L E A R N I N G O U T C O M E S

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Lorin Henderson
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L E A R N I N G O U T C O M E S What is charge? How does a charge form?

Y E A R 1 0 C H A P T E R 1 2 What are conductors, insulators and semiconductors?

What are electrical current, voltage, resistance, energy and power? Are they related?

1 L E A R N I N G O U T C O M E S What is charge? How does a charge form? Electricity What is an electric current? Y E A R 1 0 C H A P T E R 1 2 What are conductors, insulators and semiconductors? How does electricity flow in a battery? What are electrical current, voltage, resistance, energy and power? Are they related? What are the two ways of connecting electrical components? What factors affect the current flowing though a conductor? What is Ohm s Law? What are diodes, thermistors, LDRs and rheostats? How is the electricity bill worked out? What are the features in a house circuit? Name and Surname: Class:

P A G E 2 12.1 Static Electricity Matter around us consists of atoms joined together. An atom has a dense, positively charged nucleus surrounded by light, negatively charged electrons orbiting it.

When electrons flow from one atom to another, the atoms become oppositely charged A normal atom is neutrally charged as it has the same number of protons (the positive particles in the nucleus) and

2 P A G E Static Electricity Matter around us consists of atoms joined together. An atom has a dense, positively charged nucleus surrounded by light, negatively charged electrons orbiting it. The nucleus of an atom cannot be changed except in nuclear reactions; however electrons can easily change their location. When electrons flow from one atom to another, the atoms become oppositely charged A normal atom is neutrally charged as it has the same number of protons (the positive particles in the nucleus) and electrons. However, if electrons flow from one material to another, the materials obtain a net charge. The material which gains electrons becomes negatively charged, while the material which loses electrons becomes positively charged. Charge is measured in Coulombs, C. Some materials, like polyethene and hard rubber, have a tendency to gain electrons. Other materials, like cellulose acetate, glass and nylon, have a tendency to lose electrons. On rubbing a polyethene rod or a balloon with wool, electrons are transferred from the wool to the polyethene or balloon which acquires a net negative charge. On rubbing a cellulose acetate rod, electrons are transferred from the rod to the wool. The rod thus acquires a net positive charge. Two charged balloons or rods can be used to show that same charges repel. On the other hand, unlike charges attract. Electrons transfer to polythene and from acetate on rubbing Note that only the negative charged electrons can move. It is incorrect to say that a neutrally charged object has no charges. A neutrally charged object has the same number of positive and negative charges. A negative balloon attaches itself to a wall even though the wall is neutrally charged. This is due to electrostatic induction. Electrons in a neutrally charged object are attracted or repelled depending on the charge of the object used for induction. This results in an attraction due to a temporary separation of charge on the neutral object as shown in the picture on the left hand side. Separation of charge by induction resulting in an electrostatic attraction

Practice Exercise 12A P A G E 3 12.2 Charging by Induction Some materials, like polythene and acetate, can be charged by rubbing (friction). Other materials can be charged by induction.

3 Practice Exercise 12A P A G E Charging by Induction Some materials, like polythene and acetate, can be charged by rubbing (friction). Other materials can be charged by induction. For example, in the picture below: The metal sphere is neutral at first. It is hung by a string made with an insulating material so no charge can flow to or from the sphere A charged polythene rod is brought next to the metal sphere. The negative charges move away from the charged rod The metal sphere is touched with a conductor so that the electrons, which are still being repelled by the charged rod, are conducted elsewhere The conductor is removed so that the metal sphere remains positively charged A negatively charged sphere can be obtained with the same technique using a charged acetate rod The figure to the right shows a positively charged rod being brought close to two metal spheres that are touching each other. The spheres are standing on insulating supports. a. Suggest a material that the charged rod could be made of. b. Suggest a material that the insulating supports could be made of. c. Why must the person holding the charged rod be wearing thick rubber shoes? d. What happens, in terms of electrons and charges, when the charged rod is brought close to the spheres as shown in the diagram? Draw a diagram showing the distribution of charges on both spheres. e. After bringing the charged rod close to sphere A, sphere B is slowly moved apart so that the spheres are no longer touching each other. What charges are acquired by sphere A and sphere B?

P A G E 4 12.3 Current Electricity If a charged acetate rod and a charged polythene rod are connected using a wire, electrons will flow from the negative region to the positive region.

The size of the electric current is directly proportional to the electrical energy, which energy can be converted into other forms of energy (light, sound, mechanical work, heat).

4 P A G E Current Electricity If a charged acetate rod and a charged polythene rod are connected using a wire, electrons will flow from the negative region to the positive region. This flow of electrons is called an electric current and carries an amount of energy. The size of the electric current is directly proportional to the electrical energy, which energy can be converted into other forms of energy (light, sound, mechanical work, heat). The Italian physicist Alessandro Volta invented the battery in 1800 using zinc, copper and sulfuric acid. Current is the rate of flow of electric charge Q = It Batteries (or cells) have a positive (+) region and a negative (-) region. The negative region is electron rich, while the positive region is electron poor. When these regions are connected by, for example, a copper wire, electrons flow from the negative region to the positive region. When the first cell was invented, scientists guessed that something was moving however they did not know the charge of the moving particle. Thus, current flow was wrongly marked from positive to negative. This is called the conventional current and is still used since some theories and rules were based on this theory. For a current to flow through a circuit, the circuit must be a complete circuit, i.e. it should have no gaps. Electric circuits are drawn using symbols. A simple example is provided below. A table with the symbols required for SEC physics is provided on the next page (the original, larger version can be found on page 46 of the physics SEC syllabus). An average lighting bolt carries 30,000 A and 15 C. Can you calculate the average time it takes to reach ground? An electric current is defined as the rate of flow of an electric charge. Charge (Q, in Coulombs, C) and current (I, in Amperes, A) are related by the equation: Q = It

P A G E 5, S A I N T A L B E R T T H E G R E A T C O L L E G E Practice Exercise 12B 1. Look at the circuit on the right hand side. Which bulb/s will light when the switch is closed? Why? (2) 2.

Calculate the time for which the current flows. (2) 4. Draw circuits showing: a. Two batteries connected to a switch and a bulb (2) b.

5 P A G E 5, S A I N T A L B E R T T H E G R E A T C O L L E G E Practice Exercise 12B 1. Look at the circuit on the right hand side. Which bulb/s will light when the switch is closed? Why? (2) 2. Calculate the total electric charge when 0.5 A flows through a bulb for: a. 30 s (2) b. 5 minutes (2) 3. A large lighting bolt can carry up to 120 ka and 350 C. Calculate the time for which the current flows. (2) 4. Draw circuits showing: a. Two batteries connected to a switch and a bulb (2) b. A battery connected to two bulbs which can be controlled separately by two different switches (3) 5. A polythene strip is rubbed with a piece of cloth. Explain: a. How, in terms of charges, is the strip charged? Mention the charge gained by the strip. (3) b. What happens when the strip is brought close to a rubbed acetate rod (2) c. Why, in terms of charges, can be strip attract neutral paper clips. (2)

P A G E 6 12.4 Conductors and Insulators An electric current occurs due to movement of electrons. However, electrons, although present in all materials, cannot flow through all materials.

Examples include graphite and all metals (iron, copper, gold, magnesium, lead, aluminium, and many others). Electric wires are made of copper, which is a metal with good conductivity.

Examples include plastic, wood, expanded polystyrene (jablo), rubber and air. Some materials have a resistance to an electric flow which lies between that of conductors and insulators.

6 P A G E Conductors and Insulators An electric current occurs due to movement of electrons. However, electrons, although present in all materials, cannot flow through all materials. Electrical conductors are materials that have loosely bound electrons and thus allow electricity to pass through. Examples include graphite and all metals (iron, copper, gold, magnesium, lead, aluminium, and many others). Electric wires are made of copper, which is a metal with good conductivity. Wires are insulated with rubber to decrease the danger of electric shock Insulators are materials that have strongly bound electrons and thus do not allow electricity to pass through. Examples include plastic, wood, expanded polystyrene (jablo), rubber and air. Some materials have a resistance to an electric flow which lies between that of conductors and insulators. These are called semi-conductors. Examples include silicon and germanium. In order to test whether a material is a conductor or an insulator: Incandescent lamps contain a filament of tungsten. Tungsten has relatively high resistance and it heats up and glows when a large current is passed through. Tungsten does not melt under these conditions since it is the metal with highest the melting point, 3422 C A circuit with a battery, switch, a bulb (or ammeter to measure the current) and crocodile clips is set up. The material is connected between the crocodile clips. The circuit is switched on. If the bulb lights (or the ammeter gives a reading), then the material is a conductor. If the bulb does not light (or the ammeter does not give a reading), then the material is an insulator. Using an ammeter not only indicates whether a material is a good conductor or not, but also indicates how good it is as a conductor. The resistance to a flow of an electric current in a wire depends on four factors: Silver is the best conductor of electricity Length: the longer the wire, the larger the resistance Thickness: the thinner the wire, the larger the resistance Material: some materials have a higher resistance than others Temperature: as the temperature increases, the resistance increases

7 Practice Exercise 12C P A G E Electrical Energy, Power and Resistance Batteries and cells have chemical energy which, in a closed circuit, is converted into electrical energy. Voltage, which is also called the electromotive force (emf) and potential difference (pd), is related to the electrical energy in a circuit. Specifically, voltage (in volts, V) is the energy (in Joules, J) per unit charge: Thus, voltage and current can also be related to the power (in Watts, W) of a circuit: Electrical resistance of a medium measures its opposition to the passage of an electric current. This means that insulators have a very high resistance while good conductors have a low resistance. Resistance (R, in Ohms, Ω) is related to voltage and current by the equation below: Copy and complete the table below and then answer the questions that follow Object Voltage Current Power Resistance LED Strip 3.5 V 0.24 A Electric Blanket 2.4 A 200 W Washing Machine 75 V 11.2 Ω Incandescent Light Bulb Calculate the electric energy used by: a. The fan if it is left running for 6,000 s b. The electric blanket if it is switched on for 30 minutes c. The laptop if it is left running for 8 hours 0.42 A 570 Ω Fan 3 A 20W Laptop 12 V 42W Challenge: A toaster has a power rating of 1,000W and a resistance of 500 Ω. Calculate the power and voltage flowing through it.

Practice Exercise 12D P A G E 8 12.6 Series Circuits Components are said to be connected in series when they are connected within the same loop.

The same current flows through both bulbs, but the voltage is divided between the two. Passes through the second component, where it will use some of its remaining energy.

8 Practice Exercise 12D P A G E Series Circuits Components are said to be connected in series when they are connected within the same loop. When two electrical components are connected in series, the electric current Leaves the battery Passes through the first component, where it will use some of its energy Two bulbs connected in series. The same current flows through both bulbs, but the voltage is divided between the two. Passes through the second component, where it will use some of its remaining energy. Finally enters the positive side of the battery. This means that, for components in series: If one component blows, no current will flow through the circuit The current passing through the first component is the same as the current passing through the second component. As a result, ammeters are always connected in series Cheap, Christmas tree lights used to connected in series. In such a setup, if one bulb blew off, all the circuit will stop functioning. Ammeters are always connected in series. The voltage provided by the battery (V tot ) is divided among components (assuming no energy is used to pass a current through a wire) The total resistance of the circuit increases on adding resistors. Thus, bulbs get dimmer on adding more components (which could be other bulbs) in series Look at the diagram on the right hand side showing three resistors connected in series. Calculate the i. Total resistance ii. Current flowing through each component iii. Voltage across each component

Practice Exercise 12E P A G E 9 12.7 Parallel Circuits Components are said to be connected in parallel when they are connected to separate loops in the same circuit.

energy is used Joins again at the junction and enters the positive side of the battery. Two bulbs connected in series.

9 Practice Exercise 12E P A G E Parallel Circuits Components are said to be connected in parallel when they are connected to separate loops in the same circuit. When two electrical components are connected in parallel, the electric current: Leaves the battery Divides at the first junction Passes separately through the first and second components, where energy is used Joins again at the junction and enters the positive side of the battery. Two bulbs connected in series. The same voltage is present across both bulbs, but the current is divided between the two. This means that, for components in parallel: If one component blows, current will still flow through the other component The voltage passing through the first component is the same as the voltage passing through the second component. Bulb will not get dimmer on adding more components (which could be other bulbs) in parallel The current provided by the battery (I tot ) is divided among components: Home lights are connected in parallel. Voltmeters are always connected in parallel A 12V battery is connected in parallel with two resistors of resistances 3Ω and 4Ω i. Draw the circuit ii. What is the voltage across each resistor? iii. Calculate the current flowing through each resistor iv. By using your answer in iii, calculate the total current flowing through the circuit v. Calculate the total resistance in the circuit vi. By using your answer in v, calculate the total current flowing through the circuit

10 P A G E Ohm s Law Ohm s law states that the current flowing through a metal wire is directly proportional to the voltage across it (providing the temperature is constant). For an ohmic conductor (one which obeys Ohm s law), a graph of current (y-axis) against voltage (x-axis) gives a straight line graph passing through the origin. The current-voltage graph obtained for an Ohmic conductor at constant temperature Conductors that do not obey Ohm s Law are called non-ohmic conductors. These include the following: A filament lamp is a non-ohmic conductor because as more current flows through its resistance wire, it gets hotter and thus the resistance increases. As a result the graph flattens (resistance increases) on increasing the current. The current-voltage graph obtained for a filament lamp. As the current increases, the resistance increases and as a result the graph flattens. Thermistors are resistors whose resistance varies significantly with temperature. They can be used in self-regulating heating elements and temperature sensors. There are two types of thermistors, those whose resistance increases with increasing temperature and those whose resistance decreases with increasing temperature. The current-voltage graph for a thermistor whose resistance increases with temperature is identical to that of a filament lamp, while the current -voltage graph for a thermistor whose resistance decreases with temperature is curved on the opposite side. A diode (or specifically, a semiconductor diode) is an electronic component which allows current to flow through it in one particular direction only. Also, for the current to flow through a diode, it must exceed a certain voltage (threshold voltage). Diodes can be used for lighting (light emitting diodes, LEDs), to regulate voltage and to convert an alternating (varying in size and direction) current to a direct (constant) current. The current-voltage graph obtained for a diode. Current only flows through the diode above a certain voltage

P A G E 11 12.9 Other Electrical Components There are different meters that one can connect to a circuit. An ammeter is used to measure current.

A voltmeter is used to measure voltage (potential difference between two points). It is always connect in parallel.

An ohmmeter is connected in parallel and measures the resistance. A galvanometer measures the current and its direction.

It consists of a long, thin resistance wire round a barrel. A metal jockey can be moved so as to include as much of the wire as needed.

11 P A G E Other Electrical Components There are different meters that one can connect to a circuit. An ammeter is used to measure current. It is always connected in series. An ammeter has a low resistance in order not the affect the size of the current. A voltmeter is used to measure voltage (potential difference between two points). It is always connect in parallel. A voltmeter has a very high resistance to assure that it draws no (or a very low) current. An ohmmeter is connected in parallel and measures the resistance. A galvanometer measures the current and its direction. A multimeter (shown in the picture) can be set to read a variety of measurements with different sensitivities A rheostat is a variable resistor. It consists of a long, thin resistance wire round a barrel. A metal jockey can be moved so as to include as much of the wire as needed. The current flows through the resistance wire, through the jockey and then out of the rheostat to the rest of the circuit. A light dependent resistor (LDR) is a resistor whose resistance decreases (and thus more current flows through the circuit) on increasing light levels. These can be found in camera light meters and street lights amongst other products and uses.

P A G E 12 12.10 Direct and Alternating Currents Batteries produce a constant flow of current in one direction due to a flow of charges from the negative region to the positive region.

When electricity is generated, it is generated in the form of an alternating current (AC) and finally arrives to our homes as an AC.

12 P A G E Direct and Alternating Currents Batteries produce a constant flow of current in one direction due to a flow of charges from the negative region to the positive region. The voltage is of constant size and direction unless the battery starts dying out. This type of current is called a direct current (DC). When electricity is generated, it is generated in the form of an alternating current (AC) and finally arrives to our homes as an AC. In an AC the voltage is not constant but alternates between a zero and a maximum voltage (called peak voltage). The current also changes direction and thus the voltage of an AC is distributed according to the graph shown. Usually an AC is of much higher voltage than a DC. While a large DC battery would have a voltage of 12V, the mains supply used at home has a peak voltage of 230V. An AC also has frequency and periodic time since its distribution follows a wave-like pattern. The Maltese mains supply has a frequency of 50Hz.

A 3-pin plug P A G E 13 12.11 Electricity at Home The mains power supply at home carries an AC of peak voltage 230V. Electricity enters the home and appliances through the live wire (L).

The neutral wire usually carries less voltage than the live wire and is considered to be safer. However neither part of a mains circuit should be touched without first switching off the mains switch.

13 A 3-pin plug P A G E Electricity at Home The mains power supply at home carries an AC of peak voltage 230V. Electricity enters the home and appliances through the live wire (L). This wire is coloured brown. Electricity exits appliances and our homes through the neutral wire (N) which is coloured blue. The neutral wire usually carries less voltage than the live wire and is considered to be safer. However neither part of a mains circuit should be touched without first switching off the mains switch. Each wire must be insulated as if the wires had to touch, a large current would flow through them (a short circuit). A fuse is a thin wire used to prevent over-currents and damage to certain appliances. The thin wire heats up depending on how high the current passing through it is. If the current is too high, the fuse melts, thus breaking the circuit. A fuse is always connected to the live wire since it must break before the current enters the appliance! Fuses have different fixed ratings, usually 1A, 3A, 5A, 10A and 13A. This indicates the maximum current that can flow through the fuse without melting it. A fuse must always be of the closest higher rating to the current needed for the appliance. For example, a fuse of 5A is used with a car radio that operates with 3.5A. A fuse The earth wire (E) is painted yellow and green. This wire is used for safety in appliances which are not doubly insulated and only carries a current when there is a fault in the system. If a fault develops in an appliance and the live wire touches with the metal casing, a large current will flow through the casing (due to low resistance) and from the earth wire to where the wire is earthed. This will cause the fuse in the live wire to melt and make the appliance safe. The Earth wire is connected to the metal casing of appliances. If a current flows through the case, the current will flow through the earth wire and the circuit will break Maltese mains also have an automatic-circuit breaker (salvavita) which protects against short circuits and overcurrents without the need of changing fuses all the time.

14 Practice Exercise 12F P A G E Paying for Electricity People have to pay for the electrical energy they use. The prices in Malta vary according to three different classes (domestic, residential, non-residential) and according to how much electrical power is used (the more used, the higher the price per unit). The unit of electrical energy used for this purpose is the kilowatt hour (kwhrs). Energy is worked out using the usual equation, but power is given in kw and time in hours. E Pt VIt For 2010, the price per unit for a domestic residence using between 10,001 and 20,000 units was 0.36 per unit (kwhrs). Let us consider a laptop computer rated at 3.5A and 18.5V running for eight hours a day. What is the annual electrical cost? P VI 3.5* W P kW E Pt E *(8*365) E kWhrs Cost * Euro 1. Calculate the cost of each of the following appliances if one unit costs 0.38 a. A microwave oven of power 1.4kW is used to heat some soup. It takes it four minutes to do so. b. A 20W fan is left running overnight (approximately 8 hours) c. A washing machine of voltage 75V with a resistance of 11.2Ω is used for one hour a day for a whole week. d. An LED strip with voltage 3.5V and current 0.24A is left running for a whole year. e. An incandescent light bulb with a current of 0.42A and a resistance of 570Ω is left running for a whole year, eight hours daily f. A small immersion heater with a current of 5A and a current of 12V that is left running for 30 minutes 2. Suggest a suitable fuse for the washing machine, LED strip, incandescent light bulb and immersion heater in the above question. You may choose from fuse ratings of 1A, 3A, 5A, 10A and 13A.

PAPER 2 THEORY QUESTIONS

PAPER 2 THEORY QUESTIONS 1 A plastic rod is rubbed with a cloth and becomes negatively charged. (a) Explain how the rod becomes negatively charged when rubbed with a cloth... [2] (b) An uncharged metal-coated