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OCR A-Level Physics > Electricity > Flashcards

Electricity Flashcards

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1
Q

Define current.

A

Rate of flow of charge.

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2
Q

What is the unit for charge and its definition?

A

Coulomb (C), the amount of charge that passes in 1 second when the current is 1 ampere.

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3
Q

How do you measure current?

A

Ammeter in series.

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4
Q

Define potential difference.

A

Work done per unit charge.

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5
Q

What is the unit for p.d. and its definition?

A

Volt (V), the potential difference across a component when you do 1 joule of work moving 1 coulomb of charge through a component.

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6
Q

How do you measure p.d.?

A

Voltmeter in parallel.

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7
Q

Work done on a charge equals what?

A

The Kinetic energy it gains.

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8
Q

What is Mean Drift Velocity?

A

Average velocity of all charge carriers.

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9
Q

What happens if you double the number of electrons?

A

Current doubles.

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10
Q

What happens if you double the area?

A

Current doubles.

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11
Q

What happens if the electrons move twice as fast?

A

Current doubles.

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12
Q

How do metals, semiconductors and insulators differ in terms of n (number of electrons)?

A

For a metal n is large , drift velocity is therefore small.
For a semiconductor n is smaller, so drift velocity needs to be higher for same current.
For a insulator n = 0 therefore you get no current. (In reality n is very small)

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13
Q

What is special about ionic crystals?

A

Insulators when solid, however when they are molten they conduct.

Also happens to ionic solutions.

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14
Q

What is the name given to a substance that contains ions and conducts electricity like ionic crystals?

A

Electrolyte.

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15
Q

How are gases special?

A

They are insulators, but if you apply a high enough voltage electrons get ripped out of atoms, giving you ions along a path. Resulting in a spark.

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16
Q

What happens when you put a p.d. across an electrical component?

A

A current will flow.

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17
Q

What is the unit for resistance and its definition?

A

The resistance if a potential difference of 1V makes a current of 1A flow through it.

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18
Q

What three things determine resistance?

A

Length, longer the wire the more difficult it is to make a current flow.

Area, the wider the wire the easier it will be for electrons to pass through it.

Resitivity.

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19
Q

What is the definition for resitivity?

A

The resistance of a 1m length with a 1m^2 cross-sectional area, so p=RA/L.

20
Q

What are the units for resitivity?

A

ohm metres

21
Q

For an Ohmic Conductor, what is constant?

A

R (Resistance)

22
Q

What is Ohm’s Law?

A

Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it (V = IR).

23
Q

What is an I-V characteristic?

A

A graph which shows how the current flowing through a component changes as the potential differences across it increased.

24
Q

Describe the I-V characteristic for a metallic conductor.

A

Straight line through the origin.

25
Q

Describe and explain the I-V characteristic for a filament lamp.

A

Curved, starts steep but gets shallower as p.d. rises. Current flowing through the lamp increases its temperature so the resistance increases.

26
Q

Why does resitivity of a metal increase with temperature?

A

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 as the ions vibrate more so electrons collide with them more often losing energy to other forms.

27
Q

Why are semiconductors used in sensors?

A

Semiconductors have a higher resitivity than metals because less charge carriers. If energy is supplied to them more charge carriers released so resitivity decreases.

Therefore they make good sensors for detecting changes in their environment.

28
Q

What is a thermistor?

A

A resistor with a resistance that depends on its temperature.

29
Q

What does NTC mean?

A

Negative Temperature Coefficient, resistance decreases as temperature increases.

30
Q

What is the I-V graph for a NTC thermistor?

A

Curve upwards.

31
Q

Explain NTC.

A

Warming the thermistor gives more electrons enough energy to escape from their atoms. This means more charge carriers available so resistance is lower.

32
Q

What is an LDR?

A

Light Dependent Resistor

33
Q

How does an LDR work?

A

Greater the intensity of light, lower its resistance. More light means more energy so more charge carriers therefore lower resistance.

34
Q

What is a diode?

A

Diodes only let current flow in one direction.

35
Q

What is the forward bias of a diode?

A

The direction in which the current is allowed to flow (direction triangle points in circuit symbols).

36
Q

What is the threshold voltage of a diode?

A

Voltage required before they conduct. About 0.6V

37
Q

What happens at the reverse bias of a diode?

A

Resistance is very high so current is very small.

38
Q

What is power?

A

Rate of transfer of energy.

39
Q

What is the unit for power and its definition?

A

Watt (W), 1 watt is equivalent to 1 joule of work done per second.

40
Q

What is EMF?

A

The total amount of work the battery does on each coulomb of charge

41
Q

What is the terminal p.d.?

A

The terminal p.d. across the load resistance is the work done when one coulomb of charge flows through the load resistance.

42
Q

What is load resistance/external resistance?

A

Total resistance of all components in the external circuit.

43
Q

What are lost volts?

A

Energy wasted per coulomb overcoming the internal resistance.

44
Q

What does the conservation of energy tell us?

A

P.d. supplied by source = P.d. used in load resistance + P.d. wasted in internal resistance.

45
Q

How do you work out e.m.f. of multiple cells in series or parallel?

A

Series, add them together

Parallel, all equal

46
Q

How can you investigate the internal resistance and e.m.f. with this circuit?

A
  1. Vary current of circuit by changing load resistance using variable resistor. Measure p.d. for several different values of current.
  2. Plot v against i
  3. Rearrange e = V + Ir to give V = -rI + e
  4. Y intercept gives e
  5. gradient is -r
47
Q

What is kirchoff’s second law?

A

The total e.m.f. around a series circuit = the sum of the p.d.s across each component.

OCR A-Level Physics (33 decks)

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