Name…………………..
Class………….
science-spark.co.uk
G482
Electrons, Photons and Waves Module 2.2:
Resistance
Module 2.2 ResistanceCircuits / 2.2.1 Circuit symbols
Candidates should be able to:
(a) recall and use appropriate circuit symbols as set out in SI Units, Signs, Symbols and Abbreviations (ASE, 1981) and Signs, Symbols and Systematics (ASE, 1995);
(b) interpret and draw circuit diagrams using these symbols. / As circuit symbols
PD/Voltage and EMF / 2.2.2 E.m.f. and p.d.
Candidates should be able to:
(a) define potential difference (p.d.);
(b) select and use the equation W = VQ;
(c) define the volt;
(d) describe how a voltmeter may be used to determine the p.d. across a component;
(e) define electromotive force (e.m.f.) of a source such as a cell or a power supply;
(f) describe the difference between e.m.f. and p.d. in terms of energy transfer. / Energy, Work Done, Energy types, transformations and transfers, voltage, p.d., e.m.f, coulombs, charge, current
IV for a resistor / 2.2.3 Resistance
Candidates should be able to:
(a) define resistance;
(b) select and use the equation for resistance R = V / I
(c) define the ohm;
(d) state and use Ohm’s law;
(e) describe the I–V characteristics of a resistor at constant temperature, / Resistance, resistor, pd, current, ohm, ampere, volt, temperature
I/V characteristics for lamps and light emitting diodes / (f) describe an experiment to obtain the I–V characteristics of a resistor at constant temperature, filament lamp and light-emitting diode (LED);
(g) describe the uses and benefits of using light-emitting diodes (LEDs). / Current, Voltage, Resistance, Temperature, filament, LED, Diode
Resistivity / 2.2.4 Resistivity
Candidates should be able to:
(a) define resistivity of a material;
(b) select and use the equation R = ρL /A / Resistance, Ohms, PD, Current, resistivity, (rho), micrometer
Effect of temperature on resistance / (c) describe how the resistivities of metals and semiconductors are affected by temperature;
(d) describe how the resistance of a pure metal wire and of a negative temperature coefficient (NTC) thermistor is affected by temperature. / Resistance, Resistivity, semiconductor, conductor, insulator, thermistor
Power / 2.2.5 Power
Candidates should be able to:
(a) describe power as the rate of energy transfer;
(b) select and use power equations P = VI, P = I2 R and P = V2/ R
(c) explain how a fuse works as a safety device (HSW 6a);
(d) determine the correct fuse for an electrical device;. / Power, Rate, Energy, Watt, PD, Current, Resistance, Fuse
Electrical energy / (e) select and use the equation W = IVt;
(f) define the kilowatt-hour (kW h) as a unit of energy;
(g) calculate energy in kW h and the cost of this energy when solving problems (HSW 6a). / Power, Rate, Energy, Watt, PD, Current, Resistance.
G482 Module 1: 2.2 Resistance Test / Review and assess their knowledge and understanding
Lesson 6 notes – circuits
Objectives
(a) recall and use appropriate circuit symbols as set out in SI Units, Signs, Symbols and Abbreviations (ASE, 1981) and Signs, Symbols and Systematics (ASE, 1995);
(b) interpret and draw circuit diagrams using these symbols.
Wires and connections
Component / CircuitSymbol / Function of ComponentWire / / To pass current very easily from one part of a circuit to another.
Wires joined / / A 'blob' should be drawn where wires are connected (joined), but it is sometimes omitted. Wires connected at 'crossroads' should be staggered slightly to form two T-junctions, as shown on the right.
Wiresnotjoined / / In complex diagrams it is often necessary to draw wires crossing even though they are not connected. I prefer the 'hump' symbol shown on the right because the simple crossing on the left may be misread as a join where you have forgotten to add a 'blob'!
Power Supplies
Component / CircuitSymbol / Function of ComponentCell / / Supplies electrical energy.
The larger terminal (on the left) is positive (+).
A single cell is often called a battery, but strictly a battery is two or more cells joined together.
Battery / / Supplies electrical energy. A battery is more than one cell.
The larger terminal (on the left) is positive (+).
DC supply / / Supplies electrical energy.
DC = Direct Current, always flowing in one direction.
AC supply / / Supplies electrical energy.
AC = Alternating Current, continually changing direction.
Fuse / / A safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value.
Transformer / / Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection between the coils.
Earth
(Ground) / / A connection to earth. For many electronic circuits this is the 0V (zero volts) of the power supply, but for mains electricity and some radio circuits it really means the earth. It is also known as ground.
Output Devices: Lamps, Heater, Motor, etc.
Component / CircuitSymbol / Function of ComponentLamp(lighting) / / A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.
Lamp(indicator) / / A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator, for example a warning light on a car dashboard.
Heater / / A transducer which converts electrical energy to heat.
Motor / / A transducer which converts electrical energy to kinetic energy (motion).
Bell / / A transducer which converts electrical energy to sound.
Buzzer / / A transducer which converts electrical energy to sound.
Inductor
(Coil, Solenoid) / / A coil of wire which creates a magnetic field when current passes through it. It may have an iron core inside the coil. It can be used as a transducer converting electrical energy to mechanical energy by pulling on something.
Switches
Component / CircuitSymbol / Function of ComponentPush Switch
(push-to-make) / / A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.
Push-to-Break Switch / / This type of push switch is normally closed (on), it is open (off) only when the button is pressed.
On-Off Switch
(SPST) / / SPST = Single Pole, Single Throw.
An on-off switch allows current to flow only when it is in the closed (on) position.
2-way Switch
(SPDT) / / SPDT = Single Pole, Double Throw.
A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as 'on-off-on'.
Dual On-Off Switch
(DPST) / / DPST = Double Pole, Single Throw.
A dual on-off switch which is often used to switch mains electricity because it can isolate both the live and neutral connections.
Reversing Switch
(DPDT) / / DPDT = Double Pole, Double Throw.
This switch can be wired up as a reversing switch for a motor. Some DPDT switches have a central off position.
Relay / / An electrically operated switch, for example a 9V battery circuit connected to the coil can switch a 230V AC mains circuit.
NO=NormallyOpen, COM=Common, NC=Normally Closed.
Resistors
Component / CircuitSymbol / Function of ComponentResistor / / A resistor restricts the flow of current, for example to limit the current passing through an LED. A resistor is used with a capacitor in a timing circuit.
VariableResistor
(Rheostat) / / This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.
VariableResistor
(Potentiometer) / / This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.
VariableResistor
(Preset) / / This type of variable resistor (a preset) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than normal variable resistors so they are often used in projects to reduce the cost.
Capacitors
Component / CircuitSymbol / Function of ComponentCapacitor / / A capacitor stores electric charge. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.
Capacitor, polarised / / A capacitor stores electric charge. This type must be connected the correct way round. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.
VariableCapacitor / / A variable capacitor is used in a radio tuner.
Trimmer Capacitor / / This type of variable capacitor (a trimmer) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment.
Diodes
Component / CircuitSymbol / Function of ComponentDiode / / A device which only allows current to flow in one direction.
LED
LightEmittingDiode / / A transducer which converts electrical energy to light.
Zener Diode / / A special diode which is used to maintain a fixed voltage across its terminals.
Photodiode / / A light-sensitive diode.
Transistors
Component / CircuitSymbol / Function of ComponentTransistorNPN / / A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.
TransistorPNP / / A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.
Phototransistor / / A light-sensitive transistor.
Audio and Radio Devices
Component / CircuitSymbol / Function of ComponentMicrophone / / A transducer which converts sound to electrical energy.
Earphone / / A transducer which converts electrical energy to sound.
Loudspeaker / / A transducer which converts electrical energy to sound.
PiezoTransducer / / A transducer which converts electrical energy to sound.
Amplifier
(generalsymbol) / / An amplifier circuit with one input. Really it is a block diagram symbol because it represents a circuit rather than just one component.
Aerial
(Antenna) / / A device which is designed to receive or transmit radio signals. It is also known as an antenna.
Meters and Oscilloscope
Component / CircuitSymbol / Function of ComponentVoltmeter / / A voltmeter is used to measure voltage.
The proper name for voltage is 'potential difference', but most people prefer to say voltage!
Ammeter / / An ammeter is used to measure current.
Galvanometer / / A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less.
Ohmmeter / / An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting.
Oscilloscope / / An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.
Sensors (input devices)
Component / CircuitSymbol / Function of ComponentLDR / / A transducer which converts brightness (light) to resistance (an electrical property).
LDR = Light Dependent Resistor
Thermistor / / A transducer which converts temperature (heat) to resistance (an electrical property).
Lesson 7 notes – Potential difference and EMF
Objectives
(a) define potential difference (p.d.);
(b) select and use the equation W = VQ;
(c) define the volt;
(d) describe how a voltmeter may be used to determine the p.d. across a component;
(e) define electromotive force (e.m.f.) of a source such as a cell or a power supply;
(f) describe the difference between e.m.f. and p.d. in terms of energy transfer.
Voltage
Voltage is defined as the amount of work done or the energy required (in joules) in moving a unit of positive charge (1 coulomb) from a lower potential to a higher potential. Voltage is also called potential difference (PD). When you measure voltage you must have two points to compare, one of them being the reference point.
1 volt = 1 joule/coulomb
V=ΔW/ΔQ
Voltage in an electrical system can be thought of as the same thing as pressure in a water system; the Cell being the pump.
Two important pieces of terminology for you:
The voltage across the source of electrical energy is the E.M.F. (or electromotive force)
The voltage across a component is the p.d. (potential difference)
Lesson 7 questions – Potential difference and EMF
1)a)Put a tick in the box for an alternative unit for Voltage
JCJs-1
JC-1
(1)
b)A 1.2kW water heater is switched on for 1500s. During this time, a charge of 7.5kC passes. Calculate the p.d. across the heater.
…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… (3)
Total [4]
2)a)i)State the unit of electric charge
………………………………………………………………………………………… (1)
ii)Name an instrument that measures the p.d. across a component.
………………………………………………………………………………………… (1)
b)A lamp uses 36 Joules every second and draws a constant current of 3.0A over a period of 600s from a battery. Calculate:
i)the total amount of energy transferred to the lamp,
…………………………....…………………………………………………………… (2)
ii)the charge passing through the lamp in one second,
…………………………....…………………………………………………………… ……………………....………………………………………………………………… …………………………....…………………………………………………………… (2)
iii)the total charge passing through the lamp,
…………………………....…………………………………………………………… …………………………....…………………………………………………………………………………………....………………………………………………………(3)
iv)The total number of electrons passing through the lamp,
…………………………....…………………………………………………………… …………………………....………………………………………………………………………………………....………………………………………………………… (2)
v)The potential difference across the lamp. …………………………....…………………………………………………………… …………………………....…………………………………………………………… …………………………....………………………………………………………… (3)
Total [14]
Lesson 8 notes – I/V characteristics of thin wires and resistors.
Objectives
(a) define resistance;
(b) select and use the equation for resistance R = V / I
(c) define the ohm;
(d) state and use Ohm’s law;
(e) describe the I–V characteristics of a resistor at constant temperature,
Starter
Let’s imagine a simple circuit with a power pack, ammeter, resistor and bulb in series. A voltmeter is placed across the bulb.
If we increase the resistance, the current will go down and the brightness of the bulb will go down too. The voltage will stay the same.
The point to be made is simple: more resistance means less current for the same voltage.
This illustrates that increased resistance reduces the flow of current around a circuit and should leads us to the idea that it is sensible to measure resistance in terms of ‘volts per amp’. The larger the resistance of the circuit the greater the electrical ‘push’ needed to make a particular current flow (the more resistance the more volts needed per amp).
This leads to the equation R = V/I.
This is the ratio of the pd across a component to the current flowing through it.
So Resistance = Volts per Amp
Writing that out: Resistance = Voltage / Current
And 1 Ohm is One Volt per Amp
1 Ω = 1 V A-1
Kilo-ohms (k Ω) and mega-ohms (M Ω) are commonly used:
1 k Ω = 1000 Ω 1 M Ω = 1000 k Ω = 1 000 000 Ω
Worked example:
Calculating resistance
Calculate the resistance of a lamp when a pd of 10 V makes a current of 2 mA flow through it. (This will give practice in handling powers of 10.)
R = V/I = 10 / 2 x 10-3 = 5 x 103 = 5000 W = 5 k Ω
A straight line graph through the origin shows that the current is proportionalto the potential difference. This result is known as Ohm's law, which applies to metal or metal alloy wires as long as their temperature remains constant.
For any point on the graph the resistance R can be found by calculating V/I. If the graph is a straight line then the resistance is constant – the same for every value of current or potential difference. Under these conditions finding 1/gradient gives the average resistance of the wire. (Graphs using best-fit lines are often the best way of averaging results.)
The resistance stays the same when the current is reversed).
Ohm’s law
Historically, Ohm showed that the resistance of a metal under constant physical conditions (particularly temperature) is constant. The experiment of passing a varying current through a wire and measuring the voltage across it demonstrates this by generating a straight line graph that passes through the origin: if I is directly proportional to V (or the other way around) then Ohm’s law is obeyed. Any conductor (metallic or otherwise) that behaves in this way is described as an ‘ohmic conductor’.
Plenary
Wires are known as ohmic conductors because of the straight line through the origin. Resistance is constant at constant temperature.
If temperature varies, the resistance will vary also. In metals there is a lattice of ions with free electrons that conduct the electricity. If it is harder for the electrons to move along the wire because the lattice is vibrating, the resistance will increase. This is the case in a filament bulb. As the voltage increases, the temperature rises in the wire and the ions in the lattice start to vibrate making it harder for the electrons to move along the wire, therefore increasing the resistance.
Lesson 9 notes - Ohm’s law and I/V characteristics for filament bulbs and light emitting diodes.
Objectives
(f) describe an experiment to obtain the I–V characteristics of a resistor at constant temperature, filament lamp and light-emitting diode (LED);
(g) describe the uses and benefits of using light-emitting diodes (LEDs).
Filament Lamps
A filament lamp(non-ohmic).
As temperature changes, resistance changes. In metals, as temp goes up, the atoms have more kinetic energy and get in the way of flowing free electrons and so the resistance goes up.
Diode characteristics
Non-Ohmic conductor
Plenary
Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.
Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.6V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).
When a reverse voltage is applied a perfect diode does not conduct, resistance is infinite, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.
Thermistors at constant temperature behave like a wire and they are ohmic conductors. But if the temperature increases their resistance will decrease.
Lesson 9 questions – Wires, resistors, bulbs and diodes
1) a)Define Resistance
…………………………………………………………………………………………
………………………………………………………………………………………(2)
b) Fig 1.1 shows the I/V characteristics of a filament lamp.
State how, and explain why the resistance of the filament lamp changes as the PD across it changes
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
………………………………………………………………………………………(2)
Total [6]
2) Fig 2.1 shows how the potential difference V varies with the resistance R of a tungsten filament lamp.
fig 2.1