# Physics

Revision Cards for Physics

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What is static electricity?

The build up of electric charge

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Uses of static electricity:

- Photocopiers and Laser Printers

- Spray-painting cars

- Filtering factory smoke

- Heart defibrillators

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What causes charge imbalance?

Loss of electrons (which creates a positive charge) or a gain of electrons (for a negative charge). These electrons are added and removed by the atom being rubbed against other materials.

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Opposite charges attract/repel.

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Conductors hold onto their electrons tightly, whereas insulators hold onto them more loosely.

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The coulomb is the unit of charge.

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Charge flowing through a resistor transfers energy to the resistor.

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Current is a flow of electrical charge around a circuit. It is measured with an ammeter that is connected in a series circuit with the component. The unit of current is amps (A).

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Potential difference or voltage is a measure of the difference in electric potential energy per unit charge between two charged objects. It is measured with a voltmeter that is connected in a parallel circuit with the component. The unit of voltage is volts (V).

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When voltage doubles/triples, current also doubles/triples.

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- At a constant temperature; the voltage and current are directly proportional. We say it follows ohm's law.

- With a filament lamp; this heats up as the I (current) passes through it. This causes R (resistance) to increase. The I and V are not directly proportional.

- With a diode; this component has a very high R in one direction. This means that I can only flow in the other direction. Diodes are used to regulate V in circuits and to make logic gates.

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The resistance of a wire depends on several factors:

- Material

- Length

- Thickness

- Temperature

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- Current stays the same throughout a series circuit, however many components you have

- Current is shared between each component connected in a parallel circuit.

- Voltage is shared between the components in a series circuit.

- When two or more components are connected in a parallel circuit, the voltage across them is the same.

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Fuses are in plugs to prevent electric shocks. Electric shocks are caused by surges in current. If the current in a circuit gets too big, the fuse wire heats up and breaks, which causes the circuit to break and therefore prevents an electric shock.

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The power of an appliance (in Watts, W), is the energy it transforms per second.

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Alternating current is found in mains electricity, and flows back and forth continuously. The number of complete cycles per second is called the frequency, for mains electricity this is 50 Hz.

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Direct current is found in batteries, and only flows in one direction.

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The area under the line in a velocity-time graph represents the distance travelled.

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What is a resultant force?

An object may have several different forces acting on it, which can have different strengths and directions. They can be added together to give the resultant force. This is a single force that has the same effect on the object as all the individual forces acting together.

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What is friction?

Force caused by two surfaces rubbing against each other, and opposes the forward motion of an object. Friction also causes heat between the surfaces.

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What factors affect the acceleration of an object?

- Air resistance

- Mass

- Resultant  force

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What factors affect the air resistance of a mains object?

- Shape

- Speed

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A moving object decelerates when a force acts on it in the opposite direction to its forward thrust.

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The greater the mass of an object, the less its acceleration when a force acts on it.

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The velocity of a moving object increases when a force acts on it in the same direction.

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What factors affect stopping distance of a car?

- Tiredness

- Alcohol

- Drugs

- Speed of car

- Poorly maintained car

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What is thinking distance?

The thinking distance is the distance travelled by the vehicle in the time it takes the driver to react

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What is breaking distance?

The breaking distance is the distance travelled by the vehicle under the breaking force.

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What is the value of the Earth's gravitational field strength?

Is the force of gravity on an object of mass 1 kg (10 N/kg)

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What is mass?

A measure of how much 'stuff' is in an object (never changes).

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What is weight?

A force acting on that 'stuff' (changes).

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What are the three stages of a falling object?

1. Accelerating (no air resistance).

2. Still Accelerating (but now has air resistance).

3. Terminal Velocity (constant speed as gravity and air resistant forces are equal).

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What is terminal velocity?

- The velocity reached by an object when the drag force on it is equal and opposite to the force making it move

- The upward and downward force are equal

- The object is not accelerating or decelerating

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When an object is moved by a force, work is done on object by the force (work done=energy transferred)

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Why do nuclei decay?

An unstable nucleus becomes stable by emitting alpha, beta or gamma radiation

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Alpha

- Helium nucleus/nuclei

- 2 protons, 2 neutrons

- When alpha radiation occurs, the atomic number goes down by 2, and the mass number goes down by 4

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Beta

- Electron

- A neutron in the nucleus of an unstable atom spontaneously decays to form a proton and electron

- When beta radiation occurs, the atomic number goes up by 1, and the overall mass number stays the same

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Gamma

- Electromagnetic Wave

- If a nucleus still has energy after releasing alpha and beta particles

- Uncharged and has no mass

- It does not change the number of protons or neutrons in the nucleus

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speed (m/s) = distance travelled (m) / time taken (s)

acceleration (m/s2) = change in velocity (m/s) / time taken (s)

force (N) = mass (kg) x acceleration (m/s2)

weight (N) = mass (kg) x gravitational field strength (N/kg)

work done (J) = force (N) x distance (m)

kinetic energy (J) = 1/2 x mass (kg) x speed2 (m/s)2

momentum (kg m/s) = mass (kg) x velocity (m/s)

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force (N) = change in momentum (kg m/s) / time taken (s)

resistance ( ) = potential difference (V) / current (A)

power (W) = energy (J) / time (s)

power (W) = current (A) x potential difference (V)

charge (c) = current (A) x time (s)

energy (J) = potential difference (V) x charge (c)

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