Electricity and magnetism Flashcards
What is the right-hand grip rule used for?
wires and Lenz’s Law
What is the motor effect?
When current flows through a wire in a magnetic field that isn’t parallel with the current, a force acts on the wire.
In a motor, how can the direction of the force be reversed? (Two points)
Two other:
- Reversing the direction of the current
- Reversing the direction of the field, by turning magnet around
How does Flemings left hand rule work?
thumb-motion (forces)
first- field
second- current
Electric motors are designed to…to..
…use the motor effect
…to create a turning movement.
At what position is there no forces acting on the wire?
Vertical position. Reliant on momentum to carry itself further round.
The forces have no turning effect. Turning effect changes direction past vertical.
How can the turning effect be increased in motors?
FaCT:
-turns
-current
-field
What does a commutator do?
A commutator reverses the current in the coil every 180° by reversing the power suply connections,
allowing the coil to keep moving in the same direction.
What is Fleming’s left-hand rule used for?
motors and beam of electrons
What should you remember when using the rule for beams of electrons?
Electron flow is in the opposite direction of conventional current.
How can the force on the beam of electrons be reversed?
By reversing the field.
What is the electron beam effect used in?
Particle accelerators, to focus and divert beams of charged particles.
These particles have a lot of kinetic energy, so travel at enormously high speeds. Huge fields are used to divert them.
What do all generators have in common?
- a magnetic field (prod by magnets or electromagnets)
- a coil of wire (fixed or moving)
- movement (coil and field move relative to each other)
If the generator is connected to a complete circuit…
If it is not…
..induced current flows
..induced e.m.f/voltage will be at its ends ready to make current flow around the circuit.
What is induced e.m.f?
The production of emf across an electrical conductor when it cuts through field lines
What is electromagnetic induction?
the production of an e.m.f across an electrical conductor when there is relative movement between the conductor and the field.
In terms of electromagnetic induction, a current flows when the wire is moved downwards. Explain what happens when the wire is moved upwards.
current flows in an opposite direction
In terms of electromagnetic induction, describe how to make a current flow.
move the wire or magnet up and down
A centre-zero meter
Needle to the left, the current flows one way, to the right the other way
How do you increase induced e.m.f?
-Stronger magnet
-move wire more quickly
-turns
[Induction and field lines] What does the cutting rule tell us?
further away –> fewer lines –> smaller e.m.f
quicker–> cut more quickly–> bigger e.m.f
more turns–> bigger e.m.f
What is Fleming’s right-hand rule used for?
generators and electromagnetic induction
What does a slip ring do in an a.c generator?
Slip rings provide
a continuous connection
as they rotate with the coil.
What are slip-rings?
Device used to allow current to and from the a.c generators’ coil. Induced current can flow to where it is needed.
How do you increase the voltage generated in an a.c generator?
ratm:
-turn more rapidly
-turns of wire
-coil with bigger area
-stronger magnets
Explain the direction of the turning effect as seen by an observer at O.
clockwise
force on left of coil up OR..
Which two structures does a motor have?
brush and commutator
Which two structures does a generator have?
slip rings and brushes
What is a commutator?
device used to allow current to flow to and from coil of d.c motor or generator
Suggest one reason why the DC motor cannot operate without a split-ring commutator
The coil cannot continue to rotate in the same direction.
How can you increase electromagnetic induction?
Are tm:
- stronger magnet
- wire/coil more rapidly
- more turns
How can you reverse the current when working with electromagnetic induction?
reverse magnet
How AC generators work? / Explain how rotating the coil continuously causes the galvanometer needle to show an alternating current.
- Axle is turned so coil rotates. Coil cuts field between the magnets, inducing an e.m.f. in the coil.
- This induces a current in the coil which is transferred to the galvanometer via the slip rings and brushes.
Direction of current flow changes with each 180 degree rotation of coil.
What is Lenz’s law?
the direction of an induced current
always opposes the change in the circuit
or the magnetic field that produces it
N brought close, N induced
(in the direction opp the magnet movement)
in=field opposes, out=field attracts
State the position of the rotating coil when the alternating output voltage is at a
maximum value and explain why the maximum output occurs at this position.
Horizontal. Maximum number of magnetic field lines are cut at fastest rate.
How does a transformer work?
A.C in primary generates changing magnetic field. Iron core transfers field to the secondary coil.
Secondary coil cuts magnetic field and an e.m.f. is induced in secondary coil.
Why transformers do not work on DC current?
An unchanging magnetic field passes through the secondary coil, so no voltage is induced.
Explain why the power losses in transmission cables are lower when electrical energy is transmitted at higher voltages.
The same power with a smaller current and higher voltage, means less resistance. Reduces the loss of energy due to cables heating up (cables heat up less)
How are transformers used in high voltage transmissions?
Step-up transformers increase the v of a.c supply to reduce energy loss by heating in cables over long distances. Step-down transformers decrease to a safer level before delivering to homes and businesses.
Why are transformers used in high voltage transmission.
The same power with a higher voltage, means a smaller current. Reduces the loss of energy due to cables heating up (cables heat up less)
Very efficient= reduced energy loss
What is the equation relating voltage in coils and turns in coil?
Vp/Vs=Np/Ns
Calculating power in transformer.
P=IV
Calculating power loss.
P=I^2 x R
Finding the current or voltage in a transformer equation.
Ip x Vp = Is x V s
A positive rod is brought near object A. A wire is connected from conducting object A to the Earth. State and explain any movement of charge.
Electrons flow from Earth to object to balance the positive charge on the object.
An electrical device has a metal case. Explain the benefit of earthing the metal case.
Earthing provides a low resistance path for the current to flow to the ground. A high current will cause the fuse to melt which stops current to flow to the appliance.
Explain the dangers of touching a metal case that has a live wire touching the case.
fatal electric shock (electrocution)
What is the reason for choosing a particular fuse rating?
The current rating should be just above the value of current that flows when an appliance is operating normally.
Too low -> melt during normal operation
Too high -> cables can burn out + fires, dangerous
If there is high current why is it necessary to use thick wire?
Have smaller resistance. Heated less, so less risk of insulation melting or fire.
Explain why a high voltage increases the efficiency of transmission even with thinner wires.
Power loss equation. High voltage allows low current. Thin wires have high resistance. Current has greater effect than wire resistance.
Name the process by which a changing current in the primary coil P causes a changing
current in the secondary coil S
Electromagnetic induction
State the component in the transformer to which the a.c. generator is connected.
primary coil
State the other main component of a transformer and the material from which it is made.
iron core
State the effect on the motor of reversing the connections to the battery
rotation reversed
The battery in Fig. 9.1 is replaced with a resistor. The coil is made to rotate by an external
mechanism.
Explain why there is a current in the resistor.
Electromagnetic induction. Magnetic field is cut and current is induced.
State two ways in which the graph would be different if the coil spins at a faster rate.
higher frequency and greater amplitude
The magnet is pushed into the solenoid, and then brought to rest with its N-pole just inside the solenoid.
Describe the movement of the pointer of the meter M.
pointer deflects then returns to zero
The magnet is now pulled to the left out of the solenoid, at a higher speed than in (i).
Compare the movement of the pointer of the meter with that seen in (i).
greater deflection in opposite direction for a short time
State an assumption you made in your calculation for (b)(i).
transformer is 100% efficient
The variable resistor is adjusted so that its resistance changes quickly.
State and explain what is seen to happen in the circuit of the second solenoid.
Ammeter needle deflects. Electromagnetic induction: changing field is cut and current induced.
The variable resistor is adjusted much more slowly.
State and explain the difference in what is seen to happen in the circuit of the solenoid. (3 points)
slower rate of cutting field lines, deflection is smaller
reading on ammeter is smaller
Explain why the forces on AB and
CD cause the loop to rotate about the axis.
forces opposite, causes a moment, rotates
State and explain the effect on the output from the secondary coil of when increasing the voltage across the primary coil.
Greater field. Turns and voltages equation. Output power increases.
When is Earthing useful?
when live wire touches metal case (not when excess current flows into the circuit)
When is a fuse useful?
When excess current flows into the circuit (not when live wire touches metal case)
Describe and explain what is observed on the ammeter as the magnet falls from the middle
point of the solenoid to the outside.
Meter deflects in opposite direction.
Deflection is greater initially.
magnet moves faster
more field lines cut per second
Features: motors Vs generators
motor-> motion prod, battery
generator-> crank, current prod
A very small negatively-charged oil drop in the air between a pair of oppositely charged horizontal metal plates.
Suggest, in terms of forces, why the oil drop does not move up or down.
upward force due to charge of plate=downward force due to weight of drop
no resultant force, forces balanced
Without losing any of its charge, the oil drop begins to evaporate.
State and explain what happens to the oil drop.
Weight decreases. Upward force is now greater. Moves upwards.
The hanging ball is attracted towards the charged rod.
Explain why this happens, given that the ball is uncharged.
Negative charges are close. Attraction between opposite charges is greater than repulsion between like charges.
An earthed metal wire is touched against the surface of the sphere and the sphere
is discharged. State what happens in the wire as the sphere is discharged.
electrons move through wire from the sphere to the Earth
A negative point charge Z placed near to sphere S, (full of electrons). The charge experiences a force due to the electric field surrounding S. Draw an arrow to show the direction of this force on the charge.
line from sphere, pointing towards Z, through Z
Voltage in series and parallel.
V=V1+V2+V3 (series)
voltage is the same everywhere (parallel)
Current in series and parallel.
current is the same everywhere (series)
current is split between branches, adding them gives current in power supply (parallel)
Resistance in series and parallel.
R=R1+R2+R3 (series)
(fractions for parallel)
An LDR and LED are in parallel. How does the reduced resistance of the LDR effect the p.d. across the LED?
Resistance of parallel combination of LDR and LED increases, so pd increases.
Greater proportion of p.d. across parallel combination of LDR and LED
What do brushes do in generators?
Rub against slip rings as they turn, ensuring that brushes have the same emf as the coil sides. They maintain the current.
Changing the resistance using area and length.
R is directly proportional to L, twice as long means twice R
R is inversely proportional to A, half cross-sectional area means twice R
