BMAT Physics Knowledge PDF

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BMAT PHYSICS KNOWLEDGE Static electricity Insulating materials Metals are good conductors, which means that electric charges move easily through them. Materials such as plastic, wood, glass and polythene are insulators. This means they do not allow electric charges to move through them. Some insulators can become electrically charged when they're rubbed together.

Charged objects How can you tell if an insulator is charged?

 

If a plastic rod is rubbed with a duster it attracts small pieces of paper. When a balloon is rubbed on a jumper it can stick to a wall. Some dusters are designed to become charged so that they attract dust.

Positive and negative charges Objects can be positively charged, negatively charged or neutral (no charge). A substance that gains electrons becomes negatively charged, while a substance that loses electrons becomes positively charged. When a charged object comes near to another object they will either attract or repel each other.   

If the charges are the same - they repel If the charges are opposite - they attract If one is charged and the other is not - they attract

Higher tier only An atom is made up of two parts - a positively charged nucleus surrounded by negatively charged electrons. In an uncharged (neutral) atom there are the same number of positive and negative charges.

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An atom

When an acetate rod is rubbed with a duster, electrons are transferred from the rod onto the duster, making the rod positively charged.

Problems with static Static is a nuisance when:

 

Dust and dirt is attracted to insulators such as TV screens and computer monitors. Clothes made from synthetic materials often cling to each other and to the body, especially just after they've been in a tumble drier.

Higher tier only Anti-static sprays, liquids and cloths prevent the build up of charge by allowing it to conduct away.

Dangers of static Static is dangerous when:

 

There are inflammable gases or vapours or a high concentration of oxygen. A spark could ignite the gases and cause an explosion. You touch something with a large electric charge on it. The charge will flow through your body causing an electric shock. This could cause burns or even stop your heart. The chance of receiving an electric shock can be reduced if:

  

An object that might become charged is connected to the Earth by an earth wire, so any charge immediately flows down the earth wire. In a factory, machinery operators stand on insulating mats or wear shoes with insulating soles. This stops any charge flowing through them to the Earth. Lorries containing inflammable gases, liquids and powders are connected to the Earth by an earth wire before being unloaded. This means charge immediately flows down the earth wire preventing a spark causing an explosion.

Electric current Electric current is a flow of electric charge. No current can flow if the circuit is broken - for example, when a switch is open.

Measuring current Current is measured in amperes (which is often abbreviated to amps or A). The current flowing through a component in a circuit is measured using an ammeter. This must be connected in series with the component.

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Calculating current The size of an electric current is the rate of flow of electric charge. You can calculate the size of a current using this equation: I=Q÷t

  

I is the current in amperes (amps), A Q is the charge in coulombs, C t is the time in seconds, s



For example, what is the current if 20 C of charge passes in 5 s?



Current = 20 ÷ 5 = 4 A

Potential difference (voltage) A potential difference, also called voltage, across an electrical component is needed to make a current flow through it. Cells or batteries often provide the potential difference needed.

Measuring potential difference Potential difference is measured in volts, V. The potential difference across a component in a circuit is measured using a voltmeter. This must be connected in parallel with the component.

Calculating potential difference The potential difference between two points in an electric circuit is the work done when a coulomb of charge passes between the points. You can calculate the size of a potential difference using this equation: V=W÷Q

  

V is the potential difference in volts, V W is the work done (energy transferred) in joules, J Q is the charge in coulombs, C

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

For example, what is the potential difference if 48 J of energy is transferred when 4 C of charge passes?



Potential difference = 48 ÷ 4 = 12 V

Cells and circuits You should know what happens to the potential difference and current when the number of cells in a circuit is changed.

Potential difference A typical cell produces a potential difference of 1.5 V. When two or more cells are connected in series in a circuit, the total potential difference is the sum of their potential differences. For example, if two 1.5 V cells are connected in series in the same direction, the total potential difference is 3.0 V. If two 1.5V cells are connected in series, but in opposite directions, the total potential difference is 0V, so no current will flow.

Current When more cells are connected in series in a circuit, they produce a bigger potential difference across its components. More current flows through the components as a result.

Series circuits You should know the characteristics of the current and potential difference in series circuits.

Current When two or more components are connected in series, the same current flows through each component.

Potential difference When two or more components are connected in series, the total potential difference of the supply is shared between them. This means that if you add together the voltages across each component connected in series, the total equals the voltage of the power supply.

Circuit symbols You need to be able to draw and interpret circuit diagrams.

Standard symbols The diagram below shows the standard circuit symbols you need to know.

Open Switch

Closed Switch

Lamp

Cell

Battery

Voltmeter

Resistor

Fuse

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Ammeter

Variable resistor

Light emitting diode

Diode

Thermistor

Light dependent resistor (LDR)

(LED)

Circuit diagrams Two things are important for a circuit to work:

 

There must be a complete circuit There must be no short circuits To check for a complete circuit, follow a wire coming out of the battery with your finger. You should be able to go out of the battery, through the lamp, and back to the battery.

Series and parallel connections You should know the difference between series and parallel connections in circuits.

Series connections Components that are connected one after another on the same loop of the circuit are connected in series. The current that flows across each component connected in series is the same.

Two lamps connected in series

The circuit diagram shows a circuit with two lamps connected in series. If one lamp breaks, the other lamp will not light.

Parallel connections Components that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel.

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Two lamps connected in parallel

The circuit diagram shows a circuit with two lamps connected in parallel. If one lamp breaks, the other

Transformers A transformer is an electrical device that changes the voltage of an alternating current (ac) supply, such as the mains electrical supply. A transformer changes a high-voltage supply into a low-voltage one, or vice versa.

 

A transformer that increases the voltage is called a step-up transformer. A transformer that decreases the voltage is called a step-down transformer.

The National Grid Electricity is transferred from power stations to consumers through the wires and cables of the National Grid. When a current flows through a wire some energy is lost as heat. The higher the current, the more heat is lost. To reduce these losses, the National Grid transmits electricity at a low current. This needs a high voltage. Power stations produce electricity at 25,000V. Electricity is sent through the National Grid cables at 400,000V, 275,000V and 132,000V. Step-up transformers are used at power stations to produce the very high voltages needed to transmit electricity through the National Grid power lines. These high voltages are too dangerous to use in the home, so step-down transformers are used locally to reduce the voltage to safe levels. The voltage of household electricity is about 230V.

The cost of using electricity You should be able to calculate the cost of using an electrical appliance when given enough information about it.

The unit The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1kWh. The equation below shows the relationship between energy transferred, power and time: energy transferred (kWh) = power (kW) × time (h) Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1000. For example, 2000W = 2000 ÷ 1000 = 2kW.

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Also note that time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3600. For example, 1800s = 1800 ÷ 3600 = 0.5 hours.

The cost Electricity meters measure the number of units of electricity used in a home or other building. The more units used, the greater the cost. The cost of the electricity used is calculated using this equation: total cost = number of units × cost per unit For example, if 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.

Principle of Transformers A transformer is a device that changes (transforms) and alternating potential difference (voltage) from one value to another value be it smaller or greater using the principle of electromagnetic induction. A transformer consists of a soft iron coil with two coils wound around it which are not connected to one another. These coils can be wound either on separate limbs of the iron core or be arranged on top of each other. The coil to which the alternating voltage is supplied is called the primary coil or primary winding. When an alternating potential difference is supplied the resulting alternating current in the primary coil produces a changing magnetic field around it. This changing field induces an alternating current in the secondary coil. The size of the induced voltage resulting from the induced current in the secondary coil depends on the number of turns in the secondary coil.

The relationship between the voltage and the number of turns in each coil is given by:

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Transformers can be of two types:

Step-up Transformer On a step-up transformer there are more turns on the secondary coil than the primary coil. The induced voltage across the secondary coil is greater than the applied voltage across the primary coil or in other words the voltage has been “stepped-up”.

Step-down Transformer A step down transformer has less turns on the secondary coil that the primary coil. The induced voltage across the secondary coil is less the applied voltage across the primary coil or in other words the voltage is “stepped-down”.

Transformers are very efficient. If it is assumed that a transformer is 100% efficient (and this is a safe assumption as transformers may be up to 99% efficient) then the power in the primary coil has to be equal to the power in the secondary coil, as per the law of conservation of energy.

Power in primary coil = Power in secondary coil Remember, power = potential difference x current

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Thus, Primary coil p.d. x primary coil current = Secondary coil p.d. x secondary coil current VP x IP = VS x IS So if the potential difference is stepped up by a transformer then the current is stepped down by roughly the same ratio. In the case of the potential being stepped down by the transformer then the current is stepped up by the same ratio.

The Motor Effect Before discussing the motor effect it is important to gain an understanding in magnets and magnetic field. Magnets are materials normally with iron in them that produce a magnetic field. They attract other pieces of iron bought close to them with a magnetic force. The region around a magnet where a magnetic effect can be felt is called the magnetic field. A magnet has two poles: 1.

North seeking pole or North Pole

2.

South seeking pole or south Pole

The magnetic field is strongest at its poles. The field around a magnet can be represented by lines with arrows on them. The arrows show the direction of the lines of force. Each field line starts at the North Pole and finishes at the South Pole. Magnets affect a wire conducting electricity; this is because an electric current in a wire produces a magnetic field. If a wire carrying a current is placed in a magnetic field of a magnet it will experience a force due to the interaction between the magnetic field of the magnetic and the magnetic field of the current in the wire. This force the electric wire experiences is called the motor effect and only happens when the wire is carrying electricity. The direction of the lines of force around a wire carry a current can be determined using the Right-hand Grip Rule. If you were to image gripping a wire carrying a Page 9 of 25

current so that your right thumb pointed in the same direction as the flow of electrical current then the fingers of your right hand curl in the direction of the magnetic field lines.

A Simple Electric Motor An electrical motor is a device that converts electrical energy to mechanical energy. It works on the principle of the interactions between the magnetic fields of a permanent magnet and the field generated around a coil conducting electricity. The attractive and repulsive forces between the magnet and the coil create rotational motion. A simple electric motor consists of the following parts. 1.

A permanent magnet

2.

Armature or rotor This consists of a thin copper wire coiled around an iron core, hence when electric current flows it acts as an electromagnet. In the case of a simple motor this is a wire loop.

3.

Commutator A Commutator is a copper ring split in two halves. In a simple electric motor each half is connected to the ends of the wire loop. In practise they are connected to the axle.

4.

Brushes The brushes connect the wire loop or armature to the power supply

5.

Axle In electric motors the commutator is attached to the axle. The axle transfers the rotational motion.

6.

Power supply (battery)

Generating electricity Generators are the devices that transfer kinetic energy into electrical energy.

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Turning generators directly Generators can be turned directly, for example, by:

wind turbines hydroelectric turbines wave and tidal turbines

  

When electricity is generated using wave, wind, tidal or hydroelectric power (HEP) there are two steps:

1. 2.

The turbine turns a generator. Electricity is produced.

Turning generators indirectly Generators can be turned indirectly using fossil or nuclear fuels. The heat from the fuel boils water to make steam, which expands and pushes against the blades of a turbine. The spinning turbine then turns the generator.

Speed, distance and time You should recall from your Key Stage 3 studies how to calculate the speed of an object from the distance travelled and the time taken.

The equation When an object moves in a straight line at a steady speed, you can calculate its speed if you know how far it travels and how long it takes. This equation shows the relationship between speed, distance travelled and time taken:



For example, a car travels 300 metres in 20 seconds.



Its speed is 300 ÷ 20 = 15m/s.

Distance-time graphs You should be able to draw and explain distance-time graphs for objects moving at steady speeds or standing still.

Background information The vertical axis of a distance-time graph is the distance travelled from the start. The horizontal axis is the time from the start.

Features of the graphs When an object is stationary, the line on the graph is horizontal. When an object is moving at a steady speed, the line on the graph is straight, but sloped.

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The diagram shows some typical lines on a distance-time graph.

Distance - time graph

Note that the steeper the line, the greater the speed of the object. The blue line is steeper than the red because it represents an object moving faster than the one represented by the red line. The red lines on the graph represent a typical journey where an object returns to the start again. Notice that the line representing the return journey slopes downwards.

Velocity-time graphs You should be able to explain velocity-time graphs for objects moving with a constant velocity or constant acceleration.

Background information The velocity of an object is its speed in a particular direction. This means that two cars travelling at the same speed, but in opposite directions, have different velocities. The vertical axis of a velocity-time graph is the velocity of the object. The horizontal axis is the time from the start.

Features of the graphs When an object is moving with a constant velocity, the line on the graph is horizontal. When an object is moving with a constant acceleration, the line on the graph is straight, but sloped. The diagram shows some typical lines on a velocity-time graph.

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Speed - time graph

The steeper the line, the greater the acceleration of the object. The blue line is steeper than the red line because it represents an object with a greater acceleration. Notice that a line sloping downwards - with a negative gradient - represents an object with a constant deceleration - slowing down.

Acceleration You should be able to calculate the acceleration of an object from its change in velocity and the time taken.

The equation When an object moves in a straight line with a constant acceleration, you can calculate its acceleration if you know how much its velocity changes and how long this takes. This equation shows the relationship between acceleration, change in velocity and time taken:



For example, a car accelerates in 5s from 25m/s to 35m/s.



Its velocity changes by 35 - 25 = 10m/s.



So its acceleration is 10 Ă· 5 = 2m/s2.

Distance-time graphs The gradient of a line on a distance-time graph represents the speed of the object. Study this distance-time graph.

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Distance - ti...