Topic 7- Electric and magnetic fields Flashcards
What is coulombs law?
The further charges are apart, the weaker the force.
F=Q1 * Q2
———–
4πεr^2
What is electric field strength defined as?
Force per unit +ve charge
E= F/Q
E=V/d
What is electric field strength inversely proportional too?
E∝1/r^2
What does the time taken to discharge a capacitor depend on?
-Capacitance, this effects the amount of charge that can be transferred at a given PD.
-Resistance of the circuit. This effects the current.
What is the time constant?
t= τ=RC
The time constant is the time taken for the charge, potential difference or current of a discharging capacitor to fall to 37% of its value when fully charged.
What happens to the time constant as the resistance in series with the capacitor increases?
It gets bigger
What is magnetic flux density? (B)
A vector quantity measuring the strength and direction of the magnetic field around a magnet or an electric current. AREA MATTERS
What is magnetic flux? Φ
Magnetic flux is quantity of magnetism(magnetic field) AREA DOESNT MATTER
Φ=BA
What is the Force on a wire proportional too?
The force on a current carrying wire is proportional to B, the magnetic flux density (B)
F=BIl If perpendicular
F=BIl sinθ if not perpendicular
If not perpendicular find the perpendicular component
What is the force on a charged particle?
F=Bqv sinθ
What happens when you move a magnet near a coil?
An EMF will be induced in the coil, this is because the electrons in the coil will experience a force
What is faradays law?
The induced emf is directly proportional to the rate of change of flux linkage.
ε= flux linkage change d(NΦ)
———————– =. ————-
time taken dt
What is Lenz’s Law?
The induced emf is always in such a direction as to oppose the change that caused it.
ε= -flux linkage change - d(NΦ)
———————– =. ————-
time taken dt
Which direction do electric field lines point?
Away from positive charge
What is electromagnetic induction?
When there is relative motion between the coil and a magnet, the flux linkage changes, this induces an emf in the coil
What is Faradays law?
The size of an induced emf is directly proportional to the rate of change of flux linkage.
dt
Equation for electric field strength?
E= F/Q =V/d
Describe the method of Millikans Oil Drop to find the charge on an electron
Millikan sprayed ionised drops of oil into a chamber. They were then allowed to fall into a uniform electric field.
Once in the uniform electric field, the strength of the field was adjusted in order to keep an oil drop stationary. In a stationary position, the gravitational force and the electric force were balanced - there was no net force on the oil drop. So, at this point:
qV
—- =mg
d
The voltage at which the drops were stationary was measured. The charge on each drop was then calculated. Millikan found that these charges were all multiples of 1.6 x 10−19C, thus showing that the charge of each drop was made up of smaller charges with a charge of 1.6 x 10−19C.
What does a capacitor do?
- Store charge
Capacitance= Charge stored
———————
pd across capacitor
Describe the capacitor discharging Curves, for I, V and Q
Decreasing exponentially as time goes on
Describe the capacitor charging Curves, for I, V and Q
I decreases expenentially with time
Q and V increase exponentially and has a tangent when y= (max capacity)
