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A Level Physics > Electricity > Flashcards

Electricity Flashcards

Hader

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1
Q

power supply symbol

A

two filled-in dots with a gap between them

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2
Q

voltage/power rating meaning

A

the maximum p.d./power that a device can operate at safely

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3
Q

threshold voltage meaning

A

A value of p.d., above which allows current to flow easily in a diode

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4
Q

potential difference meaning

A

The work done per unit charge.

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5
Q

1V = ? (alternative units)

A

1 J/C

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6
Q

Ideal voltmeters/ammeters meaning

A

Ideal voltmetes have infinite resistance so no current flows through them - to measure the p.d. all the current should pass through the resistor and none through the voltmeter

Ideal ammeters have no resistance so have no p.d. across them

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7
Q

emf meaning

A

the terminal potential difference (p.d. across the power supply e.g. cell) when the current is 0

the amount of electrical energy provided by a power supply per coulomb of charge passing through (ε = E / Q)

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8
Q

What happens in a wire when a complete circuit is formed with a cell?

A

when a p.d. is induced, electrons move towards the positive charge

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9
Q

How do you calculate the resistance for a non-ohmic conductor from an IV graph?

A

Read off the values for current and p.d. then use the resistance formula

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10
Q

Explain why an IV graph of a filament lamp looks like that.

A
  • As the current increases, its temperature also increases
  • This causes the particles in the metal to vibrate more
  • This makes it harder for the charge-carrying electrons to travel through
  • This increases the bulb’s resistance as the current can’t flow as easily
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11
Q

Why do filament bulbs’ IV graphs plateau?

A

As the current and so temeprature increases, the resistance increases, which decreases the current.

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12
Q

What does a V-I graph for a filament lamp look like?

A

look online

quadrant 1 = x2 graph
quadrant 3 = -x2 graph

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13
Q

forward bias meaning

A

The direction in a diode which allows current to flow

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14
Q

reverse bias meaning

A

The direction in a diode where the resistance is high so the current is small

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15
Q

resistivity meaning

A

The property of a material that describes how much it opposes the flow of electric current through it.

The resistance of a 1m length of a material with a 1m2 cross-sectional area.

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16
Q

What is the A in the resistivity formula?

A

The cross-sectional area - usually a circle or rectangle

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17
Q

charge carrier meaning

A

A particle which carries an electric charge (i.e. electrons)

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18
Q

semiconductor meaning

A

A material which conducts electricity (but not as well as metals because it has fewer charge carriers). When energy is transferred to it, its temperature increases so it can release more charge carriers and its resistance decreases

19
Q

Give three examples of semiconductors.

A

LDRs, thermistors and diodes

20
Q

NTC thermistor meaning

A

Negative temperature coefficient - as the temperature increases, the component’s resistance decreases.

21
Q

Explain why thermistors’ resistance increase as the temperature decreases.

A

As the temperature decreases, the electrons lose energy so can’t escape their atoms as easily. This means there are fewer charge carriers available, so the resistance increases.

22
Q

How do you investigate the resistance of a thermistor?

A
  • Set up circuitwith thermistor and ammeter in series with a voltmeter in parallel
  • Control the temperature by placing the waterproof thermistor in a waterbath (e.g. pouring boiling water into a beaker with a thermistor inside to cover it)
  • Measure water temperature with a thermometer and current with the ammeter (as the p.d. needs to be kept constant)
  • Let the water cool. Every drop in 5°C record the p.d. current and temperature
  • Use R = V / I to calculate the resistance of the thermistor at each temperature
23
Q

risk assessment for investigating the resistance of a thermistor

A

Components and thermistor that come into contact with the water bath must be waterproof.

24
Q

superconductor meaning

A

A property of certain materials which
have zero resistivity at & below a critical temperature (this depends on the material)

25
Q

superconductor example applications

A
  • the production of strong magnetic fields (e.g. in medicine & Maglev trains)
  • the reduction of energy loss in transmission of power (power cables)
26
Q

How do you find the resitivity of a material?

A
  • Measure the diameter of the test wire three times with a micrometer to calculate the mean in metres. Then halve this to get the radius
  • Calculate the cross-sectional area using A = πr2
  • Set up a circuit with an open switch, length of wire clamped with crocodile clips at the 0cm end of a meter ruler, a flying lead, an ammeter in series and a voltmeter across the wire in parallel
  • Attach the flying lead to the end of the test wire and measure its length with the ruler
  • Close the switch and measure the current through the circuit and the p.d. across the wire
  • Open the switch
  • Use R = V / I to calculate the resistance
  • Reapeat twice more and calculate the mean resistance at this length of wire
  • Move the flying lead to 10cm below the end of the test wire and repeat to calculate the average resistance for each length of test wire
  • Plot a graph of resistance against length
  • Draw a line of best fit . The gradient = R/l = ρ / A
  • Therefore resistivity = the gradient * the cross-sectional area of the wire
27
Q

formula for resistance, energy, time and current

A

E=I2Rt

28
Q

formula for p.d., resistance, time and energy

A

E=V2Rt

29
Q

formula for time, energy, current and p.d.

A

E=VIt

30
Q

potential divider formula

A

Vout = R2 / (R1 + R2) * Vs

where:
Vout = the output p.d.
R1 = the resistance of the resistor(s) in the main series circuit
R2 = the resistance of the resistor that you’re measuring the p.d. over; the potential divider
Vs = the p.d. supply

31
Q

Kirchoff’s first law (+ give an example of a time this applies)

A

The total current entering a point is equal to the total current leaving that point e.g. a series circuit setup

32
Q

Kirchoff’s second law

A

In a closed circuit, the total of the electromotive forces is equal to the total of the potential differences

33
Q

What is the unit of emf?

A

volts

34
Q

symbol representing emf

A

epsilon - ε

35
Q

In parallel circuits, what is the total of the emfs equal to?

A

The p.d. in each loop (as they’ve got the same p.d.)

36
Q

One loop of a parallel circuit has a resistance of 4 ohms while another has 6 ohms. The total current that flows though the battery is 5A. What current flows through each loop?

A

The currents are shared in the inverse ratio of the resistance – a smaller resistance has a bigger current

4 ohms loop => 5 x (4/10) = 2A
6 ohms loop => 5 x (6/10) = 3A

idk if this is even how you work it out anymore

37
Q

effective resistance

A

the single resistor which replaces two or more resistors which gives the same effect on the circuit by allowing the same amount of current to flow (i.e. a resistor that has the total resistance of all the resistors)

38
Q

internal resistance formula not on formula sheet (with labels)

A

ε = Ir + V

r = internal resistance (resistance across the power supply)
V = terminal p.d.

39
Q

internal resistance

A

A resistance to the flow of current inside a cell due to the chemicals it is composed of; the resistance created in a power source when electrons collide with atoms inside it and lose energy.

40
Q

terminal p.d.

A

The p.d. across the terminals of a cell when a current flows

41
Q

difference between EMF and terminal p.d.

A

EMF = the p.d. across the terminals of a cell when no current flows
terminal p.d. = the p.d. across the terminals of a cell when a current flows

42
Q

How do you find the EMF from a graph of p.d. against current?

A

y-intercept

43
Q

How do you find the internal resistance from a graph of p.d. against current?

A

-gradient (modulus of p.d. / current)

A Level Physics (4 decks)

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