Topic 7- Electric and magnetic fields

Question 1

What is coulombs law?

Answer 1

The further charges are apart, the weaker the force.

F=Q1 * Q2
———–
4πεr^2

Question 2

What is electric field strength defined as?

Answer 2

Force per unit +ve charge

E= F/Q
E=V/d

Question 3

What is electric field strength inversely proportional too?

Answer 3

E∝1/r^2

Question 4

What does the time taken to discharge a capacitor depend on?

Answer 4

-Capacitance, this effects the amount of charge that can be transferred at a given PD.
-Resistance of the circuit. This effects the current.

Question 5

What is the time constant?

Answer 5

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.

Question 6

What happens to the time constant as the resistance in series with the capacitor increases?

Answer 6

It gets bigger

Question 7

What is magnetic flux density? (B)

Answer 7

A vector quantity measuring the strength and direction of the magnetic field around a magnet or an electric current. AREA MATTERS

Question 8

What is magnetic flux? Φ

Answer 8

Magnetic flux is quantity of magnetism(magnetic field) AREA DOESNT MATTER

Φ=BA

Question 9

What is the Force on a wire proportional too?

Answer 9

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

Question 10

What is the force on a charged particle?

Answer 10

F=Bqv sinθ

Question 11

What happens when you move a magnet near a coil?

Answer 11

An EMF will be induced in the coil, this is because the electrons in the coil will experience a force

Question 12

What is faradays law?

Answer 12

The induced emf is directly proportional to the rate of change of flux linkage.

ε= flux linkage change d(NΦ)
———————– =. ————-
time taken dt

Question 13

What is Lenz’s Law?

Answer 13

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

Question 14

Which direction do electric field lines point?

Answer 14

Away from positive charge

Question 15

What is electromagnetic induction?

Answer 15

When there is relative motion between the coil and a magnet, the flux linkage changes, this induces an emf in the coil

Question 16

What is Faradays law?

Answer 16

The size of an induced emf is directly proportional to the rate of change of flux linkage.

dt

Question 17

Equation for electric field strength?

Answer 17

E= F/Q =V/d

Question 18

Describe the method of Millikans Oil Drop to find the charge on an electron

Answer 18

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.

Question 19

What does a capacitor do?

Answer 19

• Store charge

Capacitance= Charge stored
———————
pd across capacitor

Question 20

Describe the capacitor discharging Curves, for I, V and Q

Answer 20

Decreasing exponentially as time goes on

Question 21

Describe the capacitor charging Curves, for I, V and Q

Answer 21

I decreases expenentially with time
Q and V increase exponentially and has a tangent when y= (max capacity)