Ohms Law & Resistance Flashcards
(47 cards)
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.
