# OCR AS/A Level Physics A

# Electricity

Navigate to resources by choosing units within one of the unit groups shown below.

## Introduction

### Overview

Delivery guides are designed to represent a body of knowledge about teaching a particular topic and contain:

- Content: a clear outline of the content covered by the delivery guide
- Thinking Conceptually: expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject
- Thinking Contextually: a range of suggested teaching activities using a variety of themes so that different activities can be selected that best suit particular classes, learning styles or teaching approaches.

## Curriculum Content

### Overview

**4.1 Charge and current**

This short section introduces the ideas of charge and current. Understanding electric current is essential when dealing with electrical circuits. This section does not lend itself to practical work but rather to introducing important ideas. The continuity equation (I = *Anev*) is developed using these key ideas. This section concludes with categorising all materials in terms of their ability to conduct.

**4.1.1 Charge**

Students should be able to demonstrate and apply their knowledge and understanding of:

(a) electric current as rate of flow of charge; \(\displaystyle I=\frac{\Delta Q}{\Delta t}\)

(b) the coulomb as the unit of charge

(c) the elementary charge \(\displaystyle e\) equals 1.6 × 10^{–19}C

(d) net charge on a particle or an object is quantised and a multiple of \(\displaystyle e\)

(e) current as the movement of electrons in metals and movement of ions in electrolytes

(f ) conventional current and electron flow

(g) Kirchhoff’s first law; conservation of charge.

**4.1.2 Mean drift velocity**

(a) mean drift velocity of charge carriers

(b) I = *Anev*, where *n* is the number density of charge carriers

(c) distinction between conductors, semiconductors and insulators in terms of *n*.

**4.2 Energy, power and resistance**

This section provides knowledge and understanding of electrical symbols, electromotive force, potential difference, resistivity and power. The scientific vocabulary developed here is a prerequisite for understanding electrical circuits in **4.3**.

Electrical billing is done in kWh. This energy unit is easy to understand. There is a desire to use energy-saving devices, such as LED lamps, in homes. Students have the opportunity to understand the link between environmental damage from power stations and the impetus to use energy-saving devices in the home (HSW10) and how customers can make informed decisions when buying domestic appliances (HSW12).

There are many opportunities for students to use spreadsheets in the analysis and presentation of data (HSW3), to carry out practical activities to understand concepts (HSW4) and to analyse data to find relationships between physical quantities (HSW5).

**4.2.1 Circuit symbols**

(a) circuit symbols

(b) circuit diagrams using these symbols.

**4.2.2 E.m.f. and p.d.**

(a) potential difference (p.d.); the unit *volt*

(b) electromotive force (e.m.f.) of a source such as a cell or a power supply

(c) distinction between e.m.f. and p.d. in terms of energy transfer

(d) energy transfer; \(\displaystyle W=VQ; W=\mathcal{E} Q\)

(e) energy transfer \(\displaystyle e\)*V* = ½ *mv ^{2}* for electrons and other charged particles.

**4.2.3 Resistance**

(a) resistance; \(\displaystyle R = \frac{V}{I}\); the unit ohm

(b) Ohm’s law

(c) (i) *I–V* characteristics of resistor, filament lamp, thermistor, diode and light-emitting diode (LED)

(ii) techniques and procedures used to investigate the electrical characteristics of a range of ohmic and non-ohmic components

(d) light-dependent resistor (LDR); variation of resistance with light intensity.

**4.2.4 Resistivity**

(a) (i) resistivity of a material; the equation \(\displaystyle R = \frac{\rho L}{A}\)

(ii) techniques and procedures used to determine the resistivity of a metal.

(b) the variation of resistivity of metals and semiconductors with temperature

(c) negative temperature coefficient (NTC) thermistor; variation of resistance with temperature.

**4.2.5 Power**

(a) the equations \(\displaystyle P = VI\text{, } P=I^2R\text{ and } P = \frac{V^2}{R}\)

(b) energy transfer; \(\displaystyle W=VIt\)

(c) the kilowatt-hour (kWh) as a unit of energy; calculating the cost of energy.

**4.3 Electrical circuits**

This section provides knowledge and understanding of electrical circuits, internal resistance and potential dividers. LDRs and thermistors are used to show how changes in light intensity and temperature respectively can be monitored using potential dividers.

Setting up electrical circuits, including potential divider circuits, provides an ideal way of enhancing experimental skills, understanding electrical concepts and managing risks when using power supplies (HSW4). Students are encouraged to communicate scientific ideas using appropriate terminology (HSW8). This section provides ample opportunities for students to design circuits and carry out appropriate testing for faults and there are opportunities to study the many applications of electrical circuits (HSW1, 2, 3, 5, 6, 9, 12).

**4.3.1 Series and parallel circuits**

(a) Kirchhoff’s second law; the conservation of energy

(b) Kirchhoff’s first and second laws applied to electrical circuits

(c) total resistance of two or more resistors in series; \(\displaystyle R=R_1 + R_2 +.\)

(e) analysis of circuits with components, including both series and parallel \(\displaystyle \frac{1}{R} = \frac{1}{R_1}+\frac{1}{R_2} +\cdots\)

(f) analysis of circuits with more than one source of e.m.f.

**4.3.2 Internal resistance**

(a) source of e.m.f.; internal resistance

(b) terminal p.d.; ‘lost volts’

(c) (i) the equations \(\displaystyle \mathcal{E} =I(R+r) \text{ and } \mathcal{E}=V+Ir\)

(ii) techniques and procedures used to determine the internal resistance of a chemical cell or other source of e.m.f.

**4.3.3 Potential dividers**

(a) potential divider circuit with components

(b) potential divider circuits with variable components e.g. LDR and thermistor

(c) (i) potential divider equations e.g. \(\displaystyle V_{out} = \frac{R_2}{R_1+R_2} \times V_{in}\) and \(\displaystyle \frac{V_1}{V_2}=\frac{R_1}{R_2}\)

(ii) techniques and procedures used to investigate potential divider circuits which may include a sensor such as a thermistor or an LDR.

### Current as the flow of charge

The students need to appreciate current as a flow of charged particles.

The experiment described on the Institute of Physics website is a convenient starting point (see link 'Current as a flow of charge').

The Georgia State University 'Hyper Physics' website provides a suitable summary.

The 'Physics Classroom' website provides a bit more detail.

'Physics Net' has some nice worked examples to lead students into using the charge, current and time formula with the charge on the electron.

### Kirchhoff’s laws

It is necessary here to develop the students’ understanding from GCSE as they will all have encountered this in one form or another.

The 'About Education' and 'S-cool' websites provide simple summaries.

Learner Resource 1 provides a summary of Kirchhoff’s first law and some sample questions. It is important to expose the students to the many different forms of electric circuits as many have had limited prior exposure.

The 'All About Circuits' website has some interesting questions on Kirchhoff’s second law.

###
*I* = *Anev*

Having looked at electron and ion flow, the drift velocity equation has some conceptual bedrock on which to stand.

P.F. Nicholls’ School Science and Technology has this useful resource which leads through current to the description of the drift velocity equation with some sample calculations and answers.

The resource in the 'Physics Classroom' also gives students the opportunity to think what changes in resistance or potential difference mean in terms of variables in the equation.

Learner Resource 2 supplies some notes and a few questions.

The 'School Science' website also gives students a chance to check their understanding.

### Ohm’s law

The link between current, potential difference and resistance should be intuitive for students if they have fully assimilated their model of electricity. The University of Colorado PhET site provides some interactive practice.

This resource on the 'NASA' website also provides some explanation and questions.

The 'Quizlet' app is readily available on all devices and computers. This resource provides flashcards and a test to reinforce the material.

An experiment available on the 'Nuffield Foundation' website could also be used to reinforce this.

### Combining resistances

The formulae for these can be seen to follow on from the work on Kirchhoff’s laws, and deriving them can help reduce issues caused by parallel combinations.

The 'Electronics Tutorials' website has a nice introduction to this topic.

An animation on the 'Walter Fendt' site also illustrates series and parallel combinations.

A presentation on 'SlideShare' also covers the series and parallel combinations as well as recapping the concepts covered earlier.

The Institute of Physics has an experiment on 'Combining resistors' to help reinforce this concept.

### Potential dividers

This brings together the work on Ohm’s law and Kirchhoff’s laws and leads into circuit theory.

The resource on 'Physics Net' provides a reasonable summary.

The 'Furry Elephant' site has a nice animation.

The 'S-cool' site has another summary of the theory with some sample calculations.

The 'Institute of Physics' has a practical approach to embedding this understanding.

There is a potential calculator available on Georgia State University’s 'Hyper Physics' site.

## Thinking Conceptually

### Overview

**Approaches to teaching the content**

This theme builds on the students’ knowledge and understanding of electricity developed from previous learning. Students should be given opportunities to integrate a model of electricity into their understanding in order to aid their explanation of what is happening in various processes. Experimental work is important here and the ultimate goal would be the understanding of how the potential divider is used in sensor circuits. The analysis of data and use of nonlinear graphs give opportunities to reinforce and develop graph plotting skills. The discussion of how the resistance of materials depends on many different factors and the difficulty this creates in selecting materials to be used in varying environments allows links to be made to the materials theme.

**Common misconceptions or difficulties students may have**

Although earlier work will have covered the use of prefixes and standard notation, many students continue to find conversions difficult – for example, from mm^{2} to m^{2} and mm^{3} to m^{3} . As in the materials theme it is useful practice to use a micrometer to measure diameter in mm and thus calculate cross-sectional area in m^{2}; thus the resistivity values calculated can be compared with book values. The use of multimeters allows for a discussion on the properties of the different meters and a deeper understanding of the concepts of current and potential difference.

Whilst the terminology is mostly familiar to the students, many will come with an incomplete conceptual model of the current, resistance and potential difference. In particular, potential difference is often a sticking point and continued emphasis on the definition of work done per unit charge can eventually yield results. Series and parallel circuits will be familiar to students but they may need practice in implementing Kirchhoff’s laws to relatively simple circuits. Most students will use the equation *V* = * IR* with relative confidence but its application to e.m.f., internal resistance and potential dividers often catches them out. Often students find it easier to work from the current flowing in the circuit to then calculate the p.d. between points or across components. The memorisation of the potential divider formula can often be detrimental to their deeper understanding of what is happening in the circuit and can leave the students struggling if there are more than two components in the circuit.

**Conceptual links to other areas of the specification**

The main thing to consider here is that this theme sets the foundations for much of the work which will be done on electric fields. Thus the overarching importance of building a strong conceptual understanding. The practical work allows for the reinforcement and further development of students’ understanding of the limitations of results and data. The variety of data means that there are opportunities for using negative gradients and the *y*-intercept for internal resistance and e.m.f. Also students could be introduced to logarithmic charts to represent the range of resistivities of materials. This use of logarithms could be further enhanced by using the data from a sensor on a logarithmic scale. The work on electricity in specific electron flow is also significant in the understanding of quantum processes.

### Model of electricity

The topic usually starts by describing electricity in terms of a model or an analogy. This may involve a water model as illustrated on the Georgia State University’s 'Hyper Physics' website.

There is similar material available on the 'Furry Elephant' website.

More details of possible models are available from the 'Nuffield Foundation'.

It is particularly important for the students to have some model or analogy that they can use for potential difference as this is the concept that causes the most significant difficulties. It is worth getting students to describe what is happening in simple circuits in terms of their model.

### Flow of ions

The appreciation that electricity is not just the flow of electrons is an important one, both in terms of conceptual understanding and in terms of linking together students’ understanding of the physical world with processes that are covered in Chemistry and Biology.

The 'Nuffield Foundation' and 'Institute of Physics' have two interesting experiments that could be demonstrated.

The 'Mindset Network' has an animation which shows how the flow of ions occurs in electrolysis.

###
*I-V* characteristics

The measurement of dependence of current on applied potential difference for a variety of components provides a useful practical link to the earlier conceptual work. The 'Nuffield Foundation' website describes how this would be carried out for a lamp here.

The 'Physics Net' website provides some details on the diode, lamp and resistor.

### Factors affecting the resistance of a wire

This is an area where the model of electricity developed by the students earlier can be used to build the familiar formula involving resistivity, resistance, cross-sectional area and length.

The 'Cyber Physics' website talks through the factors that affect the resistance of a wire.

The 'Nuffield Foundation' website provides a suitable practical method, although there are many others available.

Using conducting putty or conductive paper are alternatives to the usual wire experiment and these are detailed in this Institute of Physics resource (see link 'Resistivity document').

### Measuring the internal resistance of a cell

### Electric power

The link between mechanical power and electrical power can be usefully explored using experiments such as two found on the Nuffield Foundation website (see links 'Using an electric motor to raise a load' and 'Measuring the power of a motor'). Both these experiments use electric motors.

Alternatively the power output of a light bulb could be investigated using an experiment outlined by the 'Argonne National Laboratory' on their website.

There are a number of different takes on the bulb efficiency experiment but generally they measure the heat given out by the bulb. So it can be viewed as an extension exercise using the heat capacity formula.

### Potential dividers

The application of Ohm’s law and Kirchhoff’s first law to calculating the potential difference between two points really helps develop the students’ ability to apply circuit theory. A useful summary is provided on the 'School Physics' website.

The potentiometer is another potential divider and a page from the 'All About Circuits' website describes a possible experiment.

As extension work for the most able, the 'Wheatstone Bridge' provides a good challenge. This page from the Electronics Tutorials website explains how it works.

Alternatively, this 'YouTube video' on the Wheatstone Bridge also explains how it woks.

### Thermistors

The variation of resistance with the temperature of a thermistor is a useful concept to explore in terms of cementing students’ understanding of the drift velocity equation. A simple summary of the properties can be found on the 'School Physics' site.

It is also possible to observe the change in an experiment from the 'Nuffield Foundation' website.

The 'Maths Physics' site has a flash application that allows the experiment to be carried out virtually. It is also possible to combine this experiment with work on potential dividers.

## Thinking Contextually

### Overview

### Effect of temperature on resistance

In real life, wires warm up as the electricity passes through them. An experiment from the Nuffield Foundation allows students to measure the temperature coefficient of a wire (see link 'Temperature change and resistance').

Georgia State University has an explanation of the theory on its 'Hyper Physics' website.

There is a similar explanation on the 'All About Circuits' website, along with the opportunity of applying some of the knowledge.

If you have access to liquid nitrogen, it is also possible to buy a cheap superconductor and demonstrate superconductivity, or you can try and make one by following the instructions on the Futurescience website (see link 'Making high-temperature superconductors').

### Resistance and resistivity

Resistance is used to measure our percentage of body fat in some sets of weighing scales. A resource from the 'All About Circuits' website talks about body fat measurements before discussing the risks of electric shocks.

Resistance can also be used by archaeologists to look for features hidden underground. They carry out the survey before working out where to place trenches. The features show up because the remains have a different resistance from the soil. An experiment from the Nuffield Foundation website models a resistive survey (see link 'Modelling a resistive survey')..

### Internal resistance

Students may well be familiar with the idea that fruit can be used to make wet cells in conjunction with the correct metal electrodes.

These experiments detailed by the Nuffield Foundation measure the internal resistance of homemade cells (see links 'Internal resistance of a potato cell' and 'Internal resistance of a shoe box cell').

A resource from the Institute of Physics looks at measuring the internal resistance of a number of common power supplies (see link 'Internal resistance of power supplies')

### Measuring intensity using an LDR

When using a light dependent resistor as a light sensor it is necessary to calibrate the circuit. The experiment detailed in Learner Resource 4 allows students to use the flux equation to plot a suitable graph to calibrate their LDRs. The University of Reading has created a virtual version of a similar experiment (see link 'Virtual experiment').

Peter Vis has created a site with an interactive potential divider incorporating an LDR (see link 'Potential Divider Circuit with LDR').

### Measuring the thickness of paper

It is useful for students to see practical applications of the work they have studied on e.m.f. and understand how a calibration graph can be used to determine the optical transmission properties of an unknown piece of paper. There are different methods of carrying out experiments like this; the one described in Learner Resource 5 uses a solar cell, although an LDR could be used instead.

The 'Science Buddies' website details an extension project to look at how the output of a solar cell varies with light intensity.

### Sensor circuits

Many modern applications of electricity revolve around how the technology responds to external stimuli. From touch-sensitive screens to the accelerometer that is in so many modern phones, how sensors are used to create feedback circuits is an interesting application of potential divider circuits.

A resource from the Institute of Physics lists a series of potential divider experiments (see link 'Potential dividers document'). The final one looks at putting a sensor in part of the potential divider. This could be extended by looking at how the output could be fed into simple digital circuits.

This 'water sensor experiment' from the University of California, San Diego would make an interesting extension project.

The All About Circuits site gives details on how to make a 'static electricity sensor'.

### Electric corn starch

The non-Newtonian fluid properties of corn starch are well known. This is an interesting extension to this and shows the effect a charge balloon can have on the fluid. A version of the experiment is detailed on Steve Spangler’s website (see link 'Steve Spangler science').

The polarity of water molecules can easily be demonstrated with a stream of water and the applications to Chemistry and Biology can be discussed. Georgia State University has some information on its 'Hyper Physics' website.

The orientation of liquid crystals is also controlled using electric charges. A worksheet from Nanoyou on 'Slideshare' could be an interesting extension activity.

### Determining the resistivity of a metal

### Investigating Electrical Characteristics

### Determining the maximum power available from a cell

### Investigating Resistors and their use in Potential Divider Circuits

### Investigating circuits with more than one source of e.m.f.

### Using non-ohmic devices as sensors

## Acknowledgements

### Overview

OCR’s resources are provided to support the teaching of OCR specifications, but in no way constitute an endorsed teaching method that is required by the Board and the decision to use them lies with the individual teacher. Whilst every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions within these resources. We update our resources on a regular basis, so please check the OCR website to ensure you have the most up to date version.

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