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A LEVEL PHYSICS > 9 - Energy, Power and Resistance > Flashcards

9 - Energy, Power and Resistance Flashcards

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1
Q

Define Voltage

A

The change in electrical energy transferred or work done per unit charge (when the electricity travels through component into another form of energy such e.g. heat.)

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2
Q

Voltage Equation

A

V (Voltage) = W (work done) / Q (charge)

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3
Q

How is a voltmeter connected in circuit and why?

A

It is connected in parallel as it measures the difference in energy between two points, measuring the difference in potential across a device.

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4
Q

State the SI unit for Voltage

A

V / Volts

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5
Q

Define one volt

A

The energy transferred per unit charge as the charge moves between two points in a circuit. 1V = 1JC^-1

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6
Q

What should the resistance of voltmeters and multimeters be and why?

A

They should have a very high resistance (infinite resistance) in order to minimise the amount of current passing through.

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7
Q

What is the difference between electromotive force and potential difference?

A

E.m.f is the energy transferred (chemical to electrical) to the charged carriers by the cell/battery (ε=W/Q). P.d is the energy transferred by the charged carriers.

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8
Q

State some sources of e.m.f

A

Cell, battery or power supply

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9
Q

Define resistance

A

The ratio between the potential difference across a component and the current flowing through it.

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10
Q

What is resistance measured in?

A

Ω (ohm)

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11
Q

Resistance equation

A

R (Resistance) = V (Voltage) / I (current)

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12
Q

Describe resistance in relation to the current

A

The higher the resistance of a component, the more energy it takes to push electrons through the component. The higher the resistance, the lower the current flow.

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13
Q

State Ohm’s Law

A

For a metallic conductor at a constant temperature, the potential difference across the conductor is directly proportional to the current flowing the conductor (V=IR).

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14
Q

How is Ohm’s law demonstrated on a V-I graph?

A

On a V-I graph, the gradient represents the resistance. The straight line (constant linear gradient) and the fact that the line goes through the origin demonstrates that voltage and current are directly proportional.

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15
Q

Why does Ohm’s law only apply to metallic conductors at constant temperatures?

A

Resistance changes with temperature meaning current won’t be directly proportional to voltage therefore ohm’s law is not obeyed.

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16
Q

How does an increases temperature affect the resistance?

A

The heat energy from an insulated wire causes the temperature in the wire to increase. This causes the ions in the metal conductor to vibrate faster, making it harder for the electrons to pass through resulting in a decreased current and increased resistance.

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17
Q

I-V Characteristic for resistor

A

The I-V graph for a resistor shows a straight line (constant resistance) and it going through the origin, showing that current and voltage are directly proportional to each other, obeying Ohm’s law meaning it is an ohmic conductor. steeper, lower R shallow, higher R

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18
Q

I-V Characteristic for filament lamp

A

The I-V graph for a filament lamp has the line still going through the origin. However, the line has a changing gradient and curves. This is because of the changing temperature of the filament lamp, which changes the resistance (9.4) therefore a filament lamp is a non-ohmic conductor as current and voltage are not directly proportional to one another

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19
Q

Polarity of diodes

A

Diodes are made from semiconductors which only allow a current in one particular direction.

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20
Q

I-V characteristics for Diodes and LEDS

A

At Point A, the resistance of the diode is very high (infinite). Here, the pd is in reverse direction therefore the diode does not conduct. At B, the resistance starts to drop as the pd increases. This point is known as the threshold pd. As the pd starts to increase from this point, the resistance drops increasingly, resulting in the diode with a very little resistance as number density of charged carriers increases. The potential difference across the diode and the current running through it are not directly proportional therefore is a non-ohmic component and the resistance is not constant.

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21
Q

What are LEDs?

A

LEDs are diodes that are made from a material that lights up whenever it conducts. When a current runs through, they light up to show that that part of the circuit is live.

22
Q

Effect of different threshold pds for LEDs

A

Different LEDs have different threshold pds, relating to the colour of light that they emit.

23
Q

What else should be placed in a circuit when an LED is placed?

A

Resistor in series with the LED to limit the flow of current so the LED does not overheat and melt.

24
Q

LEDs vs Filament lamps

A

LEDs are more efficient than filament lamps as they give little to no amounts of heat whereas filament lamps do give off heat, therefore LEDs draw a lower current.

25
Q

What factors effect resistance? (4)

A

Resistance depends on the type of material the wire is made of, the length of the wire, the cross-sectional area of the wire and the temperature

26
Q

Define resistivity

A

The measure of how much a particular material opposes electron flow (a constant to link the resistance of the material with its area and length)

27
Q

Units of resistivity

A

Ωm

28
Q

Resistivity equation

A

ρ = RA/L
Resistivity = (resistance x cross sectional area) / length

29
Q

Effect of temperature on resistivity

A

resistance increases due to increased vibrations of metal ions in wire meaning resistivity also increases

30
Q

Effect of adding impurities into semiconductors

A

In semiconductors, impurities present in them such as silicon significantly increase conduction.

31
Q

Resistivity for Conductors vs Insulators

A

Conductors have lower values for resistivity (around 10^-8Ωm) whereas insulators have higher values for resistivity (around 10^16Ωm)

32
Q

What is superconductivity?

A

When some materials are cooled their resistivity drops but then at a critical temperature, the resistivity suddenly drops to zero. This is Superconductivity.

33
Q

Superconductors

A

Any components made from a superconducting material have no electrical resistance. No energy is lost when there is a current in the material so huge amounts of charge can pass through a superconductor without getting warm.

34
Q

Superconductors development in the future

A

Typically, wires become superconducting at very low temperatures which is why room-temperature superconductors are being developed. The advantages of room-temperature superconductors are that they allow very high currents, the temperature won’t have to be controlled and no energy will be wasted as heat when current passes through the wires.

35
Q

thermistors and IV characteristics

A

made from a semiconductor with a negative temperature coefficient (NTC) As the temperature of the thermistor increases, its resistance drops. As temp increases, number density of charge carriers increases. The change in resistance is sudden and dramatic so is useful in temperature-sensing circuits.

Thermistors are non-ohmic components. The I-V characteristic has similar features to a filament lamp but curves the other way due to the negative temperature coefficient.

36
Q

What is an LDR made from typically?

A

A typical LDR is made from a semiconductor in which the number density of charge carriers changes depending on the intensity of the incident light.

37
Q

Changing conditions effect on LDRs

A

In dark conditions, the LDR has very high resistance and the number density of the free electrons inside the semiconductor is low, so the resistance is high. When there is alright shining on, the number density of the charge carriers increases dramatically, leading to a decrease in resistance.

38
Q

I-V Characteristic for LDR

A

As light intensity increases, resistance decreases due to change in number density of charge carriers.

39
Q

Define electrical power

A

Electrical power is the rate of energy transfer by each electrical component. This depends on the current in the component and potential difference across it

40
Q

How do you calculate electrical energy transferred?

A

W=ItV
electrical energy transferred = current x time x voltage

41
Q

Define 1kWh

A

The energy transferred by a device with a power of 1kW operating for a time of 1 hour.

42
Q

kWh to MJ conversion

A

1kWh = 3.6MJ

43
Q

Energy transferred =

A

power x time

44
Q

How does the electron gun work?

A

A small metal filament is heated (pd applied across), known as a cathode. Some electrons gain enough KE to escape the surface of the metal. This process is called Thermionic emission. A high PD is applied between the filament and the anode inside a vacuum. The free electrons accelerate towards the anode gaining KE. A small hole in the anode allows a beam of electrons to pass through, with specific KE.

45
Q

Electron gun equation

A

eV=0.5mv^2
charge of electron x pd = …

46
Q

What is the work done by an electron equal to?

A

work done on electron is equal to kinetic energy gained due to conservation of energy

assuming negligible kinetic energy at cathode and no energy lost to heat during acceleration

47
Q

Effect of changing accelerating pd on speed of electrons

A

The greater the pd, the more energy is transferred to the electrons and so the faster they move.

48
Q

semiconductors only allow

A

current to flow in one direction

49
Q

current increases so does..

A

temperature

50
Q
A

A LEVEL PHYSICS (21 decks)

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