Physics- Magnetism and electricity
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- Created by: Kitsune
- Created on: 09-03-17 07:35
Like poles ... and unlike poles ...
repel, attract
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What materials are magnetic?
Iron, nickel, cobalt, steel
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Describe hard magnets
Difficult to magnetise, retain their magnetism for a long time
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What are the uses of hard magnets?
Compass needles and loudspeakers
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Describe soft magnets
Easy to magnetise and don't retain magnetism well
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How to magnetise something?
Stroke with a permanent magnet in one direction only, place the material in a field of direct current
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How to demagnetize something?
Hammer it, heat it, place it in a field of an electromagnet with alternating current
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How can permanent magnets attract unmagnetized materials?
By induced magnetism
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In what direction to magnetic field lines go?
From north to south
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What do magnetic field lines show?
Direction and strength of the field
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What metal is usually used for electromagnets?
Copper
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Why doesn't the coil have to be magnetic in an electromagnet?
Because the electric current that passes through it produces a field
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What are the ways of increasing an electromagnet's strength?
Increase number of coils on a wire, increase current, add a soft iron core
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What is the great thing about electromagnets?
They can be switched off
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What are the uses of electromagnets?
Electromagnetic cranes, doorbells, loudspeakers, transformers
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Describe how a ballon can induce a charge into a wall
Rub the balloon on cloth and make it positive. The wall is neutral but its atoms have positive and negative particles. The balloon is brought to the wall and the electrons in the wall are attracted to the balloon
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What do charged objects have around them?
An electric field
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What is charge measured in?
Coulombs
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What do you need for a current to flow?
A complete circuit, a conductor, a power supply
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Direct current flows in...
one direction
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Why do our bodies conduct?
Because water in our tissues is a good conductor
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What is current?
Flow of electric charge
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What device do we use to measure current?
Ammeter
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Where can you place an ammeter?
Anywhere in the circuit
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What is current measured in?
Amperes
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What is conventional current?
Flow of positive charge from negative to positive
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What is electron flow?
Movement of conduction electrons from negative to positive
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What is current?
The rate at which electric charge passes a point in a circuit
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What is potential difference?
The difference of electrical potential between two points
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What is p.d. measured in?
Volts
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What is electromotive force?
The p.d. across a cell, it's the energy supplied by a source to drive a charge around a complete circuit
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What is resistance measured in?
Ohms
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What is the difference in how a voltmeter and an ammeter are connected?
A voltmeter is connected in parallel and an ammeter is connected in series
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The longer the wire, the....
Higher the resistance
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The large the diameter of a wire, the ...
lower the resistance
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When is a resistor ohmic (judging by a graph)?
When the graph is straight
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When is a resistor not ohmic (judging by a graph)?
When the graph is curved
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Resistance of a wire is ... to its length
proportional
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Resistance is ... to it's cross section
inversely proportional
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What happens to emfs when they are connected in series?
They add up
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What is power measured in?
Watts
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What does a resistor do?
Control the amount of current flowing
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What is another name for a variable resistor?
Potentionmeter
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What does a viable resistor do?
Alter the current
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What is an input transducer?
Any device whose electrical properties change when its environment changes
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Input transducer-> circuit-> ...
output transducer
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What is an LDR?
Light dependent resistor
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How does LDR's resistance change?
In the dark its resistance is high but in light its resistance is low
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What does a thermistor's resistance depend on?
Temperature
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What does a thermistor do?
Prevent over heating
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What is a relay?
A switch that works using an electromagnet
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Describe how a relay works.
There are two complete circuits. Current flows through the first circuit and magnetizes the coil. The coil pulls the switch on the second circuit, it closes and starts a current in the second circuit.
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What does a diode do?
Allows current to flow in one direction only
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What is LED?
Light emitting diode
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What is rectification?
When diode acts as a rectifier and converts alternating current into direct current
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Mains electricity uses ... current but objects use ... current
alternating , direct
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How are LEDs useful?
They show wether a system is on
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Where are LDRs used?
Night alarms and digital clocks
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How are light in a house connected and why?
In parallel so that if one dies the others don't
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What does a potential divider circuit do?
It allows the person to obtain a smaller p.d. by splitting up the fixed p.d.
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How can you control and vary the output of a circuit?
Bu using a variable resistor
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What happens to current in a parallel circuit?
Current divides
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In series circuit p.d. is ... between the resistors
Shared
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What is a logic gate?
A device that receives 1 or more electrical input signals and produces an output signal that depends on those
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When is an output of an AND gate ON?
When input 1 and 2 are ONN
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When is an output of an OR gate ON?
When 1 / 2 / 1&2 are ON
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When is a NOT gate ON?
When the input is OFF
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When is an output of a NAND gate is OFF?
When 1 and 2 are both ON
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When is an output of a NOR gate is ON?
When neither 1 or 2 are ON
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What does damaged insulation lead to?
Electric shock
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What is done to reduces chances of electrical shock?
Metal cases of electrical appliance are earthed by connecting them to the earth wire
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What can overheating of cables lead to?
Damaged insulation, poisonous fumes emitted and fire danger
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What are the two circuit breakers?
Trip-switch and a residual- current device (RCD)
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How does a trip-switch work?
Kinda like a fuse. When a current exceeds a certain value the switch trips and the circuit is broken
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How does a fuse work?
When the temperature gets too high the wire melts and breaks the circuit
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What does the right hand grip rule show?
It tells the direction of field lines of an electric current
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Describe the first step in activating an electric bell
Bell push is pressed. Current flows through the electromagnet. i
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Describe the second step in activating an electric bell
Magnetised coil attracts springy ***** and the hammer strikes the gong, breaking the circuit
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Describe the third step in activating an electric bell
The current stop and the coil is no longer magnetized so the ***** springs back into original shape
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Describe the fourth step in activating an electric bell
The circuit is complete again and the whole thing repeats again
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Describe how a relay works?
The switch is closed so a small current flows through the circuit. The electromagnet attracts the iron armature and the armature tips, pushing the 3 contacts in another circuit together
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Relays are used to make...
a small current switch a larger one on/off
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The magnetic field lines are ... inside a solenoid
parallel
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With what current does an electric motor work?
d.c.
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What does the split ring commutator do in a d.c. motor?
It allows the coil to turn past the vertical position
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Describe how a d.c. motor works in 4 steps.
Current flows through the right hand brush, around the coil and through the other brush. Coil becomes an electromagnet so the north pole is attracted to the south pole magnet. Brush connections are reversed, current flows the other way. Wire turns.
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How do you make a d.c. motor more powerful?
Increase current, increase turns of wire on the coil, make the magnets stronger
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What is the motor effect?
When a current carrying conductor carrying an electric current is placed in a magnetic field it will experience a force provided that the conductor isn't placed parallel to the lines
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How do you reverse the direction of force in the motor effect?
Reverse direction of current, reverse direction of magnetic field
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What rule do you use to work out the motor effect?
Fleming's left hand rule
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What is a motor?
A device that transfers electrical energy into kinetic
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What is electromagnetic induction?
Generating electricity from motion
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When you move a magnet past a wire you...
induce a current
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What are the 3 ways of increasing e.m.f.
Use stronger magnets, move wire more quickly, use coil with more turns
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Which current do generators always produce?
A.c. current
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How does an a.c current move?
Back and forth
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How can you determine the frequency of an a.c. supply?
See the number of cycles it produces per second
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What rule is used to determine where a current will flow in an a.c. motor?
Fleming's right hand rule
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How to increase the voltage generated by an a.c. motor?
Turn the coil faster, add more turns, use stronger magnet.
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Why does the peak of a graph of an a.c. motor corresponds to the coil being in a horizontal position?
More magnetic field lines are cut
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Where does the field around a current push?
Against the field inducing it
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Why do power lines have huge voltages?
To avoid losing energy
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What is a transformer?
A device used to step-up or step-down the voltage of an electrical supply
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Why do transformers have to be super efficient?
Because energy has to pass through many transformers before it reaches us
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What is the voltage in transformers?
Alternating
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What does a step-up transformer do?
Increases the voltage
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In a step-up transformer, which coil has more turns?
Secondary coil
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In a step-down transformer, which coil has more turns?
Primary
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What does a step-down transformer do?
Decrease the voltage
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Describe how a transformer works.
Alternating current passes through the primary coil, creating and alternating field around d it, turning it into a magnet. Core transports the field to secondary coil. Sec. coil becomes a conductor by electromagnetic induction
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Why does the iron core of a transformer have to be soft?
It has to be magnetized easily
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Doe the coils in the transformer touch?
No
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Other cards in this set
Card 2
Front
What materials are magnetic?
Back
Iron, nickel, cobalt, steel
Card 3
Front
Describe hard magnets
Back
Card 4
Front
What are the uses of hard magnets?
Back
Card 5
Front
Describe soft magnets
Back
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