CHAPTER 17 Current Electricity

CHAPTER 17 Current Electricity
© 2013 Marshall Cavendish International (Singapore) Private Limited

Chapter 17 Current Electricity
17.1 Electric Current 17.2 Electromotive Force and Potential Difference 17.3 Resistance 17.4 Resistivity

17.1 Electric Current Learning Outcomes
At the end of this section, you should be able to: define the term current and state its SI unit; differentiate between conventional current and electron flow; apply the formula charge = current × time to solve problems; draw electric circuit diagrams. Optional (Activity) The next slide, Slide 4 (hidden), contains directives for a class demonstration that teachers may use to start the lesson. The activity requires teachers to prepare a simple electric circuit to show students. *To un-hide slides, go to ‘Normal View’ or ‘Slide Sorter’ and right click on the slide. Unselect ‘Hide Slide’.

17.1 Electric Current Class Demonstration Questions
What is the function of the battery in the circuit? Why does the bulb light up? Why does the bulb only light up when the switch is closed? What flows in the wires when the switch is closed? Materials needed: Simple electric circuit comprising a battery, connecting wires, bulb, resistor, switch Show the class the assembled circuit and switch it on. Ask students the questions on the slides. Encourage them to tackle the questions in pairs. Use question 4 to help students link the contents of this chapter to what has been learnt in the previous chapter. Answers 1. The battery provides energy to the circuit in the form of chemical potential energy. 2. The battery lights up because an electric current flows through it. The electric current (electrical energy) comes from the chemical potential energy of the battery. 3. Electric current is only able to flow in the circuit when the circuit is closed. 4. Electric charges (specifically, electrons)

Electric Current What is Electric Current?

So how do Electrons Flow?
Video on Lightning & Thunder So how do Electrons Flow?

Why does the lightning hit the tree?
So how does Lightning work? Why does the lightning hit the tree? Why is the rod good?

What conducts Electricity? How do you protect yourself?
Electric Shock! What conducts Electricity? How do you protect yourself?

Good Electrical Conductors
Things that have free electrons Metals (copper & zinc? What’s the best conductor? How about water? Good Insulators Things that do not allow electrons to flow rubber

17.1 Electric Current What is Electric Current?
An electric current is formed by moving charges. Electric current is a measure of the rate of flow of electrons through a conductor. where I = current; Q = charge; t = time taken. The SI unit of electric current is the ampere (A).

17.1 Electric Current Electron Flow
Electric current is actually caused by the flow of electrons from the negative terminal to the positive terminal. Explain to students that the idea of current as the movement of positive charges remains because the discovery that electric current is caused by the movement of electrons did not affect the basic understanding of electric current. Use the diagram on the right to illustrate that the movement of conventional current is opposite to the direction of electron flow. Electron flow - the movement of electrons from a negatively-charged end towards a positively-charged end. Conventional current – the movement of positive charges from a positively-charged end to a negatively-charged end. Emphasise this point to students. Tell students that unless otherwise stated, all references to ‘current’ in this chapter will refer to conventional current.

17.1 Electric Current How do We Measure Electric Current?
We make use of an ammeter to measure current. The ammeter should be connected in series to the circuit.

17.1 Electric Current Main Components of a Circuit
A typical electric circuit consists of four main components. A source of electromotive force that drives electric charges around the circuit. Dry cell Wires Switch Bulb Conductors that connect the components together. A load in which moving charges can do a useful job. Get students involved by asking them to match the functions listed on the left to the components listed on the right, before revealing the answers. A method of opening or closing the circuit.

17.1 Electric Current Drawing Circuit Diagrams
Electric circuits can be represented by circuit diagrams. Can you identify what the symbols in the circuit diagram below represent? ammeter cell bulb connecting wires switch

17.1 Electric Current Drawing Circuit Diagrams
Some common components and their symbols are listed in the tables below. The tables in this slide are a subset of Table 17.1 in the Textbook. Ask students to refer to the Textbook for the symbols of more components.

17.1 Electric Current Interpreting Circuit Diagrams Open circuit
Explain to students that the break in the circuit shown is caused by the open switch. Highlight to them that breaks in the circuit can also occur due to loose connections, and missing or broken wires. Point out that in an open circuit, no electric current flows through the circuit. Hence, the ammeter shows no reading. Tell students that when the switch in the circuit shown is closed, the circuit is without breaks and hence is called a closed circuit. Open circuit A circuit in which current is unable to flow due to breaks in the circuit.

17.1 Electric Current Interpreting Circuit Diagrams Short circuit
Ask students the following questions in groups/pairs before revealing what type of circuit is shown on this slide. Predict whether the light bulb will light up. (Answer: No. The electric current flows through the path of least resistance (through wire X).) Will there be a reading on the ammeter? (Answer: Yes, the circuit is a closed circuit. Electric current will flow through a closed circuit.) 2. Teachers may want to provide a few sets of apparatus for students to check their predictions, before discussing the answers. 3. If students ask why the current chooses to flow through the path with lower resistance, the teacher can give an analogy of two parallel streets that lead to the same destination. One street is under construction and has heavy vehicles parked along it, while the other is unoccupied. Naturally, you would choose to walk through the street that is unoccupied as it would allow you to get to your destination with the least amount of effort. Tell students that the concept of resistance will be covered later in this chapter. Short circuit An alternative path of lower resistance is present and hence, current flows through wire X instead of the bulb.

17.2 Electromotive Force and Potential Difference
Learning Outcomes At the end of this section, you should be able to: define electromotive force (e.m.f.) and potential difference (p.d); state the SI unit of e.m.f. and p.d.; calculate the e.m.f. when a few sources are arranged in series.

Recall We saw this circuit in Section 17.1. What is the role of the battery in the circuit? Spend two to three minutes on the questions on this slide to get responses from the students. (Possible answer: The battery is the source of electrons. Students may have the misconception that the electrons are formed by the battery or stored in the battery. This is incorrect. Tell students that a whole column of electrons moves in the wires. Use the analogy of the water pump and water to explain (on next slide). The pump does not produce the water but it pushes the water around the pipe.) Why do you need a battery to make the bulb light up?

What is Electromotive Force? A battery functions like a water pump. A water pump does work (providing energy) to drive the water around the pipe. Likewise, a battery does work to drive electrons around the circuit. Tell students that a simple electric circuit consisting of just wires and a battery is analogous to a closed water pipe with a pump. Ask them to try to explain why. Explain to students that: the water is analogous to the electrons; the water pump is analogous to the battery; the pipes are analogous to the wires. Highlight to students that without the water pump, the water in the pipes will not circulate/flow around the pipe. The pump provides the energy to move the water. In the same way, without a battery/cell, the electrons in a wire will not flow. The battery/cell provides the energy to move the electrons.

What is Electromotive Force? The electromotive force (e.m.f) of an electrical energy source is defined as the work done by a source in driving a unit charge around a complete circuit. where ε = e.m.f. of electrical energy source; W = work done (amount of non-electrical energy converted to electrical energy); Q = amount of charge. SI unit of e.m.f is the joule per coulomb (J C–1) or volt (V).

How do We Measure E.m.f.? We use a voltmeter to measure e.m.f. The positive and negative terminals of a voltmeter should be connected to the positive and negative terminals respectively of the electrical source. Optional (Activity) The next slide, Slide 19 (hidden), contains directives for a group activity. Teachers may use this activity to encourage students to discover how the arrangement of dry cells in a circuit affects the resultant e.m.f. in an electric circuit. Students also learn experimental skills such as verification by comparison, and the need to repeat experiments if results differ. For this activity, teachers need to prepare for each group the following materials: a voltmeter, two 1.5 V dry cells, connecting wires *To un-hide slides, go to ‘Normal View’ or ‘Slide Sorter’ and right click on the slide. Unselect ‘Hide Slide’.

Activity (Group) Objective Measure and observe the e.m.f. of different arrangements of dry cells. Instructions In groups, set up the circuits shown below. − − + − − − Materials needed (per group): A voltmeter, two 1.5 V dry cells, connecting wires This activity will test students’ ability to translate circuit diagrams to actual electric circuits. This helps students make a mental link between circuit diagrams and actual set-ups. Once students have obtained the voltmeter readings for all three circuits, ask them to compare their readings with those obtained by other groups. If there are differences in the readings, ask students to repeat the experiment and obtain a second set of readings. Students should observe that when the cells are arranged in series, the resultant e.m.f. is the sum of the individual cells’ e.m.f.; when the cells are arranged in parallel, the resultant e.m.f. is equal to that of a single cell. Check students’ understanding by asking them to predict what will happen if more cells are added to the circuits. Record the voltmeter readings.

Arrangement of Cells Series arrangement When cells are arranged in series, the resultant e.m.f. is the sum of all the e.m.f.s of the cells. Parallel arrangement Tell students that the resultant e.m.f. of the dry cells in the series arrangement is (1.5 V V) = 3.0 V, while the resultant e.m.f. of the dry cells in the parallel arrangement is 1.5 V. When cells are arranged in parallel, the resultant e.m.f. is equal to that of a single cell.

What is Potential Difference? The potential difference (p.d.) across a component in an electric circuit is the work done to drive a unit charge through the component. where V = p.d. across a component; W = work done (amount of electrical energy converted to other forms); Q = amount of charge. Teachers may wish to share with students an alternative definition of potential difference: The potential difference (p.d.) between two points in an electric circuit is defined as the amount of electrical energy converted to other forms of energy when one unit of charge passes between the two points. The SI unit of potential difference is the volt (V).

How do We Measure P.d.? We make use of a voltmeter to measure p.d. The voltmeter should be connected in parallel with the component. Highlight to students that while the ammeter (instrument used to measure current) is connected to the circuit in series, the voltmeter is connected in parallel to the component.

E.m.f. versus P.d. Electromotive force Potential difference Associated with an electrical energy source (e.g. a dry cell) Associated with two points in an electric circuit It is the work done by the source in driving a unit charge around a complete circuit. It is the work done to drive a unit charge through two points. Help students grasp the difference between e.m.f. and p.d. by highlighting to them that e.m.f. is associated with the source, while p.d. is associated with two points in the circuit.

Question What will the voltmeter reading show? A The e.m.f. of the electrical source (the three cells in series) B The e.m.f. of the bulb C The potential difference across the bulb Ask students to tackle these questions in pairs, before sharing the answer with the class. Answer A and C For the circuit shown, the voltmeter measures both the e.m.f. of the battery as well as the p.d. across the bulb; the e.m.f. of the battery and the p.d. across the bulb is the same in this case. As the bulb is not an electrical source, it does not have e.m.f.

17.3 Resistance Learning Outcomes
At the end of this section, you should be able to: define the term resistance; apply the formula resistance = to solve problems; describe an experiment to determine resistance; state Ohm’s Law; understand and draw the I−V characteristic graphs for ohmic and non-ohmic conductors; describe the relationship between the resistance of a metallic conductor and its temperature. p.d. current

17.3 Resistance Recall Earlier, we used the water pump analogy to help us understand e.m.f. obstacle Use the analogy of water flowing in a closed pipe to help students understand the concept of resistance. Get a few students to voice out their predictions before moving on to the next slide. (Answer: The flow of the water will slow down.) Question Predict what will happen if a porous plate (an obstacle) is placed in the path of the water flow.

17.3 Resistance What is Resistance?
resistor Rate of flow of electric charges reduced Ammeter reading will be reduced Resistor added Current is reduced Use the analogy to help students see how a resistor affects the flow of current in a circuit. Remind students of the definition of electric current: An electric current is the rate of flow of electric charge through a given cross-section of a conductor. Resistance is the difficulty for an electric current to pass through a material. It restricts the movement of free electrons in the material.

17.3 Resistance What is Resistance?
The resistance of a component is the ratio of the potential difference across the component to the current flowing through the component. where R = resistance of a component; V = p.d. across a component; I = current flowing through component. The SI unit of resistance is the ohm (Ω).

17.3 Resistance Activity (Group) Objective
Design an electric circuit that can be used to measure the resistance of a component. Instructions p.d. current Resistance = In groups, design a circuit that can be used to determine the resistance of a bulb. Using the materials given, check if your design works. Materials (per group): One 1.5 V cell, connecting wires, a voltmeter, an ammeter, a bulb If students are struggling with the design, teachers may want to give them the following tips: The voltmeter should be connected in parallel with the bulb. The ammeter should be connected in series with the bulb. The answer is revealed in the next slide.

Circuit for Measuring Resistance
Note that: The ammeter is connected in series with the bulb. The voltmeter is connected in parallel with the bulb. Highlight to students how the circuit can be drawn to look differently (e.g. the ammeter can be placed in various positions and still be in series with the bulb, the wire connections of the voltmeter can be at the circuit corners, etc). Optional (information on the resistance of ammeters and voltmeters) The next slide, Slide 32 (hidden), contains information on the resistance of ammeters and voltmeters. This can be used to help students understand how ammeters and voltmeters work.

17.3 Resistance In the circuit shown, R = 0 Ω R = ∞ Ω
Light bulb Wires Voltmeter Ammeter R = 0 Ω R = ∞ Ω All the components in the circuit actually have resistance. However, we often take the resistance of the wires, switches and ammeter to be negligible. That is, they are assumed to have zero resistance. The voltmeter is considered to have infinite resistance. That is, the resistance of the voltmeter is so high that no current flows through the voltmeter.

17.3 Resistance What are Resistors?
A resistor is a conductor in a circuit that is used to control the size of the current flowing in a circuit. There are two types of resistors — fixed resistors and variable resistors (or rheostats). Tell students that fixed resistors have fixed resistances, whereas variable resistors can be adjusted to give a range of resistances.

17.3 Resistance Ohm’s Law Ohm’s Law states that the current passing through a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions remain constant. An example of a physical condition that needs to remain constant is temperature. where I = current; V = potential difference.

Ohmic and Non-ohmic Conductors
17.3 Resistance Ohmic and Non-ohmic Conductors Ohmic conductors are conductors that obey Ohm’s Law. Non-ohmic conductors are conductors that do not obey Ohm’s Law. Ohmic conductors The I−V graph of an ohmic conductor is a straight line that passes through the origin.

17.3 Resistance Non-ohmic conductors
They do not obey Ohm’s Law and their resistance R can vary. Their I−V graphs are not straight lines, which means the ratio V/I is not a constant. Use Table 17.2 in the Textbook to further elaborate on non-ohmic conductors. The I−V graphs of a filament lamp (top) and a semiconductor diode (bottom) are shown on this slide. Tell students that as current increases, there is a tendency for the components to generate more heat, and hence result in an increase in the component’s temperature. Filament lamp – The resistance of a lamp’s filament increases as the temperature of the filament increases. (That is, resistance is not constant. It increases with increasing current.) Semiconductor diode – This device only allows current to flow in one direction. Using the second graph, show students that when p.d. is applied in the reverse direction, almost no current flows through this component.

17.4 Resistivity Learning Outcome
At the end of this section, you should be able to: apply the relationship of the proportionality of resistance to the length and cross-sectional area of a wire to solve problems.

17.4 Resistivity Recall Ohm’s Law states that the current passing through a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions remain constant. Other than temperature, what physical conditions affect the resistance of a component? Remind students what they have learnt in the previous section. Remind them about the non-ohmic conductors (filament lamp) whose resistances are affected by temperature. Ask students to consider the other physical conditions that may affect resistance.

17.4 Resistivity Other than temperature, the resistance R of a conductor also depends on its length l; its cross-sectional area A; the material it is made of (i.e. resistivity ρ). conductor A Tell students that increasing the length of the conductor increases its resistance, whereas increasing the cross-sectional area of the conductor decreases its resistance. l

17.4 Resistivity Resistivity
Rewriting , we can obtain the formula for resistivity. where ρ = resistivity of conductor; R = resistance of conductor; A = cross-sectional area of conductor; l = length of conductor. Tell students that resistivity is a property of the material used to make the conductor. The SI unit of resistivity is the ohm metre (Ω m).

17.4 Resistivity Resistivity
Different materials have different resistivities. Resistivity is a property of the material and it is independent of the dimensions of the material. The lower the resistivity of a material, the better it is at conducting electricity. Tell students that since copper has low resistivity, it has a low resistance. This means that current can flow through copper easily. Inform students that most of the wires used to conduct electricity are made of copper. Ask students to consider why silver is not used in place of copper since it has an even lower resistivity. (Answer: Silver is much more expensive as compared to copper.) Ask students to pair up and think about the potential uses of materials with high resistivity. Allow several students to share their answers with the class. (Answer: They are used in heating elements such as those found in electric kettles. Since resistance is high, more energy is dissipated as heat when current flows through these materials.)

17.4 Resistivity Worked Example
A length of resistance wire 50 cm long is connected in series with an ammeter and a 3 V battery. The ammeter reading is 0.15 A. Determine the resistance of the wire. The length of the wire is doubled and its cross-sectional area halved. Determine the new resistance and hence ammeter reading. Given that the diameter of the 50 cm long wire is 5 mm, determine its resistivity.

17.4 Resistivity Solution (b) R = V ÷ I = 3 ÷ 0.15 = 20 Ω (c)

Electric current I (SI unit: A) Resistance R (SI unit: Ω) related to defined as rate of flow of (continued on next slide) where t = time I = Q t Charge Q (SI unit: C) where related to Electromotive force ε (SI unit: V) Potential difference V (SI unit: V) where where Ask students to use the concept map (Map It) in their Textbooks to consolidate the points taught in this chapter. where W = work done by source to drive a unit charge around the circuit ε = W Q where W = work done to drive a unit charge through a component V = W Q

Electric current I (SI unit: A) related to Resistance R (SI unit: Ω) related to resistivity ρ (continued from previous slide) where where l = length A = cross-sectional area ρ = RA l where V = potential difference I = current R = V I if constant if not constant Click on the URL button to be directed to a website where a short revision video and interactive activity can be found. The video covers the following points: The difference between an open and closed circuit (shown by the flicking of a switch in a simple circuit). The difference between a series circuit arrangement and a parallel circuit arrangement. (This helps students understand how the ammeter and voltmeter should be connected.) The SI units of current, potential difference and resistance. Electric current is the movement of electrons in a conductor. The concept of resistance and how it is affected by the length and cross-sectional area of the conductor. The relationship between potential difference, current and resistance (Ohm’s Law). The activity consists of four simple questions to check students’ understanding. Get class participation in answering the questions. Revise concepts where necessary. Obeys Ohm’s Law I α V Does not obey Ohm’s Law Ohmic conductors Non-ohmic conductors URL

The URLs are valid as at 15 October 2012. Acknowledgements (slides 1−47) plasma globe © Stuartkey | Dreamstime.com (slides 6, 15) battery © Vladwitty | Dreamstime.com (slides 6, 15) bulb © Monsieurpix | Dreamstime.com (slide 7) ammeter © Arsty | Dreamstime.com (slide 18) voltmeter © Arsty | Dreamstime.com (slide 22) circuit © Marshall Cavendish International (Singapore) Private Limited (slide 33) fixed resistors © Sergpet | Dreamstime.com (slide 33) rheostat © Arsty | Dreamstime.com

CHAPTER 17 Current Electricity

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