Ohmic (wire or resistor at constant temperature)4P 07/12/07
We sat that test.
Revision for the electricity test on Friday was done by doing past paper GCSE questions.
Things you need to know for the test:
The equations.
Current (Amps) = Charge (Coulombs) / Time (secs)
Voltage (Volts) = Energy (Joules) / charge (Coulombs)
Power (Watts) = Current (Amps) * Voltage (Volts)
Power (Watts) = Energy (Joules) / Time (secs)
Resistance (Ohms) = Voltage (Volts) / Current (Amps)
The graph shapes
Ohmic (wire or resistor at constant temperature)
Filament bulb
Diode
Thermistor
And electrical safety
also fuses and the reason for the Earth wire.
Electrostatic charge - dangers, uses.
We learned about themistors.
Thermistors lose resistance as they heat up. The opposite of a filament bulb.
They do this because extra free charge carriers are released by the thermal energy. They are made of semi-conductors, not metals.
A high current passing through a thermistor will cause it to heat up, just like in a filament bulb, so when a larger voltage is put across a thermistor, its resistance goes down.
The graph curves the opposite way to the filament one.
Then we did some revision of all the concepts covered so far in electricity.
You must be able to use these formulae:
Current (Amps) = Charge (Coulombs) / Time (secs)
Voltage (Volts) = Energy (Joules) / charge (Coulombs)
Power (Watts) = Current (Amps) * Voltage (Volts)
Power (Watts) = Energy (Joules) / Time (secs)
Resistance (Ohms) = Voltage (Volts) / Current (Amps)
Also, the circuit for testing voltage and current variation and the shape of the V/I graph for a wire at constant temperature, a bulb, a diode and a thermistor.
HW Revise for a test on electricity (including electricity in the home and electrostatics) for next time.
We looked at more current/voltage characteristics. Bulbs gain in resistance as more current is passed through them because they heat up. Greater particle vibrations in the metal get in the way of electrons trying to pass through the filament as it heats up, increasing its resistance.
Diodes only allow current to pass through them in one direction. They are normal conductors, obeying Ohm's law in one direction but they have a huge resistance in the opposite direction, not allowing current to flow.
HW P71 Qs 1 + 2
We did some revision of all the concepts covered so far in electricity.
You must be able to use these formulae:
Current (Amps) = Charge (Coulombs) / Time (secs)
Voltage (Volts) = Energy (Joules) / charge (Coulombs)
Power (Watts) = Current (Amps) * Voltage (Volts)
Power (Watts) = Energy (Joules) / Time (secs)
Resistance (Ohms) = Voltage (Volts) / Current (Amps)
We first of all looked at the types of questions you may be asked in exams on electrical safety. They were pretty easy as it turns out.
Then we did a practical on simple direct current electricity to confirm the relationship between the voltage across a component and the current through it.
Those who failed to get a decent set of results can use these.
HW Plot a graph of V/I and put the sets of data for both the 2.2k and the 4.7k resistors on the same axes. You should get a straight line through the origin in each case. Calculate the gradient of both lines.
Forces and motion test came back eventually - mostly OK, remember that the area under a speed/time graph gives you the distance travelled.
We decided to wire some plugs and look at the topic of electrical safety I believe.
We also wired some plugs. A fuse is a fire prevention device - it stops large currents flowing accidently causing heating which might start a fire.
We saw where the fuses actually go. They are put into the plug of a device attached to the "live" pin.
The other 2 pins of a plug are the neutral and the Earth. The live and the neutral connection are the 2 that join the circuit in normal operation (just like connecting to both ends of a battery really.)The Earth pin is a safety measure which stops the outer case of an electrical device becoming charged by the live wire accidently. If ever the live wire were accidently to touch the outer case, a large current would flow down the Earth wire and the fuse would melt. This is described well on the BBC website here
We tried to wire some plugs ourselves, some of you are much better than me at it. Others may need to practise a bit before doing it for real....
At home, electricity is supplied as alternating current. This happens because the current is being pushed by an alternating voltage. Electrons actually go backwards and forwards within the wires 50 times a second.
Electricity we started.
Electric current is a word describing moving electrical charges. It is the rate of flow of electrical charge. The unit which is used to measure charge is called the Coulomb. (The numbers of individual electrons moving would be very large indeed.)
Current is therefore measured in Coulombs per second. Another word for 1 Coulomb per second is an Ampere
. Electrical conductors are materials that allow the passage of electric current. In order to do this they must have charge carriers (usually electrons) which are free to move.
Metals have free electrons as part of their structure which can conduct electricity.
Although electrons move slowly through metal wires, each electron repels its neighbour causing a knock on effect which means that electrical energy is transferred at the speed of light.
The dissolved ions in a solution are also free charge carriers and so can be used to conduct electricity.
Current: the rate of flow of electric charge (Amps)
1 Amp refers to 1 Coulomb (the unit of charge) of charge flowing past a point in 1 second.
Voltage: the size of electrical "push" trying to cause a current to flow (Volts)
1 Volt refers to 1 Joule of potential energy being given to each Coulomb of charge.
An electrical conductor has charge carriers (usually electrons) which are free to move, thus they allow an electrical current to flow when a voltage is applied to them.
You built and tested a circuit that can be used to see how the current and voltage across an electrical component vary in comparison to each other.
You found that voltage and current were proportional for both the components we tested. These components obey Ohms law: Current is proportional to current, the constant of proportionality is called resistance.
V = IR (Your graph may have had different axes, allowing the gradient to equal the resistance of the component.)
HW Calculate the gradient of your straight line of best fit for each unknown resistor. This should equal the resistance of the components you tested.
We sat the new concise forces and motion test. We'll move on to electricity in the home next time, and learn how to wire a plug.
Well here I was again. I showed you the Van der Graaf generator - it worked rather well. Cold dry conditions help to allow static charge build up rather than leaking away quickly. 10cm sparks were pretty awesome.
Our sparks were at least this long...
The sparks can only form when the electric field is strong enough to ionise the air between the Van der Graaf dome and a suitable Earthed object.
This site has a good summary of the things you need to know about electrostatics.
This clip shows a trick we didn't do.
This shows a couple we did.
I gave you back your forces and motion tests from last time. They were a failure in many ways due to the large variation in questions attempted. We'll do another test next time on the same topic to get a decent and fair mark for all of you.
HW Revise forces and motion for the new test. This document contains all you need to know.
We too saw the electrical safety video. The important points you need to know are:
1. Earth wires are needed to prevent the outside of an electrical appliance from accidentally becoming live.
2. The main danger from poor wiring is fire, not electrocution.
3. The correct fuse must be fitted to each appliance, but also the correct thickness of power cable must be used.
4. Don't do any of the internal wiring in your house yourself, call a qualified electrician.
4P 07/11/07
RM present, surprisingly. We decided to wire some plugs and look at the topic of electrical safety I believe.
We also wired some plugs. A fuse is a fire prevention device - it stops large currents flowing accidently causing heating which might start a fire.
We saw where the fuses actually go. They are put into the plug of a device attached to the "live" pin.
The other 2 pins of a plug are the neutral and the Earth. The live and the neutral connection are the 2 that join the circuit in normal operation (just like connecting to both ends of a battery really.)The Earth pin is a safety measure which stops the outer case of an electrical device becoming charged by the live wire accidently. If ever the live wire were accidently to touch the outer case, a large current would flow down the Earth wire and the fuse would melt. This is described well on the BBC website here
We tried to wire some plugs ourselves, some of you are much better than me at it. Others may need to practise a bit before doing it for real....
At home, electricity is supplied as alternating current. This happens because the current is being pushed by an alternating voltage. Electrons actually go backwards and forwards within the wires 50 times a second.
HW
Many millions of years later, another lesson! You had managed to already do the set of questions on electrostatics that I had lined up for you. We saw a little video on electrostatics.
Protons are heavy and positive and live in the nuclei of atoms. Electrons are light and mobile and negative and surround the nucleus at some distance, held in by the electrostatic attractive force.
Chemical reactions can cause atoms to lose or gain electrons and become charged. They are then known as positive or negative ions. However, we'll leave them to the chemists. Another way that materials can become charged is by friction.
You must make sure that you always refer to the movement of negative electrons when discussing the charging of an insulator by friction.
Alike charges attract and unlike charges repel.
A charged object will also attract something that is neutral. Think about how you can make a balloon stick to the wall. If you charge a balloon by rubbing it on your hair, it picks up extra electrons and has a negative charge. Holding it near a neutral object will make the charges in that object move. If it is a conductor, many electrons move easily to the other side, as far from the balloon as possible. If it is an insulator, the electrons in the atoms and molecules can only move very slightly to one side, away from the balloon. In either case, there are more positive charges closer to the negative balloon. Opposites attract. The balloon sticks. (At least until the electrons on the balloon slowly leak off.) It works the same way for neutral and positively charged objects.
You must also be aware of the attraction of neutral objects to charged objects by induced dipoles.
Above shows how it works with an individual atom. The same effect can be seen overall on lumps of neutral matter.
We looked at uses and dangers of electrostatics. You need to know about paint spraying, photocopying, inkjet printing and smoke precipitators. Also, how sparks can be a nuisance or dangerous in some situations.
This page has more uses of electrostatics.
We looked at sparks caused by electrostatic charging. If an object becomes highly charged enough, then it will try to discharge in any way it can. Sometimes, the electrical force has become so large that the air itself becomes ionised, allowing it to conduct. This is when a spark is seen.
Interesting site on electrostatic weather......
Van der Graff generator. It puts such a large charge on a metal dome by friction that it can cause a spark to fly through the air. Air does not usually conduct electricity, but it becomes ionised with the large electric field present meaning that the charge can escape to Earth if a suitable Earthed object is near enough the dome.
All objects which touch it and are not Earthed become charged too, causing the famous vertical hair effect.
Lightning is dangerous due to the collosal amount of charge being moved. Other, much smaller sparks can be dangerous if there are volatile chemicals about.
N.B. Mr Mackrell! This class haven't seen the Van der Graaf generator yet! Also, the test must be gone through next time! (mark it while waiting to do Jury service!). Learn to punctuate properely!
I have been away for millions of years, but I'm back walking now. I realised that you hadn't had your forces and motion test yet.
You had done a little work on electrostatics while I was away, but we started from scratch.
Protons are heavy and positive and live in the nuclei of atoms. Electrons are light and mobile and negative and surround the nucleus at some distance, held in by the electrostatic attractive force.
Chemical reactions can cause atoms to lose or gain electrons and become charged. They are then known as positive or negative ions. However, we'll leave them to the chemists. Another way that materials can become charged is by friction.
You must make sure that you always refer to the movement of negative electrons when discussing the charging of an insulator by friction.
We rubbed polyethene rods with dusters, and found that 2 such rods slightly repelled each other. (we used a low friction watch glass device to establish this). Electrons had been "scraped" from the duster onto the polyethene, leaving both negative and therefore repelling each other. Other materials were found to charge positive when rubbed by the duster (they attracted the polyethene).
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.
A charged object will also attract something that is neutral. Think about how you can make a balloon stick to the wall. If you charge a balloon by rubbing it on your hair, it picks up extra electrons and has a negative charge. Holding it near a neutral object will make the charges in that object move. If it is a conductor, many electrons move easily to the other side, as far from the balloon as possible. If it is an insulator, the electrons in the atoms and molecules can only move very slightly to one side, away from the balloon. In either case, there are more positive charges closer to the negative balloon. Opposites attract. The balloon sticks. (At least until the electrons on the balloon slowly leak off.) It works the same way for neutral and positively charged objects.
You must also be aware of the attraction of neutral objects to charged objects by induced dipoles.
Above shows how it works with an individual atom. The same effect can be seen overall on lumps of neutral matter.
We looked at uses and dangers of electrostatics. You need to know about paint spraying, photocopying, inkjet printing and smoke precipitators. Also, how sparks can be a nuisance or dangerous in some situations.
This page has more uses of electrostatics.
We looked at sparks caused by electrostatic charging. If an object becomes highly charged enough, then it will try to discharge in any way it can. Sometimes, the electrical force has become so large that the air itself becomes ionised, allowing it to conduct. This is when a spark is seen.
Interesting site on electrostatic weather......
Van der Graff generator. It puts such a large charge on a metal dome by friction that it can cause a spark to fly through the air. Air does not usually conduct electricity, but it becomes ionised with the large electric field present meaning that the charge can escape to Earth if a suitable Earthed object is near enough the dome.
All objects which touch it and are not Earthed become charged too, causing the famous vertical hair effect.
Lightning is dangerous due to the collosal amount of charge being moved. Other, much smaller sparks can be dangerous if there are volatile chemicals about.
HW Books in - email me pictures of lightening, particularly interesting types of lightening with some information to [email protected].
I have been away for millions of years, but I'm back walking now. I realised that you hadn't had your forces and motion test yet.
You had done a little work on electrostatics while I was away, but we started from scratch.
Protons are heavy and positive and live in the nuclei of atoms. Electrons are light and mobile and negative and surround the nucleus at some distance, held in by the electrostatic attractive force.
Chemical reactions can cause atoms to lose or gain electrons and become charged. They are then known as positive or negative ions. However, we'll leave them to the chemists. Another way that materials can become charged is by friction.
You must make sure that you always refer to the movement of negative electrons when discussing the charging of an insulator by friction.
We rubbed polyethene rods with dusters, and found that 2 such rods slightly repelled each other. (we used a low friction watch glass device to establish this). Electrons had been "scraped" from the duster onto the polyethene, leaving both negative and therefore repelling each other. Other materials were found to charge positive when rubbed by the duster (they attracted the polyethene).
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.
A charged object will also attract something that is neutral. Think about how you can make a balloon stick to the wall. If you charge a balloon by rubbing it on your hair, it picks up extra electrons and has a negative charge. Holding it near a neutral object will make the charges in that object move. If it is a conductor, many electrons move easily to the other side, as far from the balloon as possible. If it is an insulator, the electrons in the atoms and molecules can only move very slightly to one side, away from the balloon. In either case, there are more positive charges closer to the negative balloon. Opposites attract. The balloon sticks. (At least until the electrons on the balloon slowly leak off.) It works the same way for neutral and positively charged objects.
You must also be aware of the attraction of neutral objects to charged objects by induced dipoles.
Above shows how it works with an individual atom. The same effect can be seen overall on lumps of neutral matter.
We looked at uses and dangers of electrostatics. You need to know about paint spraying, photocopying, inkjet printing and smoke precipitators. Also, how sparks can be a nuisance or dangerous in some situations.
This page has more uses of electrostatics.
HW Revise for a forces and motion test next time.
We looked into car stopping distances, and free fall motion.
Without air resistance, all objects fall at the same rate with an acceleration of about 10m/s/s (on Earth). However, air resistance changes the way that some objects fall.
We looked at drag forces.
Our example was ball bearings falling through a thick and viscous fluid. The large amount of drag allowed us to see the effect on the rate that they fell.
A speed/time graph of the motion would look like this:
In air, (unless the object is very light), much higher speeds need to be reached.
The above shows how a parachutist first accelerates until the air resistance force on him is equal to his weight. He then travels at terminal velocity, a constant speed with balanced forces acting on him. He then artificially increases his air resistance by opening his parachute, so he slows down until air resistance once again equals his weight at a much slower (and safer) speed.
Terminal velocity occurs when the air resistance (sometimes called "drag") force equals the weight iof the falling object. This means that:
heavy, compact, objects will have a higher terminal velocity than light, spread out objects. Therefore, heavy objects will fall faster in air than light objects
The distance in which cars can stop is an important factor in road accidents.
First, thinking distance is the distance travelled when the driver has seen a hazard, but is yet to press the brake down.
Braking distance is the further distance travelled by a car as it slows down.
Total stopping distance = Thinking distance + Braking distance
If you double your speed, you reaction time remains the same, but you will have travelled twice as far before you press the brake down.
The time taken to brake to zero will double, and your average speed whilst braking will double. This means your braking distance will increase fourfold if you double your speed.
We also looked at the Physics of crumple zones.
The front of the car crumples when it crashes. Thie increases the time taken for it to be stopped. An increase in time means a decrease in the deceleration that the car undergoes. The people in the car also decelerate at a lesser rate, and so they experience a smaller force (F=ma). A smaller force is less likely to hurt them.
HW Revise for a test on all work covered so far.
A forces and motion test, as promised.
This sheet may still be missing for some absentees due to religious events. Download it, print it and fill it in if you haven't already please.
We looked into car stopping distances, and free fall motion.
We looked at the Physics of freefall.
The distance in which cars can stop is an important factor in road accidents.
First, thinking distance is the distance travelled when the driver has seen a hazard, but is yet to press the brake down.
Braking distance is the further distance travelled by a car as it slows down.
Total stopping distance = Thinking distance + Braking distance
If you double your speed, you reaction time remains the same, but you will have travelled twice as far before you press the brake down.
The time taken to brake to zero will double, and your average speed whilst braking will double. This means your braking distance will increase fourfold if you double your speed.
We also looked at the Physics of crumple zones.
The front of the car crumples when it crashes. Thie increases the time taken for it to be stopped. An increase in time means a decrease in the deceleration that the car undergoes. The people in the car also decelerate at a lesser rate, and so they experience a smaller force (F=ma). A smaller force is less likely to hurt them.
HW Revise for a test on all work covered so far. Use the set of questions on the back of the handout to help you revise. This sheet was meant to have been handed in today. Get it done if you haven't yet.
We did some more work on F=ma calculations. They are really easy as long as you can get the correct numbers out of the question and rearrange the formula at will.
I talked about why all objects fall at the same rate. If you double the mass of an object, it will have twice the weight, so twice the accelerating force on it as it falls. However, doubling the mass halves the acceleration caused by a particular force, so the effect is cancelled entirely.
HW This sheet was meant to have been handed in today. Get it done if you haven't yet.
We did another very similar experiment to last time.
This time we kept the accelerating force the same and changed the overall mass of the trolley.
We used the same calculation method to work out the acceleration and found that the acceleration fell as the mass was increased.
This relationship agreed with the formula postulated last time.
Force = Mass * Acceleration
HW Complete the set of questions on F=ma
We looked at the Physics of freefall.
Without air resistance, all objects fall at the same rate with an acceleration of about 10m/s/s (on Earth). However, air resistance changes the way that some objects fall.
We looked at drag forces.
Our example was ball bearings falling through a thick and viscous fluid. The large amount of drag allowed us to see the effect on the rate that they fell.
A speed/time graph of the motion would look like this:
In air, (unless the object is very light), much higher speeds need to be reached.
The above shows how a parachutist first accelerates until the air resistance force on him is equal to his weight. He then travels at terminal velocity, a constant speed with balanced forces acting on him. He then artificially increases his air resistance by opening his parachute, so he slows down until air resistance once again equals his weight at a much slower (and safer) speed.
Terminal velocity occurs when the air resistance (sometimes called "drag") force equals the weight iof the falling object. This means that:
heavy, compact, objects will have a higher terminal velocity than light, spread out objects. Therefore, heavy objects will fall faster in air than light objects
HW Set of questions on F=ma
We did another very similar experiment to last time.
This time we kept the accelerating force the same and changed the overall mass of the trolley.
We used the same calculation method to work out the acceleration and found that the acceleration fell as the mass was increased.
This relationship agreed with the formula postulated last time.
Force = Mass * Acceleration
HW Plot a graph from your results. Firstly one of acceleration vs. total mass. Then one of acceleration vs. (1/total mass). The second should be a straight line.
We held fire on the forces for a bit so we could carry on the practicals nest double lesson.
A worksheet on velocity against time graphs was attempted.
4B 24/09/07
Unbalanced forces cause the motion of objects to change. They accelerate in the direction of the unbalanced force.(acceleration is a vector)
We performed an experiment to test how the acceleration of an object varied when we changed the unbalanced force acting on it.
We found that the acceleration was proportional to the force.
The gradient of the graph depended on how hard it was to accelerate the trolley we were using - this depended on the mass. (It is harder to accelerate a more massive object.)
HW Finish you graph plotting Force against Acceleration. Draw a straight line of best fit, and then calculate the gradient of this line. This should be roughly equal to the mass of the trolley plus the extra masses you used. (2kgish) and answer qs 1-4 P35 of the Physics Matter text book.
We held fire on the forces for a bit so we could carry on the practicals nest double lesson.
A worksheet on velocity against time graphs was attempted.
HW Finish the worksheet if you haven't otherwise.
Unbalanced forces cause the motion of objects to change. They accelerate in the direction of the unbalanced force.(acceleration is a vector)
We performed an experiment to test how the acceleration of an object varied when we changed the unbalanced force acting on it.
We found that the acceleration was proportional to the force.
The gradient of the graph depended on how hard it was to accelerate the trolley we were using - this depended on the mass. (It is harder to accelerate a more massive object.)
HW Finish you graph plotting Force against Acceleration. Draw a straight line of best fit, and then calculate the gradient of this line. This should be roughly equal to the mass of the trolley plus the extra masses you used. (2kgish) and everyone needs to hand in their books next time with last time's HW complete.
I was so tired that I was only vaguely self aware. I think I told you Newton's laws.
I also took in the books. Marvellous.
Good grief. What did we do?
I remember. We used a tickertape timer to make a measuremnent of the acceleration due to gravity. You need to be aware of how a ttt works, but I promise we'll never have to get one out again... The correct value is approximately 9.81m/s/s. You got less than this due to friction.
HW Qs 8, 10, 11, 12, 13, 14 (I think)
We looked at another method of calculating g, the acceleration due to gravity. The timer ball measured how long it took to fall a known distance. From this we were able to work out its average speed. An object accelerating uniformly from rest will reach have a maximum speed of double its average speed. Therefore its change in speed will be equal to double its average speed allowing you to calculate its acceleration.
HW Qs 8, 10, 11, 12, 13, 14, 16 from P11
We used a tickertape timer to make a measuremnent of the acceleration due to gravity. You need to be aware of how a ttt works, but I promise we'll never have to get one out again... The correct value is approximately 9.81m/s/s. You got less than this due to friction.
HW Investigate how g varies around the planet Earth, and why. JB is dead.
So we finally had another lesson. I had not marked your cover work yet, so will have to before next time!
We talked about the differences between vector and scalar quantities. Distance travelled is a scalar, it just tells you how far you have travelled. Displacement tells you how far you have come from your original starting point and in what direction. Displacement is therefore a vector quantity. Vectors all have a size (magnitude) and a direction. Another example is forces, which must act in a particular direction.
Speed is a found by dividing distance travelled by time taken. It is therefore also a scalar quantity. A speed of 3m/s does not specify the direction in which it is travelling. This means that there should never be such a thing as a negative speed, as this implies backwards.
Velocity is the equivalent of speed, but with direction added too and so is a vector quantity.
Average velocity = Displacement / time
Acceleration = Change in velocity/time (this takes direction into account).
HW None - cover work for me to mark...
We talked about the differences between vector and scalar quantities. Distance travelled is a scalar, it just tells you how far you have travelled. Displacement tells you how far you have come from your original starting point and in what direction. Displacement is therefore a vector quantity. Vectors all have a size (magnitude) and a direction. Another example is forces, which must act in a particular direction.
Speed is a found by dividing distance travelled by time taken. It is therefore also a scalar quantity. A speed of 3m/s does not specify the direction in which it is travelling. This means that there should never be such a thing as a negative speed, as this implies backwards.
Velocity is the equivalent of speed, but with direction added too and so is a vector quantity.
Average velocity = Displacement / time
HW No. Books came in.
Books were handed out and numbers taken and the like.
We just started to look at ways of representing motion on graphs. The speed of an object can be found by looking at the gradient (slope) of a position/time graph.
HW None for now.
Books were handed out and numbers taken and the like.
We looked at motion graphs and what they represented.
We learned that the gradient of a distance against time graph tells you the speed of an object.
The gradient of a speed against time graph tells you the acceleration of an object.
The area underneath a speed against time graph tells you the distance travelled by an object.
HW Draw a sketch graph of the position against time for the car that you drew a speed against time graph.
I promised you the answer, so I'll make it so...
Here it is.