Electricity Flashcards
Electric current
Rate of flow of charge, measured in amperes/amps
Potential difference
The work done moving a unit charge between 2 points in a circuit
V=W/Q
Resistance
How difficult it is for current to flow through an appliance.
A component has a resistance of 1 ohm if 1 amp flows through it when a p.d of 1 V is applied across it
Ohmic conductor
A conductor that obeys Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions remain constant
Measuring current in a circuit
You can measure the current in a circuit with an ammeter connected in series with the component
Measuring p.d across a component
Using a voltmeter, connected in parallel across the component being measured
What does a gradient of a current-potential difference graph represent
Rate of change of current with respect to voltage
This is not the same as 1/R
Steeper gradient on resistance graph
A higher voltage is required for the same change in current
S shaped resistance graph
Filament lamp, as current increased the resistance also increases. A big increase in the voltage produces only a small increase in current
Why does the current increasing on a filament lamp cause an increase in the resistance
As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions (resistance has increased)
Diode
A diode is a appliance that only allows current to flow in one direction
Unless stated, should you assume the voltmeters to have zero resistance or infinite resistance
You should assume they have infinite resistance. Current takes the path of least resistance so, if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through it and all the current will flow through the appliance
Why assume an ammeter has 0 resistance unless stated
This assumption means that there would be 0 potential difference across the ammeter and no energy is lost across it, it does not affect the circuit
What is a light dependent resistor
A semi conductor that is sensitive to light
As the light intensity increases, its resistance decreases
How does a thermistor work
Similar to an LDR, but as the temperature increase, the resistance decreases (thermistors have a negative temp coefficient)
What is resistivity
p=RA/L
p = resistivity
R = resistance
A = cross sectional area
L = length
Superconductor
A material with 0 resistivity at or below critical temperature. The crit temp is an inherent property of the material
How to find total resistance of a circuit
Rtotal = R1+R2+R3+…
Add the individual resistances of each components
6 cells arranged in parallel, all with 5 V. What is voltage provided to the circuit
5V
6 cells arranged in series, all with 5 V. What is voltage provided to the circuit
30V
(5x6)
How does current vary between each component of a series circuit
The current through all of the components is the same so current does not vary
Current in parallel branches
Each branch of a parallel circuit can have different currents through them
Kirchoff’s first law
All of the current going into a junction is equal to the current leaving the junction
Kirchoff’s second law
For any path of a circuit, the sum of all the potential differences must equal to total emf of the circuit
Power
Rate of energy transfer
Equation for power
Power = current x voltage
P=IV
Purpose of potential divider
¬ To provide variable potential difference
¬ To provide a constant specific potential difference
What is emf
Electromotive force : the electrical energy transferred by a power supply per unit charge
