Magnetism

Question 1

State the Rules of Magnetism

Answer 1

-Like poles repel, opposite poles attract.

-Magnetic forces are non-contact forces because the magnets do not have to touch for the forces to act.

-The magnetic force is strongest near the magnet’s poles.

Question 2

Describe what is Meant by a Permanent Magnet

Answer 2

-A permanent magnet is often made from a magnetic material such as iron. It always causes a force on other magnets, or on magnetic materials.

-It produces its own magnetic field. The magnetic field cannot be turned on and off - it is there all the time.

-Bar and horseshoe magnets are examples of permanent magnets.

-Permanent magnets attract or repel another permanent magnet but only attract a magnetic material (not repel it). Magnetic materials include iron, nickel and cobalt.

Question 3

Describe what is Meant by an Induced Magnet

Answer 3

-An induced magnet only becomes a magnet when it is placed in a magnetic field.

-The induced magnetism is quickly lost when the magnet is removed from the magnetic field.

-They are only attracted by other magnets, they are not repelled.

Question 4

Describe what is Meant by a Magnetic Field

Answer 4

A magnetic field is a region around a magnet where a force acts on another magnet or on a magnetic material.

Question 5

Describe the Rules for Drawing Magnetic Field Lines

Answer 5

-The magnetic field lines never cross each other.

-The closer the lines, the stronger the magnetic field.

-The lines have arrowheads to show the direction of the force exerted by a magnetic north pole.

-The arrowheads point away from the north pole of the magnet towards its south pole

Question 6

Describe how a magnetic Compass can be Used to Plot Magnetic Field Lines around a Bar Magnet

Answer 6

-Place the compass near the North pole of the bar magnet. Draw a cross at the North pole of the compass.

-Move the compass so the South pole is on the cross. Draw another cross at the North pole of the compass.

-Continue moving the compass until we reach the South pole of the magnet. Connect all of the crosses with a line.

-Draw an arrow pointing from the North pole to the South pole.

-Repeat with different starting points around the North pole of the magnet.

Question 7

Explain how a Compass shows the Earth has a Magnetic Field

Answer 7

-If we place a compass away from any magnets, the compass always points in the North-South direction.

-This tells us that the compass is experiencing a magnetic field.

-This magnetic field is due to the Earth’s core.

Question 8

Describe what is Meant by an Electromagnet

Answer 8

-An electromagnet is a magnet created by an electric current.

-It consists of a solenoid with an iron core. The core cannot be made of steel as steel retains its magnetism.

-The iron core increases the solenoid’s magnetic field strength.

Question 9

Explain how the Strength of an Electromagnetic be Increased

Answer 9

-A wire that carries a current creates a magnetic field around the wire. The strength of this magnetic field is greater closer to the wire and if the current is increased.

-We can make a stronger magnetic field by forming a coil inside the wire.

-This creates a stronger magnetic field inside the loop. If we add more coils, the magnetic field strength increases even more.

Question 10

Describe what is Meant by a Solenoid

Answer 10

-A solenoid consists of a wire coiled up into a spiral shape. A solenoid with an iron core is an electromagnet.

-When an electric current flows, the shape of the magnetic field is very similar to the field of a bar magnet.

-The magnetic field inside a solenoid is strong and uniform, just like the magnetic field produced by a bar magnet.

Question 11

Describe the Right Hand Grip Rule

Answer 11

-To find the direction of a magnetic field produced by a straight wire, place your right fist so the thumb is pointing in the direction of the conventional current.

-The direction of the magnetic field is shown by the direction that your fingers are pointing in.

Question 12

Describe two Advantages of Electromagnets

Answer 12

-We can increase the strength of the magnetic field by increasing the size of the electric current.

-An electromagnet can be turned on and off, dependant on when it is needed.

Question 13

Explain why it can be Dangerous to Turn a High Voltage Appliance On and Off

Answer 13

Due to the high voltage, we could get sparking from the switch to the user.

-The user could be electrocuted and that could be fatal.

Question 14

Describe how a Relay Works

Answer 14

-Sometimes, it is dangerous to turn a switch on and off directly. An electromagnetic switch called a relay can be used to turn the switch on and off safely.

-When the switch in the input relay is closed, the magnet is switched on.

-This pulls the iron lever towards it and the contacts are closed. The motor in the circuit is now switched on.

Question 15

Explain why a High Voltage Circuit is also Off when a Low Voltage Circuit is Off

Answer 15

-The low voltage circuit is turned off so there is no magnetic field in the electromagnet.

-This means that the contacts in the high voltage circuit are not connected, so the high voltage circuit is also turned off.

Question 16

Describe what happens to an the Electromagnet when the Low Voltage Circuit is turned On

Answer 16

-When the low voltage circuit is turned on, an electric current flows through the coil of wire in the electromagnet.

-This means that there is now a magnetic field in the electromagnet.

Question 17

Explain why an Iron Bar is Attracted to the Electromagnet in a High Voltage Circuit

Answer 17

-Iron is a magnetic material.

-When the electromagnet is switched on, the magnetic field causes the iron block to become an induced magnet.

-Now there is a force of attraction between the iron block and the electromagnet

Question 18

Describe the Purpose of the Spring in a High Voltage Circuit

Answer 18

-The purpose of the spring is to ensure that the contacts move apart when the electromagnet is turned off.

-Without the spring, the contacts would simply stay together so we would not be able to turn off the high voltage circuit.

Question 19

Describe how an Electric Bell Works

Answer 19

-When the push switch is closed, the current flows through the circuit.

-The electromagnet then attracts the iron arm. The hammer moves and strikes the bell.

-As this happens, the contacts separate and the circuit is broken.

-The electromagnet is switched off and the hammer springs back.

Question 20

Explain why the Hammer is not Initially in Contact with the Bell in an Electric Bell

Answer 20

The switch in the circuit will be initially open.

-This means that no electric current is flowing around the
circuit.

-Because of this, the electromagnet is not turned on and there is no magnetic field.

-The iron contact is not attracted to the electromagnet so the hammer is not in contact with the bell.

Question 21

Describe what happens to the Electromagnet when the Circuit is Complete in an Electric Bell

Answer 21

-When the circuit is complete, an electric current flows through the electromagnet.

-The electromagnet now has a magnetic field.

Question 22

Describe the Effect of the Magnetic Field on the Iron Contact in an Electric Bell

Answer 22

-When the circuit is closed, an electric current flows and there is a magnetic field in the electromagnet.

-The iron contact now becomes an induced magnet and is attracted towards the electromagnet.

Question 23

Explain how the Circuit Controlling an Electric Bell Allows it to Ring Continuously Rather than Once

Answer 23

-When the iron contact becomes an induced magnet, it is attracted towards the electromagnet. This causes it to move towards the electromagnet and the hammer to ring the bell.

-However, when the iron contact moves, it breaks the circuit. This means that the electric current stops flowing.

-Now the electromagnet is switched off so there is no magnetic field. The iron contact is no longer an induced
magnet so it now springs back to its original position.

-This closes the circuit again and repeats the process.

Question 24

Describe how a Magnet in a Scrapyard Works

Answer 24

-In a scrap yard, electromagnets can be used to separate iron and other magnetic objects from other materials.

-A thick cable supplies current to the electromagnet.

-The current is switched on to pick the metals and then switched off to put them down.

Question 25

Describe how a Circuit Breaker Works

Answer 25

-A large surge of current flows through the circuit. This would be enough to turn on the electromagnet. -The electromagnet then attracts an iron lever to it. This causes the contacts to spring open which would break the circuit.

Question 26

Describe how a Car Ignition Works

Answer 26

-Closing the (ignition) switch causes a current to pass through the electromagnet. -The iron arm becomes magnetised. The electromagnet attracts the iron arm. -The iron arm pushes the starter motor contacts together. The starter motor circuit is now complete. -A current flows through the starter motor(which then turns).

Question 27

Describe what is Meant by the Motor Effect

Answer 27

-A wire carrying a current creates a magnetic field. -This can interact with another magnetic field, causing a force that pushes the wire at right angles. -This is called the motor effect.

Question 28

State the Calculation for the Force on a Wire (at Right Angles to the Magnetic Field)

Answer 28

Force (N) = Magnetic Flux Density (T) x Current (A) x Length (m) F= BIL

Question 29

Explain how the Strength of a Force on a Given Length of Wire in a Magnetic Field be Increased

Answer 29

-The force on a given length of wire in a magnetic field will increase when the current in the wire increases. -It will also increase when the strength of the magnetic field increases.

Question 30

Explain how is Fleming's Left Hand Rule Used to Find the Direction of the Motor Effect Force

Answer 30

-Hold your thumb, forefinger and second finger at right angles to each other: -The forefinger is lined up with magnetic field lines pointing from north to south. -The second finger is lined up with the current pointing from positive to negative. -The thumb shows the direction of the motor effect force on the conductor carrying the current

Question 31

Describe how an Electric Motor Works

Answer 31

-Current in the left hand part of the coil causes a downward force, and current in the right hand part of the coil causes an upward force. The coil rotates anticlockwise because of the forces described above. -When the coil is vertical, it moves parallel to the magnetic field, producing no force. A split ring commutator changes the current direction every half turn allows the coil to continue rotating. -Once the conducting brushes reconnect with the commutator after a half turn, current flows in the opposite direction through the wire in the coil. Each side of the coil is now near the opposite magnetic pole. -This means that the motor effect forces continue to cause anticlockwise rotation of the coil.

Question 32

Explain why the Coil Cannot Rotate Past 90° in an Electric Motor

Answer 32

-Once the coil reaches 90°, the forces act directly up and down. When the coil swings slightly beyond 90°, the forces still act directly up and down. This forces the coil to swing back to the 90° position. -To make the coil move past the 90° position, we will have to make the forces switch direction. -The force on the bottom of the coil will now have to act upwards and the force on the top of the coil will have to act downwards. -To do this, we have to switch the direction of the conventional current.

Question 33

Describe how a Split Ring Commutator Works

Answer 33

-A split ring commutator is a metal device attached to the coil. -This means that the connection to the battery supply is reversed every half turn. -This allows the current to change direction to allow the coil to keep rotating the same way.

Question 34

Describe the Role of a Split Ring Commutator in an Electric Motor

Answer 34

-The split-ring commutator ensures that the current always points in the same direction on either side of the coil. -This means that the force on the left hand side always points upwards and the force on the right hand side always points downwards. -This ensures that the rotation of the coil is always in the clockwise direction.

Question 35

Explain how a Loudspeaker Produces Sound

Answer 35

-A current in the coil creates an electromagnetic field. The electromagnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards. -The current is made to flow in the opposite direction. The direction of the electromagnetic field reverses. The force on the cone now pulls it back in. -Repeatedly alternating the current direction makes the cone vibrate in and out. The cone vibrations cause pressure variations in the air, which are sound waves.

Question 36

Describe the Generator Effect

Answer 36

-The generator effect is the induction of a potential difference in a wire which is moving relative to a magnetic field or experiencing a change in magnetic filed. -When a conductor cuts the lines of a magnetic field a current passes through it. This is because a potential difference is induced across the ends of the conductor. -This can be achieved by moving the magnet or the wire. This process is called electromagnetic induction. -The faster the movement, the higher the current. The stronger the magnetic field, the higher the current.

Question 37

Describe an Experiment Measuring Induced Potential

Answer 37

-A bar magnet rests outside a wire coil connected to an ammeter. -The magnet moves into the coil of wire and the ammeter shows a positive current flow. -When the magnet is stationary within the coil of wire, there is no current flow. -The magnet moves out of the coil and the ammeter shows a negative current flow. -Increase the number of coils, movement speed or magnetic field strength to change the induced potential.

Question 38

Explain why the Ammeter Shows a Reading when Measuring Induced Potential

Answer 38

-There is a magnetic field around the magnet, -The magnetic field changes/moves and cuts through the coil so a potential difference is induced across the coil. -The coil forms a complete circuit so a current is induced.

Question 39

Describe what happens to a Bar magnet as it is Pushed Into a Coil

Answer 39

-When we push the bar magnet into the coil a current is induced in the coil. -The induced current causes the coil to develop a magnetic field which repels the bar magnet, making it harder for the magnet to move in.

Question 40

Describe how Work is Done when a Magnet Induces a Coil of Wire

Answer 40

-Moving the magnet into the coil causes a magnetic field which repels the bar magnet, opposing the movement. -Moving the magnet out of the coil causes a magnetic field which attracts the bar magnet, again opposing the movement. -We are transferring energy from the movement of the magnet into the movement of the current. This means that we are doing work (ie energy transfer).

Question 41

Apart from Moving a Magnet or Coil, Describe Another Method to Induce Potential Difference

Answer 41

-We can also induce a potential difference and current if we switch the poles of magnet. -One way of doing this is to rotate the magnet so the North and South poles constantly switch places. -Another way is to use an electromagnet connected to an AC supply so that the North and South poles are constantly alternating.

Question 42

Describe how an Moving a Magnet End to End in a Coil Creates an Alternating Current

Answer 42

-As the magnet turns, the magnetic field through the coil changes. This change in the magnetic field induces a potential difference, which can make the current flow in the wire. -When the magnet is turned through a half turn, the direction of the magnetic field through the coil reverses. -When this happens, the potential difference reverses, so the current flows in the opposite direction around the coil of wire. -If the magnet is kept turning in the same direction, then the potential difference will keep on reversing every half turn and you will get an alternating current.

Question 43

Describe how an Alternator Creates an Alternating Current and Potential Difference

Answer 43

-Alternators rotate a coil in magnetic field. Potential difference is induced across the coil. -Because the circuit is complete, a current is induced in the circuit. -A slip ring is connected to the coil. Brushes make continuous contact between the external circuit and slip rings. -This means the contacts don't swap every half turn and current flows in the external circuit. -This produces reverses the potential difference every half turn so current also reverses.

Question 44

Describe the Potential Difference Graph Produced by an Alternator

Answer 44

-At first, The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced. -Then, the pd is maximum as the coil is sweeping through the magnetic field at the fastest possible rate. The side sweeping down connects to ring A, making this positive. The side sweeping up connects to ring B, making this negative. -Then, the pd falls to zero as the coil is moving horizontally and is not sweeping through the magnetic field. -After, the pd is maximum again. However, the side sweeping down is now connected to ring B, which is now positive. The side sweeping up is now connected to ring A so this is now negative. -Finally, the pd falls to zero again. This produces an alternating current.

Question 45

Describe how a Dynamo Creates a Direct Current

Answer 45

-A coil moves through a magnetic field. A potential difference is induced across the coil. -Because the circuit is complete, current is induced in the circuit. -Every half turn, the two ends of the coil swap from one brush to the other. -This means the direction of the current does not reverse every half turn.

Question 46

Describe the Potential Difference Graph Produced by a Dynamo

Answer 46

-At first, the coil is moving parallel to the direction of the magnetic field, so no potential difference is induced. -Then, the coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum. -After, the coil is moving parallel to the direction of the magnetic field, so no potential difference is induced. -Next, the coil is moving at 90° to the direction of the magnetic field again, so the induced potential difference is at its maximum. Here, the induced potential difference travels in the same direction as before. -Finally, the coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.

Question 47

Describe how a Moving Coil Microphone Works

Answer 47

-Sound waves in the air are tiny changes in air pressure. When these changes hit the diaphragm, they cause it to move backwards and forwards. -Because the coil of wire is attached to the diaphragm, the coil moves backwards and forwards over the permanent magnet. -Now we have a coil moving through a magnetic field. This induces a changing potential difference across the ends of the wire and induces a current. -The changing size and direction of the current matches the vibrations in coil. Electrical signal generated match pressure variations in sound waves.

Question 48

Explain the Purpose of an Amplifier and Loudspeaker

Answer 48

-The potential difference produced by a moving-coil microphone is very small. -The amplifier massively increases the potential difference. -The loudspeaker then converts the changing potential difference back into sound waves.

Question 49

Explain how a Transformer Works

Answer 49

-A transformer works because alternating current in the primary coil produces a changing magnetic field in the iron core and then in the secondary coil. -This induces an alternating potential difference across the ends of the secondary coil.

Question 50

Explain the Purpose of the Iron Core in a Transformer

Answer 50

-Transformers have a core made of iron. -The core is made of iron because iron can easily be magnetised. -The iron core increases the strength of the magnetic field.

Question 51

Explain why Transformers Only Work with an AC Power Supply

Answer 51

-Transformers only work with an AC supply. -An alternating current (AC supply) produces a changing magnetic field in the primary coil. -A changing magnetic field can induce a potential difference in the secondary coil. -If a direct current (DC) was used, then this would not produce a changing magnetic field and no potential difference would be induced in the secondary coil.

Question 52

Describe the Difference Between a Step Up and Step Down Transformer

Answer 52

-Step up transformers increase the potential difference. They have more turns on the secondary coil than the primary coil. -Step down transformers decrease the potential difference. they have more turns on the primary coil than the secondary.

Question 53

Give the Transformers Calculations

Answer 53

P.D in Primary Coil / P.D. in Secondary Coil = Number of Turns in primary Coil / Number of Turns in Secondary Coil -Vp / Vs = Np / Ns P.D, in Secondary Coil x Current in Secondary Coil = P.D. in Primary Coil x Current in Secondary Coil -Vs x Is = Vp x Ip

Question 54

Explain why Electricity is Transmitted Using a High Potential Difference Rather than a High Current

Answer 54

For a given power, increasing the potential difference decreases the current which decreases the energy lost by heating to the wires and the surroundings. -This makes the national grid an efficient way of transferring electricity.