Electromagnetism Flashcards
What type of attraction does a gravitational force and an electrostatic force have??
gravitational-always attractive
electrostatic-attractive or repulsive
What situation would F=Q1Q2 / 4(pi)(epsilon nought)(r^2) be used?
Rule for direction
-magnitude of force between 2 charges
direction is either towards each other or away from each other
-Forces acting on a charge between two other charges
direction is what way the outer charges are pulling or pushing the middle one (attractive or repulsive)(then add or subtract for total)
What is the E in F=QE?
magnitude of the electric field strength
What situation would E=Q1/ 4(pi)(epsilon nought)(r^2) be used?
Rule for direction
Either 1 or several point charges around an X
Treat X as positive. Direction is whatever way the charges pull or push X (attraction or repulsion)
1eV
1.6x10^-19J
Ferromagnetism
When these materials are exposed to a magnetic field, the dipoles align together, magnetising the material.
Magnetic induction
B
A measure of the strength of a magnetic field at a point
measured in Teslas
What does F=qvB equal?
mv^2/r (it is a centripetal force)
Explanation of helical motion of a charged particle
Charged particles entering a magnetic field at an angle will travel in a helical path due to the combination of its **circular motion perpendicular*$ to the field and its linear motion parallel to the filed
what is the r in B=uI/2(pi)r
distance from a current carrying a wire
What is L in F=BILsintheta?
length of conductor carrying the current I
What is the time constant for an RC circuit?
The time to increase the charge stored by 63% of the difference between initial charge and full charge
or
time taken to discharge the capacitor to 37% of initial charge
After how long can an uncharged capacitor be considered to be fully charged (or charged capacitor fully discharged) in RC circuits?
a time approximately equal to 5 time constants
Self-inductance
If a conductor is moved through a changing magnetic field, it causes electrons in the current to move. There is a self-induced emf in the conductor
(different to power supply emf)
Inductor
A coil of wire in a circuit that has the ability to induce an emf across itself
What is the value of the back EMF the instant the circuit is switched on
back emf=-emf of the supply.
explanation of back emf
-When the circuit is switched on, there is a large rate of change of current (large growth)
-This causes a large changing magnetic field around the inductor
-This induces a large back-emf across the coil
-This back EMF decreases the rate of change of current
-This causes the back emf to decrease
-As the current increases to its maximum value, the rate of change of current and back emf reduce to 0
Explain the shape of a graph of current against time when the switch on a circuit is closed with an inductor
The large rate of change of current produces a back emf which opposes the growth of current
Why would a capacitor be used in series with a tweeter?
To allow high frequency signals to passes (as high frequency means low reactance)
Why would in inductor be in series with a woofer?
To allow low frequencies to passes (low frequency means low reactance)
Where does an inductor store energy?
in the magnetic field around it
how is capacitive reactance different to resistance?
resistors oppose current by releasing energy in the form of heat-capacitors do not
What happens to the back emf of an inductor in an ac circuit (inductive reactance)
-current and magnetic field constantly changing (unlike dc where the inductor doesn’t oppose current when it is stable)
-At higher frequencies, the rate of change is greater and so back EMF is greater (larger opposition to ac current)
-inductive reactance
frequency increases, current decreases
What did james clerk maxwell say about electricity and magnetism
He unified them
They are really just manifestations of a single electromagnetic force, mediated by photons
What does eectromagnetic radiation exhibit?
wave properties as it transfers energy through space
What components does electromagnetic radiation have?
electric and magnetic field components
they oscillate in phase, perpendicular to each other and to the direction of energy propagation
If an inductor has a resistance, what is the total emf equal to
V(total of inductor)=V(resistive) + V(induced)=V(supply if there are no other components)
