Topic 7: Electric and Magnetic Fields Flashcards
Electric field
a region where a charged particle experiences a force
(direction of field = direction of force on positive charge)
Electric field strength (E)
the force per unit charge experienced by an object in an electric field;
constant in a uniform field, varies in a radial field
E = F/Q
for all electric field patterns
E = kQ/r^2
radial electric field from a point charge
- denser field lines = stronger field strength
- arrow point away from positive charge
- E against r graph exponential (close to 0 at the end, not touching both axis) 2 curves positive and negative
point charges
charges are considered as point charges
no radius, volume, mass
charge carriers
form a radial field
E = ∆ V/d
uniform electric field from parallel plates
- potential diff between plates
- same direction
- same density of field lines
- positive to negative
- edge effect
- E against d graph exponential (close to 0 at the end)
Explain what is meant by a uniform electric field.
A region where a force acts on a charged particle. The force is the same at all points.
Coulomb’s law = electric field
states that the magnitude of the force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them
Electric potential (V)
energy per unit charge in moving a positive test charge from infinity to the point in the electric field
(as the distance from the charge increases, the potential decreases, so electric potential at infinity is 0)
- when the charge is positive, potential is positive and the charge is repulsive
Electric potential difference (∆ V)
the energy needed to move a unit charge between two points
Electric force
uniform field: exerts same electric force everywhere in the field
radial field: magnitude of electric force depends on the distance between the two charges
Equipotential surfaces
potential on an equipotential surface is the same everywhere, so when a charge moves along an equipotential surface, no work is done
parallel plates: equipotential surfaces are planes equally spaced and parallel to plates
point charge: equipotential surfaces form concentric circles
Explain why the equipotential surfaces are not equally spaced
field strength = ∆ V/∆ r
∆ V are the same
field strength at a further point is lower
so ∆ r is larger, causing a larger gap between equipotential lines
Explain how the magnetic field maintains the charged particle in a circular orbit.
The force exerted is always perpendicular to the motion of travel, causing charged particles to follow a circular path as the force induced by the magnetic field acts as a centripetal force, causing centripetal acceleration (direction is to the centre of circle).
Explain the motion of the oil drop in terms of the forces acting on it as the p.d. is increased from 0V.
Oil drop stops falling and remains stationary as at terminal velocity, the forces on the drop are balanced (weight=drag). The p.d create an electrostatic force acting upwards on the drop. The electrostatic force increases as p.d increases. The net upward force causes the drop to have a negative acceleration. As speed decreases, the drag decreases. The drop remains stationary when the forces are balanced, when weight = electrostatic force.
How to increase strength of magnetic field induced?
- increasing the current flow
- increasing the turns on the solenoid
- wrap solenoid around iron core
Motor - useful energy transfer
motor transfers electrical energy to KE
Force on the wire (a force acts on charged particles moving in a magnetic field)
- battery causes a current flowing through the wire
- current flowing induces a magnetic field around wire
- magnetic field interacts with other magnetic field
- force exerted on wire
- movement of wire
change in magnetic field or changing magnetic flux linkage with a conductor =
induced emf
Fleming’s left hand rule
Force
Magnetic Field (N to S)
Current (+ to -)
Inducing voltage
- coil experience changing magnetic field
- coil cuts through magnetic field lines
- induces voltage in coil
Methods to increase induced voltage
- stronger magnetic fields
- increase no. of coils on solenoid
- increase rate of change of magnetic field (quicker, more per s)
- wrap wire around iron core
Generator - useful energy transfer
generators transform KE to electrical energy
- magnet moving inside solenoid (move it in and out)
- causes changing magnetic field
- induces a voltage in solenoid
- ammeter detects current
Why does the voltmeter register a reading?
- the handle turns
- which causes the coil to rotate inside the magnetic field
- which means the coil cut through the magnetic field lines
- leading to induced voltage in the coil
Galvanometer
- galvanometer detects current
- current is detected bc there is voltage
- voltage is induced bc there is a changing magnetic field
- there is a change in magnetic field bc the magnetic is moving into the solenoid
Magnetic field strength B
Force exerted on wire of unit length with unit current flowing
unit: Tesla (T)
Magnetic flux density B
a measure of the strength of the field
unit: Tesla (T)
Magnetic flux φ
value that describes the magnetic field lines passing through a given area
unit: weber (Wb)
Magnetic flux linkage Nφ
product of magnetic flux and number of coils
unit: weber turns (Wb)
rate of change of magnetic flux linkage
the amount of magnetic flux linkage that changes per time
unit: volts (v)
F = BQVsinθ
force on charged particles
v = velocity
F = BILsinθ
force exerted on wire
magnetic flux linkage = magnetic flux (no. of coil)
magnetic flux density = (magnetic flux)/ area perpendicular to field
Conditions needed for magnetic force to act
- particle must be charged
- particle must be travelling
- particle’s velocity must have a non-zero component perpendicular to magnetic field
Direction of magnetic force
magnetic force is always perpendicular to the charged particle’s velocity causing circular motion
Magnetic force and work done
- magnetic force and displacement are perpendicular
- therefore, work on the particle will be zero
- resulting in the particle’s KE to be unchanged
Faraday’s law (magnitude of induced emf)
the magnitude of induced emf is directly proportional to the rate of change of magnetic flux linkage
Lenz’s law (direction of induced emf)
the induced emf would go in the direction to oppose the change causing it
Alternating current
A current which periodically varies between a positive and negative value
Peak current (I0), or peak voltage (Vo)
The maximum value of the alternating current or voltage; determined from the amplitude of a current-time or voltage-time graph
peak-to-peak current or voltage
the distance between a positive and consecutive negative peak;
peak voltage Vo=peak−to−peak voltage ÷2
rms value of an alternating current
The equivalent direct current that produces the same power
rms value of an alternating voltage
The equivalent dc voltage that produces the same power
Magnetic flux
a measure of the magnitude of a magnetic field passing through a given area
