P4.1 Electricity Circuits Flashcards
Circuit diagrams
-cell/battery: provides the circuit with a source of potential difference. A battery is two or more cells
-switch: turn the circuit on (closed), or off (open)
-fixed resistor: a resistor limits the flow of current. A fixed resistor has a resistance it cannot change
-variable resistor: a resistor with a slider that can be used to change its resistance. These are often used in dimmer switches and volume controls
-thermistor: the resistance of a thermistor depends on its temperature. As temperature increases, its resistance decreases and vice versa
-light-dependent resistor (LDR): the resistance of an LDR depends on the light intensity. As the light intensity increases, its resistance decreases vice versa
-diode: a diode allows current to flow in one direction only. Current flows through the diode when it is in forwards bias position. They are used to convert AC to DC current
-LED: equivalent to a diode and emits light when a current passes through it. These are used for aviation lighting and displays
-ammeter: used to measure the current in a circuit. Connected in series with other components
-Voltmeter: use to measure the potential difference of an electrical component. Connected in parallel with the relevant component
Drawing and interpreting circuit diagrams
-electric circuit diagrams require the following to work effectively:
An energy source – This is a source of potential difference so a current can flow. This can be a cell, battery, or a power supply
A closed path or a complete circuit – Electrons need to flow in a complete loop for a current to flow. A circuit can be open and closed using a switch
Electrical components – These could act as sensors that respond to the environment (LDR, thermistor), or measure a value (ammeter, voltmeter), or transfer electrical energy to other forms of energy (LED, lamp). These must be drawn with the correct circuit symbol
The key rules to remember are:
An ammeter is always connected in series
A voltmeter is always connected in parallel to the component the voltage is being measured
The direction of current flow is always from the positive to the negative terminal of the power supply
Electric current
An electric current is the flow of electrons.
-each electron carries a negative charge.
-Metals contain lots of electrons that move freely between the positively charged metal ions. These electrons stop ions moving away from each other.
- the current in circuit transfers energy from cell to bulb
- The size of an electric current is the rate of flow of electric charge. This is the flow of charge per second. The bigger the number of electrons that pass through a component each second, the bigger is the current passing through it.
-In a single closed loop, the current has the same value at any point
- The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component.
Formula for charge flow
Charge flow (Q)= Current (I) x time taken(t)
Q=IT
Q=coulombs
I=amperes
T=seconds
Source of potential difference
-for electrical charge to flow through a closed circuit, it must include a source of potential difference
Includes:
-cell
-batteries
-electrical generator
-a cell makes one end of the circuit positive and other negative which sets up a potential difference across the circuit (also known as voltage)
-potential difference is defined as the energy transferred per unit charge flowing from one point to another
-energy transferred can be also know as work done
Formula for potential difference
Potential difference = work done/ charge
V=W/Q
V=volts
Work done=Joules
Q=coulombs
How is potential difference measured
-potential difference is measured using a voltemeter
-a voltmeter is always set up in parallel to the component you are measuring the potential difference across
Current in a loop and measure kf current
-electrons are negatively charged. Therefore will flow away from negative terminal of a cell towards positive terminal
-conventional current is defined as the flow of positive charge from the positive terminal of cell to negative terminal
-current is measured using an ammeter
-ammeter should always be connected in series with part of circuit you wish to measure current through
-in a circuit that is a closed-loop,such as series circuit, the current is the same value at any point- this is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part
-all components in a closed-loop have the same current
Resistance
Resistance is defined as opposition to current:
-the higher the resistance of a circuit, the lower the current
-this means that good conductors have a low resistance and insulators have a high resistance
-resistance of circuit can be increased by adding resistors to it
-High resistance means there is lower current and vice versa
Resistance and potential difference
-the greater the resistance of component, the lower the current for given potential difference across the component
-the lower the resistance of the component, the greater the current for given potential difference across the component
Formual for current, resistance and potential difference
Resistance= potential difference/current
R=V/C
R= ohms
V=volts
C=amps
Or V=IR
Resistors
-fixed resistors have a resistance that remains constant
-variable resistors can change the resistance through the circuit-therefore can vary the amount of current through the circuit
-for fixed and variable resistors, once the resistance is set, it will stay this value no matter how the current changes
-however the resistance of the components such as lamps, diodes,thermistors and LDRs changes with the current through the component
Ohm’s law and formula
States that the current through a conductor is directly proportional to the potential difference across it
Examples:
-fixed resistors
-wires
-heating elements
Potential difference= current x resistance
V=IR
If V and I are directly proportional this means thats the resistance remains constant
I-V graph
-an ohmic conductor will have a current-voltage graph that is a straight line through the origin
-reversing the potential difference makes no difference to the shape of the line, the potential difference and current values will just be negative
-If the axis labels are swapped around the graph will still be a straight line through the origin
Filament lamp graph
-a filament lamp is an example of a non-ohmic conductor
-this means that current and potential difference are not directly proportional- because the resistance of the filament lamp increases as the temperature of the filament lamp increases
-graph for filament lamp shows the current increasing at an proportionally slower rate than the potential difference
Because:
-as current increases, temperature of the filament in lmao increases
-higher temp causes atoms in metal lattice to vibrate more
-causes an increase in resistance as it becomes more difficult for free electrons to pass through
-resistance opposes the current, causing current to increase at a slower rate
Resistance and temperature
-all solid made up of vibrating atoms-the higher the temp, the faster these atoms vibrate
-electric current is flow of free electrons in a material-electrons collide with the vibrating atoms which impedes their flow, hence the current decreases
-so if current decreases, then the resistance will increase
-therefore an increase in temperature causes an increase in resistance
Dioide graphs
-dioide is a non-ohmic conductor that allows current to flow one direction only- called forward bias
-in reverse the diode has very high resistance, and therefore no current flows-called reverse bias
Graph is slightly different:
-when current is in the direction of the arrowhead symbol, this is forward bias-shown by a sharp increase in potential difference and current on the right side of graph
Linear and non linear graphs
-linear components are said to obey Ohm’s law and have a consistent resistance, whilst non-linear don’t
-some components may be linear at low currents, the become non-linear as the current increases-e.g, a fixed resistor at room temperature is linear, but when it becomes very hot it will become non-linear
Linear elements include:
-fixed resistors
-wires
-heating elements
Non-linear elements include:
-filament lamps
-diodies and LED
-LDRs
-Thermistors
How does resistance change?
does the resistance change?
With current
As current increases, electrons (charge) has more energy
• When electrons flow through a resistor, they collide with the atoms in the resistor
This transfers energy to the atoms, causing them to vibrate more This makes it more difficult for electrons to flow through the resistor
So resistance increases, and current decreases
With temperature
Normal wires - See above, the same process occurs as atoms vibrate when hot
Thermistor
In hotter temperatures the resistance is lower
These are often used in temperature detectors/thermostats
- With length
• The greater the length, the more resistance and the lower the current
• Electrons have to make their way through more resistor atoms, so it is harder than using a shorter wire
- With light
• LDR (Light Dependent Resistor)
• The greater the intensity of light, the lower the resistance
• So the resistance is greatest when it is dark
• These are used in automatic night lights.
With voltage
• Diodes
• Diode allows current to flow freely in one direction
• In the opposite direction, it has a very high resistance so no current can flow
Thermistors
-a thermistor is a non-ohmic conductor and a temperature-dependent reistor
-the resistance of a thermistor changes depending on its temperature-as temperature increases the resistance of a thermistor decreases and vice versa
Low temp-high resistance
High temp-low resistance
Application of thermistors
-regularly used as a thermostat-means it automatically regulates temperature or activates a device when the temperature reaches a certain point
Found in:
-ovens
-refrigerators
-fire alarms
-boilers
Light-dependent resistors
-a non-ohmic conductor
-the resistance of an LDR changes depending on light intensity on it-as light intensity increases the resistance of an LDR decreases and vice versa
More light-lower resistance
Less light-higher resistance
Applications of LDRs
-light sensor so it automatically regulates the amount of light intensity on it or activates a device when the light intensity reaches above or below a certain point
Found in:
-lights that switch on when its dark
-alarm clocks
-burglar alarm circuits
-light intensity meters
Investigating resistance in thermistors and LDrs
For the LDR circuit:
-Begin with the lamp turned off in dark room
-Record the reading on voltemeter and ammeter
-Slowly increase the light intensity of the lamp using the dimmer switch
-Record the reading on the voltmeter and ammeter for each increase in light intensity
Thermistor circuit:
-Begin with the heater turned off
-Record the reading on the voltmeter and ammeter
-Slowly increase the heat of the heater using dimmer switch
-record the reading on the voltmeter and ammeter for each increase in temperature of the heater
In both situations,make sure lamp and heaters are close, but not touching,LDR and thermistor respectively
-wait few seconds before taking voltmeter and ammeter readings to allow LDR and thermistor to react to change in environment
-Calculate resistance of LDR or thermistor for change in light intensity or temperature using equation:
R=V/I
