Electricity and Magnetism | ICSE Class 7 Physics Notes

This note covers magnetic poles, attraction and repulsion, the test for a magnet, electric current, cells and batteries, circuit symbols, switches, conductors and insulators, resistors, series and parallel circuits, the magnetic effect of current, electromagnets and the electric bell.

What are magnetic poles and the law of magnetism?

A magnet attracts materials such as iron. Materials attracted towards a magnet are called magnetic materials. Iron, nickel and cobalt are examples. Materials that are not attracted towards a magnet are called non-magnetic materials.

A bar magnet is a magnet shaped like a straight bar. Its poles are the regions near its ends where attraction is strongest. When iron filings, meaning very small pieces of iron, are brought near it, most collect near these ends.

Every magnet has a north pole and a south pole. The north pole is the end that points north when the magnet is freely suspended. The other end points south. The letters N and S stand for north pole and south pole respectively.

How do two poles interact?

Attraction means pulling towards each other; repulsion means pushing away from each other. Like poles are poles of the same kind. Unlike poles are poles of different kinds.

Definition: The law of magnetism states that like magnetic poles repel one another and unlike magnetic poles attract one another.

The poles occur in pairs. Breaking a magnet into smaller pieces does not produce an isolated north pole and an isolated south pole. Each smaller magnet still has both poles.

A magnet need not be a straight bar. Magnets can also be U-shaped or ring-shaped. Their shape does not remove the pairing of north and south poles. Identify the interacting poles before deciding whether two magnets will attract or repel.

Why is repulsion a reliable test for a magnet?

A magnet attracts both another magnet's unlike pole and an unmagnetised piece of iron, meaning iron that is not itself a magnet. Therefore, attraction alone does not establish that an unknown iron bar is a magnet. Repulsion provides the distinguishing observation.

When a known magnet repels an end of the unknown bar, the interacting ends are like magnetic poles. The unknown bar is therefore a magnet. This is the test by repulsion.

How should the test be carried out?

1. Take a known bar magnet with its north and south poles marked.
2. Support the unknown bar so that it can move freely when a magnetic force, meaning a magnetic push or pull, acts on it.
3. Bring the north pole of the known magnet near one end of the unknown bar without touching it.
4. Observe whether the unknown bar moves towards or away from the known magnet, then test its other end.
5. If one end is repelled by the known north pole, identify that end as the unknown magnet's north pole.

The movement can be observed provided the bar is free to move. A bar held firmly in place may not visibly move even though a magnetic force acts on it. Allow movement and avoid pushing the bars together.

Photograph: Interaction between two bar magnets (NCERT Class 6 Figure 4.8). Magnet A rests across round pencils. A hand holds magnet B near it. Photograph (a) shows unlike poles facing; photograph (b) shows like poles facing.

Both ends of an unmagnetised iron bar are attracted by either pole of a magnet. Do not identify a pole from attraction unless the object has already been established to be a magnet.

What are electric current, an electric cell and a battery?

Electric charge is a property of matter associated with electrical attraction and repulsion. Charges can be positive or negative. Electric current is the flow of electric charges. A conducting wire allows current to pass and provides a path along which charges can move.

An electric cell is a source of electricity. A terminal is a connection point on a cell or another electrical part. A cell has a positive terminal and a negative terminal. The signs + and − identify positive and negative respectively.

An electric circuit is a continuous, closed conducting path. When a suitable bulb is connected to a working cell through this path, current passes through the bulb and it glows. A cell can therefore supply a simple electrical device, but the connections must also form a complete circuit.

How are cells joined to make a battery?

A battery is a combination of two or more cells. In a series combination of cells, the positive terminal of one cell is connected to the negative terminal of the next. The cells are joined successively, with the remaining end terminals available for connection.

Cells need not physically lie in a straight row. They may be placed side by side in a device. There is usually a thick wire or metal strip joining the positive terminal of one cell to the negative terminal of the next.

A cell holder keeps cells in position and provides electrical connections. Check the terminal markings when fitting cells. Positive and negative signs are usually printed in battery compartments to help identify the required arrangement.

Torches, toys and television remote controls are examples of devices that use batteries. To recognise a battery arrangement, trace the terminal connections. The position of the cells in the holder is less informative than which terminals are actually joined.

How are electrical components represented by symbols?

An electrical component is a part used in an electric circuit, such as a cell, bulb or switch. A circuit diagram represents the components and their connections using agreed symbols rather than detailed pictures of the apparatus.

A switch, also called a key, makes or breaks the conducting path. A resistor is a component that opposes the flow of current. These components have different functions and must be represented by their own symbols.

What does each symbol show?

The filament is the thin wire inside this type of bulb. It glows when current passes through it. A contact is a point where conducting parts touch to make an electrical connection.

Component symbols can differ between books. Use a consistent set of conventional symbols. The cell's unequal lines distinguish its terminals; they do not mean that the drawing shows two separate cells.

Both drawings contain a cell, a bulb and a switch joined by lines. Figure 10.8 shows a closed switch and rays around the bulb; Figure 10.9 shows a gap at the open switch.

See Figs. 10.8 and 10.9 in your NCERT textbook

When drawing a circuit, first identify the components and then trace their connections. A neat arrangement on the page is useful, but it must preserve the conducting paths of the actual circuit.

How does a switch control a simple electric circuit?

An electric circuit is a continuous, closed path through which current can flow. A simple bulb circuit contains a cell or battery, a bulb, connecting wires and a switch. The path must connect the components back to the source.

When the switch is closed, its contacts are joined and the circuit is complete. This is the ON position. When the switch is open, the gap interrupts the path. This is the OFF position.

What changes when the switch is opened?

The switch can be placed anywhere along this single path. Opening it breaks the complete circuit regardless of whether it is drawn beside the bulb or beside the cell. The switch controls continuity; it is not the source of electricity.

A bulb that has fused has a broken filament. This break also makes the path incomplete. Closing the switch cannot repair the broken filament, so a fused bulb does not glow merely because the switch is ON.

Note: Use electric cells for these activities. Do not experiment with the mains supply, meaning the electricity supplied to household wiring. An electric shock can be dangerous.

To explain a non-glowing bulb, consider the whole path. An open switch and a broken filament are different faults with the same essential result: an incomplete circuit. A bulb's failure to glow is an observation that needs an explanation, not a complete diagnosis by itself.

How do conductors, insulators and resistors differ?

A conductor allows electric current to flow through it. An insulator does not allow current to pass through it in an ordinary simple circuit. Copper and aluminium are conductors; rubber and glass are examples of insulators.

Resistance means opposition to the flow of current. A good conductor offers relatively little resistance. A poor conductor offers greater resistance, and an insulator offers still greater resistance when comparing pieces of the same size.

A resistor opposes current but is not the same as an open switch. Current can pass through a resistor in a complete circuit. An open switch interrupts the conducting path by separating its contacts.

How can a simple circuit test a material?

1. Prepare a working cell-and-bulb circuit, with a switch and a gap where the object will be tested.
2. Check that the bulb and connections work by completing the gap with a conducting wire.
3. Open the switch, remove that connection and place the test object between the exposed wire ends.
4. Close the switch briefly and observe whether the bulb glows.
5. Open the switch before replacing the object, and record the observation.

If the bulb glows, the object has allowed enough current through to light it. If the bulb does not glow, check the cell, filament and contacts before drawing a conclusion about the object. Poor contact can also leave the circuit incomplete.

The bulb test detects current sufficient to make the bulb glow. Absence of visible light does not by itself prove that absolutely no current passes. The test is useful for comparing materials when the circuit is checked and the connections are secure.

How are components connected in a series circuit?

In a series circuit, components are connected end to end along a single conducting path. Current passes through the components successively around the same complete circuit. There is no separate branch around any one of them.

A branch is a separate conducting path between connection points. The absence of branches is the useful feature to look for when identifying a simple series arrangement. Components do not have to be drawn in a straight line to be in series.

How can a series bulb circuit be assembled?

1. Keep the switch open while arranging the components and wires.
2. Connect the cell, switch and bulbs successively along one path.
3. Connect the last part of the path back to the other terminal of the cell.
4. Trace the complete path to check that each bulb is included without a separate branch around it.
5. Close the switch briefly to observe the working bulbs, then open it again.

The same current passes through every component of a series circuit. It does not become a new current each time it reaches another component. All the components belong to the same uninterrupted path.

If one bulb's filament breaks, the single path is interrupted and the other bulbs also go out. The same result follows if a connecting wire is removed or the switch is opened anywhere along that path.

When reading a diagram, begin at a cell terminal and follow the wire. If the route passes through each component without dividing, the arrangement is series. This method identifies the electrical connection even when the drawing bends around corners.

How does a parallel circuit differ from a series circuit?

In a parallel circuit, components are connected in separate branches between the same pair of connection points. A junction is a point where conducting paths meet or divide. Current can divide between the branches and join again after them.

To connect bulbs in parallel, connect one terminal of each bulb to one common side of the supply and its other terminal to the other common side. Each bulb then has a branch through which current can pass.

What happens if one branch is broken?

A break in one bulb branch does not by itself break the other branches. The other working bulbs can remain lit if their paths and the common supply connections remain complete. This differs from breaking the single path of a series circuit.

A common switch is placed in the part of the circuit shared by all branches. Opening it interrupts the supply path for the whole arrangement. A switch within one branch interrupts that branch.

When tracing a parallel diagram, follow each branch separately from one common connection point to the other. Check that every branch returns to the supply. Drawing bulbs beside each other is not enough: the connections must actually provide separate paths.

A comparison of series and parallel circuits should therefore describe both the arrangement and the effect of a break. Avoid predicting what happens to other bulbs until the location of the break and the type of connection are known.

How does electric current produce a magnetic effect?

A magnetic compass is a device containing a small magnetic needle that can turn freely. The needle indicates the north-south direction when it comes to rest. The Earth behaves like a giant magnet, which explains this directional property.

Bringing a bar magnet near the compass changes the needle's direction. This turning away from its original direction is called deflection. A wire carrying current can also cause a nearby compass needle to deflect.

Definition: The magnetic effect of electric current is the magnetic behaviour produced when current passes through a wire.

How can the effect be observed?

1. Wrap an electric wire a few times around the cardboard tray of a matchbox and place a small compass inside the tray.
2. Connect the wire ends to an electric cell through a switch.
3. With the switch open, note the direction of the compass needle.
4. Close the switch and observe the needle's deflection as current passes through the wire.
5. Open the switch and observe the needle returning to its original direction; repeat the comparison.

The comparison links the needle's change in direction to the presence of current. A nearby magnet can also deflect the needle, so the needle responds to magnetic effects rather than to physical contact with the wire.

Wire is wound around a small rectangular tray containing a compass. Lines connect the arrangement with a cell, an open switch and a bulb in one circuit.

See Fig. 10.17 in your NCERT textbook

The compass needle is itself a magnet. Its response provides a visible way to detect the magnetic behaviour of a current-carrying wire. Electricity and magnetism are therefore linked: passing current through a suitable wire arrangement can be used to make a magnet.

How can an electromagnet be made?

A coil is wire wound into a series of turns. An electromagnet is a magnet produced by current flowing through a coil. A coil carrying current behaves like a magnet, and placing a suitable material inside it increases its strength.

The material placed inside the coil is called its core. An iron nail can act as the core of a simple electromagnet. Insulated wire is wire covered with an insulating material, such as plastic, cloth or enamel.

What are the construction and observations?

1. Take an iron nail and a length of insulated flexible wire.
2. Wind the wire tightly around the nail to form a coil.
3. Connect the free ends of the wire to a cell through a switch.
4. Place pins near the end of the nail and close the switch briefly.
5. Observe the pins being attracted, then open the switch and compare the behaviour.

While current passes through the coil, the arrangement behaves like a magnet and attracts magnetic material. When the current is switched off, the coil generally loses its magnetism.

A coil is wound around a nail. Connecting lines join it to a cell and a closed switch. Pins are shown gathered at the lower tip of the nail.

See Fig. 10.19 in your NCERT textbook

The iron nail and the wire have different roles. The wire carries current around the turns; the iron nail is the core that helps strengthen the magnetic effect. The switch controls whether the circuit is complete.

Keep the current on for only a few seconds at a time. Leaving the electromagnet connected weakens the cell quickly. Open the switch after observing the attraction instead of leaving the circuit operating between observations.

Why are electromagnets useful in devices?

An electromagnet connects an electrical action, switching current, with a magnetic action, attracting magnetic material. This makes it useful when attraction is needed during part of a device's operation. The current through the coil can be switched on and off.

Electromagnets can be made very strong and can lift heavy loads. A crane can have a strong electromagnet at its lifting end. Electromagnets are also used to separate magnetic material from junk.

What determines whether an object can be attracted?

The object's material matters. The attraction used here acts on magnetic materials such as iron. An electromagnet does not become a device for attracting every kind of object merely because electric current produces its magnetism.

Separation of magnetic material depends on this difference between materials. The electromagnet attracts the magnetic portion of a mixture, allowing it to be separated from material that is not attracted.

Electric motors and loudspeakers also contain electromagnets. An electric motor produces motion using electricity; a loudspeaker produces sound from electrical signals. The electric bell provides a clear example in which the coil and moving part can be examined together.

To explain any listed use, identify the coil, the current and the magnetic material involved where those details are known. The important connection is between the current-carrying coil and the magnetic attraction it produces.

How does an electric bell keep ringing?

An electric bell uses an electromagnet to move a hammer repeatedly against a gong, the metal part struck to produce the ringing sound. Its operation combines a conducting circuit, magnetic attraction and repeated making and breaking of contact.

Which parts are needed?

A coil of wire is wound around an iron piece to form the electromagnet. An iron strip is placed nearby, with a hammer at one end. A contact screw is a screw that touches the strip to complete an electrical connection.

The coil, contact arrangement, cell and switch form the circuit. When the strip touches the screw and the switch is closed, current can flow through the coil. The strip can move towards the electromagnet and return to its original position.

What is the repeating sequence?

1. Contact completes the circuit. With the switch closed and the iron strip touching the contact screw, current flows through the coil.
2. The coil becomes an electromagnet. It attracts the nearby iron strip, and the hammer moves with the strip.
3. The hammer strikes the gong. This impact produces the sound of the bell.
4. The contact breaks. Movement of the strip pulls it away from the contact screw, opening the circuit and stopping current through the coil.
5. The strip returns. The electromagnet no longer attracts the strip, so it returns to its original position and touches the screw again.
6. The cycle repeats. The restored contact allows current through the coil again. Repeated strikes in quick succession produce the ringing.

The drawing shows a round gong above the hammer and curved metal strip. Below them are wire coils on an iron arrangement, with the contact assembly beside the strip and connecting leads on the right.

See Fig. 10.20 in your NCERT textbook

The break in contact is essential to repeated ringing. It interrupts the current after the strip is attracted, allowing the strip to return and remake contact. The bell therefore does not depend on one uninterrupted pull that holds the hammer against the gong.

Opening the main switch stops the supply of current and ends the repeating cycle. During ringing, however, the contact at the screw repeatedly opens and closes even while the main switch remains closed. Distinguish these two places where the circuit can be interrupted.

Glossary

• Magnetic pole — A region of a magnet where its attraction for magnetic material is strongest.
• Repulsion — The pushing apart observed when like poles of magnets approach one another.
• Electric current — The flow of electric charges through a conducting path in a circuit.
• Electric cell — A source of electricity with positive and negative terminals for circuit connections.
• Battery — A combination of two or more cells connected together to supply electricity.
• Circuit diagram — A representation of electrical components and their connections using conventional symbols.
• Switch — A component that makes or breaks the conducting path in an electric circuit.
• Conductor — A material that allows electric current to flow through it.
• Insulator — A material that does not allow current through it in an ordinary simple circuit.
• Resistor — A circuit component that opposes the flow of electric current through it.
• Series circuit — An arrangement with components connected successively along a single conducting path.
• Parallel circuit — An arrangement with separate branches connected between the same two connection points.
• Electromagnet — A magnet produced by current flowing through a coil of wire.
• Core — Material placed inside a coil to increase its magnetic strength.
• Contact screw — The screw that touches the iron strip to complete an electric bell's circuit.

Common errors and misconceptions

• Misconception: Attraction proves that an iron bar is a magnet. Correct: An unmagnetised iron bar is also attracted. Repulsion by a known magnet establishes that the bar is a magnet.
• Misconception: Breaking a bar magnet separates its north and south poles. Correct: Each smaller magnet has both north and south poles.
• Misconception: Cells are joined positive to positive to make the series battery described here. Correct: Connect the positive terminal of one cell to the negative terminal of the next.
• Misconception: Closing a switch makes every bulb glow, even a fused bulb. Correct: A fused bulb has a broken filament, so the conducting path remains incomplete.
• Misconception: A resistor stops current in exactly the same way as an open switch. Correct: A resistor opposes current within a conducting path; an open switch breaks that path.
• Misconception: Breaking any one bulb branch switches off every parallel bulb. Correct: Other complete branches can still work if the common supply path remains complete.
• Misconception: An electromagnet always loses every trace of magnetism immediately when switched off. Correct: The coil generally loses its magnetism when the current is switched off.
• Misconception: An electric bell rings because its electromagnet holds the hammer permanently against the gong. Correct: Repeated breaking and remaking of contact produces repeated hammer strikes.

Exam-style questions with model answers

Q1. State what happens when like magnetic poles and unlike magnetic poles are brought near one another. [2 marks]

1. Like poles repel one another: this applies to north facing north or south facing south.
2. Unlike poles attract one another: this applies to a north pole facing a south pole.

Q2. An unknown bar is either an unmagnetised iron bar or a bar magnet. A marked bar magnet is available. Explain how to identify the unknown bar by repulsion and why attraction alone is insufficient. [3 marks]

1. Support the unknown bar so that it can move. Bring the marked magnet's north pole near each end in turn, without pushing the bars together.
2. If an end is repelled, the unknown bar is a magnet and that end is its north pole, because like poles repel.
3. Attraction alone is insufficient because both a magnet's unlike pole and an unmagnetised iron bar can be attracted by the known magnet.

Q3. Two cells are to be joined in series to make a battery. Explain the terminal connection and identify the positive and negative terminal lines in a cell symbol. [3 marks]

1. Join the positive terminal of one cell to the negative terminal of the other. This gives a series combination of cells, with the remaining end terminals available for the external circuit.
2. The longer line in the cell symbol represents the positive terminal, identified by the positive sign.
3. The shorter, thicker line represents the negative terminal. A battery symbol repeats the long and short line pairs.

Q4. In a simple circuit, a suitable working cell, working bulb and sound connecting wires form a single path through an open switch. Predict the bulb's behaviour before and after closing the switch, with reasons. [4 marks]

1. With the switch open, the bulb does not glow because the switch leaves a gap in the single conducting path.
2. The open circuit prevents current from flowing through the bulb, even though the cell and bulb are working.
3. Closing the switch joins its contacts and completes the path through the bulb and back to the cell.
4. Current then passes through the bulb's filament, so the bulb glows in the completed circuit.

Q5. Compare series and parallel bulb circuits in terms of connections, available paths and the effect of one broken bulb filament. Assume suitable working cells and bulbs, closed switches and sound wires, with no other fault. [3 marks]

1. Series bulbs are connected end to end along one path. Parallel bulbs are connected in separate branches between the same two connection points.
2. In series, the same current passes through each bulb. In parallel, current divides between branches and joins again afterwards.
3. A broken filament interrupts the whole series path. In parallel, the other complete branches can still carry current because the stated fault affects only one bulb branch.

Q6. Insulated wire, an iron nail, pins, a cell, connecting wires and a switch are available. Describe making and testing an electromagnet, its behaviour when switched off and one precaution about connection time. [5 marks]

1. Wind the insulated wire tightly around the iron nail to make a coil. The nail is the core inside the coil.
2. Connect the free wire ends to the cell through the switch so that closing the switch allows current through the coil.
3. Place pins near the nail's end and close the switch briefly. The current-carrying coil behaves like a magnet and attracts the pins.
4. Open the switch and compare the behaviour. When the current is switched off, the coil generally loses its magnetism.
5. Keep the current on for only a few seconds at a time, because leaving the electromagnet connected weakens the cell quickly.

Q7. An electric bell has a working cell, a closed main switch, a coil around an iron piece, an iron strip carrying a hammer, a contact screw and a gong. Initially the strip touches the screw. Explain one complete repeating cycle of ringing. [6 marks]

1. Because the main switch is closed and the strip touches the contact screw, the circuit is complete and current flows through the coil.
2. The coil becomes an electromagnet and attracts the nearby iron strip, moving the hammer attached to that strip.
3. The moving hammer strikes the gong. This impact produces the sound, while the strip's movement also carries it away from the screw.
4. The strip loses contact with the screw, breaking the circuit. Current through the coil stops, so the electromagnet no longer attracts the strip.
5. The iron strip returns to its original position and touches the contact screw again, restoring the conducting path through the coil.
6. Current flows again, and the attraction and hammer strike repeat. Rapid repetition of this cycle produces the continuing ringing while the main switch remains closed.

Q8. In a cell-powered test circuit, a test object fills a gap in a single path containing a working bulb and a closed switch. The bulb glows. What does this show? Would a non-glowing bulb alone prove that another object is an insulator? Explain. [3 marks]

1. The glowing bulb shows that the test object permits enough current through the completed circuit to light the bulb, so it conducts electricity.
2. A non-glowing bulb alone does not establish that the replacement object is an insulator. A poor connection, weak cell or broken filament can also prevent visible light.
3. Check the cell, bulb and contacts with a known conducting connection before interpreting the replacement object's result. The bulb test detects current sufficient to make the bulb glow.

Key takeaways

• Like magnetic poles repel and unlike poles attract; repulsion distinguishes a magnet from an unmagnetised iron bar.
• An electric cell supplies electricity, and a series battery joins the positive terminal of one cell to the next cell's negative terminal.
• A closed switch completes a conducting path; an open switch or broken filament interrupts a simple series circuit.
• Conductors allow current through, insulators prevent passage in a simple circuit, and resistors oppose current within a conducting path.
• Series components share a single path, whereas parallel components occupy separate branches between the same two connection points.
• A nearby compass needle can reveal the magnetic effect produced by a wire carrying electric current.
• A current-carrying coil forms an electromagnet; an iron core strengthens it, and the coil generally loses magnetism when switched off.
• An electric bell rings through repeated magnetic attraction, hammer strikes, breaking of contact and restoration of the circuit.

Test yourself

Why does attraction to a magnet fail to identify an unknown iron bar as a magnet?

An unmagnetised iron bar is also attracted. Repulsion from a known magnetic pole is the distinguishing test.

What do the long and short lines in a cell symbol represent?

The long line represents the positive terminal; the shorter, thicker line represents the negative terminal.

What is the difference between a resistor and an open switch?

A resistor opposes current in a conducting path. An open switch breaks the conducting path at its separated contacts.

Why does one broken filament stop all the bulbs in a series circuit?

The bulbs share one path, and the broken filament makes that path incomplete.

Why can other parallel bulbs remain lit when one bulb branch is broken?

The other branches still provide complete paths, provided the common supply connections remain complete.

What does a nearby compass needle's deflection show when current is switched on?

It shows the magnetic effect of the current-carrying wire on the compass needle, which is itself a magnet.

What generally happens to an electromagnet when its current is switched off?

The coil generally loses its magnetism. Its magnetic action depends on the current through the coil.

Why must the electric bell's strip break contact with the screw?

The break stops current, ending the attraction so the strip can return and remake contact for the next strike.