Ohms Law & Resistance Flashcards
What is ohms law?
Current going through a metallic conductor is proportional to the voltage applied, provided physical conditions (e.g. Temp) are constant.
V=IR
In a series circuit, how do you calculate total resistance?
RT=R1+R2+R3+R4+R5
What is Kirchoff’s Voltage Law?
In a series closed circuit, the sum of the voltage drops are equal to the EMF
What is the Polarity of potential differences in a series circuit?
Potential difference is opposite polarity to EMF but same amplitude. They cancel each other out.
In a parallel circuit, how do you calculate total current?
IT = I1 + I2 + I3 + I4
- Each arm has its own current independent of other arms
In a parallel circuit, how do you calculate total resistance?
(1/RT) = (1/R1) + (1/R2) + (1/R3) …….
- The equivalent resistance of two or more resistors is always smaller than the smallest resistance value.
In a series circuit, how do you calculate total current?
I = (EMF / RT)
In a parallel circuit, how do you calculate EMF drop?
Each parallel arm is supplied with the same EMF voltage.
What is Kirchoff’s current law?
The sum of currents entering a junction is equal to the sum of the currents leaving.
or…
The algebraic sum of currents at a junction is zero.
What is a bridge circuit?
It has 4 components in a diamond shape, feeding the voltage between 2 corners for an output from the 2 opposing corners.
What does a balanced wheatstone bridge mean?
The ratio of resistances and voltages in both arms (r1/r2 = r3/r4) are equal. No voltage exists between arms and no current is shown on the ammeter.
What does an unbalanced wheatstone bridge mean?
If the resistance ratio across one arm changes, the voltage will drop.
- Current flow will trigger the ammeter
What are applications for wheatstone bridges?
Often used in temperature control/indicating circuits (e.g. thermistors, strain gauges, pressure sensors).
- 1 indicator is measuring; the rest are stable.
What is the main disadvantage of using wheatstone bridges?
The accuracy is affected by changed to the power supply. If voltage deviates, bridge becomes unbalanced.
What is a power supply’s internal resistance?
Batteries have an internal resistance that causes a voltage drop and limits current. As current increases, the voltage drop across the internal resistance increases meaning less voltage is available for load.
For high load operations, is low or high internal resistance preferred?
For high current operations, a low internal resistance is needed.
- For batteries, the larger the battery the lower the internal resistance.
What are 4 factors that affect resistance?
- Length (Proportional)
- Cross-sectional area (Inversely proportional)
- Material
- Temp
What equation is used to calculate resistance of a conductor with uniform shape and dimensions?
R = (pL) / A
R= Resistance
p= Resistivity
L= Length (m)
A= Cross-sectional area (m^2)
What is a positive and negative temperature co-efficient?
PTC - Resistance increases with temp (gold, silver, copper)
NTC - Resistance decreases with temp (Carbon, silicone, germanium)
Is NTC or PTC preferred?
PTC is preferred due to the increase in resistance limiting the current, which limits overheating.
- PTC and NTC materials can be combined to cancel out and maintain constant stable resistance.
What are characteristics of Carbon composition resistors?
- Cheap and multi-purpose
- carbon granules mixed with low conductivity bonding agents which is formed into a rod.
What are advantages and disadvantages of Carbon Composition resistors?
Advantage: - Cheap and multipurpose
Disadvantage: - Not suitable for continuous high-power operations where highly stable characteristics are required but can handle high pulses.
What are characteristics of Carbon Film resistors?
- Ceramic tube with a spiral cut on carbon film. Thickness of the spiral determines resistance.
- Inexpensive, good tolerance (3-5%)
What are characteristics of metal film resistors?
- Similar to carbon film, but tighter tolerance
- Very stable and low noise.
What applications are metal film resistors used for?
Specialized use in RF receivers to not interfere with reception or Transmisson of signals due to electrical noise.
What are characteristics of Wire Wound resistors?
- Nichrome wire wound on ceramic
- Used for high power uses to dissipate Watts of power.
What do the 4 resistance bands mean?
1st two bands - resistance value
3rd band - multiplier
4th band - tolerance value
What is an acronym that can be used to remember the colour codes?
BBROYGBVGW
- Better be right or your great big venture goes wrong.
- 0-9
What do silver and gold mean on the resistors when it is used as a multiplier?
-Silver ( x10^-2)
-Gold (x10^-1)
Why are resistors manufactured to Prefered Values?
- Resistors are manufactured to a limited range for mass production and tolerances.
- The higher the tolerance, the more widely spread the preferred values are.
What is a rheostat and how is it connected in a circuit?
A variable resistor with 2 leads.
- Connected in SERIES with the load. Directly controls CURRENT.
How are most variable resistors constructed?
A rotary shaft that moved a slider arm around in a circular path.
What are the two main types of variable resistors?
- Carbon Track: A circular track of carbon layer.
- Wire wound: A high resistance nichrome wound on a ceramic former. The rotating contact runs along the top of the wire coil.
What is the advantage of using a carbon track variable resistor?
Self-lubricating
What is a Potentiometer?
A variable resistor with 2 fixed legs (connected to EMF) and a variable lead in the middle (Changes the voltage).
Total leads = 3
What are most variable resistors constructed as?
They are constructed as potentiometers because they can be used as either a potentiometer or a rheostat.
What is a thermistor and what are its applications?
Constructed of metal oxides that are sintered together. These change resistances significantly with a small change in temperature (NON-LINEAR).
- Applications: Low fuel warming where the thermistor experiences change in temperature when not submerged in fuel.
What is a varistor and what are its applications?
A variable resistor which rapidly changes resistance at a given voltage.
- Used for surge protection to absorb and conduct voltage spikes in delicate circuits.
Describe how a varistor works
- The varistor consists of granular zinc oxides which has a high resistance due to its grain boundary.
- When voltage exceeds the breakdown voltage, the resistance drops sharply and begins to conduct high current.
- Under normal operation, the varistor remains in high pressure and prevents conduction.
How does heating occur in a resistor?
When electrons travel between valence shells, energy is given off each time and converted to heat and during atomic friction with current.
- In a good conductor: Very little energy is given off
- In a poor conductor: Much more energy is needed to release electron, gives off more heat.
In terms of heating, how can corrosion points cause failure and overheating?
The corrosion points are high resistance, resulting in severe localized heating.
Does the size of the resistor matter in terms of handling power (wattage)?
- The smaller the resistor, the less power it can handle. Can result in overheating, smoke and fire. Larger resistors spread heat more rapidly and evenly.
What is work, and what is the equation for it?
Work = Voltage x Charge
- Work is done when a force on a body causes motion.
Define energy
Energy is the capacity to do work
Define power and give the equation for it.
Power is the average rate of work (Watts). 1 Watt = 1 Joule per second.
P=IV
P=(I^2) x R
P= (V^2) / R
Do the Total Power Consumption calculations account for whether the circuit is series or parallel?
No
What is the Maximum Power Transfer Theorem?
For a given internal resistance, the maximum power transfer occurs when the Load Resistance = Internal Resistance
- OR -
Max Power is provided when the supply and load impedances are matching.
