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Physics > Chapter 9 - Energy, Power And Resistance > Flashcards

Chapter 9 - Energy, Power And Resistance Flashcards

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1
Q

How do you draw a cell or battery?

A

Longer terminal is positive side

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2
Q

What is potential difference?

A

Measure of the transfer of energy by charge carriers

One volt is the potential difference across a component when 1J of energy is transferred per unit charge

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3
Q

What is the equation for potential difference (v)?

A

V=W/Q (same as emf) (energy transferred/charge)

V=IR

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4
Q

What is a voltmeter?

A

Used to measure p.d.
Connected in parallel across a particular component
Have resistance of several million ohms (so that minimum current passes through the voltmeter itself)

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5
Q

What is electromotive force (emf)?

A

Used to describe when work is being done on the charge carriers

Energy transferred from one form of energy to electrical energy per unit charge

Charges gain energy as they pass through cells or batteries

(Voltage Added)

Sum of E.M.F = Sum of P.D.

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6
Q

How does an electron gun work?

A

A small metal filament is heated by an electric current and the electrons gain kinetic energy and escape from the surface of the metal. This is called thermionic emission. (Emission of electrons through the action of heat)

There is a high p.d between the filament and the anode so the electrons accelerate towards it. The anode has a small hole in it so electrons with a certain kinetic energy (in line with it) can pass through it

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7
Q

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

A

Work done on electron = Gain in kinetic energy

eV=1/2mV^2 assuming electrons have negligible kinetic energy at the cathode

More accelerating p.d. means more energy transferred so faster movement

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8
Q

What is resistance?

A

High resistance means more energy is needed to push electrons through that component

Measured in ohms

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9
Q

How do you determine resistance?

A

Measuring current (I) and p.d (V)

R=V/I

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10
Q

What is an ohm?

A

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

1ohm = 1VA^-1

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11
Q

What is 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

Double the p.d means double the current

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12
Q

What is the relationship between temperature and resistance?

A

High temperature = High Resistance

Positive ions within the wire have more energy so vibrate with a greater amplitude so the number of collisions with free electrons increases

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13
Q

What is it called when a resistor obeys or disobeys Ohm’s law?

A

An ohmic conductor (obeys)

A non-ohmic conductor (disobeys)

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14
Q

How does an IV graph look for:
a fixed resistor
a filament lamp

A

Fixed Resistor: Straight Line with fixed gradient (shallow gradient means greater resistance.

Filament Lamp: Not a straight line, more like a subtle S shape, not a constant resistance

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15
Q

What is a diode?

A 
A component that only allows current to flow in one direction
Can be light emitting 
Non-Ohmic
Not a constant resistance
Its behaviour depends on polarity
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16
Q

What factors affect resistance?

A 
Temperature (High = High resistance)
Material
Length (Long = High Resistance)
Cross sectional area (Large = Low resistance)
17
Q

What is the difference between resistance and resisitivity?

A

Resistance refers to one component whereas resistivity describes the electrical property of a material.

Resistance is ohms
Resistivity is ohm metre

18
Q

What is the equation of resistance using resistivity and resistivity?

A

R = pL/A

Resistance = resistivity x length / cross sectional area

p = RA/L

Resistivity = Resistance x Area /Length

19
Q

What is a negative temperature coefficient?

A

When temperature increases the resistance drops. (because number density increases) Thermistors have this

20
Q

What is a thermistor?

A

An electrical component made from a semi conductor with a negative temperature coefficient
Can monitor small changes in temperature (thermostats, engine temp, kettles, toasters)

Temperature increases = Resistance decreases
Non-Ohmic

21
Q

What is an LDR?

A

A component made from a semiconductor in which the number density of charge carriers increases when light shines on it

light intensity increases = resistance decreases

22
Q

What is electrical power and what is its equation?

A

Rate of energy transfer by each electrical component

P=VI p.d. x current

P=I^2R current^2 x resistance

P=V^2/R p.d. ^2 / R

P=VQ/T p.d. x charge / time

23
Q

What factors affect how much a device costs to run?

A

The power of the device and how long it is used for

Physics (9 decks)

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