9. Energy, power, and resistance Flashcards by Eden Baxendale

Define potential difference

The energy transferred from electrical energy to other forms, per unit charge

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What is the equation for potential difference?
(include units)

V = W / Q

V = potential difference / V
W = energy transferred by charge / J
Q = charge / C

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What is one volt?

The p.d. across a component when 1J of energy is transferred per unit charge passing through the component
(1V = 1JC^-1)

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What is a voltmeter used for and how is it connected?

used to measure potential difference
connected in parallel across a particular component

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How does resistance affect a voltmeter?

The higher the resistance, the better the voltmeter

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Define electromotive force

The energy transferred from chemical energy (or another form) to electrical energy per unit charge

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What is the difference between pd and emf?

pd is used to describe when work is done by the charge carriers
emf is used to describe when work is done on the charge carriers

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What is the equation for emf?

e(3) = W / Q

e(3) = emf / V
W = energy transferred by charge / J
Q = charge / C

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What is an electron gun?

An electrical device used to produce a narrow beam of electrons
These electrons can be used to ionise particles by adding or removing electrons from atoms

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How does an electron gun work?

A metal filament (cathode) is heated by an electric current
The electrons in the piece of wire gain kinetic energy
Some of the electrons gain enough kinetic energy to escape from the surface of the metal via thermionic emission
Free electrons accelerate towards the anode gaining kinetic energy
If the anode has a small hole in it, then the electrons in line with this hole can pass through a beam of electrons with a specific kinetic energy

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What is the equation for energy transfer for electrons and other charged particles?

work done on electron = gain in kinetic energy
eV = 0.5mv^2
(this assumes that electrons have negligible kinetic energy at the cathode)

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What is the equation for resistance?

R = V / I

R = Resistance of component / Ω
V = p.d. across component / V
I = current in component / A

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What is the relationship between resistance and energy?

The higher the resistance of a component, the more energy required to push electrons through the component

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Define the ohm

The resistance of a component when a p.d. of 1V is produced per ampere of current

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State ohms law

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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What is the relationship between temperature and resistance?
Explain it

increase in temperature = increase in resistance

When temperature of the wire increases the positive ions inside the wire have more internal energy and vibrate with greater amplitude about their mean positions.
The frequency of the collisions between the charge carriers and the positive ions increases
So the charge carriers do more work - transfer more energy as they travel through the wire

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Draw the I-V graph for a resistor

straight line through the origin

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What can we conclude from the I-V graph of a resistor?

The potential difference across the resistor is directly proportional to the current in the resistor. Therefore:
- a resistor obeys ohms law and can be described as an ohmic conductor
- the resistance of the resistor is constant

The resistor behaves in the same way regardless of the polarity

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Draw the I-V graph of a filament lamp

s shape through origin

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What can we conclude from the I-V graph of a filament lamp?

The potential difference across a filament lamp is not directly proportional to the current through the resistor.
- a filament lamp does not obey ohms law and so can be described as a non-ohmic conductor
- the resistance of a filament lamp is not constant

The filament lamp behaves in the same way regardless of the polarity

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For a filament lamp what is the relationship between resistance and pd?
Explain

Resistance increases as pd increases

As current increases, rate of flow of charge increases, so more electrons pass through the filament lamp per second.
So more collisions occur between electrons and positive ions per second
When electrons collide with ions they transfer energy to the ions, causing the, to vibrate more

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What is special about a diode?

Only allows current in one particular direction

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What do LEDs do?

Emit light of a single specific wavelength

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Draw the I-V graph of a diode

flat on negative x-axis
increases on positive x and y quadrant

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What can we conclude from the I-V graph of a diode?

Potential difference is not directly proportional to current. Therefore: - diode does not obey ohms law and so can be described as a non-ohmic conductor - resistance of diode is not constant - Diodes behaviour depends on the polarity

Describe A, B, and C points on a I-V graph of a diode

At A: resistance is high with pd in the reverse direction, the diode does not conduct At B: As the pd increases, resistance gradually starts to drop At C: Above the threshold value the resistance drops sharply for every small increase in pd. Above this point the diode has very little resistance

What is the threshold pd?

The minimum pd at which a diode begins to conduct (around 0.7V for a silicon diode)

What is the equation for resistivity? include units

R = pL / A R = resistance / Ω p = resistivity / Ωm L = length / m A = cross-sectional area / m^2

How can you determine the resistivity of a material?

- Set up a circuit with a variable d.c. supply, an ammeter, and a voltmeter around a wire (draw image). - Record the current and pd across each length of the wire - Use R = V/I to calculate resistance - Plot a graph of R against L (should be straight line through origin) - R = pL / A , so gradient = p/A, multiply the gradient by A to obtain resistivity. more detail is needed eg what you measure length with and how

What are the factors that affect resistance?

- temperature of the wire - material of the wire - length of the wire - cross-sectional area of the wire

Define resistivity

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

How does resistivity vary in conductors, semiconductors, and insulators?

Conductors ~ 10^-8 Ωm Insulators ~ 10^16 Ωm

What does it mean if an electrical component has a Negative Temperature Coefficient (NTC)?

Resistance decreases as temperature increases

Why do some electrical components have a NTC?

In some semiconductors, as the temperature increases, the number density of the charge carriers also increases

What is a thermistor?

An electrical component made from a semiconductor with a negative temperature coefficient

What are thermistors used in?

- Simple thermometers - Thermostats - to control heating and air conditioning units - To monitor engine temperatures - To monitor temperatures inside electrical devices

What are the I-V characteristics for a thermistor?

Graph is s shape but not s thermistor is a non-ohmic conductor As current increases temperature increases - however, number density of charge carriers also increases which leads to a drop in resistance Resistance decreases as temperature increases

Draw an open switch

—o_ o—

Draw a closed switch

—o/o—

Draw a cell

—| I—

Draw a battery

—| I- - -| I—

Draw a diode

sideways triangle and line with a line going all the way through

Draw a resistor

rectangle with line going in an out

Draw a variable resistor

rectangle with line going in and out and a diagonal arrow going through

Draw a lamp

circle with cross in it line going in and out

Draw a fuse

a rectangle with a line going all the way through

Draw a voltmeter

circle with a ‘v’ in it line going in and out

Draw an ammeter

circle with an ‘A’ in it line going in and out

Draw a thermistor

rectangle with line going in and out hockey stick going through it

Draw and LDR

Smaller rectangle with line going in and out with 2 arrows point at rectangle at an angle

Draw and LED

sideways triangle with line with line going all the way through 2 arrows point away from it at an angle

Draw a capacitor

—| |—

Wait is the basic principle of an LDR?

The number density of charge carriers changes depending on the incident light

What happens in an LDR in dark conditions?

Number density of free electrons is low high resistance

What happens in an LDR in light conditions?

Number density of free electrons is high Low resistance

Define Electrical power

The rate of energy transfer by each electrical component

What are the 3 equations for electrical power?

P = VI P = I^2 R P = V^2 / R

Derive P = VI

V = W/Q —> W = VQ P = W/t P = VQ/t Q/t = I P = VI

What is the equation for energy transfer?

W = VIt W = Energy transferred / J V = pd / V I = current / A t = time / s

What is the defining equation for power?

P = W / t P = power / W W = work done / J t = time / s

Define kilowatt-hour

The energy transferred by a device with a power of 1kW operating for a time of 1 hour (1kWh =3.6MJ)

9. Energy, power, and resistance Flashcards

(61 cards)