Beatty 6091 WS Electromagnetism teacher (Updated 2021

1 A. Prior Knowledge Before T / F Statement After T / F 1. All metals (Al, Cu, Fe) can be magnetized. F 2. Iron is attracted to magnet, so attraction is a sure test of magnetism. F 3. The strength of the electromagnet can be increased by changing the direction of the current. F 4. A force is acting on a current-carrying wire if it is in a magnetic field. T 5. A circuit breaker should be made by a permanent magnet. F 6. A circuit breaker and an electric bell make use of magnet in their operations. T Beatty Secondary School Secondary 4E Pure Physics Electromagnetism Name: __________________________ ( ) Class: __________ Date: _____________ Learning Outcomes (a) draw the pattern of the magnetic field due to currents in straight wires and in solenoids and state the effect on the magnetic field of changing the magnitude and/or direction of the current (b) describe the application of the magnetic effect of a current in a circuit breaker (c) describe experiments to show the force on a current -carrying conductor, and on a beam of charged particles, in a magnetic field, including the effect of reversing (i) the current (ii) the direction of the field (d) deduce the relative directions of force, field and current when any two of these quantities are at right angles to each other using Fleming’s left-hand rule (e) describe the field patterns between current s in parallel conductors and relate these to the forces which exist between the conductors (excluding the Earth’s field) (f) explain how a current -carrying coil in a magnetic field experiences a turning effect and that the effect is increased by increasing (i) the number of turns on the coil (ii) the current (g) discuss how this turning effect is used in the action of an electric motor (h) describe the action of a split -ring commutator in a two -pole, single-coil motor and the effect of winding the coil on to a soft-iron cylinder

2 A) Magnetic effect of current When current flows through a wire, a magnetic field is produced around it. B) Magnetic field of current-carrying conductors • The magnetic field of a straight , current -carrying wire consists of concentric circles. • The magnetic field is stronger closer to the wire. Method to identify direction of magnetic field in a wire: Right hand grip rule • The right-hand grip rule can be used to determin e the direction of the magnetic field created by a straight wire carrying a current. • The thumb points in the direction of the current. • The other fingers point in the direction of the magnetic field around the wire. When no current flows through XY, the compass needles point to the north. When current flows through XY, the needle of compass A points to the east. The needle of compass B points to the west.

3 The symbols below are used to represent the directions of magnetic fields or currents. Practice Points to note: • The direction of the magnetic field of a current -carrying wire reverses when the direction of the current is reversed. • The strength of the magnetic field of a current -carrying wire increases when the current is increased. Represents the current flowing into the plane of the paper or the magnetic field pointing into the plane of the paper Represents the current flowing out of the plane of the paper or the magnetic field pointing out of the plane of the paper In the diagrams below, draw the magnetic field lines around the conductor. On your diagrams, indicate clearly the direction of the magnetic field lines. Side view Side view Top view Top view

4 C) Interaction of magnetic fields between two current-carrying conductors Top view Top view Top view Top view • Both current going in the opposite direction (REPULSION) • Both current going in the same direction (ATTRACTION) Practice Sketch the magnetic field pattern around each of the current-carrying wires shown below and indicate the interaction between 2 current-carrying conductors.

5 D) Magnetic field of a flat coil • The magnetic field strength of a flat coil can be increased by increasing the current flowing in the coil or increasing the number of turns of the coil • The magnetic field is stronger in the region inside the coil because the fields from each part of the wire are in the same direction and the field lines are confined to a small space. Practice The diagram (right) shows a flat coil. The switch is closed and a current flows through the wire. (a) Mark clearly on the wire the direction of current flow. (b) Sketch the magnetic field caused by a current flowing in a flat coil. E) Magnetic field of a solenoid Three methods to increase the magnetic field strength of a solenoid: Method Reason Increase the current in the solenoid Magnetic field strength is proportional to the amount of current flowing through the conductor. Increase the number of turns of the coil The magnetic effect is multiplied by the number of turns in a coil. Place a soft iron core into the solenoid A soft iron core focuses magnetic field lines within the coil.

6 Method to identify poles in a solenoid: Right hand grip rule • The direction of the magnetic field of a solenoid can be determined using the right-hand grip rule. • The fingers point to the direction of current in the solenoid. • The thumb points to the direction of N-pole of solenoid. F) Application of Electromagnetism The first diagram below shows a circuit breaker with the contact closed when the circuit is operating normally. The next diagram shows the same circuit breaker after a large current passes through the circuit. Normal operating current Large current Describe in detail how the circuit breaker switches off the current when the current becomes too large. During normal operating, the current flowing through the solenoid causes a magnetic field in the iron core to form an electromagnet . However, the electromagnet is not strong enough to exert an attraction force to pull the iron armature. When there is an increase in current flow , the magnetic field in the iron core increases, leading to a stronger electromagnet. The iron armature undergo magnetic induction and becomes attracted and move about the pivot. The iron armature becomes out of contact with the springy metal, breaking the circuit and the current flow stops. When the current flow stops, the electromagnet loses its magnetism since iron is a s oft magnetic material . The iron armat ure remains out of contact with the springy metal until the reset button is pushed.

7 G) Motor effect – Force due to magnetic fields A current-carrying conductor experiences a force when it is placed in a magnetic field. The force is a result of the interaction between the magnetic fields of the conductor and the magnetic field of the magnet. Method to determine the direction of force: Fleming’s left hand rule

8 Practice 1. Using arrows, dots and crosses, indicate and stat e the direction of the force acting on the following current carrying wires. x x x x x x x x x x x x Direction of force: Up x x x x x x x x x x x x Direction of force: Left x x x x x x x x x x x x Direction of force: