Electricity 2

    Cards (27)

    • Conservation of energy states that energy cannot be created or destroyed but changed from one form to another.
    • Due to the law of conservation of energy, when a charge, Q, flows through the circuit energy converted by battery = energy dissipated in resistors (Qe = QV1 + QV2), so e = V1 + V2 as V = IR (e = IR1 + IR2).
    • The internal resistance of a cell opposes the flow of charge through the cell. Some of the energy converted by the cell, or power supply will be used up inside the cell to overcome the resistance
    • The difference between the e.m.f of a cell and the potential difference, V is at its terminals is e - V = Ir and is sometimes called the lost volts
    • To find the current of the circuit with internal resistance is through I = e/(R + r)
    • A charge carrier is either an electron detached from its atom or an ion in a conducting material that is free to flow in order to create a current
    • The network of resistors has divided the pd, which this arrangement is described as a potential divider
    • Pd has been divided in the ratio of the resistances (1:3). The resistance of the load should be at 10x greater then the output resistances
    • I = V2 / R2 and I = V/R1 + R2. So, V2 / R2 = V / R1 + R2. So V2 = V x R2 / R1 + R2 and similarly V1 = V x R1 / R1 + R2
    • A three-terminal resistor used as a potential divider is called a potentiometer. A potentiometer such as that it is a rheostat bent to complete a circuit. It has a fixed contact at each and a rotating arm that forces the sliding contact
    • Rheostat controls current
    • A potentiometer controls p.d
    • A thermistor can be used in a potential divider circuit to control the output the voltage
    • If the temperature rises, the resistance R1 of the thermistor will decrease and so V out will decrease
    • If R1 decreases the circuit resistance will get less and so the circuit current will increase. As V out = IR + V out will increase
    • An ohmmeter is used to measure the resistance R1 of the thermistor at room temperature. The resistor is selected such that its resistance R2 = R1 / 2. The output voltage V out for temperatures ranging from room temperature
    • A light-dependent resistor (LDR) is a component that is sensitive to light when light is incident on an LDR the value of its resistance decreases as the level of light increases. An LDR is made from a semiconductor material with a high resistance such as sulfide. It has a high resistance because there are very few electrons that are free and able to conduct the vast majority of the electrons are loaded into a crystal lattice and unable to move.
    • As light falls on the semiconductor, the light photons are absorbed by the semiconductor lattice, and their energy is transferred to the electrons. This gives some of the electrons sufficient energy to break free from equation I = nAvq, the number of conducting electrons per cubic metre increases as light shines on the LDR.
    • This results in a lowering of the resistance of the semiconductor and hence the overall LDR resistance. The process is progressive, as more light shines on the LDR semiconductor, more electrons are released to conduct electricity and resistance falls further. The resistance of an LDR can be several M ohms in darkness and fall to a few hundred ohms in bright light.
    • An LDR can be used in a potential divider circuit to control the output voltage. V out = V in x R2 / R1 x R2
    • If light incident on the LDR falls the resistance R2 if the LDR will increase and so V out will increase. This can be used to activate a switch to turn on a light when it gets dark (and off again when it gets light again)
    • I = V/R but in parallel circuit: I = I1 + I2 + I3. So V/R = V/R1 + V/R2 + V/R3. As V is the same across each resistor, in parallel: 1/R = 1/R1 + 1/R2 + 1/R3
    • Battery with emf and internal resistance
    • When light strikes the photocell, it gives some of its energy to free electrons in the semiconductor material of the cell (silicon)
    • Multimeter
    • Photocell
    • LDR