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OCR A-Level Physics > 4.2 Energy, Power & Resistance > Flashcards

4.2 Energy, Power & Resistance Flashcards

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1
Q

what does a switch do?

A

turns an electric current on and off

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2
Q

what does a cell do?

A

provides the circuit with a source of energy or emf

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3
Q

what does a lamp do?

A

transforms electrical energy to light energy as a useful form

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4
Q

what does a resistor do?

A

controls the amount of electric current flowing in a circuit or through a component

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5
Q

what does a variable resistor do?

A

controls the amount of electric current flowing in a circuit or through a component but different to a normal resistor because the resistance can be controlled

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6
Q

what does a fuse do?

A

acts as a safety device - will melt or ‘blow’ if the current gets too high

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7
Q

what does a battery do? how is it different to a cell?

A

provides the circuit with a source of energy or emf, a battery is two or more cells

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8
Q

what is an ammeter?

A

measures the amount of electrical current flowing in a circuit or through an electrical component, measured in amps

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9
Q

what is a voltmeter?

A

measures the size of the potential difference in a circuit or through an electrical component, measured in amps

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10
Q

what is a thermistor?

A

a electric component that responds to the temperature of the environment and changes its resistance as a result

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11
Q

what is a diode?

A

a component that allows current to flow only in one direction, often used to protect delicate components from having large currents through them

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12
Q

what does LED stand for?

A

light-emitting diode

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13
Q

what does LDR stand for?

A

dependent resistor

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14
Q

what does a LED do?

A

like a diode, allows electrical current to flow through it in only one direction, when current flows through it, it emits light (lamp)

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15
Q

what is the unit for potential difference?

A

volts, V

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16
Q

define potential difference or voltage (p.d)

A

the p.d measured across a component is the energy lost per unit charge by the charges passing through the component (transfers electrical energy to other forms of energy), is it measured in volts

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17
Q

define electromotive force (e.m.f)

A

the e.m.f of a supply is the energy gained per unit charge by charges passing through the supply, when a form of energy is transferred to electrical energy carried by the charges, measured in volts

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18
Q

what is an example of a source of emf?

A

a cell or battery

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19
Q

what is the distinction between emf in terms of energy transfer?

A 
p.d = transfers electrical energy into other sources
emf = transfers energy from other sources into electrical energy
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20
Q

what is the formula for emf?

A

emf or voltage = energy transferred to charge / charge

V = E / Q

21
Q

how must you connect an ammeter in a circuit?

A

in series

22
Q

how must you connect a voltmeter in a circuit?

A

in parallel across the component

23
Q

what happens to the electrons in a conductor when a potential difference is applied across the conductor?

A

the electrons are accelerated and gain kinetic energy

24
Q

what is the equation for the energy transferred to each charged particle? (kinetic energy)

A 
eV = 0.5mv^2
where e = charge
V = potential difference
m = mass of the electron
v = velocity of the electron
25
Q

what is Ohm’s law?

A

ohm’s law states that the current through a conductor is directly proportional to the potential difference across it, provided that the physical conditions, such as temperature, remain constant

26
Q

what is the equation for resistance?

A

resistance = voltage / current

or V = IR

27
Q

what is resistance measured in?

A

ohms, Ω

28
Q

what factors affect resistance? (think of the equation)

A

length - the longer the wire the more difficult it is to make a current flow
cross-sectional area - the wider the easier it will be for the electrons to pass along it
resistivity - this depends on the material

29
Q

what does it mean if a component as a resistance of 1Ω?

A

a component has a resistance of 1Ω if a potential difference of 1V makes a current of 1A flow through it

30
Q

what is resistivity affected by?

A

temperature
-charge is carried through metals by free electrons in a lattice of positive ions
-heating up a metal makes it harder for electrons to move about, the ions vibrate more when heated, so the electrons collide with them more often losing energy to other forms
RESISTIVITY OF A METAL INCREASES AS THE TEMP INCREASES

31
Q

what does an I-V graph look like for an ohmic conductor (at a fixed temp)?

A

straight line with constant gradient through the origin

(obeys Ohm’s Law, current and voltage are directly proportional), RESISTANCE IS CONSTANT

32
Q

what does an I-V graph look like for filament lamp?

A

-line starts straight, through the origin then starts to curve and flatten as gradient gets close to 0 (S shape)
-at low temp, filament lamp behaves in accordance to Ohm’s
-as the current increases, its temperature increases due to self-heating affect which in turn increases its resistance
(resistance of a filament lamp increases as its temp increases)

33
Q

what does an I-V graph look like for LED or diode?

A
  • the line is flat until threshold voltage (usually around 0.6V) and then shoots up straight with constant gradient
  • diodes and LEDs only let current flow in one direction (from positive to negative, this is called the FORWARD BIAS direction)
  • REVERSE BIAS is the reverse direction where the resistance is almost infinite meaning no current, regardless of voltage
34
Q

what does an resistance-temp graph look like for NTC thermistor? and why does it look like this?

A
  • exponential curve downwards with negative gradient
  • an NTC thermistor changes its resistance as temperature changes
  • at LOW temperatures, resistance is HIGH, as temperature increases resistance decreases
  • this is because an NTC is a semiconductor and warming it up gives more electrons enough energy to be released, this means that there are more charge carriers available so resistivity and resistance is lower
35
Q

what does an I-V graph look for a LDR (light dependent resistor)?

A

-exponential curve downwards with negative gradient
-at low light intensity (DARK), resistance is high
- at high light intensity (BRIGHT), resistance is low
(useful for switching components on/off under certain conditions of brightness)
-this is because an LDR is a semiconductor and warming it up gives more electrons enough energy to be released, this means that there are more charge carriers available so resisitvity and resistance is lower

36
Q

what does an NTC thermistor stand for?

A

negative temperature coefficient reisistor

37
Q

give some examples of semiconductors?

A

thermistors, LDRs, and diodes

38
Q

what is the equation for resistance?

A 
R = ρL / A
where R = resistance of the component
ρ = the resistivity of the component
L = length of the component
A = cross sectional area
39
Q

what is resistivity? and what are the units?

A

an intrinsic property that isn’t affected by physical dimensions of the material (only affected by temp.) its units are ohm metres Ωm

40
Q

outline an investigation to determine the resistivity of a metal wire

A

since R = ρL / A, ρ = RA / L

  • we can find rho by making measurements of current and p.d for different lengths of wire, to minimise errors from the heating effect its best to adjust the variable resistor to keep a low fixed current flowing through the wire (remember temp affects resistivity)
  • use a micrometer to measure the diameter (at a number of different places to reduce error) and find cross-sectional area of wire
  • create a circuit with a power supply, a variable resistor, an ammeter in series, a length of test wire attached to a crocodile clip and a flying lead to be able to change the length and a voltmeter in parallel across the wire
  • record the length of wire and record voltage and current readings, repeat for several different lengths, use these values to work out the different resistances of the wire at different lengths and plot a graph of resistance (y axis) and length (x axis)
  • gradient = R/L so multiply the line of best fit by the cross-sectional area to get resistivity
41
Q

what is the equation that links resistivity of a material and its temperature?

A

ρT = ρo ( 1 + α ( T - To )
where ρT = the resistivity of a material at a temperature T
ρo = the resistivity value at To (usually room temp.)
α = the temp. coefficient
T = the temperature of the material (in degrees or celcius)
To = the reference temperature at which the resistivity of the material is quoted

42
Q

what are the equations for power?

A 
P = IV 
P = WD / t
by substituting V = IR
P = I^2 R
P = V^2 / R
43
Q

what is power defined as?

A

the rate of doing work

44
Q

what is the unit for power?

A

watts, W

45
Q

what is the kilowatt hour a unit of?

A

energy, energy = power x time

1000W for 1 hour

46
Q

what is 1kWh equal to in joules?

A

1000joules x 3600 seconds (in 1 hour)

= 3.6 million joules

47
Q

what is the common unit for energy used in households when charging for electricity?

A

kilowatt hours

48
Q

why are kilowatt hours used?

A

more sensible, appropriate unit as large amounts of energy are used in households

OCR A-Level Physics (24 decks)

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