Additional electromagnetism for separate science Flashcards
Explain how electromagnets are used in an electric bell
The hammer of the magnet forms part of the circuit, and the circuit includes an electromagnet.
When the switch is turned on the electromagnet moves the hammer towards the bell. When the hammer moves it breaks the circuit.
This stops the flow of current so the electromagnet stops working, causing the hammer to swing back towards the start position where it reconnects the circuit.
The cycle then repeats itself
Explain how electromagnets are used in a circuit breaker
A circuit breaker contains an electromagnet which is influencing a hinged metal contact.
When the current in the neutral and live wires are the same the contact remains in the neutral position and the circuit is complete
if there is a short circuit then the current stops passing down the neutral wire and passes down the earth wire instead and the current increases. This means that the magnetic field of the electromagnet increases removes the iron lever causing the switch to hinge open and break the circuit
Explain how electromagnets are used in a relay switch
In a relay switch an electromagnet on a potential difference circuit is attached to an electromagnet.
When the electromagnet is switched on it pulls a switch in a high potential difference circuit. This closes the switch and allows the high potential difference circuit to work.
When the electromagnet is turned off the switch on the high potential difference circuit swings open. This stops the operation of that circuit
Explain what electromagnetic induction is
Why does the wire and magnet need relative movement for induction to occur?
Electromagnetic induction is the generation of a potential difference in a wire if the wire moving relative to a magnetic field. If the wire is in a complete circuit this potential difference will drive a current through the circuit
Relative motion is required becuase electromagnetic induction is caused by the wire cutting the field lines of the magnet. This can be caused by either moving the wire through the field lines of the magnet, or by moving the magnet so that the field lines cross the wire
What are the factors that influence the size of the current generated by electromagnetic induction
Factors that affect current by affecting the potential difference generated:
- The length of wire in the magnetic field - the longer the wire the larger the induced PD, and therefore current
- The speed the wire is moved - the faster the wire is moved the larger the induced PD
- The strength of the magnet - the stronger the magnet, the greater the induced PD and therefore the greater the current produced
Factors that affect current after the potential difference has been generated:
The resistance of the circuit - the lower the resistance the higher the current will be for the same PD generated
State Lenz’s law
Explain how this can be used to find the directino of the induced current in a solenoid
Lenz’s law states that an induced current will induce a magnetic field that will oppose the change causing the current
This means that if there is a north pole moving towards a solenoid, the current induced in the solenoid will make a north pole on that side to stop it moving. Alternatively, if the north pole is moving away from the solenoid, that side of the solenoid will generate a south pole to stop the magnet from moving.
Once you know the direction of the magnetic field use the right hand screw rule to find the current. Lay your thumb along the solenoid pointing north, and the current is curling in the same direction as your fingers
Explain how to investigate the magnitude and direction of the magnetic field induced in a solenoid when a magnet is moved into it.
- Create a solenoid and attach it to an ammeter
- slowly move the magnet into the soleniod and note the size and direction of the current. The direction of the current can be seen from whether the current is positive or negative
- Slowly remove the magnet from the solenoid and note the direction and size of the current
- comment on the effect that changing direction has on the current (it will change direction)
- Repeat the investigation but this time move the magnet quickly in and out of the solenoid.
- Comment on the effect that changing the speed has on the current (the higher the speed the higher the current. This is becuase a larger potential difference is being generated)
Explain how to find the direction of the induced potential difference (and therefore current) when a straight wire is moved through a magnetic field
The right hand rule is used to find the direction of the current. Lay your first finger in the direction of the magnetic field from north to south, then turn your hand around that axis so the thumb is laying in the direction of the motion. Your middle finger will now be pointing in the direction of the current.
Describe how a loudspeaker works
In a loudspeaker, a membrane is attached to a coil of wire which is in the magnetic field of a bar magnet. The sound wave enters the loudspeaker as pulses of alternating electric current, which travel round the coil of wire.
When the current moves through the coil, it generates a magnetic field which interacts with the magnetic field of the bar magnet. This causes the membrane to move, which in turn moves the air particles and generates a sound wave.
Explain how a microphone works
In a microphone there is a coil of wire attached to a membrane inside a magnetic field. When a sound wave hits the membrane it causes the coil to move. The movement of the coil releative to the magnetc causes a potential difference to be generated across the coil, which results in a current.
