Resistance And Resistors Flashcards
The resistance of a material is
the reciprocal of its conductance.
Resistance is measured using what unit of temperature?
Absolute.
A resistor has 4 bands on it coloured blue, yellow, yellow, gold. It’s value is
640 kΩ ± 5%.
The electron flow through a conductor will be decreased the most if the cross sectional area
is decreased and the length is increased.
Small resistors too small or mis-shapen for the application of colour are marked instead using
a letter code.
Total resistance in a parallel resistor circuit, of R1 and R2 is
1/RT = 1/R1 + 1/R2.
If the resistance of a resistor which is in series with two other resistors is doubled
the volts drop across that resistor increases.
The current will reduce, but not halved, since it depends on how big the other two resistors are in relation to it. It will however take a greater share of the voltage drop in the circuit.
The total resistance of the circuit shown is
3 Ohms.
Series 3 + 3 = 6 ohms. Then, two 6 ohm resistors in parallel, total is half = 3 ohms.
Potentiometers are used as a
variable resistor.
Potentiometers are not a ‘source’ of voltage.
In a Wheatstone Bridge, the ammeter reads zero, and the variable resistor is adjusted to 5 ohms. Resistor P has a value of 10 ohms and Q has a value of 100 ohms. What is the value of the unknown resistor?
0.5 ohms.
R = Rv * P/Q.
In general, increasing the cross sectional area of an electrical cable
enables it to carry more current.
Get the formula for resistance of a cable. If you increase the cross sectional area, its resistance will decrease and it can carry more current. ‘Carrying voltage’ is not technically correct terminology, and besides, Any size cable can ‘carry’ any size voltage, but it is the current which will destroy it.
Six resistors each of 6 ohms would be
1 ohm in parallel.
Get the formulas for resistors in parallel and resistors in series. Or, remember that resistors in series, total is greater than the largest, and resistors in parallel, total is less than the smallest.
A potentiometer varies
voltage.
A potentiometer is basically a variable voltage splitter.
The formula for resistance in series is
RT = R1 + R2 + R3 …..Rn.
What is the combined value of resistances of 4 ohm and 6 ohm connected in parallel?
2.4 ohm.
Resistors in parallel, the total resistance will be less than the smallest (but only slightly less).
If a number of resistors are connected in parallel, the total resistance is
smaller than the lowest.
Resistors in parallel. Total resistance is smaller than the lowest single resistor.
The resistance to electrical flow in a wire depends on
the diameter, length, material of wire and temperature.
When resistors are in parallel the total current is equal to
the sum of the currents.
Which of these will cause the resistance of a conductor to decrease?
Decrease the length or increase the cross-sectional area.
If 2 resistors, one red, yellow, black, gold and the other violet, blue, black, silver were replaced by a single resistor. Ignoring tolerance, what would the colour coding be?
Brown, black, brown.
The red/yellow/black resistor is 24 ohms (get a colour code chart). The violet/blue/black resistor is 76 ohms. Total is 100 ohms. A 100 ohm resistor is brown/black/brown.
A 47 kilohm resistor has the following colour code
Yellow, Violet, Orange.
The bridge circuit shown will be balanced when the value of the unknown resistor R is
2 ohms.
72 / 24 = 6 / R, R = 2 ohms.
A 300 ohm resistor would have a colour code of
orange, black brown.
In this circuit, Rx is equal to
R3 * R2 ÷ R4.
Assuming the diagram is incomplete (no cross branch with a zeroed ammeter as per a Wheatstone bridge) Take Rx/R3 = R2/R4 and transpose.
In a parallel circuit containing resistors
the voltage is the same in all parts of the circuit.
In a parallel circuit, the voltage across each resistor is the same.
Five different value resistors all have the same voltage dropped across them. How are the resistors connected?
In Parallel.
Conductance of a circuit is 2 milliMhos. What is its resistance?
500 Ohms.
Conductance is the ease at which current flows. It is the inverse of resistance. (The unit ‘Mho’ is ‘Ohm’ backwards). Conductance = 2/1000 Mhos. Therefore Resistance = 1000/2 Ohms.
When light hits a photodiode, its resistance
decreases.
Resistance decreases with light intensity.
A 2 Megohm resistor can be written
2M0F.
Sometimes the multiplier, like M (for meg), or K (for kilo) or R (for 1) is put in place of the decimal point. The last letter is the tolerance.
In a Wheatstone bridge at balance the galvanometer reads zero
amps.
The unit for resistivity is the
ohm.meter.
A 47 Kilohm resistor with a 10% tolerance has the following colour code
Yellow, Violet, Orange, Silver.
The total resistance in a circuit is greater than the least resistor. This is true for a
series circuit only.
The unknown resistance R in the Wheatstone bridge shown is
4 ohms.
R = 2 * 8/4. Read up on Wheatstone bridges.
This is a symbol for
a voltage dependant resistor.
VDR = Voltage Dependant Resistor.
A potentiometer has which of the following properties?
3 terminals.
This is a symbol for.
a voltage dependant resistor.
An increase in operating temperature in most electrical devices carrying current results in.
an increase in resistance and a decrease in current.
Copper is an inferior conductor to aluminium when comparing.
weight for weight.
What happens to the resistance of a copper conductor when the temperature increases?
It increases.
Carbon has a.
negative temperature coefficient.
The 5th coloured band on a resistor represents the.
tolerance.
If the temperature of a pure metal is reduced to absolute zero, its resistance will be.
practically zero.
What is represented by this diagram?
Potentiometer.
If a resistor is too small or misshapen to fit the colour code bands, how else is its value indicated?
Letter code.
This is a diagram of a.
Light Dependant Resistor.
This is a diagram of a.
Variable Resistor.
Resistors required to carry a comparatively high current and dissipate high power are usually of.
Wire wound ceramic type.
A thyristor.
if energized on, will switch on a circuit.
The temperature of a pure metal can greatly affect the resistance of it. What temperature scale is used?
Centigrade.
