P4 Electric Circuits
Electrical Charges and Fields
Insulating materials that become positively charged when rubbed lose electrons
Insulating materials that become negatively charged when rubbed gain electrons
Electrons are transferred when objects become charges
Like charges repel, unlike charges attract
Some insulating materials become charged when rubbed together
The force between two charged objects is a non-contact force
Current and Charge
A battery consists of two or more cells connected together
The size of an electric current is the rate of flow of charge
A circuit diagram shows how components are connected together
Electric current = Charge / Time
Every component has its own agree symbol
Potential Difference and Resistance
Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential differences across the resistor
Reversing the potential difference across a resistor reverses the current through it
Resistance = Potential Difference / Current
Potential Difference = Energy Transferred / Charge
Component Characteristics
Diode: forward resistance low; reverse resistance high
A thermistor's resistance decreases if its temperature increases
A filament lamp's resistance increases if the filament's temperature increases
An LDR's resistance decreases if the light intensity on it increases
Resistance = Potential Difference / Current
Series Circuits
Adding more resistors in series increases the total resistance because the current through the resistors is reduced and the total P.D across them is unchanged
R total = R1 + R2 ....
For cells in series, acting in the same direction, the total potential difference is the sum of their individual potential differences
For a component in series:
The total potential difference is shared between the components
Adding their resistance gives the total resistance
The current is the same in each component
Parallel Circuits
The bigger the resistance of a component, the smaller the current that will pass through that component
Current = Potential Difference / Resistance
For components in parallel:
Adding more resistors in parallel decreases the total resistance because the total current through the resistors is increased and the P.D. across them is unchanged
The potential difference across each component is the same
The total current is the sum of the currents through the separate components