PHY 10 - Electrical Circuits Flashcards
What is electrical current (I) ?
The amount of charge (Q) that flows past a point in a given amount of time (t)
What are the units of electrical current ?
Amps (amperes)
OR
Coulombs/second
How is an electrical current caused ?
By the movement of electrons between two points of significant potential difference of an electrical circuit.
How is energy lost by an electrical circuit ?
Free electrons accelerate towards the positive connection. As they move, they collide with atoms in the substance, losing energy which we observe as heat.
What is an electric current ?
The motion of electrons
What happens to an electrical circuit if the magnitude of the electric potential is increased ?
The current will increase because the electrons will accelerate faster
Describe the concept of joule heating ?
Collisions between electrons and atoms transfer energy to the atoms, which is released as heat.
What are Ohmic conductors ?
Materials such as joule heaters that obey Ohm’s law, V=IR
What is Ohm’s law ?
Describe how Ohm’s law works ?
What is an Ammeter ?
Used to measure the flow of current
What is this ?
An ammeter - measures the flow of current
What is resistance (R) ?
The measure of opposition to the flow of electrons in a substance
What is resistivity (p) ?
The inherent resistive property of a substance, which varies with temperature
What is the equation that includes resistance and resistivity ?
Why does electrical resistance vary with temperature ?
The thermal motion of molecules increases with temperature, resulting in more collisions between electrons which impede their flow.
What are the units of resistance ?
Ohm’s
1 Ohm = 1 volt/ampere
What happens when a positive current flows across a resistor ?
There is a voltage decrease and a power loss.
P = VI = V2/R
Describe how to calculate electrical power ?
What does a resistor look like in a circuit diagram ?
What is this ?
A bulb.
(N.B. This also acts like a resistor!)
Why is a bulb in an electrical circuit treated like a resistor ?
The filament inside a light bulb resists the flow of current, and becomes bright and hot.
The brightness of the bulb depends on how much power it loses.
What are the units of Q (charge) ?
Coulombs (C)
What are Capacitors ?
They are used in circuits to store charge, they are similar to batteries but can discharge much more quickly.
E.g. A flash gun on a camera
What is the unit of a capacitor ?
Farad
Describe what is meant by a series circuit ?
One component follows another…
How would you calculate the resistance of two resistors in series ?
Rtotal = R1 + R2 etc…
Describe what is meant by a parallel circuit ?
How would you calculate the resistance of two resistors in parallel ?
1/Rtotal = 1/R1 + 1/R2
What does EMF stand for ?
Electromotive Force
E.g. A battery
Units: Volts
Symbol:
What is the SI unit of power ?
Watts
What is this ?
A capacitor
What is a Dielectric ?
A capacitor that contains an insulator
What is this ?
A battery represented in a circuit diagram
How do you assign the direction of Current through a circuit diagram ?
Due to convention, charge flows from the positive end of the battery
What is a junction in a circuit diagram ?
A point in the circuit where the current can flow a number of ways.
What happens when a current meets a capacitor in a circuit ?
The capacitor becomes charged at the end of the current, the opposite end would become negatively charged, the charges are seperated by the capacitor which stores the current.
How does a capacitor store charge ?
When wired to a circuit, one plate of the capacitor will gain a positive charge, and the other a negative.
This can then be connected to a second circuit and the charge will flow.
Name two sources of EMF ?
- Generators
Mechanical power → Electrical Power - Batteries
Chemical Power → Electrical Power
What are the units of EMF ?
Volts (V)
Why is actual voltage delivered to the circuit not the same as the voltage of the source ?
The source itself has internal resistance (r) which causes a loss of voltage.
What happens when two sources of EMF are connected in oppostion (+ve pole to +ve pole) ?
The charge loses energy when passing through the second EMF source
What is Kirchoff’s first law ?
This is also known as the Junction theorem.
When different currents arrive at a point, the sum of the current equals zero.
N.B. We can then arbitrarily define all current arriving at the junction as positive, and all current leaving as negative.
What is Kirchoff’s second law ?
Also known as the loop theorem.
The sum of voltage changes in one continuous loop of a circuit is zero.
How are voltage changes in a circuit arbitrarily defined ?
Positive in a clockwise direction.
What is capacitance (C) ?
It is an inherent property of a conductor.
C = Q/V = Charge/Electrical potential = coulomb/volt
Therefore, capacitance is the number of coulombs that must be transferred to a conductor to raise its potential by 1 volt.
What happens are the dielectric strength of a medium increases ?
Dielectric strength is the electrical field strength at which a substance ceases to be an insulator and becomes a conductor.
The greater this value, the greater the capacitative strength of the medium.
What do dielectric substances do ?
They set up an opposing electrical field to that of the capacitor, which decreases the net electrical field, increasing the capacity of the capacitor.
The molecules of the dielectric are dipoles which line up in an eletrical field.
Explain the trend in the way capacitors discharge ?
The charge drains rapidly at first, then decreases gradually in an exponential decay pattern.
How would you calculate the potential energy of a capacitor ?
PE = 1/2 QV
Which end of the battery is positive ?
The larger line
