Electricity Flashcards
Current
Rate of flow of charge (I = Q/t)
Voltage
Work done per unit charge (V = W/Q)
Resistance
Opposition to the flow of charge (R = V/I)
Ohm’s Law
- current in a conductor is proportional to the potential difference applied to it
- provided physical conditions such as temperature remain constant
- constant of proportionality given by R, resistance
Describe I-V graph for ohmic conductor
- current is directly proportional to potential difference
- since resistance remains the same
- passes through origin
Describe I-V graph for filament lamp (non-ohmic conductor)
- s-shaped curve passing through origin
- current directly proportional to potential difference initially
- then resistance increases with current due to increasing temperature
Describe I-V graph for semiconductor diode
- n shaped curve passing through origin
- forward biased diode allows current to pass through with low resistance (steep gradient) when threshold voltage is reached
- reverse biased diode does not allow current to pass through but there is some leaking current
- if potential difference is high enough in reverse then diode can break at break down voltage allowing infinite current to pass
How to draw V-I graph based on I-V graph
inverse graphs so reflection in y=x
State resistance of ammeter
zero
State resistance of voltmeter
infinite resistance
Suggest why voltmeters are not connected in series when measuring voltage through a component
- voltmeter has infinite resistance
- impedes the flow of charge
- smaller current would flow through the component so measured voltage would be lower than true value
Suggest why an ammeter connected in parallel would stop a circuit from working
- ammeter has zero resistance
- if connected in parallel a large current would flow
- since current moves through the path of least resistance
- wire in the ammeter would burn out resulting in short circuit
Resistivity
- quantifies how strongly a given material opposes the flow of electric current
- ρ = RA/L where ρ= resistivity (Ωm), R=resistance (Ω), A=cross-sectional area (m2), L=length (m)
Metallic Conductors
A substance through which current flows due to movement of delocalised electrons
Insulator
A substance through which electrons cannot flow since all electrons are fixed in outer shells of atoms
Semiconductor
A substance through which more current flows at higher temperatures as electrons break free from atoms and are free to move
Explain why resistance of metallic conductors increases with temperature
- positive temperature coefficient
- positive metal ions vibrate more at higher temperatures
- higher frequency of collisions between them and charge carriers
- so charge cannot pass through easily
Explain why resistance of ntc thermistor decreases as temperature increases
- negative temperature coefficient
- semiconductor
- number of charge carriers increases since more electrons break free at higher temperatures
Give application of thermistors
temperature sensors
Superconductor
- zero resistance
- when critical temperature reached
- resistivity decreases with temperature
Explain why superconductors do not heat up over time
- current passes through with no resistance
- no voltage across it thus no heating effect
Suggest uses of superconductors
- high power electromagnets in MRI scanners or maglev trains
- supercomputers due to reduction in energy loss in transmission of electrical power so high efficiency
Kirchhoff Current Laws
- Current passing through components in a series circuit is the same
- At a junction total current in = total current out
Kirchhoff Voltage Laws
- Total potential difference across components is equal to the sum of potential difference across each component in series
- Potential difference is constant across parallel branches
- For any complete loop sum of emfs equals sum of potential difference drops
How to calculate sum of resistance for resisters in series
RT=R1+R2+R3+ …
How to calculate sum of resistance for resisters in parallel
1/RT=1/R1+1/R2+1/R3+ …
How to calculate terminal pd of cells in series when internal resistance is not negligible
- add potential differences of cells arranged in same direction
- minus potential differences of cells arranged in opposite directions
- total internal resistance is sum of individual internal resistances
- V = e - Ir
How to calculate terminal pd of identical cells in parallel when internal resistance is not negligible
- total emf of cells = emf of each individual cell
- V = e - Ir/n where n is number of identical cells
Diode
Component which only allows current to flow in one direction
Potential divider
- two or more resistors in series
- connected to a source of emf
- supplies constant or variable potential difference that is a fraction of voltage between its ends
State formula for output voltage of potential divider
Vout = Vin [R2 / (R1 + R2)]
Suggest an advantage of using potential dividers (potentiometers) to variable resistors in a sensor circuit
- potential difference can be reduced to zero with potential dividers
- only be reduced to a minimum with a variable resistor
Electromotive Force (EMF)
Electrical energy per unit charge supplied by a power source
e = E/Q
e = V + Ir
Internal Resistance
Resistance due to materials within a source of electrical energy
Suggest why parallel circuits have a lower resistance relative to series
- more paths for current to flow through
- less resistance to the flow of charge
State and explain whether two cells connected in a series or in a parallel combination would run out first
- series cells run out first
- double current through them compared to parallel cells
Cons of internal resistance
- lost volts
- energy wasted
- limits current of circuit
