Fields and their Consequences Flashcards
If alternating current is passed through a wire perpendicular to a magnetic field, why will the wire vibrate?
Force on wire perpendicular to B and I, magnitude of F changes as size of I changes, current constantly changing direction so force on wire will constantly change direction
What is the effect on an oscilloscope trace (showing voltage with ac supply) if the time base is switched off?
Vertical line of length equal to twice peak voltage as trace no longer moves horizontally
Why is the energy supplied by a capacitor more than the energy stored in a capacitor?
50% of the energy supplied is wasted due to resistance in the circuit as it is transferred to the surroundings when charge flows in the circuit
How can the initial charge on a capacitor be determined from a graph of current against time?
Area under graph
Give two reasons why the all the energy stored in a capacitor will not be transferred to a motor when it discharged?
Energy lost through heating of wires and in overcoming frictional forces in the motor
Explain why there is an increase in energy stored by a charged capacitor when a dielectric is pulled out from between its plates?
In the polar dielectric, molecules align in the field with the positive charged end towards the negative plate and work is done on the capacitor is separating the positively charged surface of the dielectric from the negatively charged plate
Which way do electrons flow when a capacitor is charging?
Electrons are forced from the negative terminal of the cell to the negative plate and from the positive plate to the positive terminal of the cell
When a capacitor is charged by a cell through a resistor, explain the energy changes which take place?
50% of the chemical energy stored in the cell is transferred to electric potential energy stored in the capacitor and 50% is transferred to thermal energy due to resistance in the wires and resistor
When a capacitor charges or discharges, describe how the rate of flow electrons changes with time
Rate of flow of electrons is greatest at start and decreases to zero when the capacitor is fully charged or discharged
Explain the flow of electrons when a capacitor discharges through a resistor?
Electrons flow from the negative plate of the capacitor to the positive plate of the cell
Describe the energy changes which take place when a capacitor discharges through a resistor?
Electric potential energy stored by capacitor is all converted to thermal energy by resistor as the electrons flow through it
Why is gravitational field strength the negative of the potential gradient?
Gravitational potential is always negative (zero at infinity)
Why does the gravitational force decrease as you move from the surface of a planet towards its centre?
Only the mass in sphere of radius r contributes to g
What way do electrons transfer when an uncharged Perspex rod is rubbed with an uncharged dry cloth
Rod to cloth
Explain why the time taken for an object to fall vertically is independent of its mass
Force on a particle = mg and acceleration = F/m so a=g, time taken to fall depends only on distance and acceleration
