Electromagnetic Effects (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Physics): Flashcards

The purpose of the slip rings is to allow the alternating current to flow between the rotating coil and the external circuit.

False.

A maximum e.m.f. is induced when the coil is in a horizontal position and its motion is perpendicular to the field.

This is because the greatest number of field lines are cut when the coil moves perpendicular to the field.

True.

No e.m.f. is induced when the coil is in a vertical position and its motion is parallel to the field.

This is because no field lines are cut when the coil moves parallel to the field.

The direction of the induced e.m.f. (and current) reverses every half rotation of the coil.

The graph of e.m.f. against time for an a.c. generator is a sine or cosine curve, depending on the starting position of the coil:

• starting from a horizontal position (maximum e.m.f.), the graph is a cosine curve

• starting from a vertical position (zero e.m.f.), the graph is a sine curve

In an a.c. generator, the size of the induced e.m.f. can be increased by:

• increasing the frequency of rotation of the coil

• increasing the number of turns on the coil

• increasing the strength of the magnet

• inserting a soft iron core into the coil

The magnetic field due to a current in a straight wire forms concentric circles centred on the wire, which get further apart with increasing distance from the wire.

True.

The circular field pattern around a current-carrying wire indicates that the field has no poles.

The right-hand grip rule is used to determine the direction of the magnetic field around a current-carrying wire.

Point the right-hand thumb in the direction of the (conventional) current; the direction of the curled fingers represents the direction of the magnetic field lines.

The field lines around a solenoid are similar to those of a bar magnet:

• one end of the solenoid becomes a north pole and the other a south pole

• the field lines emerge from the north pole and return to the south pole

False.

The north pole forms at the end where the current flows anticlockwise, and the south pole at the end where the current flows clockwise.

The strength of the magnetic field around a straight wire can be increased by increasing the amount of current flowing through the wire.

The field is strongest near the wire and becomes weaker further away, shown by the field lines becoming further apart.

The strength of the magnetic field around a solenoid can be increased by:

• increasing the amount of current flowing through the coil

• increasing the number of turns on the coil

• inserting an iron core into the coil

Reversing the direction of the current reverses the direction of the magnetic field produced.

The iron core becomes an induced magnet when a current flows through the coils.

The magnetic field produced by the solenoid and the iron core together creates a much stronger magnet overall — this is how an electromagnet works.

A current-carrying conductor produces its own magnetic field.

When this interacts with an external magnetic field, the conductor experiences a force, which makes it move.

False.

A current-carrying conductor will only experience a force if the current through it is perpendicular to the direction of the magnetic field lines.

The direction of the force on a current-carrying conductor can be reversed by:

• reversing the direction of the current

• reversing the direction of the magnetic field

Fleming's left-hand rule is used to determine the direction of the force (thrust) on a current-carrying conductor in a magnetic field, given the directions of the current and the magnetic field.

In Fleming's left-hand rule:

• the thumb points in the direction of the force (thrust) on the conductor

• the first finger points in the direction of the magnetic field

• the second finger points in the direction of the current (from positive to negative)

True.

The direction of the force on a current-carrying wire depends on the direction of the current and the direction of the magnetic field.

In Fleming's left-hand rule, the force, field and current are all perpendicular to each other.

The motor effect can be used to create a simple d.c. electric motor.

The force on a current-carrying coil causes it to rotate in a single direction.

A simple d.c. motor consists of:

• a coil of wire (which is free to rotate) between the poles of a permanent magnet

• a split-ring commutator and brushes connected to a source of d.c.

As current flows through the coil, it produces a magnetic field which interacts with the external magnetic field.

Forces act in opposite directions on each side of the coil, causing a turning effect.

The split-ring commutator reverses the direction of the current in the coil every half turn, so the forces on the coil continue to cause rotation in the same direction.

True.

When the coil is vertical, the split ring is no longer in contact with the brushes, so no current flows and no forces act.

The momentum of the coil causes it to continue rotating until the split ring reconnects with the brushes.

The speed at which the coil of a d.c. motor rotates can be increased by:

• increasing the current

• using a stronger magnet

The direction of rotation of the coil in a d.c. motor can be changed by:

• reversing the direction of the current supply

• reversing the direction of the magnetic field by reversing the poles of the magnet

The force supplied by a d.c. motor can be increased by:

• increasing the current in the coil

• increasing the strength of the magnetic field

• adding more turns to the coil

To determine the direction of rotation of the coil in a d.c. motor:

• draw arrows for the direction of the magnetic field (north to south) and the current (positive to negative)

• use Fleming's left-hand rule to find the direction of the force on each side of the coil

• use the force directions to determine the direction of rotation

A transformer is a device used to change the size of an alternating voltage or current.

This is achieved using the generator effect.

A basic transformer consists of:

• a primary coil

• a secondary coil

• a soft iron core

Iron is used for the core of a transformer because it is easily magnetised.

A step-up transformer:

• increases the voltage of a power source

• has more turns on the secondary coil than on the primary coil

False.

A step-down transformer has fewer turns on the secondary coil than on the primary coil .

This decreases the voltage of the power source .

The transformer in the diagram is a step-down transformer.

The secondary voltage (180 V) is lower than the primary voltage (300 V).

The ratio of the voltages across the primary and secondary coils of a transformer is equal to the ratio of the number of turns on each coil:

Where:

• = voltage across the primary coil, in volts (V)

• = voltage across the secondary coil, in volts (V)

• = number of turns on the primary coil

• = number of turns on the secondary coil

The output (secondary) voltage of a transformer is calculated using:

This shows that the output voltage depends on:

• the number of turns on the primary and secondary coils

• the input (primary) voltage

An ideal transformer is a transformer which is 100% efficient, so the input power in the primary coil is equal to the output power of the secondary coil.

The equation for an ideal transformer is:

Where:

• = primary current, in amps (A)

• = primary voltage, in volts (V)

• = secondary current, in amps (A)

• = secondary voltage, in volts (V)

False.

Although transformers can increase the voltage of a power source, due to the law of conservation of energy, they cannot increase the power output.

Since , a high voltage means electricity can be transmitted at a low current.

A smaller current flowing through the power lines results in less heat being produced due to resistance in the wire.

This reduces the energy loss in the power lines, making the energy transfer more efficient.

The power dissipated in a wire due to resistance is given by:

Where:

• P = power, in watts (W)

• I = current, in amps (A)

• R = resistance, in ohms (Ω)

A step-up transformer is used to increase the voltage and decrease the current of electricity before transmission.

This ensures the same power transfer with a smaller current, so less thermal energy is lost due to resistance in the wire.

A step-down transformer is used to decrease the voltage and increase the current of electricity after transmission, because high-voltage electricity is dangerous for use in homes.