2Boardworks AS Physics Using Electricity Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
3Boardworks AS Physics Using Electricity Sensing devicesBoardworks AS PhysicsUsing ElectricityTeacher notesThe variable resistor is not normally used as a sensor, but to deliberately alter the resistance in a circuit, usually by increasing the length of a resistive coil.
4Light dependent resistors Boardworks AS PhysicsUsing ElectricityA Light Dependent Resistor (LDR) is an input transducer, converting light energy to a change in electrical properties. Its resistance decreases as light intensity increases.resistance (Ω)cadmium sulfide tracklight intensity (lux)As photons of light hit a cadmium sulfide track, they give bound electrons enough energy to jump into the conduction band.LDR symbolThe resistance can fall from 1 MΩ in darkness to 500 Ω in light.
5Boardworks AS Physics Using Electricity ThermistorsBoardworks AS PhysicsUsing ElectricityNegative temperature coefficient (NTC) thermistors are input transducers that have a decreasing resistance as temperature is increased.resistance (Ω)temperature (°C)As the surrounding temperature increases, the electrons in the metal oxide of the thermistor gain energy. This increases the number of charge carriers, decreasing resistance.thermistorsymbol
6Boardworks AS Physics Using Electricity Sensors summaryBoardworks AS PhysicsUsing Electricity
7Boardworks AS Physics Using Electricity Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
8Boardworks AS Physics Using Electricity Sharing voltageBoardworks AS PhysicsUsing ElectricityTeacher notesIn this activity the simple nichrome wire acts as a resistor; sliding the contact along the wire moves the voltmeter’s input. This splits the wire’s resistance, forming a rudimentary potential divider. Thus the voltmeter receives a different proportion of the voltage dependent upon the position of the contact. This basic activity could act to provoke discussion about the link between resistance and voltage, and help to give a basic understanding of the principles behind the potential divider.
9Boardworks AS Physics Using Electricity Potential dividersBoardworks AS PhysicsUsing ElectricityPotential dividers reduce voltage. Varying the ratio of a pair of resistors changes the output voltage of a circuit.VINR1R2VOUTVOUT=VIN×R1 + R2R20 V0 VVOUT will be a fraction of VIN. The magnitude of VOUT is dependent upon the ratio of the two resistors R1 and R2.
10Using the potential divider equation Boardworks AS PhysicsUsing Electricity
11Sensors and potential dividers Boardworks AS PhysicsUsing ElectricityTeacher notesVOUT, R1 and R2 can all be hidden, allowing the activity to be used to pose a range of potential divider calculation questions. Each component has a different range of resistances and sets the value of the fixed resistor at a different level.
12Potential divider questions Boardworks AS PhysicsUsing Electricity
13Potential dividers summary Boardworks AS PhysicsUsing Electricity
14Boardworks AS Physics Using Electricity Teacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
15Boardworks AS Physics Using Electricity Electrical powerBoardworks AS PhysicsUsing ElectricityThe power, or rate of energy transfer, of a device is a product of the voltage and current passing through the component.power (W) = voltage (V) × current (A)What is the power of a bulb which uses a 230 V mains supply and has a current of 0.44 A passing through it?P = V × I= 230 × 0.44= WWhat is the voltage across a microchip if it has a normal operating power of 0.5 W and draws a current of 0.1 A?V = P ÷ I= 0.5 ÷ 0.1= 5 V
16Different forms of the power equation Boardworks AS PhysicsUsing ElectricityElectrical power can also be calculated using resistance.The equations linking power to resistance are found by substituting the equation V = I × R into the power equation:P = V × IP = V × Iand… V = I × Rand… I = V ÷ RTherefore, using substitution:Therefore, using substitution:P = I × R × IP = V × V ÷ RP = I2 × RP = V2 ÷ R
17Boardworks AS Physics Using Electricity Energy in circuitsBoardworks AS PhysicsUsing ElectricityTeacher notesStudents should note that power can be substituted into this equation giving e = Pt, as P = VI.
18Boardworks AS Physics Using Electricity EfficiencyBoardworks AS PhysicsUsing ElectricityEfficiency is a measure of how well a device transforms energy into useful forms.A light bulb converts electrical energy to useful light and wasted heat.lightelectricalheatWhat is the efficiency of the bulb if it converts 50 J of electrical energy into 45 J of heat energy?useful energy outefficiency =× 100total energy in5=× 100= 10 %50
19Boardworks AS Physics Using Electricity Efficiency of a motorBoardworks AS PhysicsUsing ElectricityWhat is the efficiency of this system, if the motor takes 5 seconds to lift the weight?(take gravity to be 9.81 N/kg)6 V2 Apulleymotor1.5 menergy into system:electrical energy = I × t × V= 2 × 5 × 6= 60.0 J1.4 kgenergy used:gravitational potential energy = m × g × h= 1.4 × 9.81 × 1.5= 20.6 Juseful energy out20.6efficiency =× 100=× 100= 34.3 %total energy in60.0
20Electricity in the home Boardworks AS PhysicsUsing Electricity
21Boardworks AS PhysicsUsing ElectricityTeacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
22Current and drift velocity Boardworks AS PhysicsUsing ElectricityCurrent is a flow of charge. Electrical devices activate almost instantly once they are supplied with power, however the electrons actually move around a circuit quite slowly. Their velocity is called drift velocity.Current and drift velocity are linked by the following equation:I = current (amps)n = charged particles per unit volumeA = cross-sectional area (m2)v = drift velocity (m/s)e = charge on an electron (1.6 x C)I = nAve
23Understanding I = nAveBoardworks AS PhysicsUsing Electricity
24Alternating current and direct current Boardworks AS PhysicsUsing Electricity
25RMS voltageBoardworks AS PhysicsUsing ElectricityThe voltage of AC can be viewed using an oscilloscope. There are three common voltage measures, namely peak, peak-to-peak and RMS (root mean squared) voltage.RMS voltagepeak voltagepeak-to-peak voltagezero voltsTeacher notesRMS is the average magnitude of the voltage over a whole cycle, i.e. the average voltage if negative voltages are treated as positive.RMS is a measure of the average magnitude of the voltage.VPEAKVRMS=√2
26RMS current and RMS power Boardworks AS PhysicsUsing ElectricityTo investigate voltage we use an oscilloscope connected across a resistor. As V I, the equation for calculating RMS current is similar to the equation for RMS voltage:IPEAKIRMS=√2The equation for RMS power is a little different:PPEAK = IPEAK × VPEAKIPEAKVPEAKPRMS = IRMS × VRMS =×√2√2PPEAKPRMS =2
27AC calculationsBoardworks AS PhysicsUsing Electricity
28AC/DC summaryBoardworks AS PhysicsUsing Electricity
29Boardworks AS PhysicsUsing ElectricityTeacher notesIn ‘Slide Show’ mode, click the name of a section to jump straight to that slide.
30Glossary Boardworks AS Physics Using Electricity Teacher notes alternating current (A.C.) – Electric current in which the direction of electron flow constantly alternates direction, producing a sine wave voltage trace.current (I) – A flow of charge, measured in amperes (A).direct current – Electric current in which the electrons flow in one direction and voltage is constant.drift velocity – The average velocity of electrons if an e.m.f. is applied. A low resistivity tends to lead to a low drift velocity.efficiency – A measure of how well a device transforms energy into useful forms. efficiency = (useful energy out / total energy in) × 100electromotive force (e.m.f) – The amount of electrical energy that is transferred to each unit of charge when one form of energy is converted to electrical energy. E.m.f is measured in volts (V).input transducer – A device that converts a specific type of energy into electrical energy, allowing the original energy source to act as an input for a circuit.kilowatt hour – A unit of energy used to measure the large quantities of electrical energy used domestically.Light Dependent Resistor (LDR) – A semiconducting input transducer with a cadmium sulfide track that causes resistance to decrease as light intensity increases.lux – The standard units of illuminance.ohmic conductor – A conductor with a linear current-voltage (I–V) graph.oscilloscope – A device for visualizing electrical signals on a screen, with voltage on the y-axis and time on the x-axis.peak-to-peak voltage – The difference between the voltage peaks and troughs in an AC signal.peak voltage – The voltage measured from 0 V to the highest voltage of an AC signal.potential divider – A pair of series resistors that divide the input voltage in proportion to their resistance. This produces an output voltage lower than the input voltage.power – The rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt (W).resistance – The amount by which a material limits the flow of electrons through it. Resistance is affected by the length and cross-sectional area of a material as well as its resistivity. The SI unit of resistance is the ohm (Ω).resistivity – A measure of the particulate matter opposing electron flow in a material.root mean squared voltage – The average voltage of an AC signal if negative voltages are made positive. It is calculated by dividing the peak voltage by √2. Given the symbol VRMS.sensor – A device that measures a physical quantity and converts it into a signal that can be analyzed by a processor.thermistor – A semiconducting input transducer formed from a metal oxide. Its resistance decreases as temperature increases.voltage – The difference in potential across an electrical component, measued in volts (V).
31What’s the keyword?Boardworks AS PhysicsUsing Electricity
32Multiple-choice quizBoardworks AS PhysicsUsing Electricity