Physics Electrical circuits

For a thermistor - At a constant temperature, the line is straight so its resistance is constant. If the temperature is increased, its resistance decreases.

For a light dependant resistor - LDR - Resistance decreases if the light intensity on it increases. 

Ohms law - States that a current through a resistor at constant temperature is directly proportional to the potential difference across the resistor. 

A filament bulb - The line curves away from the current axis. So the current is not directionally proportional to the potential difference. The filament bulb is a non-ohmic conductor. 

The resistance (=potential difference/current) increases as the current increases. So the resistance of a filament bulb increases as the filament temperature increases.

The resistance increases - the ions in the metal filament vibrate more, as the temp increases. They resist the passage of the electrons through the filament. 

Reversing the potential difference makes no difference to the shape of the curve. The resistance is the same for the same current, regardless of its direction. 

Open switch
Closed switch
Lamp
Cell
Voltmeter
A resistor
Fuse
Ammeter
Variable resistor
Thermistor
Light dependent resistor (LDR)
Light emitting diode (LED)
Diode

Electric charge is measured in coulombs (c) - Electric current is measured in amperes (A). An electric current - 1 ampere, is a rate of flow of charge of 1 coulomb per second.

The current (A) = charge flow (C) or I = Q/T when I is current, Q is coulombs, and
--------------- T is time in seconds.
Time taken (S)
E.g. I = Q/T — 8.0 C / 4.0S = 2.0A
The size of an electric current = is the rate of flow of charge.

A (LED) - emits light when a circuit passes through it.
Variable Resistor - is used to allow the current to be varied.

In the forward direction, the line curves towards the axis. So the current is not directly proportional to the potential difference. A diode is not an ohmic conductor. 

In the reverse direction, the current is negligible. So its resistance in the reverse direction is much higher than in the forward direction.

Light Emitting Diode - emits light when a current passes through it in the forward direction. 

Some insulators, charged by rubbing with a dry cloth.
Rubbing a polyethene rod with a dry cloth transfers electrons to the surface atoms of the rod, from the cloth.
Rubbing a perspex rod with a dry cloth transfers electrons from the surface atoms of the rod to the cloth.
The perspex rod is positively charged.

It makes your hair stand on end. The dome charges up when the generator is on. The Van de Graff - the belt rubs against a felt pad and becomes charged.
The belt carries the charge onto an insulated metal dome.
Sparks are produced when the dome can no longer hold any more charge.

When a charge flows steadily through a component -
The potential difference across component (volts) = Work done (joules)/charge in (coulombs)

OR V = W/Q        V=(V)     W=(J)    Q= (C)

Resistance (ohms) = Potential difference/ Current (amperes)

OR R=V/I        R = Resistance(Ohms)      V= Potential difference (Volts)   I = Current(amperes)

The current through a resistor is  constant temperature is proportional to the  potential difference across the resistor.

If the current through a resistor is decreased,the potential difference across the resistor decreases. 

If the current through a resistor is reversed, the potential difference across the resistor reverses and the resistance of the resistor stays the same. 

Protons and neutrons make up the nucleus of the atom. Electrons move about in the space around the nucleus.
Protons have a positive charge (+)
Electron has an equal negative charge (-)
Neutrons are uncharged
Uncharged atom = no. electrons + protons
Only electrons transferred, to / from the atom - a charged atom is called an ion.
Adding electrons to an uncharged atom - negative atom has more electrons than protons)
Removing electrons from an uncharged atom, makes them positive - atom has fewer electrons than protons.

In a series circuit, the same current passes through each component. In a series circuit, the total potential difference of the power supply is shared between the components.

Cells in Series - As long as the cells act in the same direction - The total potential difference of cells in series is the sum of the potential difference of each cell.

E.G - suppose the current through the bulb is 0.1A when the bulb is dim - the resistance of the bulb would be, 9 ohms = (0.9V/0.1A).

The resistance of a varieble resistor at this setting would be 6 ohms = (0.6V/0.1A. 

The total resistance of two (or more) components in series is equal to the sum of the resistance of each compoment -

Total resistance, R (total. Ohms) = R1 + R2 ...... RN 

Resistance of the first and second component (R1 and R2)

A 4.5V battery is connected to a 1.0 Ohms resistor and a 5.0 Ohms in a series with each other.

Caluclate : (a)  The total resistance (b) The current through the resistors

Solution = (a) Total resistance = 1.0 Ohms + 5.0 Ohms

(b) Current = battery potential difference/total resistance

= 4.5V/6.0 Ohms = 0.75A

The total current through the whole circuit is the sum of the currents through the seperate branches.

For components in parallel, the potential difference across each component is the same.

The bigger the resistance of the component, the smaller the current through it.

I = V/R which  means Current (amperes) = potential difference(Volts)/Component resistance (ohms)

The total resistance of two (or more) components in parallel is less than the resistance of the resistor with the least resistance.

There are three resistors - R=1 Ohm, R = 2 Ohm and R = 6 Ohms - all are connected in parallel to a 6V battery.

Calculate - (a) The current through each resistor (b) The current through the battery

Solutions - (a) 1I = V1/R1 - 6/1 = 6A ------- 2I = 2V/R2 = 6/2 = 3A-------- 3I = V3/R3 = 6/6 = 1A

(b) The total current form the battery = I1 + I2 + I3 = 6A+3A+1A = 10A