Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• discuss dynamic equilibrium, where nothing seems to be happening
• develop precise enough physical descriptions of processes to enable discussions about power and energy to be well grounded
• connect the insights from physics to the lived-in world
• maintain a strategic view when developing multi-step arguments
• link actions with apparatus to accounts with words and diagrams

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• emphasise what many different devices have in common
• make appropriate use of quantification, as being essential to arguments relying on energy and power
• show the uses for ideas such as field
• exploit the idea of compensation

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Practical devices can be thought about using power in pathways

This episode takes us into the realms of practical devices such as motors, transformers and generators. How can the working of these devices be described and explained in terms of energy, energy in stores and power in pathways?

Electrical working shifts energy at a particular rate, thereby depleting stores.

Magnetic fields introduce their own challenges, of action at a distance, which is dealt with in the SPT: Forces topic. The interaction of magnetic fields with electric fields and currents sets more challenges, also detailed here.

What's happening in terms of energy?

Wrong Track: Chemical energy in the battery is transferred to kinetic energy of the car, and this goes to heat energy in the surroundings: chemical energy (battery)  →  kinetic energy (car)  →  heat energy (surroundings: air, road, car).

Right Lines: When the eco–friendly electric car is going at a steady speed along the road, energy is shifted from the chemical store of the battery to the thermal store of the surroundings: energy (chemical store: battery)  →  energy (thermal store: surroundings)

Matching the teaching to the challenge

Thinking about the teaching

In thinking about this question it's helpful to start by distinguishing between the physical (real-world) and energy descriptions of the electric car.

The physical picture to have in mind here is one of the car travelling along at a steady speed with the car battery gradually going flat.

In energy terms we'd say that the chemical store of the battery is gradually being depleted of energy. If energy is being taken from the chemical store, the question is, where is it going to balance the energy books?

It's very tempting to take the wrong track approach here and to argue that the chemical energy from the battery is being transferred continuously to the kinetic energy of the car. However, the energy in the kinetic store of the car is not changing – it's not building up or accumulating, because the car is moving at a steady speed.

In thinking carefully about the situation, it becomes clear that the energy being shifted from the chemical store of the car battery can only be shifted to the thermal store of the surroundings. As the car moves along the road at a steady speed, it simply warms up the surroundings.

You might be thinking that the energy description seems to miss out the key practical point – that the car is moving. In some ways we'd agree with you. Energy descriptions focus attention on the energy in the stores at the beginning and end of processes and in so doing draw attention to some features of each process and not to others. In this example, the car's speed is constant, there is no change in the energy in the kinetic store associated with the car and so it doesn't feature in the energy description.

In terms of pathways, the chemical store of the battery is depleted, or gradually empties, along an electrical working pathway, while the thermal store of the surroundings is filled through a heating by particles pathway.

What's the energy story when an electric car is increasing its speed?

Wrong Track: Chemical energy in the battery is transferred to kinetic energy of the car and this then goes to heat energy in the surroundings. Chemical energy (battery)  →  kinetic energy (car)  →  heat energy (surroundings: air, road, car).

Right Lines: When the electric car is speeding up, energy is shifted from the chemical store of the battery both to the kinetic store of the car and to the thermal store of the surroundings: energy (chemical store: battery)  →  energy (kinetic store: car)  →  energy (thermal store: surroundings).

Using multiple levels of description explicitly

Thinking about the teaching

Once again, it's helpful to think of this process in terms of physical, energy and pathway descriptions.

The physical picture to have in mind here is one of the car building up speed (accelerating) and also warming up the air around it, with the car battery gradually going flat.

In energy terms we'd say that the chemical store of the battery is gradually emptying while the kinetic store of the car is filling and the thermal store of the surroundings is also filling.

A different perspective is portrayed when thinking about pathways. Here the car's electric motor is the central device. On one side of the motor, there's an electric circuit – an electrical working pathway along which energy is shifted from the battery to drive the motor. On the other side of the motor, there are mechanical linkages that enable the motor to drive the car forward – there is a mechanical working pathway along which energy is shifted as the motor drives the car forward. Furthermore, there's a heating by particles pathway along which the thermal store of the surroundings is filled. The power in the input pathway (electrical working) is switched to both output pathways (mechanical working and heating by particles).

Practical limitations on electric motors are related to energy and to power

What's the big drawback to using electric motors in cars? This is a question worth addressing with students and it provides a further opportunity to practise a simple power in pathways calculation.

The BMW Mini E has an electric motor rated at 150 kW.

Teacher: Can you explain what rated at 150 kW tells us about the motor of the car?

Martha: The power of the Mini is 150 kW.

Teacher: There's more to it…

Martha: The maximum possible power is 150 kW.

Teacher: Yes, that's it! When the Mini is moving at its top speed the electrical store of the battery is being emptied at a rate of 150,000 J each second. So if the battery stores about 100,000 kJ how can we calculate how long the car will run for at this top steady speed?

Az: Divide 100,000 kilojoule by 150,000 joule / second.

Teacher: That's exactly right, and if we do the calculation it comes to 100 000 joule150 000 joule / second. That's 667 seconds, or just over 11 minutes.

The message from the calculation is clear: the car doesn't run for long at top speed before needing a recharge.

Quantification is essential if you want to be convincing

Save it campaigns that are based on switching off electric lights or changing from incandescent (filament) to compact fluorescent bulbs are often met by the public with a shrug of the shoulders and It'll make no difference.

Experience has shown that students can become very motivated by carrying out energy surveys for their home or school. The approach is the same in both cases and involves the following steps:

1. Track down all of the incandescent bulbs in current use in the house/school. (For the school survey, split the class into teams that take a different building each.)
2. Make the assumption that each incandescent bulb has a power output of 75 watt. This is an interesting and helpful assumption to make – talk it through with the class. This simplifying assumption avoids having to inspect each and every bulb to find its power rating. The figure of 75 watt seems reasonable, lying somewhere between high (100–150 watt) and low (30–50 watt) power bulbs.
3. For each bulb there is a saving of 60 % of the rate at which energy is shifted from the supply. That is for each 75 watt bulb there's an energy saving at the rate of 45 watt or 45 joule every second.
4. Multiply the total number of incandescent bulbs used by 45 watt to find the rate at which energy is being dissipated through bulbs warming the surroundings, for the school or home. In a large public building, such as a school, this rate of wastage can become significant and, if this is the case, the class may wish to prepare a report for the school governors or local authority. But take care in interpreting your results – what you save on warming by light bulbs you may need to spend on extra warming by other means during the winter. In the summer you might counter the excess temperatures caused by the inefficient light bulbs by opening the windows or doors.

Keeping the three directions in mind: current; field; force

Hans Christian Ørsted (1777–1851) was a Danish physicist and chemist who is most widely known for observing that electric currents produce a magnetic field. In the case of a straight conducting wire, the magnetic field lines are circular in shape, centred on the line of the wire.

When a current-carrying wire is placed in the space between two attracting magnets a force acts on the current-carrying wire. This is perhaps not surprising since the current-carrying wire is a magnet and it therefore experiences a force when placed in a magnetic field. What is surprising is the direction of the force on the wire.

The force on the current carrying wire:

• Is not in the direction of the current in the wire.
• Is not in the direction of the magnetic field between the attracting magnets.
• Is in a third direction: at right-angles to both the wire and the field.

Transformers use the idea of compensation

Wrong Track: The changing potential difference across the primary coil is 12 volt and this gives 120 volt across the secondary coil. So, since volts tell us the number of joule / coulomb, there must be a 10 times gain in energy from the transformer!

Right Lines: The point to remember is that, if the potential difference across the secondary coil goes up by 10 times, the current through the secondary coil goes down by 10 times.

Working through an account using a numerical example

Thinking about the teaching

The basic assumption in thinking about the working of a transformer is that there must be a power balance across it. The input power and output power must be equal. In other words, it is impossible to get more energy out than is put in. With this in mind, let's consider the following transformer set up: input turnsoutput turns = 110

The potential difference across the primary coil is 12 volt.

The current in the primary coil is 2 ampere.

Teacher: OK! So what's the power input to the input coil of the transformer?

Al: Is it 12 times 2, that's 24 watt?

Teacher: Exactly: 12 volt  ×  2 ampere! Well done! And what does 24 watt tell us?

Susan: 24 joule inverse second is delivered to the input side of the transformer.

Teacher: Excellent! Now, there are 10 times the number of coils on the output coil as compared to the input coil, so what must be the potential difference across the output?

Greg: 120 volt.

Teacher: Good! Now, if we assume that the transformer is 100 % efficient, what does that mean?

Shapi: That the power out is the same as the power in.

Teacher: OK. If we assume that there is 100 % efficiency, who can tell me what the current in the output coil must be? The power output must be 24 joule inverse second, the potential difference is 120 V, so what's the current?

Gail: 0.2 ampere?

Teacher: Yes, exactly right! 0.2 ampere. The potential difference goes up by 10 times and the current goes down by ten times to keep the balance of power across the transformer. The same number of joule second^-1 come out as go in.

Generators do not make charge!

Wrong Track: Electric generators are the opposite of electric motors. Instead of changing electricity into movement, they change movement into electricity.

Right Lines: A common form of generator switches power from a mechanical to an electrical pathway.

Getting charge moving by working

Thinking about the learning

The wrong track thinking set out here takes us back to the world of forms of energy, where the working of a generator is described in terms of movement (or kinetic) energy being changed into electrical energy. Let's think about a simple hand-turned generator, running at a steady rate and connected to an electric bulb, in terms of energy being shifted along pathways.

The physical description is simple:

Teacher: I turn the handle and the bulb lights.

The energy description:

Teacher: Energy is shifted from the chemical store associated with my body to the thermal stores of the surroundings.

The power description:

Teacher: Power is switched from the mechanical working pathway to the electrical working pathway.

Seen in these terms it's clear that the generator is a very useful device because of the way in which it switches the power from one pathway to another (from mechanical working to electrical working).

Electricity is not made by generators

Wrong Track: Electric generators make electricity.

Right Lines: Electrical generators make energy available using an electrical pathway. Electrical generators switch power to the electrical pathway.

The charge is already in the loop: the generator sets them in motion

Thinking about the learning

The idea that electric generators make electricity is quite a common one and portrays similar thinking to the idea that electric cells provide electricity. The image here is one of the generator or the cell acting as a store of something called electricity. In fact, the cell or generator provides a potential difference that sets the charge in motion, creating an electrical pathway.

There's a good chance you could improve your teaching if you were to:

Try these

• developing physical descriptions before accounts that use energy and power
• using apparatus, words and diagrams together to develop accounts
• emphasising the practical consequences of calculations with power and energy
• putting new theoretical ideas to work to imbue them with meaning

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• replacing the physical arguments with mnemonics
• arguing about power and energy without quantifying
• conflating different steps of multistep arguments
• not making links between different devices, all of which switch power

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.