Boardworks AS Physics Using Electricity

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Boardworks AS Physics Using Electricity

Boardworks AS Physics Using Electricity
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

Boardworks AS Physics Using Electricity
Sensing devices Boardworks AS Physics Using Electricity Teacher notes The 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.

Light dependent resistors
Boardworks AS Physics Using Electricity A 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 track light intensity (lux) As photons of light hit a cadmium sulfide track, they give bound electrons enough energy to jump into the conduction band. LDR symbol The resistance can fall from 1 MΩ in darkness to 500 Ω in light.

Boardworks AS Physics Using Electricity
Thermistors Boardworks AS Physics Using Electricity Negative 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. thermistor symbol

Boardworks AS Physics Using Electricity
Sensors summary Boardworks AS Physics Using Electricity

Boardworks AS Physics Using Electricity
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

Boardworks AS Physics Using Electricity
Sharing voltage Boardworks AS Physics Using Electricity Teacher notes In 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.

Boardworks AS Physics Using Electricity
Potential dividers Boardworks AS Physics Using Electricity Potential dividers reduce voltage. Varying the ratio of a pair of resistors changes the output voltage of a circuit. VIN R1 R2 VOUT VOUT = VIN × R1 + R2 R2 0 V 0 V VOUT will be a fraction of VIN. The magnitude of VOUT is dependent upon the ratio of the two resistors R1 and R2.

Using the potential divider equation
Boardworks AS Physics Using Electricity

Sensors and potential dividers
Boardworks AS Physics Using Electricity Teacher notes VOUT, 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.

Potential divider questions
Boardworks AS Physics Using Electricity

Potential dividers summary
Boardworks AS Physics Using Electricity

Boardworks AS Physics Using Electricity
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

Boardworks AS Physics Using Electricity
Electrical power Boardworks AS Physics Using Electricity The 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 = W What 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

Different forms of the power equation
Boardworks AS Physics Using Electricity Electrical 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 × I P = V × I and… V = I × R and… I = V ÷ R Therefore, using substitution: Therefore, using substitution: P = I × R × I P = V × V ÷ R P = I2 × R P = V2 ÷ R

Boardworks AS Physics Using Electricity
Energy in circuits Boardworks AS Physics Using Electricity Teacher notes Students should note that power can be substituted into this equation giving e = Pt, as P = VI.

Boardworks AS Physics Using Electricity
Efficiency Boardworks AS Physics Using Electricity Efficiency 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. light electrical heat What is the efficiency of the bulb if it converts 50 J of electrical energy into 45 J of heat energy? useful energy out efficiency = × 100 total energy in 5 = × 100 = 10 % 50

Boardworks AS Physics Using Electricity
Efficiency of a motor Boardworks AS Physics Using Electricity What 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 V 2 A pulley motor 1.5 m energy into system: electrical energy = I × t × V = 2 × 5 × 6 = 60.0 J 1.4 kg energy used: gravitational potential energy = m × g × h = 1.4 × 9.81 × 1.5 = 20.6 J useful energy out 20.6 efficiency = × 100 = × 100 = 34.3 % total energy in 60.0

Electricity in the home
Boardworks AS Physics Using Electricity

Boardworks AS Physics Using Electricity Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

Current and drift velocity
Boardworks AS Physics Using Electricity Current 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 volume A = cross-sectional area (m2) v = drift velocity (m/s) e = charge on an electron (1.6 x C) I = nAve

Understanding I = nAve Boardworks AS Physics Using Electricity

Alternating current and direct current
Boardworks AS Physics Using Electricity

RMS voltage Boardworks AS Physics Using Electricity The 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 voltage peak voltage peak-to-peak voltage zero volts Teacher notes RMS 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. VPEAK VRMS = √2

RMS current and RMS power
Boardworks AS Physics Using Electricity To 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: IPEAK IRMS = √2 The equation for RMS power is a little different: PPEAK = IPEAK × VPEAK IPEAK VPEAK PRMS = IRMS × VRMS = × √2 √2 PPEAK PRMS = 2

AC calculations Boardworks AS Physics Using Electricity

AC/DC summary Boardworks AS Physics Using Electricity

Boardworks AS Physics Using Electricity Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

Glossary 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) × 100 electromotive 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).

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Multiple-choice quiz Boardworks AS Physics Using Electricity