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Physics: Electrons, Waves and Photons > Chapter 9, Energy, Power and Resistance > Flashcards

Chapter 9, Energy, Power and Resistance Flashcards

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1
Q

Describe the circuit symbol for an open switch.

A

Two circles with a line from one to just above the other circle.

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2
Q

Describe the circuit symbol for a cell.

A

Two lines with one longer than the other.

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3
Q

Which line on a cell shows the positive terminal?

A

The longer one.

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4
Q

Describe the circuit symbol for a diode.

A

An equilateral triangle with a straight line perpendicular to the wire at the corner which touches the wire.

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5
Q

Describe the circuit symbol for a battery.

A

An amount of cells in series or two cells connected with a dotted line.

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6
Q

Describe the circuit symbol for a resistor.

A

A rectangle.

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7
Q

Describe the circuit symbol for a variable resistor.

A

A resistor with an arrow going through it from bottom left to top right.

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8
Q

Describe the circuit symbol for a filament lamp.

A

A circle with a cross through it.

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9
Q

Describe the circuit symbol for a fuse.

A

A resistor but the wire continues through the centre of the rectangle.

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10
Q

Describe the circuit symbol for a voltmeter

A

A circle with a V in the centre.

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11
Q

Describe the circuit symbol for an ammeter.

A

A circle with an A in the centre.

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12
Q

Describe the circuit symbol for a thermistor.

A

A resistor but with a diagonal line through it which is flat past the bottom left corner of the rectangle.

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13
Q

Describe the circuit symbol for a LDR.

A

A resistor with arrows pointing towards it from the top left.

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14
Q

Describe the circuit symbol for a LED.

A

A diode with arrows pointing away from it to the top right.

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15
Q

Describe the circuit symbol for a capacitor.

A

Two parallel lines with a gap between.

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16
Q

Defining equation for potential difference?

A

V= W/Q (p.d. = work done per unit charge)

17
Q

What property does an ideal voltmeter have, what does it measure and how is it connected to a circuit?

A

A voltmeter measures p.d. and an ideal one has infinite resistance as it is connected in parallel to the part of the circuit you want to measure the p.d. across.

18
Q

What is the electromotive force?

A

ε = W/Q (e.m.f. = work done per unit charge on the charge carriers)

19
Q

How does an electron gun work?

A

A small metal wire is heated by a current, this wire is placed in a vacuum with a high p.d. between the wire (acting as a cathode) and an anode. Free electrons gain K.E. and fly towards the anode, passing through a small hole to great a beam of electrons.

20
Q

How do you derive the electron gun equation?

A

Set the equation W = QV (or in this case W = eV as it is one electron) equal to the K.E. equation and solving for velocity.

21
Q

What is the defining equation for resistance?

A

R = V/I (resistance is the ratio of p.d. to current).

22
Q

Unit of resistance?

A

The ohm where 1Ω= 1VA^-1.

23
Q

State Ohm’s law.

A

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

24
Q

Why does temperature need to be constant for Ohm’s law to apply?

A

Because increasing temperature increases resistance as there is an increase in the chance for an electron to collide.

25
Q

What is the I - V characteristic of a fixed resistor?

A

A straight line through the origin.

26
Q

What is the I - V characteristic of a filament lamp?

A

Curve which is steeper the closer it is to the origin (similar to an inverse cubic graph).

27
Q

What is the unique property which defines a diode?

A

Current can only travel in one direction across them.

28
Q

What is the I-V characteristic of a diode?

A

Resistance drops for high voltages (but diode will explode if p.d. is too high) but is basically infinite for negative p.d.’s giving a shape like an exponential curve shifted to the right (e.g. e^(x-5)).

29
Q

Resistivity equation?

A

R = ρL/A (resistance is proportional to length/area, constant of proportionality is ρ for resistivity).

30
Q

What is a thermistor with a positive temperature coefficient and what is the I - V characteristic?

A

Resistance increases as p.d. increases, looks like the I - V of a filament lamp. (this isn’t required as spec only covers ntc’s)

31
Q

How does a thermistor with a negative temperature coefficient work?

A

By increasing temperature the number density of the material increases, decreasing resistance.

32
Q

What does a graph of temperature against resistance look like for an NTC thermistor?

A

Looks like an e^-x curve, to use a thermistor as a sensing device you want the main drop off for resistance to be in the range you are measuring the temperature of.

33
Q

What is the I - V characteristic of an NTC thermistor?

A

Looks like a cubic graph.

34
Q

What is an LDR?

A

A Light Dependant Resistor (LDR) decreases its resistance from an increase of light intensity.

35
Q

What’s the equation for electrical power?

A

P = VI (power = p.d. * current), this can also be re-arranged to give P = V^2/R = I^2R

36
Q

What is the kilowatt-hour and why is it used?

A

The kilowatt-hour is a larger unit of energy than the joule which is used to measure energy used. 1kWh = 3.6MJ

37
Q

Typical cost per kWh?

A

6-15p.

Physics: Electrons, Waves and Photons (5 decks)

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