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
