th-century scientist who played a crucial role in understanding electromagnetism.
In 1820 he accidentally discovered that a compass needle gets deflected when an electric current passes through a metallic wire placed nearby. This showed that electricity and magnetism are related phenomena.
His research later led to technologies such as the radio, television and fibre optics. The unit of magnetic field strength is named the oersted in his honour.
2. Magnetic Field and Field Lines
- Compass needle: a compass needle is a small bar magnet. Its end pointing towards north is the north-seeking (north) pole, and the end pointing towards south is the south-seeking (south) pole.
- Poles: like poles repel; unlike poles attract.
- Why a compass deflects near a bar magnet: the compass needle is itself a small magnet. The poles of the bar magnet attract or repel the poles of the needle, so the needle turns.
Magnetic field: the region surrounding a magnet in which the force of the magnet can be detected.
Magnetic field lines: the lines along which iron filings align themselves in a magnetic field. A field line is the path along which a hypothetical free north pole would tend to move.
Magnetic field lines: the lines along which iron filings align themselves in a magnetic field. A field line is the path along which a hypothetical free north pole would tend to move.
Field lines leave the north pole, curve round and enter the south pole. Inside the magnet they run from south to north, so they are closed curves.
Drawing field lines with a compass (Activity 12.3)
1. Place a bar magnet on a sheet of paper and mark its boundary. Put a small compass near the north pole of the magnet. The south pole of the needle points towards the north pole of the magnet.
2. Mark the positions of the two ends of the needle. Move the needle so that its south pole occupies the position where its north pole was. Repeat step by step until you reach the south pole of the magnet.
3. Join the marked points with a smooth curve. This curve is one magnetic field line. Repeat from other starting points to draw as many lines as you can.
4. As the compass moves along a field line, its deflection increases as it moves towards the poles.
- Iron filings (Activity 12.2): iron filings sprinkled around a bar magnet and gently tapped arrange themselves in a pattern. The magnet exerts a force on them, and the pattern shows the field lines.
- Magnetic field is a vector: it has both direction and magnitude.
Properties of magnetic field lines
1. The direction of the field at a point is the direction in which the north pole of a compass needle would move there.
2. Outside the magnet, field lines emerge from the north pole and merge at the south pole. Inside the magnet they run from the south pole to the north pole. So they are closed curves.
3. The strength of the field is shown by how close the lines are: crowded lines mean a stronger field (a greater force on the pole of another magnet).
4. No two field lines cross each other. If they did, a compass needle at the crossing point would have to point in two directions at once, which is impossible.
2. Outside the magnet, field lines emerge from the north pole and merge at the south pole. Inside the magnet they run from the south pole to the north pole. So they are closed curves.
3. The strength of the field is shown by how close the lines are: crowded lines mean a stronger field (a greater force on the pole of another magnet).
4. No two field lines cross each other. If they did, a compass needle at the crossing point would have to point in two directions at once, which is impossible.
3. Magnetic Field due to a Current through a Straight Conductor
What the activities show
Direction of deflection (Activity 12.4): a straight wire placed parallel to and above a compass deflects the needle. If the current is reversed, the needle deflects the opposite way. So the direction of the magnetic field reverses when the current reverses.
Shape of the field (Activity 12.5): a thick wire passes vertically through a cardboard sprinkled with iron filings. When a current flows, the filings form concentric circles around the wire. These circles represent the field lines.
Direction at a point: a compass placed at a point on a circle shows the field direction there, given by the direction of the north pole of the needle.
The circles grow larger and fainter away from the wire: the field is weaker at $Q$ than at $P$.
- Effect of current: if the current is increased, the compass deflection increases. The magnitude of the field at a given point increases as the current through the wire increases (it depends directly on the current).
- Effect of distance: if the compass is moved away from the wire (current unchanged), the deflection decreases. The field produced by a given current decreases as the distance from the wire increases (it depends inversely on the distance). The concentric circles get larger and larger away from the wire.
- The pattern depends on the shape of the conductor (straight wire, loop, solenoid), which is why we study these separately.
Right-hand thumb rule
Imagine holding the current-carrying straight conductor in your right hand with the thumb pointing in the direction of the current. Your fingers wrap around the conductor in the direction of the magnetic field lines.
Maxwell's corkscrew rule (same rule)
If a corkscrew is driven in the direction of the current, the direction in which it turns is the direction of the magnetic field.
- Example (power line): a horizontal power line carries current from east to west. At a point directly below or above the wire, the field turns clockwise in a plane perpendicular to the wire when viewed from the east end, and anticlockwise when viewed from the west end.
4. Magnetic Field due to a Current through a Circular Loop
- Field lines: bend a straight wire into a circular loop. At every point of the loop, the field lines are concentric circles that become larger and larger away from the wire.
- At the centre of the loop: the arcs of these big circles look like straight lines. The field lines at the centre are parallel straight lines.
- Every part adds in the same direction: applying the right-hand rule to each section of the loop shows that all sections contribute field lines in the same direction inside the loop, so their effects add up.
- Coil with $n$ turns: the current in each turn has the same direction and the field of each turn adds up. So the field is $n$ times that produced by a single turn.
- Strength: the field depends directly on the current in the wire. More current or more turns gives a stronger field.
- Direction for a clockwise loop: for a loop lying on a table with the current clockwise (seen from above), the field points into the table inside the loop and out of the table outside the loop.
- Activity 12.6: a circular coil with many turns passes through a cardboard sprinkled with iron filings. When current flows and the board is tapped, the filings show the pattern of the field around the coil.
5. Magnetic Field due to a Current in a Solenoid; Electromagnet
Solenoid: a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
One end of the solenoid acts as a north pole and the other as a south pole.
- Like a bar magnet: the field pattern around a current-carrying solenoid is similar to that of a bar magnet. One end behaves as a magnetic north pole and the other as a south pole.
- Uniform field inside: inside the solenoid the field lines are parallel straight lines. So the magnetic field is the same at all points inside a solenoid, that is, it is uniform.
- Representing a uniform field: draw equally spaced parallel straight lines, all pointing the same way.
Electromagnet: a piece of magnetic material (such as soft iron) magnetised by placing it inside a current-carrying solenoid. It consists of a core of soft iron wrapped with a coil of insulated copper wire.
- Why it works: the strong magnetic field inside a solenoid magnetises the soft iron placed in it.
- Making it stronger: more current and more turns give a stronger field.
6. Force on a Current-Carrying Conductor in a Magnetic Field
- The idea (Ampère): a current-carrying conductor produces a field that exerts a force on a magnet. The French scientist André-Marie Ampère (1775–1836) suggested that the magnet must also exert an equal and opposite force on the current-carrying conductor.
Activity 12.7: displacement of a rod
1. A small aluminium rod $AB$ (about 5 cm) is suspended horizontally between the poles of a strong horse-shoe magnet, with the field directed upwards (north pole below, south pole above the rod). It is connected in series with a battery, key and rheostat.
2. Pass current from $B$ to $A$: the rod is displaced (towards the left). So a force acts on a current-carrying conductor in a magnetic field.
3. Reverse the current: the rod is displaced the opposite way (towards the right).
4. Reverse the field (interchange the poles): the force is again reversed.
Conclusion: the direction of the force depends on both the direction of the current and the direction of the magnetic field.
- Largest force: the force (the displacement of the rod) is largest when the current is at right angles to the magnetic field.
- Perpendicular to both: when current and field are perpendicular, the force is perpendicular to both of them.
- Size of the force (Question 2): the rod is displaced more if the current is increased, if a stronger magnet is used, or if the length of the rod in the field is increased.
Fleming's left-hand rule: forefinger for the field, middle finger for the current, thumb for the force.
Fleming's left-hand rule
Stretch the thumb, forefinger and middle finger of your left hand so that they are mutually perpendicular.
If the first (fore) finger points in the direction of the magnetic field and the second (middle) finger points in the direction of the current, then the thumb points in the direction of motion or force on the conductor.
If the first (fore) finger points in the direction of the magnetic field and the second (middle) finger points in the direction of the current, then the thumb points in the direction of motion or force on the conductor.
Solving direction problems with Fleming's left-hand rule
1. Fix the directions you know: the field, the current, or the force. Use the rule to find the third.
2. Current direction: for a positive charge (such as a proton or an alpha particle) the current is along its motion. For a negative charge (an electron) the current is opposite to its motion.
3. Electron entering a field at right angles: take the current opposite to the electron's motion; the force then comes from the rule. (In Example 12.2, an electron moving down the page in a field pointing to the right is pushed into the page.)
4. Alpha particle moving west, deflected north: the current is westward and the force is northward, so the field must be upward.
5. Electron beam moving from the back wall to the front wall, deflected to your right: the current is towards you (backward) and the force is to the right, so the field is directed vertically downward.
- A moving charge in a field: the force on a moving charge is perpendicular to its velocity, so it changes the direction of the velocity (so velocity and momentum can change), but not the mass or the speed.
- Devices that use this effect: electric motors, electric generators, loudspeakers, microphones and measuring instruments.
- Induced current (Exercise 7): the direction of the current induced in a coil rotating in a magnetic field is given by Fleming's right-hand rule (thumb: motion, forefinger: field, middle finger: induced current). It belongs to electromagnetic induction.
7. Domestic Electric Circuits
One domestic circuit: mains (live and neutral) pass through the main fuse, the meter and the distribution box; appliances are connected in parallel, and the earth wire protects metal-bodied appliances.
- Mains supply: power comes through overhead poles or underground cables. It is AC at 220 V with a frequency of 50 Hz.
- Live wire: one wire, usually with red insulation (also called positive).
- Neutral wire: another wire, with black insulation (also called negative). The potential difference between live and neutral is 220 V.
- Earth wire: a third wire with green insulation, connected to a metal plate deep inside the earth near the house.
- Path of the supply: the wires enter the house at the meter board, pass through the main fuse into the electricity meter, then through the main switch (distribution box) to the line wires that supply separate circuits.
- Two separate circuits: a 15 A circuit for appliances of high power rating (geysers, air coolers) and a 5 A circuit for bulbs, fans and similar low-power appliances.
- Appliances in parallel: in each circuit, appliances are connected across the live and neutral wires in parallel, so each gets the same potential difference. Each has its own switch to turn it on and off.
Why earth a metal-bodied appliance?
Appliances with a metallic body (electric press, toaster, table fan, refrigerator) are connected through the earth wire to the earth.
The earth wire gives a low-resistance conducting path for current.
If current leaks to the metal body, it flows to the earth, which keeps the body at the potential of the earth. The user then does not get a severe electric shock.
Fuse, overloading and short circuit
Short circuit: when the live and neutral wires touch directly (damaged insulation, or a fault in an appliance), the current in the circuit increases abruptly and heavily.
Overloading can also happen because of an accidental rise in the supply voltage, or because too many appliances are connected to a single socket.
Fuse: a fuse in the circuit stops an unduly high current. The Joule heating produced by the large current melts the fuse and breaks the circuit, protecting the appliances and the wiring.
- Two safety measures: the earth wire (earthing) and the electric fuse.
- Avoiding overloading: do not connect too many appliances to a single socket; use the right circuit for high-power appliances; use fuses of proper rating.
- Checking an appliance against a circuit's rating: current $I = \dfrac{P}{V}$. A 2 kW oven on a 220 V supply draws $\dfrac{2000}{220} \approx 9.1\ \text{A}$. On a circuit rated 5 A this exceeds the rating, so the fuse would melt and the circuit would break (it would need the 15 A circuit).
8. Magnetism in Medicine
- Every current has a magnetic field: even the weak ion currents that travel along nerve cells produce magnetic fields. A nerve impulse sent to the muscles (for example, when we touch something) produces a temporary magnetic field.
- Very weak: these fields are about one-billionth of the Earth's magnetic field.
- Significant in two organs: the heart and the brain.
- MRI: the magnetic field inside the body is the basis of Magnetic Resonance Imaging (MRI), which produces images of different body parts. Analysing these images helps in medical diagnosis.
9. Summary and Quick Revision
| Situation | What happens / rule |
|---|---|
| Bar magnet's field lines | Emerge from N, merge at S (outside); S to N inside; closed curves; never cross; crowded = strong |
| Straight current-carrying wire | Concentric circles around the wire; direction by the right-hand thumb rule; stronger with more current, weaker with distance |
| Circular loop / coil of $n$ turns | Concentric circles, straight lines at the centre; all sections add inside; $n$ turns give $n$ times the field |
| Solenoid | Field like a bar magnet; uniform, parallel lines inside; one end N, other end S |
| Electromagnet | Soft iron core inside a solenoid of insulated copper wire |
| Conductor in a field | Force, largest when current $\perp$ field; direction by Fleming's left-hand rule |
| Fuse | Melts due to Joule heating on overload or short circuit, breaking the circuit |
Rule
Right-hand thumb rule
Thumb along the current, fingers curl along the magnetic field lines around the wire.
Rule
Fleming's left-hand rule
Forefinger: field. Middle finger: current. Thumb: force or motion. All three mutually perpendicular.
Safety
Earth wire and fuse
Earthing keeps a metal body at earth potential; the fuse breaks the circuit when the current is unduly high.
| Exercise | Answer |
|---|---|
| 1. Field near a long straight wire | (d) concentric circles centred on the wire |
| 2. Current at the time of a short circuit | (c) increases heavily |
| 3. True or false | (a) True: at the centre of a coil the field lines are parallel straight lines. (b) False: green insulation marks the earth wire, not the live wire |
| 4. Two ways of producing magnetic fields | A permanent magnet; a current-carrying conductor (for example a coil or solenoid, as in an electromagnet) |
| 5. When is the force largest? | When the current is at right angles to the magnetic field |
| 6. Electron beam deflected to your right | Field is vertically downward |
| 7. Rules | (i) Right-hand thumb rule; (ii) Fleming's left-hand rule; (iii) Fleming's right-hand rule |
| 8. When does a short circuit occur? | When live and neutral wires touch directly, so the current rises abruptly |
| 9. Function of the earth wire | Gives leakage current a low-resistance path to the earth, keeping the metal body at earth potential and preventing severe shock |
Quick facts:
• A compass needle is a small magnet; north-seeking end = north pole.
• Field lines are closed curves and never intersect.
• Field of a straight wire: increases with current, decreases with distance.
• Reversing the current reverses the field and the force; reversing the field reverses the force.
• Mains: 220 V, 50 Hz AC; live (red), neutral (black), earth (green).
• A compass needle is a small magnet; north-seeking end = north pole.
• Field lines are closed curves and never intersect.
• Field of a straight wire: increases with current, decreases with distance.
• Reversing the current reverses the field and the force; reversing the field reverses the force.
• Mains: 220 V, 50 Hz AC; live (red), neutral (black), earth (green).
Common mistakes:
• Saying field lines start at the south pole (outside the magnet they go from N to S).
• Drawing field lines that cross or that are not closed.
• Using the left hand for the thumb rule or the right hand for Fleming's force rule.
• Taking the current direction along an electron's motion (it is opposite).
• Forgetting that the force is largest when current and field are perpendicular, and that it is perpendicular to both.
• Calling the green wire the live wire; the green wire is the earth wire.
• Connecting domestic appliances in series (they are connected in parallel).
• Forgetting the rating of a circuit when checking an appliance (use $I = P/V$ and compare).
• Saying field lines start at the south pole (outside the magnet they go from N to S).
• Drawing field lines that cross or that are not closed.
• Using the left hand for the thumb rule or the right hand for Fleming's force rule.
• Taking the current direction along an electron's motion (it is opposite).
• Forgetting that the force is largest when current and field are perpendicular, and that it is perpendicular to both.
• Calling the green wire the live wire; the green wire is the earth wire.
• Connecting domestic appliances in series (they are connected in parallel).
• Forgetting the rating of a circuit when checking an appliance (use $I = P/V$ and compare).