Physics AQA Unit 2 Notes

Summary of Unit 2 notes, for the AQA exam.

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  • Created by: Bexie
  • Created on: 28-05-12 16:17

Static Electricity

When two objects are rubbed together, the electrons move from one object to another. This causes one object to have an overall positive charge and the other to have a negative charge.

Like Charges Repel. Unlike Charges Attract. Neutral Charges are attracted to both.

If an object becomes highly charged then the potential difference between the object and the ground increases and the object will discharge. A spark is formed, which is the electrons jumping from the object to earthed conductor.

Static Electricity is used in Smoke Precipitators (particles cross through a grid and given a negative charge. This means they attract to positively charged plates on the side of the chimney, where they can then be collected). And photocopiers (image of the document is projected on to positively charged copying plate, electrical charge leaks away when light is reflected onto it. The negatively charged black toner particles are attracted to the remaining positive areas. Heated paper makes the toner stick to it.)

Static electricity can be dangerous. E.g. A charge can build up in fuel tankers, due to the friction of tyres. To ensure safety, tankers are earthed before opened.

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Circuits

We need to know the symbols in a circuit. (Including the less common ones. Such as a Diode, Light-Dependent Resistor, Thermistor and Variable Resistor). We should be able to recognise and draw these.

Current (measured in Amps) = The rate of flow of electrical charges or electrons.

Resistance (measured in Ohms) = Something that apposes the Current.

Voltage (measured in Volts) = Is the 'push' given to electrons in a circuit.

In a series circuit;

  • Total resistance is the sum of the resistance of each component in the circuit.
  • The current is the same all the way through each component.
  • The voltage is shared equally between ech component in the circuit.

In a parallel circuit;

  • The voltage is the same across each component.
  • The total current through the circuit is the sum of the current through each component.
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Power

Electrical devices use different amounts of power (measured in Watts). Power is the amount of energy transformed by the device every second. It is calculated in one of two ways;

  • Power = Energy Tranformed (J) / Time (s)
  • Power = Potential Difference (V) x Charge (C)

Current is the amount of charges (measured in Coulombs) that flow every second, it is represented by this equation;

Charge (C) = Current (A) x Time (s)

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Mains electricity and Safety

Direct Current = Circuits powered by cells/batteries where current flows in one direction.

Alternating Current = Current changes direction (moving back and forth in the circuit).

From an oscilloscope trace we can determine the period and frequency of the Alternating Current. In UK supplies, it is 230 volts with a frequency of 50 cycles per second.

The period is the length of time for one complete wave to pass. When we have worked out the period for one wave, we are able to work out frequency. Using the equation;

Frequency = 1/Period (Time)

Electrical cables are composed of a copper wire surrounded by a plastic insulator. The three pin plug consists of three separate wires; Neutral Wire (Blue), Earth Wire (Green and Yellow) and Live Wire (Brown).

The voltage of live wire alternates between positive and negative. Neutral wire remains close to zero. The earth pin and fuse is used for safety, by conducting the electricity to the ground (an earthed conductor).

Fuses have different current ratings. The fuse will blow is the current is too high.

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Atoms 1

Plum Pudding Theory; It was once believed that the atom was a positive fluid (the pudding) with electrons dotted inside it (the plums).

This was later disproved by Rutherford and Marsden. They did this by firing alpha particles (positively charged) at a gold leaf and observing the angles at which they got reflected. What they should have seen, was the particles passing straight through, however a number of particles got deflected at different angles.

Conclusion = The atom was mostly empty space with a small, positively charged nucleus in the centre. Electrons orbited this.

Atoms contain protons, neutrons and electrons. Nucleus is made up of protons and neutrons. All atoms of same element have the same number of protons. Some elements have same number of protons but different number of neutrons, these are called isotopes.

Protons have a relative mass of one. And a positive relative charge. (P for Positive)

Neutrons have a relative mass of one. And no charge. (N for Neutral)

Electrons have a very small relative mass. And a negative relative charge.

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Atoms 2

Ions = Atoms that have gained or lost an electron.

Mass Number = Number of Protons and Neutrons an element has. (Top number on periodic table)

Atomic Number = Number of Protons an element has. (Bottom number on periodic table)

In electrically neutral atoms. The number of protons = The number of electrons.

Isotopes with an unstable nucleus, emit radiation or decay. Two forms are beta and alpha radiation.

Beta decay is when a beta particle (an electron) is emitted from an atom.

Alpha decay is when an alpha particle (a helium nucleus) is emitted from an atom.

Background radiation comes from space in the form of cosmic rays.

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Nuclear Fission and Fusion

Fusion is where two atomic nuclei join together to form a larger one. When this occurs, energy is released. It is by this process that stars get their energy.

Fission is the opposite. It is the splitting of an atomic nuclei and it is the process nuclear power plants use. Most common fission materials are; Uranium 235 and Plutonium 239.

In order for fission to occur, the atomic nucleus must absorb a neutron. The neutron is fired at the nucleus, which causes the nucleus or split, forming two smaller nuclei. When the splitting occurs, 2 or 3 more neutrons are released, as well as energy. These neutrons are then absorbed by other nuclei causing the process to repeat (a chain reaction).

Control Rods are used to control the reaction, by absorbing neutrons if the reaction needs to be slowed down.

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Distance-Time and Velocity-Time graphs.

Distance-time graphs tell us how an objects distance is changing over time.
If the is a smooth slope on your graph, the object is moving at a constant speed.
If there is a flat line on your graph, then the object isn't moving.
A steeper slope means a faster speed.
If the slope is downwards, the object is returning to the starting position.
An upwards curve means the object is accelerating, downwards curve is decelerating.

Speed = Distance / Time

Velocity-time graph tells us how an objects velocity changes over time (Acceleration).
Acceleration = Change in Velocity / Time Taken
If there is a smooth slop on our graph, the object is accelerating. The steeper the slope, the larger the acceleration.
If there is a flat line, the object is moving at a constant speed.
Downwards slope is when the object is decelerating.
The area under the graph, tells us the distance travelled.

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Forces

Forces act in pairs. When two forces interact they are equal and in the opposite direction.

Weight is also a force measured in Newtons and is calculated by;

Weight (N) = Mass (kg) x Gravitational field strength (N/kg)

The gravitational field strength on Earth is 10N/kg.

A resultant force is he sum of forces acting on an object.

Balanced forces occur when an object is stationary or moving at a constant speed.

For falling objects, when they are balanced, the velocity at which the object is falling is called the terminal velocity. This is true for objects falling through gases and liquids. For a falling object, force is given by;

Resultant Force (N) = Mass (kg) x Acceleration (m/s2)

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Force and Energy

When a force acts upon an object, energy is tranferred and work is done.

The amount of work done = The amount of energy transferred.

Work Done (J) = Force applied (N) x Distance moved (m)

Work done against a surface with friction, results in the energy transforming to heat.

Kinetic energy is movement energy and is worked out as;

Kinetic Energy (J) = 1/2 x Mass (kg) x Velocity2 (m/s2)

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Momentum

Mimentum (kg m/s) describes how much motion an object has. Like velocity, momentum has magnitude acting in a certain direction.

Momentum (kg m/s) = Mass (kg) x Velocity (m/s)

In all situations, momentum is conserved, providing there are no external forces acting.

For collisions, the momentum before the collision = momentum after the collision.

If you apply a force to an object for a certain time, you ae giving an object momentum.

Force (N) = Change in Momentum (kg m/s) / Time Taken for Change (s)

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Car Safety

Momentum is taken into consideration when designing car safety features.

If the car is able to come to a much slower stop (longer time), then the force will be reduced then compared to a sudden stop. Some of the safety features cars have, for this purpose, include; Crumple Zones, Air Bags and Seat Belts.

How quickly a car can stop, depends on the car and driver.

Stopping Distance = Thinking Distance (Drivers Reactions) + Braking Distance (Car and Road conditions)

Thinking Distance is increased if;

  • Driver is tired
  • Driver has been drinking alcohol/taking drugs.

Braking Distance depends on;

  • Road Surface (e.g. Icy roads)
  • Weather Conditions (e.g. Fog, Rain)
  • How well car responds (e.g. Broken brakes)
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Comments

Aida Abdul-Raheem

Very helpful, thank you :)

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