Students should learn:
- that when insulating materials are rubbed together, charge can be transferred from one to the other
- that objects become electrically charged when electrons move from one material to the other
- that when charged objects arebrought together, like charges repel and unlike charges attract.
- state that there are two types of electrical charge and that electrons carry a negative charge whereas protons carry a positive charge
- draw diagrams showing how charge can be transferred from one object to another indicating the fact thatcharges are equal and opposite
- describe the forces that act between charged objects.
- provide a detailed description of thetransfer of charge in terms of electron movement.
The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge. [P2.3.1 b)]
When two electrically charged objects are brought together they exert a force on each other. [P2.3.1 c)]
Two objects that carry the same type of charge repel. Two objects that carry different types of charge attract. [P2.3.1 d)] / Chapter map: Current electricity
Teacher notes: Current electricity
Lesson structure / Support, Extend and Practical notes
Starters
Laws of attraction – Give the students a set of three cards with pictures of bar magnets on them and ask them to arrange them so that they all attract each other or all repel each other. Use real magnets to check the answers. Students should come up with some kind of triangular arrangement. (5 minutes)
Invisible force fields – Give the students two bar magnets and ask them to balance them so that the end of one is floating above the end of the other. Ask: ‘Can they balance two magnets above one another?’ Demonstrate magnet rings on a pole if you have them and ask the students to explain what is happening. You should reach the idea that there are invisible, non-contact forces. Students needing support should be able to draw diagrams demonstrating the effects. Extend students by asking them to state what factors affect the size of the forces and to consider gravitational forces too. Compare these with the electric force later in the lessons and discuss the factors that affect the size of this force. (10 minutes)
Main
Start by demonstrating the balloon-sticking effect; it should be fairly easy to get the balloon to stick to a wall or to your own body. You may also be able to show that two charged balloons repel each other. You can discuss the static build up on a TV screen by talking about the amount of dust that builds up on it.
The use of a ‘Van de Graaff’ generator is fairly essential. Students tend to get excited and some volunteer to receive a shock. Try some of the demonstrations in ‘Practical support’. Make sure that you do not shock any students who have medical problems.
The students should be familiar with the structure of the atom by now and you should be able to go through this part quickly. Highlight the idea of an ion; this will be used in later explanation around resistance.
Emphasis needs to be placed on the idea that it is only the electrons that are free to move. When electrons leave an object, it becomes positively charged and when they enter a neutral object it becomes negatively charged. Some students struggle with the idea that adding electrons makes something negative. They need to grasp that the electron has a negative charge and so if you have more electrons, you have more negative charge.
To demonstrate the effect that charged objects have on each other, the students can carry out the simple practical activity, ‘The force between two charged objects’ from the Student Book. They should have no trouble finding that like charges repel and opposites attract.
At the end, the students should be able to tell you the simple attraction/repulsion rules.
Plenaries
Static force – Support students by giving them a set of diagrams with charged objects on them, two or more on each card, and ask them to draw force arrows. Some students canbe extended by asking them to try to draw the direction of the resultant force. (5 minutes)
Forever amber – The students should write a brief newspaper report for the ancient Greek newspaper:
announcing the discovery and properties of static electricity. It was first discovered using amber or ‘electrum’ as they called it. This could be extended as homework in which the ancient Greeks might like to speculate about future uses of this mysterious ‘electricity’. (10 minutes) / Support
The concepts discussed here are best shown through a range of demonstrations, each one supported by clear diagrams showing the charge on the objects. You can provide diagrams without the charges shown and ask students to add this feature after each demonstration.
Extend
Ask: ‘Is it actually friction that charges up objects?’ Apparently objects of different materials can become charged up just by being left in contact with each other, and rubbing objects together just increases the area of contact. The students can find out about this explanation.
Practical support
The ‘Van de Graaff’ generator
This is an impressive and fun piece of equipment that can be used to demonstrate many of the aspects of static electricity. It is not just for giving shocks!
Equipment and materials needed
A ‘Van de Graaff’ generator (VDG) and accessory kit.(See CLEAPSS Handbook, Section 12.9.)
Details
A VDG is a very temperamental device. Some days it will work very well but on others you will barely get a crackle. Dry days are best, and it is advisable to polish the dome to make it shiny. Keep computers (and mobile phones) away from the VDG.
It is traditional to start by showing the sparks that the VDG can produce. Connect the discharging wand (or discharging dome) to earth and switch on the generator. Give the dome a couple ofminutes to build up charge while you explain what the VDG is doing. Bring the wand close to the dome and with luck you will get reasonably big sparks.
Hair standing on end can be demonstrated easily, and works best if the student stands on an insulating box. Make sure that the dome is discharged before the student steps off the insulator.
If you don’t want to use a student, then you may have a hair sample that can be attached to the top of the dome or use a set of polystyrene balls in a container.
Other demonstrations can include bringing a fluorescent tube close to the dome or demonstrating a current as a flow of charge.
Safety: Make sure students do not have any heart conditions.
The force between two charged objects
With this simple experiment, the students should be able to find that there are two types of charge and investigate how they affect each other.
Equipment and materials needed
For each group: retort stand with boss and clamp, cotton, twoperspex rods, two polythene rods and a dry cloth.
Details
The students first need to make a ‘hammock’ from the cotton to be able to suspend one of the rods from the retort stand; they might find this easier if they use some light card as a base. They then rub one of the rods vigorously with the dry cloth and place it in the hammock. Next, they rub one of the other rods and bring it close to the suspended one and note the interaction; the suspended rod should rotate towards or away. They continue this procedure for all of the combinations of rods. If there seems to be little movement, it is probably because the cloth is not dry enough.
Course / Subject / Topic / Pages
Additional science / Physics / P2 4.1 Electrical charges / Pages 208–209
Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn:
- that electrical circuits are drawn using standard symbols
- the difference between a cell and a battery
- the size of an electric current ismeasured in amperes using an ammeter
- the symbols used to represent common circuit components.
- recognise and draw the circuitsymbols for a cell, a battery, a switch, an indicator, a resistor, a variable resistor, a diode, a fuse, a voltmeter, an ammeter and an LED
- describe the function of each of the above components
- state the difference between a cell and a battery
- draw circuit diagrams using the above symbols.
Electric current is a flow of electric charge. The size of the electric current is the rate of flow of electric charge. The size of the current is given by the equation:
[ P2.3.2 a)]
Circuit diagrams using standard symbols. The following standard symbols should be known: (See Student Book pages 210 and 215 for symbols). [P2.3.2 c)]
Lesson structure / Support, Extend and Practical notes
Starters
It’s symbolic – Show a set of slides/diagrams to the students containing common symbols and ask them to say what they mean. Use road signs, hazard symbols, washing symbols, etc. This should help them to understand that symbols are a simple way of representing information clearly. You can use symbols used in countries where the language is very different.(5 minutes)
Describe the circuit – Give the students diagrams of two circuits containing cells, switches and bulbs, one series and one parallel, and ask them to describe them both in a paragraph. The student can demonstrate their understanding of circuit symbols this way and you can check their prior knowledge of concepts such as current, voltage, series and parallel. Support students by providing simple circuits with the minimal number of components. Students can be extended by asking them to draw a circuit when given a description of it. (10 minutes)
Main
The students should be familiar with the basic ideas of circuits and circuit symbols from KS3. You can use this topic to check the students’ circuit-building skills, so that you can be sure that they can carry out the investigations later.
Start by showing the components and asking ‘Do symbols have to look like what they represent?’ When introducing each symbol, show the students a real device represented by that symbol. You could show them that there are several physically different looking devices that match each symbol. For example, there are a range of different ammeters represented by the same symbol and a wide range of resistors.
Most students cope well with the basic symbols for bulbs, switches and batteries. You may find that they struggle more with the various types of resistors because they are so similar. The best way to describe resistors is to discuss what is added to the basic resistor symbol.
- The variable resistor has an arrow through it showing that you can adjust it.
- The LDR has arrows going towards it representing light.
- The fuse has a thin line representing the thin wire that runs inside it.
The concept of current needs to be covered in some detail. The students need to work with the formal definition shown in the book; it’s a bit like measuring the rate of flow of water in a pipe; the water current would be the mass of water passing each second. Simulation software really helps students to visualise the charges moving through the wire, discuss the charge moving around the circuit from positive to negative carrying energy with it. You will explain this energy transfer in more detail in future lessons.
With the remaining time, you should let the students build a couple of circuits. Those in the ‘Circuit tests’ practical support are fine, or you could extend students further (see ‘Extend’ idea).
Plenaries
Current calculations – The students have a few calculations based on the equation to perform. You can extend the students by asking them to perform a calculation based on rearrangements of the equation and using unusual quantities (a time of 1 hour and a current of 40 mA when recharging a mobile phone for example). Support can be provided by sticking to relatively simple quantities and calculations requiring no rearrangement. (5 minutes)
Symbol domino loop – Give the students a set of cards showing circuit symbols and descriptions of their functions. Each card would have a symbol and a description that matches the symbol on a different card but when they are placed together correctly, the whole set would make a complete loop. Can the students name them all? (10 minutes) / Support
Some students have particular difficulties with connecting up electronic circuits correctly because they cannot match the neat circuit diagrams with the jumble of wires they are given. You can support students by using fixed boards, such as the Locktronic ones. It is a good idea to write the names of the component on them until the students can match the symbol and name correctly.
Extend
For the circuit-building exercise, extend students by asking themto look at the currents through the different branches of the circuit and find any relationships. They could even look into the potential differences and see if they can come up with the relationship before you discuss it in future lessons.
Practical support
Circuit tests
This is a simple introduction to building circuits allowing the students to refresh their skills.
Equipment and materials needed
Cells (1.5 V), torch bulb (1.5 V), leads, diode, variable resistor.
Details
The students set up a simple circuit with the variable resistor and the bulb. They should find that the variable resistor can be used to alter the brightness of the bulb and be told that this is due to the current being changed. The students then include a diode in the circuit. They should then reverse the diode. This will show that the diode only allows the current in one direction.
Course / Subject / Topic / Pages
Additional science / Physics / P2 4.2 Electric circuits / Pages 210–211
Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn:
- how to use an ammeter and voltmeter
- how to measure the resistance of a component
- that a wire at a constant temperature obeys Ohm’s law
- that the resistance of a metal wiredoes not depend on the direction of the current.
- measure the resistance of a resistor using an ammeter and voltmeter
- calculate the resistance of a device from the current through it and the potential difference across it
- state Ohm’s law for a metal wire.
- perform calculations that involve rearrangement of the resistanceequation.
V = W/Q [P2.3.2 b)]
Current–potential difference graphs are used to show how the current through a component varies with the potential difference across it. [P2.3.2 d)]
The current–potential difference graphs for a resistor at constant temperature … .[P2.3.2 e)]
The resistance of a component can be found by measuring the current through, and potential difference across, the component. [P2.3.2 f)]
The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor. [P2.3.2 g)]
Calculate current, potential difference or resistance using the equation: V = I x R.[P2.3.2 h)]
Controlled Assessment: AS4.5 Analyse and interpret primary and secondary data.[AS4.5.3 a)] / Maths skills: Potential difference, current and resistance
How Science Works: What’s the potential
Practical: Resistance in a wire
Lesson structure / Support, Extend and Practical notes
Starters
Resistors – Show the students the circuit symbols for all of the different types of resistor and ask them to describe the similarities in the symbols. Ask: ‘What do they think the other parts of the symbols mean?’ This is a simple recap. (5 minutes)