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

4.2 Energy, Power and Resistance Flashcards

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1
Q

List 3 rules for drawing circuit diagrams.

A
  • Only use symbols required.
  • Don’t leave gaps in between wires.
  • Use a pencil and ruler.
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2
Q

What is the difference between a cell and a battery?

A

A battery is two or more cells connected end to end in series, a cell is singular.

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3
Q

Which side of the cell is the positive terminal?

A

The longer side represents the positive terminal.

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4
Q

How is potential difference defined?

A

Potential difference/voltage is a measure of the transfer of energy by charge carriers.

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5
Q

Define the volt.

A

One volt is the potential difference across a component when 1 Joule of energy is transferred per unit charge passing through the component.

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6
Q

What is the equation for potential difference?

A

Potential difference (V) = Energy Transferred (J)/Charge (C)

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7
Q

What instrument is used to measure potential difference and how is it used?

A

The voltmeter is used to measure potential difference and it must be connected in parallel with the device it is measuring. An ideal voltmeter should have infinite resistance so that no current passes through the voltmeter itself.

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8
Q

How is electromotive force (e.m.f) defined?

A

E.m.f is the energy transferred from chemical to electrical energy stores per unit charge.

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9
Q

What is the equation for electromotive force?

A

E.m.f (V) = Energy Transferred (J)/Charge (C)

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10
Q

What is the difference between potential difference and electromotive force?

A

E.m.f is used when energy is transferred from chemical to electrical sources, where p.d is used when energy is transferred from electrical to any other source.

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11
Q

What is an electron gun and what is it used for?

A

An electron gun is an electrical device used ton produce a narrow beam of electrons. They can be used to ionise particles by adding or removing electrons, and have precisely determined kinetic energies. They are used in electron microscopes, mass spectrometers and oscilloscopes.

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12
Q

Describe how electrons are emitted from the filament (cathode) in an electron gun, and what name is given to thus process?

A

All electron guns need a source of electrons where a small metal filament is heated by an electric current. Electrons in the wire gain kinetic energy, or enough to escape the surface of the metal. This is called thermionic emission.

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13
Q

Describe how the beam of high speed electrons is produced after emission from the cathode in electron guns.

A

If the heated filament is in a vacuum and a high p.d is applied between the filament and the anode, the filament acts as a cathode and freed electrons accelerate to the anode, gaining kinetic energy. If the anode has a small hole then electrons pass through, creating the beam of electrons.

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14
Q

What is an expression relating the work done on an electron - in an electron gun - to its increase in kinetic energy?

A

eV = 1/2mv²

e = elementary charge
V = accelerating p.d (V)
m = mass (kg)
v = velocity (ms-1)

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15
Q

How is resistance defined?

A

Resistance (Ω) = P.d (V)/Current (A)

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16
Q

What is the unit of resistance and how is it defined?

A

Ohms (Ω) - the resistance of a component when a p.d of 1 Volt is produced per Ampere of current.

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17
Q

State Ohm’s law.

A

For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d across its ends.

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18
Q

Why does resistance increase as a wire gets hotter?

A

When the temperature of a wire increases, the positive ions inside have a lot more internal energy and vibrate with greater amplitude. The collision frequency between positive ions and charge carriers increases so charge carriers do more work and therefore transfer more energy.

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19
Q

What does an I-V characteristic of an electrical component show?

A

I-V characteristics show the relationship between the current and the potential difference across a component.

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20
Q

How can data be collected for an I-V characteristic of an unknown component?

A

Either use a variable resistor, or a potentiometer.

21
Q

What does an I-V characteristic of a resistor look like, and what conclusions can be made?

A
  • Straight line through the origin.
    The p.d across the resistor is directly proportional to the current. The resistor therefore obeys Ohm’s law. Resistance is constant, and behaves the same way regardless of polarity.
22
Q

What does the I-V characteristic of a filament lamp look like, and what conclusions can be made?

A
  • Straight line through origin that eventually plateaus.
    The p.d across the lamp isn’t directly proportional to the current. The lamp does not obey Ohm’s law and is non-ohmic. Resistance isn’t constant, and the lamp behaves in the same way regardless of polarity.
23
Q

Explain the shape of the filament lamp I-V characteristic.

A

The increase in resistance causes the graph shape, and this is caused by the wire becoming very hot: as current increases, the rate of flow of charge does also, so more collisions occur due to a higher electron flow. Energy is transferred, ions vibrate more to increase the temperature, causing an increase in resistance.

24
Q

What is a diode?

A

A diode is a component that only allows current to flow in one particular direction.

25
Q

What does the I-V characteristic of a diode look like, and what conclusions can be made?

A
  • Flat line on x-axis, begins to increase in one quadrant.
    The p.d isn’t directly proportional to the current. The diode doesn’t obey Ohm’s law and is non-ohmic. The resistance is not constant and the behaviour does depend on polarity.
26
Q

What is the function and uses of a thermistor?

A

A thermistor is made of a semiconductor with a negative temperature coefficient, as the temp of the thermistor increases, resistance drops.
- Used in thermometers, thermostats, monitoring temperature in devices and engines.

27
Q

A thermistor has a negative temperature coefficient, what does this mean?

A

The resistance drops as the temperature increases, as when the temperature increases, the number density of charge carriers does also.

28
Q

What is the experiment that can be performed to investigate how resistance of a thermistor varies with temperature?

A
  • Use an ohmmeter and a water bath. alternatively an ammeter and voltmeter can be used to calculate resistance.
  • A thermistor is connected to this, and submerged into the water bath, which changes through temperature ranges 20-50°C.
  • Resistance is recorded/calculated at different temperatures and plotted on a graph, resistance against temperature.
29
Q

What does the I-V characteristic of a thermistor look like, and what conclusions can be made?

A
  • Begins as straight line through origin, gradient eventually increases.
    The current increases and so temperature does too, but resistance decreases as the number density increases. It is a non-ohmic conductor.
30
Q

What is the function and uses of LDRs.

A

LDRs are small electrical components that change resistance depending on the light intensity.
- Used in street lamps, smartphones, laptops and telescopes.

31
Q

How does resistance of an LDR change with light intensity?

A

In dark conditions, the LDR has high resistance which means the number density of free electrons is very low. When light shines onto an LDR, the number density of charge carriers increases dramatically, so resistance rapidly decreases.

32
Q

What type of material are thermistors and LDRs made of?

A

They are made of semi-conductors.

33
Q

List 3 factors that affect the resistance of a wire.

A
  • Material of the wire.
  • Length of the wire.
  • Cross-sectional area of the wire.
34
Q

How is the resistance of a wire related to its length?

A

The resistance of a wire is directly proportional to the length of the wire.

35
Q

How is the resistance of a wire related to its cross-sectional area?

A

The resistance of a wire is inversely proportional to the cross-sectional area of the wire.

36
Q

What is the equation used to calculate resistance from length, cross-sectional area and resistivity?

A

Resistance (Ω) = (Resistivity (Ωm) x length (m))/Cross-sectional Area (m2)

37
Q

How is resistivity defined?

A

The resistivity of a material at a given temperature is the product of the resistance and cross-sectional area, divided by length.

38
Q

What are the units of resistivity?

A

Resistivity is measured in ohmmeters

39
Q

What is the difference between resistance and resistivity?

A

Resistance - refers to specific component only.
Resistivity - describes electrical property of a material.

40
Q

What happens to the resistivity of a metal wire as its temperature is increased?

A

The resistivity of a material varies with temperature the same was as resistance of most components vary with temperature. As the material gets hotter, resistivity increases.

41
Q

What is an experiment that can be used to determine the resistivity of a wire?

A
  • Measure the p.d and current of a wire to determine resistance.
  • Because the resistance is directly proportional, it is a straight line through the origin when plotted.
  • The gradient is resistivity/area, and therefore resistivity can be determined by m x a.
42
Q

What is electrical power?

A

Electrical power is the rate of energy transfer by each electrical component.

43
Q

What is the generic equation for power?

A

Power (W) = P.d (V) x Current (A)

44
Q

What are three equations that can calculate electrical power?

A

1) P=VI
2) P=I²R
3) P=V²/R

45
Q

How can the energy transferred by an electrical component in a given time be calculated?

A

Energy Transferred (J) = P.d (V) x Current (I) x Time (S)

46
Q

Define the kilowatt-hour.

A

A kilowatt-hour is the energy transferred by a device with a power of 1kW operating for an hour.
1kW=3.6MJ

47
Q

How can you calculate the energy transferred to a device in kilowatt-hours?

A

Convert the energy transferred (J) into MJ, and divide by 3.6 to get kWh.

48
Q

What are alternate phrases for power?

A
  • Energy transferred per second.
  • Rate of energy transferred.
49
Q

How can you calculate the energy transferred to a device in joules?

A

Energy Transferred (J) = Power (W) x Time (s)

OCR-A A Level Physics (9 decks)

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