Thought Whakamātau 🔗
- Dragging a piece of wire through a magnetic field creates an induced voltage
- If the circuit is complete it creates an induced current
- What does a current in a magnetic field experience?
- Yes, a force! $F=BIL$
Think, pair, share: If we drag the coil to the right, what will we observe?
- A current will be induced,
- That current will experience a force ($F=BIL$)
- That magnetic force will oppose the externally applied force
Lenz’s Law 🔗
- The induced current (and magnetic force) always opposes the change/action producing it
- In our previous tauria/example the induced current causes a force which opposes the externally applied force
- This law is a result of the conservation of energy. Work must be done to generate electrical energy.
Continuing our tauria from earlier: a metal rod is moved in a magnetic field. The rod is $24cm$ long and moves at $8ms^{-1}$ through a magnetic field with strength $0.7T$. Calculate the magnitude and direction of the opposing force acting upon the rod.
Lenz’ Law in Solenoids 🔗
- Recall: A solenoid is a coil of wire used as an electromagnet.
- Question: What happens if we pass a magnet through solenoid?
- Answer: We induce a current in the coil.
- Question: What does that current do?
- Answer: It creates a magnetic field that opposes the original field.
Lenz’ Law in Solenoids 🔗
A magnet moving inside a solenoid will induce a current and therefore, a magnetic field, which will oppose the original field. We can use our right hand rule to deduce the direction of the current flow.
Understanding Check 1 🔗
A magnet is dropped into a long copper tube with its North pole facing downwards. As the North pole approaches the top end of the tube, which way will the current flow, when viewed from above the top of the tube?
The current will flow anti-clockwise, creating a magnetic field pointing upwards, opposing the North pole falling down.
Understanding Check 2 🔗
When the magnet has just left the bottom of the tube, in which sense does the current flow, again when viewed from above the top of the tube?
The current will be in the opposite sense now, i.e. clockwise. When the South pole leaves, Lenz’s Law says that there must be a South pole induced at the bottom end of the tube, producing a force on the magnet in the upward direction, opposing its downwards motion.
Practice 🔗
- Worksheet 9 Q4
- Homework booklet Q23, 21, 22
- Textbook Activity 20B Q4, 5, 6