# GCSE OCR GCSE (9-1) Combined Science A (Gateway Science)

# P3.2 Simple circuits

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 which best suit particular classes, learning styles or teaching approaches.

## Curriculum Content

### Overview

**P3.2 Simple circuits **

Mathematical learning outcomes:

PM3.2i recall and apply: potential difference (V) = current (A) x resistance (Ω)

PM3.2ii recall and apply: energy transferred (J) = charge (C) x potential difference (V)

PM3.2iii recall and apply: power (W) = potential difference (V) x current (A) = (current (A))2 x resistance (Ω)

PM3.2iv recall and apply: energy transferred (J, kWh) = power (W, kW) x time (s, h)

Assessable content statements:

P3.2a describe the differences between series and parallel circuits

P3.2b represent d.c. circuits with the conventions of positive and negative terminals, and the symbols that represent common circuit elements

P3.2c recall that current (I) depends on both resistance (R) and potential difference (V) and the units in which these are measured

P3.2d recall and apply the relationship between I, R and V

P3.2e explain that for some resistors the value of R remains constant but that in others it can change as the current changes

P3.2f explain the design and use of circuits to explore such effects

P3.2g use graphs to explore whether circuit elements are linear or nonlinear (M4c, M4d)

P3.2h use graphs and relate the curves produced to the function and properties of circuit elements (M4c, M4d)

P3.2i explain, why, if two resistors are in series the net resistance is increased, whereas with two in parallel the net resistance is decreased (qualitative explanation only)

P3.2j calculate the currents, potential differences and resistances in d.c. series and parallel circuits

P3.2k explain the design and use of such circuits for measurement and testing purposes

P3.2l explain how the power transfer in any circuit device is related to the potential difference across it and the current, and to the energy changes over a given time

P3.2 m apply the equations relating potential difference, current, quantity of charge, resistance, power, energy, and time, and solve problems for circuits which include resistors in series, using the concept of equivalent resistance (M1c, M3b, M3c, M3d)

### Revision activity

A general revision tool in a card sort activity can be found on the My Physics website.

This is an interactive task for learners to check their knowledge of terms and definitions, comprising of 20 questions.

### Answering longer answer questions

### Testing understanding of circuits

### Temperature change and resistance

The Resource link directs learners how to investigate the changing resistance of a wire as it heats up. As well as learner instructions, there are also teaching notes and health and safety procedures.

This practical is an excellent way to graph results and the same procedure can be used to test the linearity of different circuit elements, such as filament lamps, semi-conductor diodes, thermistors and LDRs.

### Resistors in circuits

## Thinking Conceptually

### Overview

**Approaches to teaching the content**

**Electric current**

It would be a good idea to revisit Key Stage 3 content in order to highlight misconceptions or gaps in knowledge and understanding.

Areas to revisit include:

- Definitions of key terms such as current, p.d, charge, static electricity, parallel circuit, series circuit, resistance (maybe some higher ability learners have covered this), conductor and insulator.
- Conditions needed for current to flow.
- The names and symbols of common components such as diodes, LDRs, thermistors, filament bulbs, ammeter, voltmeter and resistor.
- Ability to draw series and parallel circuits, using component symbols as above.

How to measure current and p.d. in a series and parallel circuit.

**Potential Difference and Resistance**

Thus far the term voltage has been used in order to establish confidence in Key Stage 3 knowledge. In order to move on to understand the various behaviours of components within a circuit, the term p.d. (potential difference) should be used. More able learners may feel comfortable using the term p.d. from the outset of the GCSE course. The p.d. is defined as the difference in energy transferred between any two points in a circuit, per coulomb of charge that passes between those points.

When a current flows through a piece of wire, for example, the free moving electrons collide with the ions on the metal. This collision causes resistance. In order to understand this, learners will need to have knowledge of metallic structures from Chemistry GCSE.

Learners should understand the equation that is based on Ohm’s Law, which states that p.d. is directly proportional to current:

potential difference (V)= current (A) x resistance (Ω)

The relationship between the current and the p.d. is a constant, as long as the temperature stays the same. This relationship can be illustrated through undertaking practical work based on Ohmic components such as wires.

Spark Fun is a great resource to understanding the basic terms of resistance and resistors.

**Linear & Non-Linear circuit elements**

Resistors are examples of circuit components that are linear and therefore referred to an ohmic. A component that is non-linear does not have a linear relationship between current and p.d. i.e. as p.d. doubles, the current does not.

Examples of components to use are:

- Resistors
- Semiconductor diodes
- Thermistors
- LDRs
- Filament lamps
- Wires

In the series circuit above:

I_{1} = I_{2} = I_{3}

The current is the same anywhere in the circuit.

Total voltage = V_{1} + V_{2} + V*3*

The p.d. is different in different parts of the circuit, yet adds up to the total p.d. input.

These concepts are not new (taught at Key Stage 3 and revisited at the start of the topic).

Total resistance = R_{1} + R_{2} + R_{3}

In the parallel circuit above:

Total current = I_{1} + I_{2} + I_{3}

V_{1} = V_{2} = V_{3}

1/ total resistance = \(\displaystyle \frac {1}{R_1}+\frac {1}{R_2}+\frac {1}{R_3}\)

Ideas surrounding energy transfers can be explored when introducing this part of the topic. This topic requires learners to move on from Key Stage 3 where they have looked at types of energy transfer, to now calculating the amount of energy transferred using the following equation:

energy transferred (J) = charge (C) x potential difference (V)

So 1 volt = 1 joule of energy per coulomb of charge

Learners can also use: energy transferred (J) = current (A) x time (s) x potential difference (V)

**Electrical Power**

The word ‘power’ may hold very different meanings for different learners, so it is important to address these and link them to the scientific meaning as much as possible.

Here we start to link different formulae together and see the connection between energy, power, p.d. and current.

So while:

energy transferred (J) = current (A) x time (s) x potential difference (V)

and

power (W) = potential difference (V) x current (A)

These can be combined to say that:

energy transferred (J) = power (W) x time (s)

For resistors we can combine

power (W) = potential difference (V) x current (A)

potential difference (V) = current (A) x resistance (Ω)

to give: power (W) = (current (A))2 x resistance (Ω)

**Common misconceptions or difficulties learners may have**

**Electric current**

The very nature of current, charge and electricity means we are unable to see it, we can only experience the effects of it. Learners therefore find electricity difficult to conceptualize. Models or analogies are good techniques to use to help learners visualize and understand elements of this topic.

The following links from the Nuffield Foundation called Models of Electric Circuits details a range of possible misconceptions, and includes models that teachers can use to help learners reconceptualise their ideas:

**Linear & Non-Linear circuit elements**

Learners can become confused when we start to discuss circuit components that do not obey Ohm’s law. For many materials the resistance is constant independent of current and p.d. These materials are ohmic. Materials which are non-ohmic can be more challenging to understand. Practical work and graphical representation should help learners understand that resistance can change.

Explaining the shape of the I-V curve for a filament bulb can be challenging. An increase in p.d. causes the temperature of the filament to rise and as a result ions begin to vibrate more. This leads to a higher frequency of collisions between electrons and ions, thus the resistance increases.

When discussing how temperature affects resistance learners may be confused when a bulb and a thermistor are compared. In a thermistor, resistance decreases as its temperature increases. The relationship between resistance and temperature is not the same for all circuit components.

**Resistors in Series and Parallel**

The differences in current and p.d. in a series and parallel circuit can be difficult to understand. In a series circuit, the current and resistance can be explained using the same idea: there is one route for the current to flow so the current going into the circuit from the cell is the same as the current returning to the cell. Therefore the current flowing through each resistor must be the same, and so the calculation R = V/I can be used quite easily. Learners find it a challenge to apply the rules they know for the behaviour of current and p.d. in parallel circuits to use to calculate resistance of components within these.

**Energy Transfers in a Circuit & Electrical Power**

This area of the topic introduces more units of measurement which are easy to confuse. An example would be the use of both J and kWh as units of energy, many learners posit kWh as a unit of power.

### Electric current

The institute of Physics have provided a vast amount of resources and information on their website.

As well as activities and diagrams, each link includes tips on how you can teach the topic to improve learner understanding, as well what difficulties learners may come across.

### Resistors in series and parallel

### Calculation revision

## Thinking Contextually

### Overview

**Approaches to teaching the content**

**Electric current**

Electricity is all around us. Asking learners in the lesson to spend a typical day without electricity makes them aware of how much we rely on electricity, and the possible implications of that. You may wish to have a small discussion about the fuels needed to supply this much electrical energy, and the consequences of that need.

**Resistance and p.d.**

Fuses are examples of how resistance is used for safety. Learners can be given a fuse and asked to look at how thin the piece of wire is. The idea of a fuse saving lives can be used as the big idea of the lesson. Investigations which demonstrate how wire thickness affects resistance will help learners to understand why fuses are designed as they are, and how they break circuits.

**Energy transfers in a circuit and electrical power**

When starting to apply the mathematical element of this topic it is important to relate the information to what learners know about electrical appliances. Ask them to find out the power of some electrical appliances they have at home. What does the word ‘power’ mean in these instances?

In the context of the home, the same equations are used as above, but with different units. This can be confusing for learners.

The following equation is used to calculate a cost of energy, note the different units.

Energy transferred (kWh) = power (kW) x time (h)

The amount of energy transferred can then be multiplied by a unit of cost, e.g. 7p per kWh and the total cost calculated.

This calculation can be applied to the cost of electricity in the home, oil prices and charges set by energy companies can be used to discuss the future of our energy supplies.

### Measuring current and p.d. in different circuits

### Resistance effects

### Creating curious circuits

The link from The Institute of Engineering and Technology requires a free registration, and directs you to an extension activity, with an overview and three worksheets for learners to follow.

The activity involves making conductive dough to explore different types of circuits. Learners build creative circuit models and then measure p.d. and current to calculate resistance.

### Energy transfers in a circuit and electrical power

### Energy

### IV characteristics

### Series and parallel circuits

## 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.

© OCR 2016 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work.