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0.10 Electric circuits

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Current

Current is the rate of flow of charge, i.e. the rate at which charges move past a fixed point in a circuit. We use the symbol I to show current and it is measured in amperes (A). One ampere is one coulomb of charge moving in one second. The relationship between current, charge and time is given by:

I = Q Δ t

When current flows in a circuit we show this on a diagram by adding arrows. The arrows show the direction of flow in the circuit. By convention we say that charge flows from the positive end ( or terminal) of a battery, through the circuit, and back to the negative end ( or terminal) of the battery. This is shown in the diagram below. We measure the current with an instrument called an ammeter.

The arrows in this picture show you the direction that charge will flow in the circuit. Note that the arrows point from the positive end of the battery, through the circuit, towards the negative end of the battery.

Interesting fact

Benjamin Franklin made a guess about the direction of charge flow when rubbing smooth wax with rough wool.He thought that the charges flowed from the wax to the wool (i.e. from positive to negative) which was opposite to the realdirection. Due to this, electrons are said to have a negative charge and so objects which Ben Franklin called "negative" (meaning a shortage of charge) really have an excess of electrons. By the time the true direction of electron flow wasdiscovered, the convention of ´positive¡ and ´negative¡ had already been so well accepted in the scientific world that noeffort was made to change it.
A battery does not produce the same amount of current no matter what is connected to it. While the voltageproduced by a battery is constant, the amount of current supplied depends on what is in the circuit.

Exercises: current

Using the relationship between current, charge, and time, calculate the current in a circuit which has 0,8 C of charge passing a point every second.

  1. I = Q Δ t I = {Q} over {%DELTA t}

  2. Given: Q = 0,8 C

    Δ t = 1 s

    Asked for: I

  3. I = 0,8 C 1 s x = 0,8 A

How much charge flows per second in a circuit with a current of 1,5 A ?

  1. Asked for: Charge, Q

    Given: Current I = 1,5 A

    Δ t = 1 s

  2. I = Q Δ t I = {Q} over {%DELTA t}

  3. Q = I × Δ t = 1,5 A × 1 s = 1,5 C

If 500 × 10 3 μC of charge flow past a point in a circuit in 1 second, what is the current in the circuit?

  1. Asked for: current, I

    Given: charge Q = 500 × 10 3 μC
    Δ t = 1 s

  2. Q = 500 × 10 3 μC x = 500 × 10 3 × 10 − 6 C x = 0,5 C

  3. I = Q Δ t

  4. I = Q Δ t x = 0,5 C 1 s x = 0,5 A

I measure the current in a circuit to be 500 mA . How much charge is flowing per second in the circuit?

  1. Asked for: Charge, Q

    Given: Current I = 500 mA ,
    Δ t = 1 s

  2. 500 mA = 500 × 10 − 3 A

  3. I = Q Δ t I = {Q} over {%DELTA t}

  4. Q = I × Δ t x = 500 × 10 − 3 A × 1 s x = 0,5 C

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Source:  OpenStax, Physics - grade 10 [caps 2011]. OpenStax CNX. Jun 14, 2011 Download for free at http://cnx.org/content/col11298/1.3
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