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PHYSICS UNIT 1 > CURRENT ELECTRICTY > Flashcards

CURRENT ELECTRICTY Flashcards

Mr Prestwich's lessons

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1
Q

In a series circuit the current…

A

…is the same at all points.

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2
Q

In a parallel circuit the current in less in…

A

…branches with more resistance.

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3
Q

The sum of all currents in each branch is…

A

…the total current entering or leaving the supply.

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4
Q

In a parallel circuit the total p.d. in each branch…

A

…is the same.

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5
Q

1/ RT = 1/ R1 + 1/R2 +1/ R3. This rule is for what type of circuit?

A

Parallel.

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6
Q

Resistance in a series circuit is calculated by?

A

Adding all the individual resistances.

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7
Q

Potential difference is shared between components, with components with bigger resistances taking more p.d. in what type of circuit?

A

Series.

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8
Q

Charge (Q) =

A

(change in) Current (I) x (change in) Time (t)

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9
Q

What is Ohm’s law?

A

Ohm’s Law states that the current through a conductor is directly proportional to the potential difference.

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10
Q

If something doesn’t apply Ohm’s Law what does it mean? Give and example?

A

It means that current and potential difference is not directly proportional to one another. Examples: Diode, LED, Filament lamp.

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11
Q

What is a coulomb?

A

A coulomb is the charge transported by a steady current of one amp in one second.

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12
Q

What is an amp?

A

An amp is what current is measured in.

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13
Q

What is potential difference? What is it measured in?

A

Potential difference is the work done per unit charge. It is measured in Volts (p.d. is also known as Voltage).

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14
Q

What is current?

A

The rate of charge passing a point in a circuit.

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15
Q

What are the 4 things does resistivity depend on?

A

Material, length, cross sectional area and temperature.

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16
Q

What does the characteristic curve of a Filament lamp look like?

A

It is an S shaped curve, as current increases voltage increases exponentially. (S curves towards the voltage axis. This means you can draw it two different ways depending whether p.d. is on the x or y axis.)

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17
Q

Resistivity is a constant for a material. What is the definition of Resistivity?

A

The resistivity of a material is the resistance of a sample which is one metre long and has a cross-sectional area of 1m^2.

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18
Q

What is the equation you can find Resistivity from?

A

Resistance (R) = Resistivity(roh/p ) x (Length (L) /Cross-sectional Area (A))

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19
Q

What are the units for all the components of the Resistivity equation?

A

R = Ohms
Resistivity = Ohm Metres
Length = Metres
Cross-sectional Area = Metres^2

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20
Q

Describe a conductor? What does a temperature increase do? (2 marks)

A
  • Has delocalised electrons, so charge can be passed.

- Increase in Kinetic Energy, increase in movement of electrons, more resistance.

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21
Q

Describe an insulator? What does a temperature increase do? (3 marks)

A
  • No free electrons (very high resistance)

- Doesn’t generally change arrangement, but very high temperature can cause an electron to escape.

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22
Q

Describe the effect of temperature on a semi-conductor?

A
  • At low temps act like insulator.
  • As temperature increases, resistance decreases.
  • Resistivity decreases as temp increases.
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23
Q

Describe a super conductor?

A
  • Have very low resistance.
  • Have to be super cooled.
  • Used in MRI scanners.
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24
Q

How do you work out cross-sectional area? (5 steps)

A
  • Measure diameter
  • Halve it to get radius
  • Convert into metres
  • Calculate using: A= pi x r^2
  • UNITS = m^2
25
Q

Power (Watts) = (3 answers)

A
  • Current (I) x Potential difference (V)
  • Current^2 (I) x Resistance (Ohms)
  • Potential difference^2 (V) / Resistance (Ohms)
26
Q

Efficiency (%) =

A

(Useful Energy (J) / Total energy (J) ) x 100

27
Q

Energy (J) = (3 answers)

A
  • Power (W) x Time (secs)
  • Potential difference (V) x Current (I) x Time (secs)
  • Charge (Q) x Voltage (V)
28
Q

Potential difference (V) =

A

Current (I) x Resistance (Ohms)

29
Q

V out (V) =

A

(V in (V) x Resistance 2 (Ohms) ) / (Resistance 1 + Resistance 1 (both is Ohms) )

30
Q

What is a potential divider?

A

It has 2 or more resistors in series and a source with a fixed p.d.

31
Q

What does a potential divider do?

A

It supplies a set p.d. which you can vary depending how much p.d. you want. You have a constant input and get a variable output.

32
Q

What special thing can potential dividers also do? (Hint: thermistor)

A

Can provide a p.d. which varies with physical condition such as pressure or temperature.

33
Q

A potential divider can be used as a “potentiometer”. What does this do?

A

Controls voltage by dividing the resistance along the coil. Has to have to have 3 points of contact.

34
Q

Define: Electromotive Force (Emf: shown by a Greek Epsilon)

A

The Emf of a source is the ELECTRICAL POTENTIAL energy transferred FROM other forms of energy to other forms PER COULOMB OF CHARGE.

35
Q

What is a simple description of Emf, with UNITS?

A

Potential energy per unit charge, measured in Volts.

36
Q

What is the internal resistance?

A

The resistance in the actual cell.

37
Q

Why do dry batteries have a high internal resistance?

A

So that they can get hot, as a high internal resistance causes a lot of energy to be dissipated over it.

38
Q

Emf (V) = (2 answers)

A
  • Terminal p.d. (V) + Volts lost across Internal Resistance (V).
  • (Current (I) x Resistance of circuit (Ohms) ) + (Current (I) x Internal resistance (Ohms) )
39
Q

How does the Emf equations apply to a graph of y = mx + c?

A

y = V
x = I
m = -r (internal resistance)
+ c = Epsilon

40
Q

You can measure the Emf across the terminals of the cell when…

A

…There is no current so no energy dissipated across the internal resistance so full emf value found.

41
Q

What does a oscilloscope plot?

A

Voltage against time.

42
Q

What is direct current? What does it look like on a oscilloscope?

A

It is where the current only flows in one direction.

A straight line of constant voltage.

43
Q

What is alternating current? What does it look like on a oscilloscope?

A

It is where the current is currently changing direction.

It is an oscillating sinusoidal wave.

44
Q

What is d.c. and a.c. current used for?

A

Direct current comes from batteries.

Alternating current is like mains current in the UK.

45
Q

Frequency (Hz) =

A

1 / Time period of one wave (secs)

46
Q

What is the V rms?

A

The root mean squared voltage, meaning the the mean a.c. voltage squared and rooted.

47
Q

Which is higher V rms or V o (Peak voltage)?

A

V o because V rms is the mean of the peak voltages.

48
Q

V rms (V) =

A

V o (V) / root2

49
Q

I o (A) / root2 =

A

I rms (A) (root mean squared current).

50
Q

What is the x axis and y axis of an oscilloscope?

A

x axis: time base (seconds or milliseconds per division)

y axis: Y-gain / volts per divison

51
Q

How does an oscilloscope work?

A
  • Beam of electrons fired at screen
  • Y plates (left and right) deflect the beam up and down.
  • X plates (above and below) deflect the beam side to side.
52
Q

What does a spot on a oscilloscope show? (2 marks)

A

Time base off, no voltage.

53
Q

What does a dot raised up/down from the centre on a oscilloscope show? (2 marks)

A

Time base off, d.c. input.

54
Q

What does a straight horizontal line anywhere on the oscilloscope screen show? (2 marks)

A

Time base on, d.c. input.

55
Q

What does a straight vertical line on the centre of the oscilloscope screen show? (2 marks)

A

Time base off, a.c. input.

56
Q

What does a sinusoidal wave on the oscilloscope screen show? (2 marks)

A

Time base on, a.c. input.

57
Q

Time period (secs) =

A

Length on one wavelength (in divisions) x Time base (sec per division).

58
Q

V pp =

A

The peak to peak distance, measured in the time base.

PHYSICS UNIT 1 (2 decks)

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