RM 08/09/04 4ths
Exercise and safety sheets were issued.
Text books: "Physics Matters" (PM) and "Physics for AQA" (PAQA) were issued.
Bring "Physics for AQA" to all your lessons please, the other may be used for setting homework questions aswell as for general help at home.
We looked at the basics of electricity. Electric current is the flow of charge around a conducting circuit. Voltage or potential difference is what pushes the charge around the circuit. Conductors must all have free charge carriers inside them which can move through their structure to carry electrical current. These free charge carriers are nearly always electrons. Electrons flowing through a conductor will collide with each other and with positive ions in the conductor's structure. This slows the flow of electrons down, and is known as electrical resistance.
HW None for today.
RM 10/09/04 4th form
We did an experiment to see how the current flowing through a resistor varied with the voltage (potential difference) put across it. It turns out that the larger the voltage (size of "push" on electrons in the circuit), the larger the current. They are proportional for a resistor; a voltage against current graph gives a straight line through the origin. A resistor is said to obey Ohms law.
The resistance of an object can be found by the following formula:
Resistance = Voltage / Current
HW Qs 1-6 from P188 of Physics matters. (Revision of current and voltage rules.)
RM 15/09/04 4th form
The experiment from last time was repeated for a 6V bulb. The bulb did not behave in the same way as the resistors. The current flowing through a bulb is not proportional to the voltage across it. Because the bulb heats up, it's resistance gets higher the more voltage you put across it. When a metal is hot, its particles vibrate more, meaning that the electrons flowing through the metal collide with positive ions and with each other more often, causing more electrical resistance.
Books were taken in, tomorrow non doers or yellow forms dispatched.
HW Finish P6+7 questions on paper.
RM 17/09/04 4th form
People had handed their books in under the wrong door! Much better to get them handed in on time please. (Rm 509). We recapped the idea of potential dividers and went through the homework question on that subject. In a series circuit, the voltage from the battery is shared between the components. The ones with the largest resistance get the largest share of the the voltage. In a parallel circuit, each parallel path has the full battery voltage accross it. In fact, just treat each parallel path like a seperate series circuit connected to the battery. More current always flows down the parallel path with the least resistance.
HW None - we'll wait until everybody is here.
RM 22/09/04 4ths
We went through the adding up of resistances in parallel. We also built a light meter briefly using an LDR, a fixed resitor and a voltmeter. This kind of circuit will be very useful when we come to look at control in electronics.
HW P191 Physics Matters (all questions) and P193 Physics Matters (all questions)
RM 24/09/04 4ths
We looked at the amount of energy which is converted by electrical components into other forms. The amount of energy that is converted by a component per second (its power) is given by the voltage across it times the current flowing through it.
P = IV
This, along with V = IR is the main formula that you need to know for simple electronics.
HW Bring along a copy of your electricity bill at home to the next lesson.
RM 29/09/04 4ths
More on electrical power. The power of an electrical component can be calculated by multiplying the current running through it by the voltage across it.
P = IV
Power is measured in Watts. A 1 Watt device can convert 1 Joule of energy per second. You should be capable of working out how many Joules of energy are used by an electrical component that you might set up in the lab.
Electrical energy used is the power of the device multiplied by the time that you have left it running for.
Energy used = Pt
In your home, you use many millions of Joules of electrical energy every day. It is more convenient to use a different unit to measure the energy that you use in a different unit, called the Kilowatt hour (kWh).
1 kWh = The energy used by a 1000 Watt device working for 1 hour.
1 kWh of energy from the electricty company tends to cost about 7p. To work out how much energy you are using, you need to multiply the power of your device ( in kiloWatts) by the amount of time you are using it (in hours).
HW Complete the set of GCSE questions which were started in the lesson.
RM 01/10/04 4ths
We did a simple set of V=IR and P=IV questions. You must know how to use those formulas fairly easily, along with adding up resistors etc.
We started electrostatics. There are 2 types of charge, positive and negative. Negative charge is carried by electrons. Positive charge is carried by protons which live in the nucleus of atoms. Electrons whizz around the outside of atoms and are much lighter and more mobile. A Van der Graff generator deposits charge (electrons) on a metal bell by friction (Electrons are literally "scraped" from a belt onto the bell. This means that there are more electrons in the metal bell than there are protons, and so it has an overall negative charge.
Like charges repel, so the electrons have an electrostatic force on them trying to leave the dome of the Van der Graff. The force gets so large that eventually the air itself becomes a viable conducting medium by ionising (turning into charged particles). At this point, a spark is formed and jumps from the dome to the nearest earthed point.
HW So many absentees that an extension on the electronics sheet was granted until Wed!
RM 06/10/04
We looked at a couple of other electrostatics demos. The gold leaf electroscope shows when it has a charge on it because the charge on the gold leaf and on the metal next to it are alike. The gold leaf is repelled by the metal next to it and therefore is held away at an angle. Materials can be charged by rubbing with a cloth. Different materials will charge differently when rubbed by the same cloth. It depends which materials "scrapes" electrons off which. The material which loses electrons gains an overall positive charge (it has more protons than electrons) and the material which has gained electrons has a negative charge.
You need to know about the uses of electrostatics in machines e.g. photocopier.
HW Revise for a test which will be on Friday, on simple electronics. This means that it will be on calculations involving V = IR and P = IV (and all their variations) as well as adding up resistances.
RM 8/10/04
The website speaks the truth. Look at it. Believe it. We sat the test.
RM 13/10/04
We went through the test, in general well done. If you scored below 65% then you have reason to be a little worried.
We need to look at electricity in the home. It is provided as alternating current, which is not like what batteries produce (direct current), but goes back and forth. The reason for this will become clear later... It is provided at about 240V - this is enough to harm a human.
We did something ever so useful and learned how to wire a plug. Live wire carries an alternaing voltage of about 240V (Brown or red), Neutral wire is at 0V (Blue or black), these 2 wires alone are required to power appliances. The Earth wire (green or yellow and green) is for safety. Metal cases of appliances are attached to the Earth wire so it cannot remain live. The fuse is fitted to the live wire so that if too much current flows at any stage, the fuse melts and the circuit is broken.
HW Qs 1,2,3,4 from P26/27
ME-S, NS, YL, KS will all see me tomorrow lunchtime.
RM 19/10/04
We watched a vid. on electrical safety before moving on to ideas of electrical cells and electrolysis. An electrical cell has 2 electrodes immersed in a chemical such that one electrode reacts with the chemical and gives up electrons into the solution, and one electrode reacts with the chemical and accepts electrons from the solution. The reactions can't take place unless one electrode has a supply of electrons, and the other has a place to put its electrons. This is done by connecting the electrodes externally into an electrical circuit.
Electrolysis is the other way round where a voltage is put across a pair of electrodes which are then immersed in a solution which contains ions. +ve ions are attracted to the negative electrode and can be neutralised there. -ve ions are attracted to the positive elctrode and can be neutralised there.
HW Electrolysis Qs 1-4 from page 25 AQA book.
RM 22/10/04
We looked at cells in a little more detail. We have almost finished the "simple" electricity part of the course and we're on to complex control in electronics next half term.
HW The electrolysis Qs set last time to be handed in after half term.
RM 03/11/04
RM absent. You looked at "control in circuits" questions from your AQA book P146-149. There was a mistake! Ignore the way this book tells you to read the colour bars on a resistor. First 2 colours represent plain numbers, the last number gives you the number of zeroes to stick on the end. Don't go inserting any decimal points.
RM 05/11/04
We went over a few of the questions that you did on Wednesday. The resistor complication was cleared up. Electrolysis questions were finally taken in.
RM 10/11/04
Potential Dividers as sensing circuits
RM 12/11/04
Transistors are switches which respond to an input which is an electrical signal. If a high enough voltage appears at the input, the rest of the transistor becomes conducting. For current to flow through the transistor, the other 2 connections must be placed in series with a power supply, such as a battery.
More complex switches can be made using combinations of transistors. They can have more than one input, and can behave in different ways when the input is on. These are called logic gates. It is important to note that logic gates must be connected to a power supply in order to be able to have an "on" output. This power supply is not always shown in circuit diagrams containing logic gates. No current every flows from the input of a transistor or logic gate to the output.
RM 17/11/04
We got some real live logic gates and connected their outputs to a lamp to test their "truth tables" (which show whether the output is on or off dependant on the signal at the inputs). ON or "logic 1" means a high voltage, OFF or "logic 0" means a low voltage. Remember: No current ever flows through the logic gates from their inputs to the outputs, their output voltage comes from a seperate power supply.
NOT gates have one input. They reverse whatever signal comes into their input.
OR gates have 2 inputs. If either one or both of them are ON, the output is ON.
AND gates require both their 2 inputs to be ON for the output to be ON.
Combinations of gates can be made to produce other types, e.g. OR then NOT = NOR etc.
HW P151 AQA book answer all questions.
RM 19/11/04
RM absent with CCF. Some comprehension exercises on control in circuits were completed by those who were present.
RM 24/11/04
A capacitor consists of 2 parallel metal plates. They are not touching each other and no charge ever flows straight from one of the plates to another.
When a battery is connected across the plates, it tries to push electrons around the circuit as normal. Electrons cannot flow through the gap in the circuit that is part of the capacitor. However, electrons can start to build up on one of the plates. They cause that plate to have an overall negative charge. The spare electrons have travelled all the way around the circuit from the other plate, which is left with an overall positive charge.
The capacitor does not charge up immediately, it takes some time to do so. During this time, the voltage across the capacitor gradually rises from zero up to the voltage provided by the battery. We plotted a graph showing how the voltage varied over time.
If you want to slow down the time taken for the capacitor to charge up to a certain voltage you can do one of 2 things:
HW Question set on board. 1. Sketch a graph of voltage againt time for a charging capacitor, and then one of current against time for the same. Then explain (4 marks worth)how current can flow around a charging capacitor circuit when it is not a complete circuit.
RM 26/11/04
An analogy (model) for a capacitor in a circuit is a large elastic membrane in a water pipe circuit. Capacitors are very commonly used electrical components, you are required to know about their use as timing devices in potential divider. If you replace the thermistor in a temperature sensor potential divider circuit with a capacitor, you have a circuit where the output voltage varies over time.
We'll finish off control in circuits on Wednesday and probably have a test on Friday. Then on to forces and motion.
HW Design a circuit that will turn on the lights in a dark room for a short while, giving the operator time to get seated before automatically turning off. (use a transistor, capacitor, variable resistor, a relay and a cell).
RM 01/12/04
Final look through control in circuits questions in preparation for a test on Friday.
HW Revise all control in circuits stuff.
RM 03/12/04
We sat the control in circuits test. You seemed to find it quite easy - we'll see whether your confidence was well founded....
Forces and Motion next - nice and easy.
RM 08/12/04
Test had mixed fortunes. We started forces and motion by looking at ways of measuring the motion of objects. Light gates and a ticker tape timer were used.
Average speed of an object = Distance travelled/time
Acceleration = Change in speed/time
HW None
RM 10/12/04
Forces and motion simple notes were made. Motion is the measurement of something's position over time. Light gates, ticker tape timers etc. can be used to do this. Speed is the rate at which position chabges over time. Acceleration is the rate at which speed changes over time. All of these things can be represented on graphs.
HW Little sheet of motion graphs - fill in the shapes of the missing lines.
RM 15/12/04
We looked at the motion of bodies falling under gravity. They all accelerate at the same rate. The more mass an object has - the greater the force acting on it. Weight = Mass times gravitational field strength.
W = mg
However, the more mass an object has, the harder it is to accelerate it. This is summarised by Newton's second law. The acceleration experienced by a body with an unbalanced force on it is equal to the size of the force divided by the mass of the object.
acceleration = Force / Mass
a = F/m