Magnets and Materials
Out of copper, aluminium, iron, and wood, state which one is classified as a magnetic material.
Iron
Define magnetic field.
A region of space around a magnet in which a magnetic pole experiences a force.
What is the standard S.I. unit used to measure electric charge?
Coulombs (C)
A plastic rod is rubbed with a dry cloth and becomes positively charged. Explain what has happened to the rod in terms of subatomic particles.
It has lost negative electrons (it did not gain positive protons).
Define the term electrical conductor and give one typical example.
A material that allows electric charge (electrons) to flow through it easily. Example: Copper, aluminium, or any metal.
State the type of force that occurs when the North pole of one bar magnet is brought close to the North pole of another bar magnet.
Repulsion
Outside a bar magnet, state whether magnetic field lines point from North to South, or from South to North.
From North to South
Define electric field.
A region of space in which an electric charge will experience a force.
Describe a simple laboratory experiment using a lightweight object (like small paper scraps or a suspended pith ball) to determine if a plastic comb is electrostatically charged.
Bring the comb close to the uncharged paper scraps. If the scraps are attracted to the comb, it is charged.
State whether glass is an electrical conductor or an insulator, and give a brief reason based on its atomic structure.
Glass is an insulator. Its electrons are tightly bound to their parent atoms and are not free to move throughout the material.
A student brings the North pole of a bar magnet close to an unmagnetised iron nail. Explain why the iron nail is attracted to the magnet, even though it was not a magnet originally.
Magnetic Induction
A small plotting compass is placed next to a bar magnet. Which pole of the compass needle points in the direction of the magnetic field?
The North pole
Two isolated small spheres, Particle A and Particle B, experience a mutual repulsive force when brought close together. If Particle A is known to be positively charged, state the type of charge on Particle B.
Positive charge
A synthetic carpet becomes statically charged when someone walks across it in rubber-soled shoes. Explain why this charging only occurs effectively when the air in the room has low humidity (is very dry).
Humid air contains moisture which allows charges to conduct away into the atmosphere (earthing). Dry air acts as a better insulator, allowing static charge to build up on the body.
A student tries to charge a copper rod by holding it in their bare hand and rubbing it vigorously with a cloth. Explain why the copper rod does not hold a static charge.
Copper is an electrical conductor. Any charge generated on the rod immediately flows through the student's body and into the Earth (it gets earthed).
A student has three identical-looking metal bars, P, Q, and R. When tested with a hanging magnet, bar P is attracted at both ends, bar Q is repelled by one end, and bar R experiences no force at all. Identify which bar is the permanent magnet and explain why.
Bar Q
Only a permanent magnet can repel another magnet. Attraction can happen with unmagnetised metal, so repulsion is the only true test.
Two unmagnetised soft iron nails are hung side-by-side from the North pole of a bar magnet. Explain why the bottom tips of the two nails push apart (repel) from each other.
Both bottom tips have a North pole induced in them.
An electron is placed in a uniform electric field that points to the right. State the direction of the electrostatic force acting on the electron and explain why.
To the left (opposite to the field lines)
Explain step-by-scale why rubbing an initially neutral polythene rod with a wool cloth causes the polythene rod to acquire a negative charge.
Friction transfers electrons from the atoms of the wool cloth to the surface of the polythene rod. Because electrons are negatively charged, the polythene gains excess electrons while the wool is left with a deficit, making the wool positive.
Explain the difference in terms of internal charge carriers between a solid metallic conductor and a solid insulating plastic.
A metallic conductor contains a "sea" of delocalized (free) electrons that can drift through the lattice structure. An insulating plastic has no free charge carriers; all electrons are localized in covalent bonds or inner shells.
You are given two identical metal bars that look completely alike. One is a permanent bar magnet, and the other is an unmagnetised piece of soft iron. Without using any other equipment or materials, describe how you can determine which bar is the permanent magnet.
Touch the end of one bar to the center of the other.
A student touches a soft iron nail and a steel paperclip to a strong magnet, then pulls them away. The iron nail immediately drops any extra filings it picked up, but the steel paperclip stays magnetised. Explain why in terms of induced magnetism.
Soft iron loses its induced magnetism easily, while steel retains it.
A uniform electric field points vertically upwards. Two separate spheres—one with a positive charge and one with a negative charge—are placed into this field. State which sphere experiences an upward force and which experiences a downward force.
The positive sphere experiences an upward force; the negative sphere experiences a downward force.
Explain how an uncharged metal sphere can be given a permanent negative charge using a positively charged rod by the process of induction, without the rod touching the sphere.
Bring the positive rod close to the metal sphere; free electrons are attracted toward the rod.
While the rod is held in place, momentarily touch the opposite side of the sphere with a finger (earthing it), allowing electrons from the earth to flow onto the sphere.
Remove the earth connection first, then remove the charged rod. The sphere is left with a net negative charge.
A negatively charged plastic ruler is brought close to (but not touching) an isolated, uncharged metal cylinder resting on an insulating stand. Describe and explain the distribution of charge on the metal cylinder.
The free electrons in the metal cylinder are repelled by the negative ruler and move as far away as possible to the far side of the cylinder. This leaves the near side positively charged due to a deficit of electrons and the far side negatively charged, demonstrating electrostatic induction while the overall cylinder remains neutral.