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A-Level Physics > Electricty > Flashcards

Electricty Flashcards

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1
Q

Give an equation and definition for current.

A

I = ΔQ/Δt. Current is the charge flow per unit time.

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2
Q

Give the equation and definition for p.d.

A

V = W/Q. Potential difference is the work done per unit charge.

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3
Q

Give the equation and definition for resistance.

A

R = V/I. Resistance is the ratio of potential difference across acomponent to the current flowing through it.

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4
Q

Give Kirchoff’s current law.

A

As charge must be conserved: the total current flowing into a junction is equal to the total current flowing out of a junction.

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5
Q

How does the current law apply to series circuits?

A

Same current at all points.

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6
Q

How does the current law apply to parallel circuits?

A

Current splits between the branches based on resistance.

I = V/R gives I ∝ 1/R

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7
Q

Give Kirchoff’s voltage law.

A

As energy must be conserved: for a closed loop the total voltage across the cells/batteries is equal to the total voltage across the components.

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8
Q

How does the voltage law apply to series circuits?

A

Voltage of the cell/battery is shared based on resistance of components.

V = IR gives V∝ R

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9
Q

How does the voltage law apply to parallel circuits?

A

Each branch gets the full voltage of the cell/battery.

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10
Q

Give the equation for total resistance in series.

A

RT = R1 + R2 + …

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11
Q

Give the equation for total resistance in parallel.

A

1/RT = 1/R1 +1/ R2 + …

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12
Q

Explain the shape of the ohmic conductor graphs.

A

Ohmic conductors obey Ohm’s law -> current is directly proportional to potential difference -> as the resistance is constant (provided that temperature remains constant) -> straight line through origin.

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13
Q

Explain the shape of the filament lamp graphs.

A

As p.d. increases -> current increases -> temperature of filament increases -> metal ions vibrate with greater amplitude -> more frequent collisions between electrons and ions -> resistance increases -> graph curves as V/I increases.

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14
Q

Explain the shape of the NTC thermistor graphs.

A

As p.d. increases -> current increases -> temperature increases -> energy of thermistor increases -> more charge carriers released -> resistance decreases -> graph curves as V/I decreases.

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15
Q

Explain the shape of the diode graphs.

A

For negative p.d. -> reverse bias -> very high resistance -> little or no current flows. For positive p.d. -> forwards bias -> no current flows until threshold voltage met -> above this, current increases as p.d increases.

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16
Q

Describe a circuit that can be used to investigate I-V characteristics.

A

Connect test component and a variable resistor in series with a power source. Connect ammeter and voltmeter. Vary resistance of variable resistor to vary V and I. Or use a potentiometer

17
Q

Give the relationship between length of wire and resistance.

A

Resistance ∝ Length

18
Q

Give the relationship between area of a wire and resistance.

A

Resistance ∝ 1/ Area

19
Q

Give the equation and units for resistivity.

A

ρ = RA/ L Units = Ωm

20
Q

Give the definition for resistivity.

A

Resistivity is the resistance of a material with a cross-sectional area of 1 m2 and a length of 1 m.

21
Q

What happens to resistivity as temperature increases?

A

As temperature increases, resistivity increases.

22
Q

How can you determine resistivity experimentally?

A

Change length of wire and measure I and V. Calculate R =V/I.

Plot a graph of resistance against length of wire.

Gradient = resistivity/area.

23
Q

Describe what a superconductor is.

A

A superconductor is a material that has zero resistivity (and zero resistance) at or below a critical temperature.

24
Q

Describe and explain the uses of superconductors.

A

High speed MagLev trains and MRI scanners -> as superconductors can be used to create very strong magnetic fields.

Electrical transmission cables -> zero resistance -> no energy wasted to surroundings -> more efficient.

25
Q

Describe a limitation of using superconductors.

A

Critical temperature is very low -> cooling with liquid helium/nitrogen is expensive and impractical.

26
Q

Total p.d. for cells in series?

A

The sum of the p.d. of the individual cells.

27
Q

Total p.d. for identical cells in parallel?

A

Equal to the p.d. of one of the cells.

28
Q

What is the definition for power?

A

Power is the work done/energy transferred per unit time.

Or: Power is the rate of work done/energy transfer.

29
Q

Give the power equations from the data sheet.

A

P = VI = I2R = V2/R

30
Q

How does a potential divider circuit work?

A

Two or more resistors in series, connected to a power source.

P.d. of power source is shared between resistors. V ∝ R.

31
Q

How are potential divider circuits used in sensory circuits?

A

LDRs used in automatic lights.

Thermistors used in thermostats.

32
Q

What is a potentiometer?

A

Long coil of wire with a moveable connection. Gives two resistors with varying resistance. Can be used to investigate I-V characteristics – allows V to be varied from 0 V to full voltage of cell/battery.

33
Q

Give an equation and definition for e.m.f.

A

ε = E/Q. E.m.f. is the work done moving 1 C of charge through a cell.

34
Q

Define internal resistance.

A

Resistance of a cell/battery caused by electrons colliding with metal ions and losing energy.

35
Q

Give the equation linking e.m.f., terminal p.d., lost volts.

A

E.m.f = terminal p.d. + lost volts

36
Q

Give the equation linking ε, I, R and r.

A

ε = I (R +r)

37
Q

How can you investigate internal resistence experimentally?

A

Measure V and I for different values of R.

Plot graph of V against I. V = - Ir + ε

Gradient = -r

y-intercept = ε

A-Level Physics (6 decks)

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