Electricity - Charge and Current
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charges: positive and negative. Like charges repel and unlike attract. Charge carries energy around a circuit. It gains energy as it moves through the battery and looses energy to the components. Charge is carried by charge carriers, these are electrons in a circuit, charge is fixed in fundemental particles like electrons, total charge must always be conserved. Positively charged particles carry charge as do ionic solutions where the ions are free to move.
Charge, Q, represents the number of electrons we group together (to reduce the very large number) into Coulombs, C
Q = 1 Coulomb = 1C = 6.25 x10^18
Electric current is the rate of flow of charge or the amount of charge per second flowing past a point. It is measured in Amperes,A, which is equivilent to C/s. Current is measured by an ammeter and placed in series in a circuit.
Current = Charge/time I = Q/t
The charge on an electron is: e = 1.6 x10^-19C
Electricity - Potential difference, Voltage and e.
Voltages - Voltage is a measure of the amount of energy a component transfers per unit of charge passing through it
V = energy transferred (J) / Charge (C) V = E / Q
Potential difference - The energy transferred per unit charge from electrical to another form by a component in a circuit
pd(V) = energy transferred (J) / Charge (C) pd = W / Q
Electromotive force - The energy transferred per unit charge converted into electrical energy by the source
emf(V) = energy transferred (J) / Charge (C) emf = E / Q
In a circuit the chemical energy in the cell is converted into electrical creating electromotive force this causes the charged particles in metal wires to be accelerated creatin a current in the circuit.
The electronvolt,eV - A unit of energy that is generally used with sub-atomic particles. If an electron is accelerated by a potential difference of 1V, the energy it will gain is:
E = V x e = 1 x 1.6 x10^-19 so 1eV = 1.6 x 10^-19
Electricity - Mean Drift Velocity
The mean drift velocity is the average extra velocity gained by the electrons when a pd. is applied accross it. (Or the average displacement per second travelled by the electrons accross a wire.) Delocalised electrons in metals move with random thermal motion in a completely random way, when an potential difference is applied to a circuit an electric field is created exerting a force on the electrons causing them to drift in the direction of the force, they still move randomly but tend to drift one way.
For a conductor, the current,I, is given by:
I = nAvq I = electrical current(A) q = charge on each charge carrier(C)
n = number density of charge carriers(m^3) A = cross-sectional area (m^2)
(number per unit volume) v = mean drift velocity (m/s)
Dift velocity = v = I / nAq
In a metal charge carriers are delocalised electrons and n is very high, this means drift velocity must be small even for high currents
Semiconductors have fewer charge carriers so the drift velocity need to be bigger to give the same current. A perfect insulator would have no charge carriers so current could not flow.
Electricity - Resistance and Resistivity
Electrical resistance is the opposition to the flow of current within a conductor. How much current you get from a particular emf depends on the resistance of the component. Resistance is mesured in ohms Ω. Resistance is the result of collisions between charge carriers and the vibrating lattice ions as they drift through the conductor. As a result of these collision the electrical energy is disipated as thermal energy and the component heats up. As the component heats up, the amplitude of the vibration of the lattice ions increases reducing the drift velocity as there are more collisions increasing the resistance
R = V / I
Three things determine resistance: Length of wire (l) - the longer the wire the more difficult it is to make current flow. Area of wire (A) - the wider the wire the easier it is to make current flow. Resistivity of conductor (ρ)- This depends on what the conductor is made from as the structure of the material may make it easy or difficult for charge to flow,
R = ρl / A
Resistivity is a general property of materials that restricts the flow of electric current through the material. Generally the higher the number density of charge carriers the lower the resistivity as I = nqvA a higher n for a given pd gives a higher keyboard.
Electricity - Series and Parallel
As charge flows through a circuit it does not get used up or lost, this is because of the conservation of charge making it impossible to lose charge as current is the flow of charge it makes sense that current can also not get used.
Kirchoff's first law - The total current entering a junction = the total current leaving it
Energy must also be conserved, in electrical circuits, energy is transferred round the circuit. Energy transferred to a unit charge is emf and energy transferred from unit charge is potential difference. In a closed loop these two quantities must be equal
Kirchoff's second law - Total emf around a series circuit = Sum of the pd across each component
Series circuits Parllel circuits
- Vtotal = V1 + V2 + V3 >Itotal = I1 + I2 + I3
- Current is constant >Voltage is constant
- Rtotal + R1 + R2 + R3 >Rtotal = 1/R1 + 1/R2 + 1/R3
Electricity - Current/Potential difference graphs
I-V graphs show how the current through a component changes as the potential difference across it changes. A straight line graph is representational of an ohmic conductor, this is one where the current is directly proportional to the potential difference across it at a constant temperature, this means the resistance is constant. A metallic conductor is ohmic and has the same straight line graph, the steeper the gradient, the smaller the resistance of the conductor.
Electricity - Current/Potential difference graphs(
Filament lamp (1)- The graph for a filament lamp is curved which starts steep then gets shallower as potential difference increases. The filament in a lamp is just a curled up wire but when current flows through it, it lights up but its resistance increases too.
Thermistor (2+3) - A thermistor is a resistor thats resistance is dependent on temperature. Negative Temperature coefficient thermistors are those in which the resistance decreases as the temperature goes up. As the voltage increases so does the current and temperature, this causes the resistance to decrease and more current can flow.
Electricity - Current/Potential difference graphs(
LDR - An LDR is a resistor that is dependent on light, the greater the intensity of light shining on the resistor, the lower its resistance is. The light provides energy that releases more electrons from the atoms which means there are more charge carriers and a lower resistance.
Diodes - Diodes are designed to only let current flow in one direction. Forward bias is the direction in which current is allowed to flow, most diodes require a threshold voltage of about 0.6V in the forward direction before they conduct. In reverse bias the resistance is very high meaning almost no current flows.
Electricity - Semiconductors
Semiconductors have a higher resistivity than metals because they have a lower number density this means they has fewer charge carriers availabe than metals. However if energy is supplied to some type of semicoductor for example by heating, then more charge carriers are released from the atoms, this increases the current and lowers the resistance. This means that semiconductors are very usefull in sensors for detecting changes in the environment.
In a battery chemical energy is used to make electrons move, as they move, these electrons collide with atoms therefore batteries must have a resistance, this is known as internal resistance. The total amount of work the battery does on each coulomb of charge is its electromotive force (e.m.f) e.m.f = W/Q
The potential difference across the resistor in the circuit (Load resistance) is the terminal p.d, this is equal to the e.m.f of the battery minus its internal resistance
e.m.f = V + v e.m.f = I(R+r) e.m.f = V + Ir
V = terminal p.d v = volltage lost to internal resistance I = current R = load resistance r = internal resistance
Series - total = 1 + 2 + 3
Parallel - total = 1 = 2 = 3
Electricity - Potential dividers
In a circuit with two or more sources of resistance, the potential difference across the voltage source for example the battery, is split in the ratio of the resistances. you can ause potential dividers to manipulate the Vout of a component.
If you replace R1 with a variable resistor you can change the value of Vout, as R1 increases Vout decreases
Electricity - Uses of potential dividers
LDR's and thermistors can be used in a circuit to control the voltage out of a circuit at a fixed resistor.
Thermistor - Could be used in an air conditioning unit, as the temperature increases the resistance of the thermistor decreases therefore increasing the Vout at the fixed resistor that would turn on the air con
LDR - Could be used in streetlights, as it gets darker, there is less light energy and the resistance in the LDR increases, Vout would be around the LDR meaning as it got darker the Vout would get larger and turn on the lamp.
A Potentiometer is a variable resistor that changes resistance the further you move the slider accross it. The potentiometer replaces R1 and R2 and the slider changes the relative sizes of R1 and R2 therefore varying Vout. This can be very useful in thing like speakers where the volume and voltage are changed continuously.
Electricity - Energy and Power
Power is the rate of transfer of energy or the rate of doing work. It is measured in Watts where 1 Watt is equivalent to 1 Joule of work done per second.
P = W/t P = VI
This is becuase - P = W / t V = W / Q
----> W = V x Q (Q= I x t )
----> W = V x I x t
----> P = (VIt / t ) = VI
Power dissipated or lost - P = I^2R
Total energy transferred - W = VIt
The efficiency of a motor could be found by adding a voltmeter and ammeter into the circut to discover the power in to the circuit. The useful power out of the circuit could then be calculated and the efficiency by
Efficiency = (Useful power output / total power input) x 100
Working as a physicist 1
Precission - How close the repeats are together
Accuracy - How close to the true value the repeats are
Working as a physicist 2
Mechanics - Scalars and Vectors
Scalars are quantities that are represented only by their magnitude, Vectors are quantities with both magnitude and direction. You can add vectors to find the resultant of them, put the vectors as arrows tip to tail then find the first tail to last tip. Vectors can also be resolved verticallly and horizontally
Mechanics - Motion Graphs
Mechanics - Motion Graphs 2
The gradient of the graph is equal to the acceleration and the area under the graph is equal to the displacement
Mechanics - Newtons Laws
1st Law - An object will maintain its state of motion (stationary or constant velocity) until an external force acts upon it. The forces on the object must be balanced so there is no resultant force. When an object is stationary on a table the reaction force and weight force are equal so the object remains stationary, at terminal velocity, the weight force and drag forces are balanced
2nd Law - The acceleration of an object is proportional to the resultant force acting upon it.
F = ma
3rd Law - Every force has an equal and opposite reaction force, If an object A exerts a force on object B, then object B exerts an equal but opposite force on object A. The law applies to all types of force but the pairs of forces must be >the same type, >act along the same line, >act on two seperate bodies >be equal in magnitude >act in opposite directions
Momentum is the product of mass and velocity of an object, assuming no external forces act, momentum is always conserved - p = mv
Impulse is the change in momentum when a force is applied to an object for a time
I = m(v-u)
Mechanics - Work and power
Work done is the amount of energy transferred from one form to another whenever a force causes a movement.-----> Work done = force x distance moved in the direction of the force
(may have to resolve vectors to find magnitude of force in direction)
Power is the rate of doing work, it is the amount of money transferred per second and is measured in Watts -------> Power = Energy transferred / time P = E / t