Mod 4 : energy power and resistance Flashcards
How does a battery make charge flow
Inside the battery it has various chemicals that create an electrical potential
How do components gain energy
Charge carriers pick up energy from the energy source and deposit it at the components
What is electromotive force
The energy per coulomb of charge that is transferred from the cell to the charge carriers (chemical to electrical)
What is potential difference
The energy per coulomb of charge that is transferred from the charge carriers to the components in a circuit (electrical to other forms)
Define a volt
The p.d across a component when 1J is transferred per coulomb of charge passing through
Why must a volt metre have a high resistance
So that it does not disrupt the circuit and so that current can not floe through and current will only flow through the component
Why must a. Voltmeter be placed in parallel to the component we want to measure
Because p.d is the same in parallel circuits
Give an equation to find work done using elementary charge
W= eV
Explain the trend of the resistance in fixed resistors
They are designed so that their resistance is always constant despite environmental changes
Give an example of a fixed resistor in a circuit with only a cell and a bulb
The wires are considered as fixed resistors but with very low resistance
Describe the i-v characteristics of a fixed resistor
A straight line through the origin, fixed resistors follow ohms away so current and p.d should be irectly proportional
When does a bulb follow ohmic law
For small values of current
Describe the i-v characteristics of a filament lamp and why
As magnitude of current increases the heating effect on metal ions also increases this causes the resistance to increase. Therefore the graph is an s shape, current first increases with p.d however current decreases after as resistance is increasd
What are diodes made from
Semi conductors that only allow current to flow in one direction.
When is resistance infinite in diodes
When the diode is reverse bias
When does a diode conduct
When the threshold value for p.d is reached as the resistance decreases rapiddaly as the p.d increases above threshold value because the number density of charge carriers increase
What is a negative temp coefficient (NCT)
A semiconductor, when temp increases the R decreases (opposite of filament lamp)
When are NCT’s used
In temp sensing circuits such as thermometers and thermostats , as it is used to electrically ensure the temp
What does a resistor do
Reduces current in the circuit
What does a light emitting diode do
Only allows current to flow in one direction and emits light when current flows throughit
What does a thermistor do
Emits heat when resistance is low
What does a capacitator do
Stores charge
What three things determine the resistance of a wire/ object
Length(L)-the longer the wire the more difficult it is to make current flow
Area (A)- the wider the wire the easier it is for current to flow as electrons can flow through easier
Resistivity (p) - this depends on the material of wire and environmental factors such as temp
Define resistivity
The resistance of a material of a 1m length with a 1m^2 cross-sectional area measured in ohm metres
Describe a method to find the resistivity of a wire
Assume that the cross sectional area of the wires is circular, so u can use the are of a circle equation.
Use a micrometer to find the diameter of the wire in three different areas and find mean
Then use the circle equation to find A
Clamp the test wire to a ruler and connect wire to rest of circuit, make sure the wire is lined up with ruler.
Then attach the flying lead to the wire and switch the circuit on.
Record the length of wire, the reading on voltmeter and ammeter.
Find the resistance using V=IR, then use
R=pL/A to find resistivity.
Repeat for several different lengths increasing in increments of 0.10m
What is terminal potential difference (v)
The potential difference accross the terminals of a cell
What would be the terminal p.d if there was no internal resistance in the cell
It would be the same as the e.m.f
Why is some of the p.d lost provided by the cell
Because of the cells internal resistance as the chemical energy of the battery causes the electrons to move the rope they collide and energy is dissipated, so p.d is lost
Why does r decrease when temp increase in a thermistor
Warming the thermistor will give more electrons enough energy to escape from their atoms. Therefore there are more charge carriers available, higher current so lower resistance
How can thermistors be used for daily appliances
Monitoring temp of devices so they don’t overheat
Monitoring the temp of fridges, hair dryers, kettles etc
Monitors engines to prevent overheating
Why are thermistors used in temp monitoring circuits
The change in R is dramatic in thermistors therefore a small change in temp can be detected by monitoring the r of thermistors
Why doLDR R decrease when LI increases
Light provides energy that releases more electrons therefore there are more charge carriers so R decreases
state some LDR uses
Street lamps
Brightness detectors in devices
What is an LDR made of
Semi conductors which n change depending on LI
State the changes in R of an LDR in a dark condition vs light
In dark conditions the LDR’s R is high therefore n in the semi conductors decreases
When light is shined on the LDR the n increases therefore resistance increases
What is threshold p.d of a diode
The voltage at which a diode begins conducting in the forward bias direction
What is the threshold p.d of a silicone diode
0.7 v
State an alternative unit for the volt
JC^-1
Explain haw the cost of using an electrical device can be determined
Cost=device power(KW) x cost per kWh x time (hours)
Define the kilowatt hour
The energy transferred by a 1 kWh appliance when used for an hour
Suggest why electricity companies use kWh instead of joules
For a more usable scale as kWh is a much larger unit than a J
Define the electrical power of a device
The rate at which an electrical device transfers electrical energy into other forms of energy
Why are fuses often used in circuits
Prevents a circuit from over heating due to a surge of current the fuse will melt if the max value for current is exceeded therefore current can not flow so it prevents a fire
Name the three factors which affect a materials resistance
Length
Area
Resistivity
What could affect a materials resistivity
The material
Temp of the wire
Describe and explain the effect that temp has on the resistivity of the metas and semiconductors
Metals
Resistivity increases as temperature increases
The ions in the metal vibrate more as they gain heat energy . This increases the frequency of collisions between metal ions and electrons
Semiconductor
The resistivity decreases as temp increases . The increase in energy allows more electrons to escape their atoms and therefore the number of charge carriers increase
Describe an experiment for finding the resistivity of a material
Measure the diameter of the wire at several points accross the wire using a micrometer and then find the average diameter
Calc the cross sectional area using pier^2
Measure the length of the wire using a metre ruler
Connect the wire to a battery and add a ammeter in series and a voltmeter in parallel
Measure the current and voltage then calc the resistance
Use resistivity formula
How to find the resistivity of a wire using a graph
Measure voltage and current of a wire at diff lengths then plot a graph
Resistivity= gradient x cross sectional area
In terms of a diode explain forward and reverse diode
Forward- the direction in which current can flow
Backwards- resistance is infinite only small amounts of current can flow
Describe the benefits of using a thermistor in a greenhouse
To sense the temp and ensure that the owner is aware of any significant changes in temp
