9 : Energy, power and resistance Flashcards
Power supplies
- A battery is two or more cells connected in series
- longer terminal is positive
Potential difference
- defined as the energy transferred from electrical energy to other forms per unit charge
- a measure of the transfer of energy by charge carriers and the work done by the charge carriers
- p.d. Across a component like a filament lamp is a result of electrical energy being transferred into heat and light as charge carriers move through a lamp
The volt
- potential difference is measured in
- one volt id the p.d. Across a component when 1J of energy is transferred per unit charge passing through the component
- V=W/Q
The voltmeter
Used to measure p.d and always connected in parallel across a component
- ideally would have infinite resistance so when connected no current passes through
Electromotive force
- the energy transferred from chemical energy to electrical energy
- Work done on the charge carriers I.e. charged gaining energy as they pass through a component like a battery
- the greater thee e.m.f. The more energy per column has been transferred into electrical energy
- E = W/Q
Calculating energy transferred in charges
Energy depends on the size of the p.d. And the charge passing through the component
- W=VQ or W=EQ
Electron gun
- a small metal filament is heated by an electric current
- electrons in piece of wire gain kinetic energy and same gain enough to escape surface of a metal = thermionic emission
- heated filament is a vacuum and a high p.d is applied between the filament (acts as a cathode) and an anode. The freed electrons accelerate towards the anode gaining kinetic energy
- the abide has a small hole in it then electrons in line with this hole can pass through it
- creating a beam of electrons with a specific kinetic energy
Energy transfers
As electrons accelerate towards the anode they gain kinetic energy - from elimination of p.d. The work done on a single electron travelling from he cathode to the anode is equal to eV by considering law of conservation the world done n electron to its increase in kinetic energy
eV=1/2 mv^2
They have negligible kinetic energy at cathode
The greater the p.d. the more energy is transferred to the electrons so the fast er they move
Resistance
Each component resists the flow of charge carriers through it.
- It takes energy to push electrons through a component the higher the resistance the more energy it takes
Determining resistance
- it is the ratio between V and I
- R = V/I
- measured in ohms which is the resistance of a component when a p.d. of 1V is produced per ampere of current
Ohm’s law
Investigates into the resistances of metallic conductors
- for a metallic conductor kept at a constant temperature the current in the wire is directly proportional to the to the p.d. across its ends
Temperature and resistance
Current in circuit changes because the temperature of the wire increases over time as a result of heating caused by the current . As wire gets hotter its resistance increases
Why does resistance increase in a wire with temp
When the temperature of the wire increase the positive ions instead the wire have more internal energy and vibrate with greater amplitude about their mean positions.
- the frequency of the collisions between the charge carriers and the positive ions increases and so charge carriers do more work and so transfer more energy as they travel through the wire
I-V characteristics of a resistor
Fixed resister where resistance is constant regardless of temp changes
- p.d. Across resting id directly proportional to the current therefore resistor obeys ohms law and thus an ohmic conductor
- resistor behaves same way regardless of polarity.
I-V characteristics of a filament lamp
- potential difference across a filament lamp is not directly proportional to the current through the resister I.e. does not obey ohms law so a non-ohmic component and resistance is not constant
- lamp behaves same way regardless off polarity
Diodes
Only allows a current in a particular direction
Light-emitting diode (LED)
- emit a single specific wavelength
- are efficient and take little energy to run they light up when there is current in them
I-V characteristics of a diode
- potential difference across a diode is not directly proportional to the current through it therefore a diode does not obey ohm’s law and therefore non-ohmic component and the resistance of diode is not constant
- behaviour depends on polarity
threshold voltage
As the p.d., increase the resistance gradually starts to drop
- different LED’s have different values for their threshold p.d. Related to the colour of light they emit
Resistivity
- at a given temperature is the product of the resistance of a component made of the material and its cross-sectional area divided by its length
- is used to describe the electrical property of a material
- resistivity of a particular material at a given temperatures is the constant or proportionality
R= ρL/A
Resistance and length
For any given current increasing the length of the wire will increase the p.d. across it. doubling the length doubles the p.d so resistance must have doubled
R is proportional to L
Resistance and cross sectional area
When area of wire increases the resistance decreases a wire withe greater cross-sectional area have a lower resistance
- for any given p.d. Doubling the cross - sectional area will double the current in the wire so the resistance must have halved
Resistance is inversely proportional to its cross-sectional area
Resistive and different materials
Good conductors have a resistivity of 10^-8 Ωm
Insulators have a value of 10^16 Ωm
Temperature and number density
Some semiconductors has a negative temperature coefficient meaning that as their resistance drops as the temperature increases
- as the temperature increases the number density of the charge carriers also increases
The thermistor
To the temperature of thermistor increases its resistance drops
- thermostats and thermometers
I-V characteristics of thermistor
Non-ohmic - like a filament lamp the current increases temperature increases but the resistance decreases
Light-dependent resistor
Made of a semi conductor in which the number density of charge carriers changes depending on the intensity of the incident light
- in dark conditions the LDR has a very high resistance the number density of the free electrons inside the semiconductors is very low so resistance is high
Transferring energy
Electrical circuits are often used to transfer energy from one place to another often from a power source
- whenever there is a current in a component, energy is transferred from the power source to that component
Electrical power
Rate of energy transfer by each electrical component is called electrical power.
- measured in watts
- P=VI
Deriving P=VI
P=W/t
W = VQ
P=VQ/t
Q/t = I
P = VI
Calculating energy transferred
The energy transferred in a given time can be determine by
W=PT
P=VI
W=VIt
Paying for energy
We pay for the total amount of electrical energy transferred to our homes
- power of the device and how long the device is used for
- W=Pt
The kilowatt hour
The energy transferred by a deceive with power of 1KW operating for a time of 1 hour
The cost of each kWh depends on company but then total cost can be calculated
