Magnetism
Induced magnetism: magnets attract materials by inducing magnetism in them, in other words the material becomes a magnet as well. The side of the material facing the magnet will become the opposite pole as the magnet.
Ferrous material: magnetic – anything which contains iron, nickel, or cobalt can be magnetised
Non–ferrous material: non-magnetic e.g. copper, grass, ketchup, butter, wood, ass-gravy (poop) etc.
Magnetisation methods:
-inducing magnetism produces a weak magnet. It can be magnetised strongly by stroking with one end of a magnet, in one direction.
-the most effective method is to place the metal in a long coil of wire (solenoid) and pass a large DC (direct current) through the coil.
Demagnetisation methods:
-SMASH IT WITH A HAMMER, dropping etc.
-heating to a high temperature
-solenoid method but with alternating current
Iron vs. steel: iron is a soft ferromagnetic material meaning it will magnetise and demagnetise easily. Steel is a hard ferromagnetic material meaning it is hard to magnetise and demagnetise. Soft ferromagnetic materials are used to create temporary magnets, for example the magnets which lift cars in a rubbish dump, or the magnet in a circuit breaker. Hard ferromagnetic materials are used to create permanent magnets like fridge magnets, horse-shoe magnets.
The magnetic field lines go from north to south. The north pole of a magnet can be found by placing a compass near the magnet. The needle will point the direction of the magnetic field line.
4.2 (a) Electric charge
Detecting charge:
You can detect an electrostatic charge using a leaf electroscope.
If a charged object is placed near the cap, charges are induced. The metal cap gets one type of charge (positive or negative) and the metal stem and gold leaf get the other type of charge so they repel each other.
There are 2 types of charges: positive and negative.
Unlike charges attract and like charges repel.
Electric field: a region in which an electric charge experiences a force
Conductors: materials that let electrons pass through them. Metals are the best electrical conductors as they have free electrons. This also makes them good thermal conductors
Insulators: materials that hardly conduct at all. Their electrons are tightly held to atoms and hardly move, but they can be transferred by rubbing
The SI unit of charge is the Coulomb (C).
Electric field lines :
Induced Charge: a charge that appears on an uncharged object because of a charged object nearby, for example if a positively charged rod is brought near a small piece of aluminium foil. Electrons in the foil are pulled towards the rod, which leaves the bottom of the foil with a net positive charge. The attraction is stronger than the repulsion because the attracting charges are closer than the repelling ones.
4.2 (b) Current
Current: a flow of charge, the SI unit is the Ampere (A).
An ammeter measures the current in a circuit. It is connected in series, the current is a rate of flow of charge.
Charge (C) = current (A) x time (s) C = I x t
The conventional current direction is the direction the positive particles would travel in. This is the opposite of what actually happens, as it is the negative particles (electrons) that move. Conventional current is indicated with arrows on the lines (wires). Conventional current goes from the positive side (long line in cell drawing) to the negative side (short
line in cell drawing). Actual current goes from the negative side (short line in cell drawing) to the positive side (long line in cell drawing).
4.2 (c) Electro-motive force
The maximum voltage a cell can produce is called the electromotive force (EMF), measured in volts. When a current is being supplied, the voltage is lower because of the energy wastage inside the cell. A cell produces its maximum PD when not in a circuit and not supplying current.
4.2 (d) Potential Difference
Potential difference, or PD for short, is also known as voltage. Voltage is the amount of energy the cell gives the electrons it pushes out. Voltage is measured in volts (V) and is measured by a voltmeter (connected in parallel). If a cell has 1 Volt, it delivers 1 Joule of energy to each coulomb of charge (J/C).
Voltage = Energy / Charge Volts = Joules / Coulomb V = E / C
4.2 (e) Resistance
Resistance (Ù) = potential difference (V) / current (A)
R = V / I
Factors affecting resistance:
–Length Double the length = double the resistance (proportional)
-Cross-sectional area Half the cross-sectional area = double the resistance (inversely proportional)
–Material Better conductor = less resistance
–Temperature For metal conductors higher temperature = more resistance For semi-metal conductors higher temperature = less resistance The V =IR law can be investigated with the following apparatus:
4.2 (f) Electrical energy
Electrical power = Voltage (V) × Current (A) P = V × I
Electrical energy (J) = power (W) × time (s) E = P × t
Electrical energy (J) = Voltage (V) × Current (A) × time (s) E = V × I × t