# Charge that capacitor!⤴

from @ Helpmyphysics

A capacitor can be used in a sensor circuit to switch on a transistor after a time interval.
Such a voltage divider circuit will act as a kind of timer.
The time the voltage across the capacitor reaches 0.7 v, to switch on the transistor, can be short or long depending on the size of the capacitance of the capacitor or the size of the resistor its being charged through.

To lengthen the time interval the resistance can be made bigger or the vakue of the capacitor’s capacitance can be made bigger.

# Ohm’s Law Sim⤴

from @ Helpmyphysics

Use this sim to determine the relationship between the voltage across a resistor and the current passing through it.
Notice the variable resistor will vary the current in the circuit.

Take current and voltage readings in a table and plot a graph current in the x-axis and voltage in the y-axis.

# Cool down with a thermistor!⤴

from @ Helpmyphysics

A thermistor is a resistor whose resistance changes with temperature.
When it becomes cold its resistance gets bigger. When it becomes hot its resistance becomes smaller.

Check the voltages across the voltage divider.
The mosfet acts like a switch. When the voltage at its gate becomes bigger than 2 volts it will switch on the relay which in turn switches the fan on.

# Share the voltage!⤴

from @ Helpmyphysics

Click the picture to go to the sim.

Some questions:

1. What do you notice about the variable resistor’s voltage as its resistance increases?
2. What do you notice about the sum of the voltages across the resistors?
3. For each resistor in turn, take the ratio of voltage across it and divide by the resistance. What do you notice?

# More on coding the normal force.⤴

from @ Helpmyphysics

In this code I have added a slider to change the value of the acceleration of the floor.
Think of the floor of an elevator.

A positive acceleration indicates that the elevator is accelerating upwards and a negative acceleration indicates a downward acceleration.

Press the play button to start the program and then the pencil button to examine the code

# Visualising the normal supporting force.⤴

from @ Helpmyphysics

Here is a vpython code to visualise the normal force acting on a block sitting on the foor.
The normal force is provided by the floor. You can think of it as a supporting force.

If the normal force was removed the box would be in freefall!

Think of standing on a trampoline. The trampoline webbing provides an upwards force to stop you falling.

This normal force would be the reading on a weighing scale.

Change the value of acceleration variable “a” in the code to see what happens to the normal force when the floor is accelerating upwards or downwards

The normal force is the reading of the weight in newtons on the weighing scale.

What is your weight reading when the floor is accelerating upwards, ie a positive acceleration and when the floor is accelerating downwards, a negative acceleration?

Press the small pencil to change the code. Then run.

# Coding the Doppler effect⤴

from @ Helpmyphysics

The black car is moving towards an observer,represented by the green ball, while sounding its horn.
The sound waves spread out from the car. Notice that the sound waves squash up as the car approaches the observer and stretch out as the car leaves the observer behind.
Press the run button to start the program.
Can you see why the observer hears a higher frequency of sound as the car approaches and a lower frequency of sound as the car moves away?