- Created by: sugarpie1080
- Created on: 27-12-18 15:25
Ohm's Law: At a constant temperature, the current through a conductor is proportional to the p.d. applied across it
- Filament Lamp
- As the temperature increases, the resistance increases, as metals have a high resistance when hot. Higher resistance causes a lower gradient
Ohm's Law 2/
- Has a low resistance when 'forward biased', i.e. when v > 0.6. But when v < 0.6, the resistance becomes so high it is virtually non-conducting.
- 0.6V = threshold voltage
Ohm's Law 3
- Shown is a NTC thermistor, with the resistance decreasing at it gets hotter.
- This is due to a semi-conducting material, whose atoms release more conduction electrons as the temperature rises
Quantity Series Rules Parallel Rules Current (I) Current is the same throughout (I1 = I2 = I3) Itotal = I1 +I2 + I3 Voltage (V) Vtotal = V1 +V2 + V3 Voltage is the same throughout (V1 = V2 = V3) Resistance (R) Rtotal = R1 +R2 + R3 1/Rtotal = 1/R1 +1/R2 + 1/R3
- Sum of the currents entering a junction = 0
- In a closed loop the emf sum = sum of p.ds (IR)
- It is a property of materials that doesn't vary with the size, unlike resistance
- R = pl/A
- Experiment: Measuring wires of different lengths to plot a graph to find resistivity
- Using a screw-gauge to measure cross-sectional area
- Graph with length on x-axis, and resistance on y-axis, gradient p/A
- You could also vary the area instead, with the current on the x-axis, and gradient pl
I = nqva
- We use the drift velocity (v) as charged particles do not travel in a straight line through a conductor, because they collide with other particles in the material
- n = carrier density, how many charge carriers in the material
- In metals, it is very large, which makes them good conductors as they have little resistivity
- Insulators have almost none, meaning they have very high resistivity
- Semiconductors have low amounts, meaning they have higher resistivity than metals
- LDRs are semiconductors, therefore is there is more light energy, more charge carriers are freed.
- As the temperature in an NTC thermistors, the charge carriers also increases. The lattice vibrations here have a very minimal effect.
- If the temperature is increased in a metal, there is an increase in lattice vibration, which reduces the drift velocity. Therefore the resistivity increases with temperature.
E.M.F and Internal Resistance
- E = V+ Ir
- E = emf (total voltage in a cell)
- V = voltage of the circuit
- I = current
- r = internal resistance
- When a cell is connected and current is drawn, some of the energy is dissipated within the cell itself.
Potential Divider Circuits
- Potential divider circuit:
- I = Vin / (R1 + R2)
- I = Vout / R1
- Vout = Vin x R1 / (R1 + R2)
- Semiconductors start with a low amount of charge carriers, which are free electrons
- As the energy increases, the amount of charge carriers increases, due to the electrons passing amount of energy needed to break from the other atoms.
- This means that the current increases, therefore the resistivity decreases