# P5- Electric circuits

## Charging an insulator

When you rub two different insulating materials against each other they become electrically charged. This only works for insulated objects - conductors direct the charge flow to earth.

When the materials are rubbed against each other:

• Negatively charged particles called electrons move from one material to the other
• The material that loses electrons becomes positively charged
• The material that gains electrons becomes negatively charged
• Both materials gain an equal amount of charge, but the charges are opposite.
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## Attraction and repulsion

If two charged objects are brought close together they are either attracted towards each other, or they are repelled away from each other. If they have the same type of charge they will repel and push away from each other. Two negative charges placed near each other will repel each other, and so will two positive charges.

If they have opposite charges they will attract each other. When a positive charge and a negative charge are placed near each other they will attract each other.

Electrically charged objects can attract small uncharged objects that are close to them. For example, an electrically charged plastic comb can pick up small pieces of paper.

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## Metals

Metal atoms release some of their electrons, and these electrons are free to move through the structure of the metal. This is why metals are good conductors of electricity. Insulators have no charges free to move, which is why they do not conduct electricity.

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## Electrical circuits

In an electrical circuit, all the components have charges that are free to move. When a circuit is made, the battery causes these free charges to move. They move in a continuous loop around the circuit.

The strength of an electric current is measured in amperes, usually shortened to amps. Amps are a measure of how much electric charge is flowing round the circuit.

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## Size of a current- Battery voltage

The size of a current in a circuit depends on the voltage of the battery and the resistance of the components in the circuit.

Voltage can be thought of as the 'push' it exerts on charges in the circuit. A bigger voltage means a bigger 'push', resulting in a larger current.

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## Resistance and Ohm's Law

An electric current flows when charged particles called electrons move through a conductor. The moving electrons can collide with the atoms of the conductor. This is called resistance and it makes it harder for current to flow. These collisions make the conductor hot. It is this that makes a lamp filament hot enough to glow.

Resistance is measured in ohms, Ω. The greater the number of ohms, the greater the resistance.

The equation below shows the relationship between resistance, voltage and current:

Resistance = voltage / current

Ohms (Ω) = volts (V) / amperes (A)

The current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor. So, if you double the voltage, the current also doubles. This is called Ohm's Law. The graph shows what happens to the current and voltage when a resistor follows Ohm's Law.

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## Resistance in a series circuit

When components are connected in series, their total resistance is the sum of their individual resistances. For example, if a 2 Ω resistor, a 1 Ω resistor and a 3 Ω resistor are connected side by side, their total resistance is 2 + 1 + 3 = 6 Ω.

If you increase the number of lamps in a series circuit, the total resistance will increase and less current will flow.

## Variable resistors

The resistance in a circuit can also be altered using variable resistors. For example, these components may be used in dimmer switches, or to control the volume of a CD player. Adjust the resistance in the simulation by clicking the + and - buttons to see the effect on the current.

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## Thermistors

Thermistors are used as temperature sensors, for example, in fire alarms. Their resistance decreases as the temperature increases:

• At low temperatures, the resistance of a thermistor is high and little current can flow through them
• At high temperatures, the resistance of a thermistor is low and more current can flow through them

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## LDR

LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:

• In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it
• In bright light, the resistance of an LDR is low and more current can flow through it

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## Circuit Diagram

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. To check for a short circuit, see if you can find a way past the lamp without going through any other component. If you can, then there is a short circuit and the lamp will not light.

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## Series connections

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

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

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## Parallel connections

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

Because a parallel circuit has more paths for charges to flow along, the current is bigger, and the resistance of the whole circuit is smaller.

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## Current

A current flows when an electric charge moves around a circuit. No charge can flow if the circuit is broken, for example, when a switch is open.

• Current is measured in amperes
• Amperes is often abbreviated to amps or A
• Current flowing in a component in a circuit is measured using an ammeter
• The ammeter must be connected in series with the component
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## Potential Difference

A 'potential difference' across an electrical component is needed to make a current flow in it. Cells or batteries often provide the potential difference needed.

'Potential difference' is often called 'voltage'.

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

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## Energy in circuits

The potential difference between any two points in a circuit is the energy transferred to, or from, a given amount of charge as it passes between those points. In the circuit on the previous card, the charges gain energy in the cell, and then transfer that same amount of energy into light and heat in the lamp. That is why the potential difference across the cell is the same as the potential difference across the lamp; it is the same amount of energy.

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## Potential difference- with added cells

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 1.5V cells are connected in series in the same direction, the total potential difference is 3.0 V. If two 1.5 V cells are connected in series, but in opposite directions, the total potential difference is 0 V, so no current will flow.

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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.

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## Series circuits

When two or more components are connected in series, the same current flows in each component. Check your understanding of this by answering the questions about the circuit below.

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.

Two identical resistors connected in series will share the potential difference. They will get half each. For example if two identical resistors are connected in series to a 3 V cell then the potential difference across each of them is 1.5 V.

If resistors connected in series are not the same then the potential difference is larger across the larger of the resistors.

For example if a 2 Ω and a 1 Ω resistor are connected in series to a cell of 3V, then the potential difference across the 2 Ω resistor will be 2 V, and across the 1 Ω resistor will be 1 V.

If there is a change in the resistance of one component then this will result in a change in the potential differences across all of the other components in the circuit.

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## Parallel Circuits

When two or more components are connected in parallel, the total current flowing in the circuit is shared between the components.

When two or more components are connected in parallel, the potential difference across them is the same. This means that if a voltage across a lamp is 12 V, the voltage across another lamp connected in parallel is also 12 V.

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## Series circuits

The potential differences across resistors in series must add up to the battery voltage. This is because the total energy transferred by the battery must equal the amount of energy transferred to the other components in the circuit. Energy is always conserved.

The energy is transferred from the cell to the electric charge moving through the circuit. The charge then transfers energy to the components (bulbs, resistors, etc).

More energy is transferred by charge flowing through a larger resistance than through a smaller one.

This is why a large and a small resistor connected in series have different voltages across them. The large resistance has a larger voltage because more energy is being transferred as the charge flows through it.

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## Parallel Circuits

In parallel circuits, the voltage across each component is the same as the voltage of the battery. Each component in parallel has the same current as it would have if it were connected to the battery without the other components present.

This means that a higher resistance in parallel with a smaller resistance would have less current in it, as the same voltage will cause less current in a larger resistance than in a smaller one.

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## Electromagnetic induction

A voltage is produced when a magnet is moving into a coil of wire. This process is called electromagnetic induction. The direction of the induced voltage is reversed when the magnet is moved out of the coil again. It can also be reversed if the other pole of the magnet is moved into the coil.

If the coil is part of a complete circuit then a current will be induced in the circuit

Notice that no voltage is induced when the magnet is still, even if it is inside the coil.

To increase the induced voltage:

• Move the magnet faster
• Use a stronger magnet
• Increase the number of turns on the coil
• Increase the area of the coile coil is part of a complete circuit then a current will be induced in the circuit.
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## Generators

It is not practical to generate large amounts of electricity by passing a magnet in and out of a coil of wire. Instead, generators induce a current by spinning a coil of wire inside a magnetic field, or by spinning a magnet inside a coil of wire. As this happens, a potential difference is produced between the ends of the coil, which causes a current to flow.

One simple example of a generator is the bicycle dynamo. The dynamo has a wheel that touches the back tyre. As the bicycle moves, the wheel turns a magnet inside a coil. This induces enough electricity to run the bicycle's lights.

The faster the bicycle moves, the greater the induced current and the brighter the lights.

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## Making AC electricity

When a wire is moved in the magnetic field of a generator, the movement, magnetic field and current are all at right angles to each other. If the wire is moved in the opposite direction, the induced current also moves in the opposite direction.

Remember that one side of a coil in a generator moves up during one half turn, and then down during the next half turn.

This means that as a coil is rotated in a magnetic field, the induced current reverses direction every half turn. This is called alternating current (AC).

It is different from the direct current (DC) produced by a battery, which is always in the same direction.

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## Continued

In practical generators, the coil is fixed, and mounted outside the magnet, and it is the magnet which moves.

The size of the induced voltage can be increased by:

• Rotating the coil or magnet faster
• Using a magnet with a stronger magnetic field
• Having more turns of wire in the coil
• Having an iron core inside the coil

The mains electricity is an AC supply. The voltage it supplies to our homes is 230V.

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## Transformers

A transformer is an electrical device that changes the voltage of an AC 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.
• Step-down transformers are used in mains adapters and rechargers for mobile phones and CD players.
• Transformers do not work with DC supplies.

A transformer consists of a pair of coils wound on an iron core. The AC in one coil produces a changing magnetic field. This changing magnetic field induces a voltage in the other coil of the transformer.

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## Transformers

A transformer needs an alternating current that will create a changing magnetic field. A changing magnetic field also induces a changing voltage in a coil. This is the basis of how a transformer works:

• The primary coil is connected to an AC supply
• An alternating current passes through a primary coil wrapped around a soft iron core
• The changing current produces a changing magnetic field
• This induces an alternating voltage in the secondary coil
• This induces an alternating current (AC) in the circuit connected to the secondary coil

It's important to know that:

• There is no electrical connection between the primary and the secondary coils.
• Transformers only work if AC is supplied to the primary coil. If DC was supplied, there would be no current in the secondary coil.
• As the current in the primary coil increases steadily or decreases steadily, there is a constant voltage induced in the secondary coil.
• As the voltage in the primary coil reaches maximum strength the voltage induced in the secondary coil is at its weakest (zero volts).
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## Calculating voltages

The ratio between the voltages in the coils is the same as the ratio of the number of turns in the coils.

Primary voltage / secondary voltage = turns on primary / turns on secondary

This can also be written as:

Vp/Vs = Np/Ns

Step-up transformers have more turns on the secondary coil than they do on the primary coil.

Step-down transformers have fewer turns on the secondary coil than they do on the primary coil.

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## Magnetic forces

A current-carrying wire or coil can exert a force on a permanent magnet. The wire could also exert a force on another nearby current-carrying wire or coil.

If the current-carrying wire is placed in a magnetic field (whose lines of force are at right angles to the wire) then it will experience a force at right angles to both the current direction and the magnetic field lines.

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## The electric motor

A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves.

The direction of the current must be reversed every half turn, otherwise the coil comes to a halt again. This is achieved using a conducting ring split in two, called a split ring or commutator. A coil of wire is used with lots of turns to increase the effect of the magnetic field.

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