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Physics Igcse > P7 > Flashcards

P7 Flashcards

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1
Q

T or F: all wires with a current running through them have magnetic fields

A

True

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2
Q

What is the pattern of magnetic fields in straight wires and solenoids?

A

Wire:
Right hand rule
Thumb -> direction of conventional current
Other fingers -> direction of magnetic field (if question mentions movement of electrons it goes the other way)

Solenoids: coil of wire
The smaller magnetic fields generated around the wire combine so that the magnetic field of the solenoid is that of a bar magnet
Can become stronger with a soft iron core
Poles: look at ends:
Current clockwise -> south
Current anti clockwise -> north

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3
Q

What can change the direction of a magnetic force in an electromagnetic field?

A

Reversing the current
Reversing the magnetic field

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4
Q

What will happen to the magnetic field if the magnitude and direction of the current is changed?

A

Magnitude:
Increased current -> increased strength of magnetic field

Direction:
Change direction of current -> change direction of magnetic field
Changing one will change the other so it’s essentially doing the same thing

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5
Q

Describe an experiment that can show the forces acting on a wire in a magnetic field (including reversing the direction)

A

Material:
Wire
Cell
Ammeter
Variable resistor
Connecting wires
Cardboard (holes for wire)
Clamp stand
compasses or iron filings

Steps:
1. Attach wire to cardboard
2. Secure to stand (vertical wire)
3. Attack wire to series circuit (with resistor and ammeter)
Compass:
4. Place compasses on cardboard
5. Draw direction of arrow and repeat around the whole wire
6. Repeating at different distance to the wire

Iron filings:
4. Pour, shake, observe

Repeat after change current or the direction of the field

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6
Q

Describe an experiment that can show the forces acting on a solenoid in a magnetic field (including reversing the direction)

A

Material:
Solenoid
Cell
Ammeter
Variable resistor
Connecting wires
Cardboard (holes for wire)
Clamp stand
compasses or iron filings

Steps:
1. Attach wire to cardboard
2. Secure to stand (vertical wire)
3. Attach wire to series circuit (with resistor and ammeter)
4. Iron filings:
Pour, shake, observe

Repeat after change current or the direction of the field

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7
Q

What is Fleming left hand rule? What is it used for?

A

Fleming LEFT HAND rule is used to determine the relative direction of force, field and current in MOTORS

Thumb -> force (or (thuM(otion)b)

First finger -> field

Second finger -> current (conventional)

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8
Q

What happens to a current-carrying coil in a magnetic field?

A

It experiences a turning affect

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9
Q

What can increased the turning effect of a dc motor?

A

Increasing the number of turns on the coil
Increasing the strength of the magnetic field

-> force increased

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10
Q

What is a motor?

A

A motor converts electrical energy into mechanical energy to produce a turning effect

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11
Q

What are dc and ac?

A

Direct current:
Pretty straightforward (hahaha)
Powered by cells or batteries
Graph for voltage -> straight horizontal line

Alternate current:
Current periodically changes direction
Powered by mains
Graph: mountain and valleys

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12
Q

Explain how a dc motor works (not including a split ring commutator)

A
  1. Current flows through the coil at 90° to the magnetic field
  2. Current -> magnetic field
  3. The already existing magnetic field and the magnetic field of the wire exert a force on the coil
  4. Turns the coil (using Fleming left hand rule figure out the direction)
  5. This is happening on the other side of the coil too -> just in the opposite direction
  6. Because of the opposite forces the coil turns until it is vertical at which point momentum keeps it turning
  7. The coil has now made a half turn and the steps are repeated again
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13
Q

What is a split ring commutator?

A

A split ring commutator swaps the contacts of the coil -> changes which side of the coil is touching the neg/pos terminal every half turn

This allows a full rotation -> or else it would flip flop and never make a full rotation

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14
Q

What factors can be changed in a dc motor to increase speed and change the direction of rotation?

A

Increase speed:
Increase current
Strong magnets

Direction of rotation:
Change direction of current
Change direction of the magnetic field

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15
Q

What is electromagnetic induction?

A

When a conductor moves perpendicular to a magnetic field, an emf (and current) is induced because the conductor has cut through the the field lines

emf and current can also be induced if a conductor is stationary and there is a constantly changing magnetic field (remember: magnet going through a solenoid counts) -> field lines cut through turns of coil

REQUIRED MOVEMENT

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16
Q

How can you find the direction of the emf/current in electromagnetic induction?

A

Right hand dynamo rule

Basically the Fleming left hand rule but on your right hand -> used for generators

17
Q

What factors affected the magnitude of an induced emf (electromagnetic induction)?

A

Increasing the speed of magnet/wire/coil

Increase number of turns on the coil

Increase the size/area of the coil

Increase the strength of the magnetic field

18
Q

What is a generator?

A

A device that converts mechanical energy into electrical energy

Mechanical energy is provided by rotating turbines via:
High pressure steam
Wind
Falling water

19
Q

How does an ac rotating coil generator work (no slip rings) ?

A
  1. Coil is force to spin in a uniform magnetic field
    (Connected to circuit via slip rings and metal brushes)
  2. When the coil cuts through a magnetic field -> induces emf and current

The ac is caused by the fact that the because of the structure of the slip rings, every half turn the side if the coil that is touching the metal brushes changes (the direction of the current NEVER ACTUALLY CHANGES)

20
Q

When is the induced emf the biggest in an ac generator?

A

Field lines and coil (horizontal) are parallel to each other-> cuts the field lines fastest

Smallest when the field lines and coil (vertical) are perpendicular -> doesn’t cut through the lines

21
Q

What is a transformer?

A

An electrical device used to increase or decrease voltage of an alternating current

22
Q

What is the structure of a transformer?

A

An iron core (block of iron with middle gone) -> magnetic field

Primary side:
Primary coil wrapped around the core-> electromagnet
Top -> north and negative
Bottom -> south and positive

Secondary side:
Secondary coil wrapped around core -> electromagnetic induction
Bottom -> south and positive
Top -> north and negative

23
Q

How does a transformer work?

A
  1. Alternating current supplied to primary coil (via a circuit)-> produces a constantly changing magnetic field around the primary coil
  2. Iron core -> magnetized because the changing magnetic field passes through it
  3. Changing magnetic field in secondary coil -> induces voltage + current (that alternate)

If secondary coil part of a circuit -> alternating current will flow

24
Q

What does a step up transformer do?

A

Increase the voltage of the power source

Used when power goes out of a power station (electricity transport at high voltage, low current -> less chance of fire and less power loss )

More turns on the secondary coil

25
Q

What does a step down transformer do?

A

Decrease the voltage of a power source

Used in appliances (most used a lower voltage than mains)

More turns on primary

26
Q

What equation relates voltage to the number of turns?

A

V(p)/V(s) = N(p)/N(s)

Primary voltage (input)/secondary voltage (output) = primary turns/secondary turns

ASSUMES 100% EFFICIENCY/NO POWER LOSS -> NO RESISTANCE

27
Q

Why are transformers important in the transmission of electricity?

A

When a grid supplies electricity, if it is transported at a high current, lots of energy is lost as heat

So, transformer converts it so there is a low current and high voltage

Then when it reaches homes, transformers -> high voltage -> low voltage

28
Q

What is an equation using current and voltage for transformers?

A

Ip x Vp = Is x Vs

Explanation:
Power = current x voltage
Power should remain constant in both primary and secondary circuits (?)

ASSUMES 100% EFFICIENCY

29
Q

Why is power loss lower when voltage is high?

A

Energy loss is a result of heat loss as the current goes through the wire (high resistance, low voltage (they are inversely proportional)

So, if the voltage is high the resistance is low, making the current also low resulting in less power loss

30
Q

What is the equation for power loss?

A

Power loss (watts) = I^2 (amps) x R (ohms)

Physics Igcse (9 decks)

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