Chapter 9, Energy, Power and Resistance Flashcards
Describe the circuit symbol for an open switch.
Two circles with a line from one to just above the other circle.
Describe the circuit symbol for a cell.
Two lines with one longer than the other.
Which line on a cell shows the positive terminal?
The longer one.
Describe the circuit symbol for a diode.
An equilateral triangle with a straight line perpendicular to the wire at the corner which touches the wire.
Describe the circuit symbol for a battery.
An amount of cells in series or two cells connected with a dotted line.
Describe the circuit symbol for a resistor.
A rectangle.
Describe the circuit symbol for a variable resistor.
A resistor with an arrow going through it from bottom left to top right.
Describe the circuit symbol for a filament lamp.
A circle with a cross through it.
Describe the circuit symbol for a fuse.
A resistor but the wire continues through the centre of the rectangle.
Describe the circuit symbol for a voltmeter
A circle with a V in the centre.
Describe the circuit symbol for an ammeter.
A circle with an A in the centre.
Describe the circuit symbol for a thermistor.
A resistor but with a diagonal line through it which is flat past the bottom left corner of the rectangle.
Describe the circuit symbol for a LDR.
A resistor with arrows pointing towards it from the top left.
Describe the circuit symbol for a LED.
A diode with arrows pointing away from it to the top right.
Describe the circuit symbol for a capacitor.
Two parallel lines with a gap between.
Defining equation for potential difference?
V= W/Q (p.d. = work done per unit charge)
What property does an ideal voltmeter have, what does it measure and how is it connected to a circuit?
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.
What is the electromotive force?
ε = W/Q (e.m.f. = work done per unit charge on the charge carriers)
How does an electron gun work?
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.
How do you derive the electron gun equation?
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.
What is the defining equation for resistance?
R = V/I (resistance is the ratio of p.d. to current).
Unit of resistance?
The ohm where 1Ω= 1VA^-1.
State Ohm’s law.
For a metallic conductor kept at a constant temperature the current is directly proportional to the p.d. across its ends.
Why does temperature need to be constant for Ohm’s law to apply?
Because increasing temperature increases resistance as there is an increase in the chance for an electron to collide.
What is the I - V characteristic of a fixed resistor?
A straight line through the origin.
What is the I - V characteristic of a filament lamp?
Curve which is steeper the closer it is to the origin (similar to an inverse cubic graph).
What is the unique property which defines a diode?
Current can only travel in one direction across them.
What is the I-V characteristic of a diode?
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)).
Resistivity equation?
R = ρL/A (resistance is proportional to length/area, constant of proportionality is ρ for resistivity).
What is a thermistor with a positive temperature coefficient and what is the I - V characteristic?
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)
How does a thermistor with a negative temperature coefficient work?
By increasing temperature the number density of the material increases, decreasing resistance.
What does a graph of temperature against resistance look like for an NTC thermistor?
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.
What is the I - V characteristic of an NTC thermistor?
Looks like a cubic graph.
What is an LDR?
A Light Dependant Resistor (LDR) decreases its resistance from an increase of light intensity.
What’s the equation for electrical power?
P = VI (power = p.d. * current), this can also be re-arranged to give P = V^2/R = I^2R
What is the kilowatt-hour and why is it used?
The kilowatt-hour is a larger unit of energy than the joule which is used to measure energy used. 1kWh = 3.6MJ
Typical cost per kWh?
6-15p.
